Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Khorsand Kazemi, Kasra
Master of Applied Science - MASc
University of British Columbia
Hazardous aqueous liquids and Volatile Organic Compounds (VOC)s could cause severe health and environmental problems. Long exposure to hazardous aqueous liquids and VOCs could cause skin, lung, liver and kidney problems and cancers. Thus, to prevent these hazards, working personnel require using personal protective equipment (PPE) in hazardous industrial environments. Lately, detecting VOCs and hazardous solutions in an environment and on protective suits has become of the highest importance for personnel safety. Previously, wireless microwave technology has demonstrated its application in VOCs and hazardous liquids sensing, owing to its ease of fabrication, low cost, and real-time response. These features highlight wireless microwave sensors as a promising candidate for use in sectors where human intervention is limited. Nonetheless, there is little evidence on developing hazardous solutions and VOC sensing antenna structures. This thesis presents an innovative approach using antennas to sense hazardous aqueous liquids and VOCs. Two planar antenna sensors were designed, fabricated and analyzed for hazardous solutions and VOC sensing applications. The initial studies were performed to evaluate the performance of an antenna sensor in detecting hazardous aqueous liquids on the surface of a superhydrophobic fabric, representing a PPE, where the designed antenna sensor operated at the frequency of 3.2 GHz. Water droplets replicating hazardous aqueous liquids were drop-cast on the fabrics to investigate the fabric-embedded antenna sensor’s performance, while a secondary transceiver antenna communicated with the fabric-embedded antenna sensor. It was observed that the liquid’s presence on the PPE caused a frequency shift in the transmitted signal where this shift in the frequency was used as a criterion for the sensing application. Additionally, a MXene-based antenna sensor was developed and investigated for wireless VOC and humidity sensing applications. The MXene-based antenna sensor sensed different VOCs (e.g., Acetone, Isopropyl alcohol, Ethanol, and Methanol) and their concentrations. Furthermore, a receiver antenna was located at a distance of 1.4 meters to communicate with the MXene-based antenna sensor and collect sensing data. It was observed that a change in the humidity or VOC concentration causes a frequency shift in the antenna sensor’s response. The results of these studies demonstrated the efficiency of using antenna sensors for detecting the presence of hazardous aqueous liquids on the superhydrophobic fabric and MXene-based antenna sensors in detecting VOCs in hazardous environments.
An investigation on copper-based andMXene-based planar antenna sensorsfor hazardous substance detectionbyKasra Khorsand KazemiB.Sc, Universtiy of Tabriz, 2019A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE COLLEGE OF GRADUATE STUDIES(Electrical Engineering)THE UNIVERSITY OF BRITISH COLUMBIA(Okanagan)January 2022© Kasra Khorsand Kazemi, 2022The following individuals certify that they have read, and recommendto the College of Graduate Studies for acceptance, a thesis/dissertation en-titled:An investigation on copper-based and MXene-based planarantenna sensors for hazardous substance detectionsubmitted by Kasra Khorsand Kazemi in partial fulfilment of the re-quirements of the degree of Master of Applied ScienceDr. Mohammad Hossein Zarifi, School of EngineeringSupervisorDr. Kevin Golovin, University of TorontoSupervisory Committee MemberDr. Shahriar Mirabbasi, Department of Electrical and Computer EngineeringSupervisory Committee MemberDr. Yasmine Abdin, Department of Materials EngineeringUniversity ExamineriiAbstractHazardous aqueous liquids and Volatile Organic Compounds (VOC)scould cause severe health and environmental problems. Long exposure tohazardous aqueous liquids and VOCs could cause skin, lung, liver and kidneyproblems and cancers. Thus, to prevent these hazards, working personnelrequire using personal protective equipment (PPE) in hazardous industrialenvironments. Lately, detecting VOCs and hazardous solutions in an en-vironment and on protective suits has become of the highest importancefor personnel safety. Previously, wireless microwave technology has demon-strated its application in VOCs and hazardous liquids sensing, owing to itsease of fabrication, low cost, and real-time response. These features high-light wireless microwave sensors as a promising candidate for use in sectorswhere human intervention is limited. Nonetheless, there is little evidence ondeveloping hazardous solutions and VOC sensing antenna structures. Thisthesis presents an innovative approach using antennas to sense hazardousaqueous liquids and VOCs. Two planar antenna sensors were designed, fab-ricated and analyzed for hazardous solutions and VOC sensing applications.The initial studies were performed to evaluate the performance of anantenna sensor in detecting hazardous aqueous liquids on the surface of asuperhydrophobic fabric, representing a PPE, where the designed antennasensor operated at the frequency of 3.2 GHz. Water droplets replicating haz-ardous aqueous liquids were drop-cast on the fabrics to investigate the fabric-embedded antenna sensor’s performance, while a secondary transceiver an-tenna communicated with the fabric-embedded antenna sensor. It was ob-served that the liquid’s presence on the PPE caused a frequency shift in thetransmitted signal where this shift in the frequency was used as a criterionfor the sensing application.Additionally, a MXene-based antenna sensor was developed and inves-tigated for wireless VOC and humidity sensing applications. The MXene-based antenna sensor sensed different VOCs (e.g., Acetone, Isopropyl alco-hol, Ethanol, and Methanol) and their concentrations. Furthermore, a re-ceiver antenna was located at a distance of 1.4 meters to communicate withthe MXene-based antenna sensor and collect sensing data. It was observediiiAbstractthat a change in the humidity or VOC concentration causes a frequencyshift in the antenna sensor’s response.The results of these studies demonstrated the efficiency of using antennasensors for detecting the presence of hazardous aqueous liquids on the su-perhydrophobic fabric and MXene-based antenna sensors in detecting VOCsin hazardous environments.ivLay SummaryHazardous substances, including aqueous liquids and VOCs, can belethal since long exposures could result in organ dysfunctions. To mini-mize this, the endangered personnel generally utilize various PPE, includ-ing masks and protective suits. Additional measures, such as detectinghazardous compounds, are necessary to enhance personnel safety. In thisregard, microwave technology has already proved its promising potential forsensing hazardous compounds. Moreover, the wireless operation feature ofmicrowave technology enables simultaneously sensing and transmitting thesensing data to a distanced receiver. This thesis proposes novel, low-cost, ro-bust copper and MXene-based antenna sensor solutions to detect hazardousaqueous liquids on a PPE and various VOCs in an environment. The pro-posed antenna sensor solution can wirelessly communicate and transmit thesensing response to a distanced receiver node. The findings of this thesisindicate that the antenna sensors have an excellent potential for wirelesshazardous substance detection in an environment.vPrefaceThis work was under the guidance and supervision of Prof. MohammadHossein Zarifi at the School of Engineering at The University of BritishColumbia. All copyrighted figures presented in chapters 1, 2 and 3 werereprinted with permission, as indicated in the caption of the figures.Chapter 3 has been published in the following publication:− K. K. Kazemi, T. Zarifi, M. Mohseni, R. Narang, K. Golovin, andM. H. Zarifi, “Smart Superhydrophobic Textiles Utilizing a Long-Range Antenna Sensor for Hazardous Aqueous Droplet Detection plusPrevention,” ACS Appl. Mater. Interfaces, vol. 13, no. 29, pp.34877–34888, Jul. 2021, doi: 10.1021/acsami.1c07880.Chapter 4 is composed of 2 journal articles where the first one is publishedand the second one is under review:− K. K. Kazemi, E. Hosseini, S. Hub, R.Narang, S. Li, M. Arjmand, M.H.Zarifi “MXene Membrane in Planar Microwave Resonant Structuresfor 5G Applications ”Appl.Mater. Today.− K. K. Kazemi, S. Hu, O. Niksan , K. K. Adhikaria,S. Li, M. Arj-mand, M.H. Zarifi “Planar Antenna sensor based on Ti3C2Tx MXenemembrane for VOC and humidity sensing ” Adv. Mater.InterfacesviTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . viiList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . xvDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xviiChapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Thesis Objectives and Aims . . . . . . . . . . . . . . . . . . . 21.3 Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Chapter 2: Literature Review: Wireless Microwave Struc-ture for Sensing Applications . . . . . . . . . . . . 52.1 Dielectric properties and the Microwave sensing principle . . 52.2 Passive wireless resonant sensors . . . . . . . . . . . . . . . . 82.2.1 Wireless communication between the passive wirelessresonant sensor and reader . . . . . . . . . . . . . . . 82.2.2 Dielectric sensing . . . . . . . . . . . . . . . . . . . . . 92.2.3 Strain and rotation sensing . . . . . . . . . . . . . . . 112.3 Antenna sensors . . . . . . . . . . . . . . . . . . . . . . . . . 14viiTABLE OF CONTENTS2.3.1 Dielectric sensing . . . . . . . . . . . . . . . . . . . . 152.3.2 Temperature sensing . . . . . . . . . . . . . . . . . . 172.3.3 Strain and Crack sensing . . . . . . . . . . . . . . . . 182.4 Wearable antenna sensors . . . . . . . . . . . . . . . . . . . . 232.5 Conductive nanomaterials/ polymers-based antennas and an-tenna sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.5.1 Conductive polymers and nanomaterials for fabricat-ing antennas . . . . . . . . . . . . . . . . . . . . . . . 282.5.2 Conductive polymers and nanomaterials for wirelesssensor and antenna sensor applications . . . . . . . . . 302.5.3 Newly emerging MXene nanomaterial for antenna ap-plications . . . . . . . . . . . . . . . . . . . . . . . . . 322.6 Summary of chapter . . . . . . . . . . . . . . . . . . . . . . . 34Chapter 3: Smart superhydrophobic textiles utilizing long-range antenna sensor for hazardous aqueous dropletdetection plus prevention . . . . . . . . . . . . . . 353.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.2 Inset-fed rectangular patch antenna design and operation prin-ciple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.1 Rectangular patch antenna dimensions . . . . . . . . . 383.3 Design, Methodology and Simulations . . . . . . . . . . . . . 403.3.1 Antenna design and the principle of operation . . . . 403.3.2 Patch antenna sensor simulation and optimization . . 423.3.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . 483.3.4 Preparation of superhydrophobic fabrics . . . . . . . . 493.3.5 Wettability analysis, surface characterization, and dura-bility evaluation . . . . . . . . . . . . . . . . . . . . . 503.4 Antenna Sensor Fabrication and Experimental Setup . . . . . 513.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . 533.5.1 Performance of Superhydrophobic Fabrics . . . . . . . 533.5.2 Antenna Performance . . . . . . . . . . . . . . . . . . 553.5.3 Liquid sensing with the antenna sensor . . . . . . . . . 583.5.4 Long-range liquid detection on the treated and un-treated fabric . . . . . . . . . . . . . . . . . . . . . . . 603.5.5 Chapter summary . . . . . . . . . . . . . . . . . . . . 64Chapter 4: MXene-based planar antenna for VOC sensingapplication . . . . . . . . . . . . . . . . . . . . . . . 664.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66viiiTABLE OF CONTENTS4.2 Experimental section for MXene fabrication . . . . . . . . . . 674.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . 674.2.2 Preparation of multilayer Ti3C2Tx . . . . . . . . . . . 674.2.3 Assembly of Ti3C2Tx membrane . . . . . . . . . . . . 684.2.4 Characterization . . . . . . . . . . . . . . . . . . . . . 684.2.5 Characterization of Ti3C2Tx powder and membrane . 684.3 Fabrication of the antenna sensor and experiment setup . . . 694.4 MXene antenna sensor . . . . . . . . . . . . . . . . . . . . . . 714.5 Gas sensing response of the MXene antenna sensor . . . . . . 754.5.1 MXene antenna sensor for Acetone vapour sensing . . 754.5.2 Response of the sensor to different VOC vapours . . . 814.5.3 Sensor’s response to different Humidity levels . . . . . 814.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.7 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . 89Chapter 5: Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 905.1 Conclusion and contributions . . . . . . . . . . . . . . . . . . 905.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . 905.3 Future works . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92ixList of TablesTable 3.1 Static water contact angle of treated fabrics . . . . . . 54Table 3.2 Impact of Water Droplets on the Antenna Sensor’s Re-sponse . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Table 4.1 Different rotations for the MXene membrane on . . . 73Table 4.2 Fitting curve data for different VOC samples . . . . . 85Table 4.3 Fitting curve data for different concentrations of Acetone 85xList of FiguresFigure 2.1 The electromagnetic spectrum and Wave-material in-teraction. . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 2.2 passive wireless sensor and an interrogator antenna . . 9Figure 2.3 Passive wireless microwave sensors for dielectric sensing 10Figure 2.4 Passive wireless microwave sensor for strain sensing . 12Figure 2.5 Passive wireless microwave sensor for rotation sensing 13Figure 2.6 Antenna sensors for dielectric sensing . . . . . . . . . 16Figure 2.7 Antenna sensor for liquid sensing . . . . . . . . . . . 17Figure 2.8 Strain sensing mechanism for antenna sensors . . . . 18Figure 2.9 Crack sensing mechanism for antenna sensors . . . . . 20Figure 2.10 Crack impact on the antenna sensor’s response . . . . 20Figure 2.11 Antenna sensor for crack sensing . . . . . . . . . . . . 21Figure 2.12 Antenna sensor for fatigue crack sensing . . . . . . . 22Figure 2.13 Antenna crack sensor . . . . . . . . . . . . . . . . . . 23Figure 2.14 Liquid metal antenna . . . . . . . . . . . . . . . . . . 25Figure 2.15 Fabric-based antenna sensor . . . . . . . . . . . . . . 27Figure 2.16 Wearable antenna sensor for liquid sensing . . . . . . 28Figure 2.17 Wireless microwave sensor for detecting bacteria onIV bag and tooth . . . . . . . . . . . . . . . . . . . . 31Figure 2.18 flexible antenna sensor for strain sensing . . . . . . . 32Figure 2.19 MXene-based antenna and transmission line . . . . . 34Figure 3.1 Various shapes of patch elements . . . . . . . . . . . 37Figure 3.2 Fringing field effect in a microstrip structure and Mi-crostrip fed patch antenna. . . . . . . . . . . . . . . . 38Figure 3.3 Effective permittivity around a microstrip structure. . 39Figure 3.4 Designed antenna sensor structure . . . . . . . . . . . 41Figure 3.5 Simulation results for the designed antenna sensorstructure. . . . . . . . . . . . . . . . . . . . . . . . . . 41Figure 3.6 Simulation results for different widths of the patchantenna. . . . . . . . . . . . . . . . . . . . . . . . . . 43xiLIST OF FIGURESFigure 3.7 Radiation pattern simulations for the patch with dif-ferent widths. . . . . . . . . . . . . . . . . . . . . . . 43Figure 3.8 Simulation results for different lengths of patch antenna. 44Figure 3.9 Gain pattern simulation results for the patch withdifferent lengths. . . . . . . . . . . . . . . . . . . . . . 44Figure 3.10 Simulation results for different sample locations . . . 45Figure 3.11 E-field concentration on the meander lines, withoutmaterial and with material. . . . . . . . . . . . . . . . 46Figure 3.12 Simulation results for the designed antenna, with andwithout sample on its surface. . . . . . . . . . . . . . 47Figure 3.13 Antenna gain. . . . . . . . . . . . . . . . . . . . . . . 47Figure 3.14 Simulation results for water with conductivity valuesranging between 0 to 4 S/m . . . . . . . . . . . . . . 49Figure 3.15 Schematic of superhydrophobic fabric preparation . . 50Figure 3.16 SEM images of the prepared fabrics . . . . . . . . . . 54Figure 3.17 The washing fastness of the superhydrophobic fabricsand The abrasion resistance of the superhydrophobicfabrics . . . . . . . . . . . . . . . . . . . . . . . . . . 55Figure 3.18 Fabricated antenna sensor . . . . . . . . . . . . . . . 56Figure 3.19 The antenna sensor covered with the fabrics). . . . . 56Figure 3.20 Measured S11 profile of the bare antenna sensor, com-pared to the treated and untreated fabrics. . . . . . . 57Figure 3.21 Antenna sensor’s one port measurement setup. . . . . 59Figure 3.22 Sensitivity comparison on the different regions overthe antenna sensor. . . . . . . . . . . . . . . . . . . . 59Figure 3.23 Comparison of S11 parameters for a bare sensor withdifferent volumes of water. . . . . . . . . . . . . . . . 61Figure 3.24 Comparison of the resonant notch frequency and theresonant amplitude of the antenna sensor covered withtreated and untreated fabrics . . . . . . . . . . . . . . 61Figure 3.25 Setup for wireless liquid detection on fabric. . . . . . 62Figure 3.26 Transmission coefficient (S21) measured results forbare antenna, antenna covered with treated fabricand covered with untreated fabric. . . . . . . . . . . . 63Figure 3.27 The resonant frequency and resonant amplitude re-sponse of the antenna sensor covered by untreatedand treated fabrics, for water volumes deposited overthe sensitive region, ranging from 4 to 20 µL. . . . . . 63xiiLIST OF FIGURESFigure 4.1 XRD pattern, SEM and digital images for the MXenemembrane . . . . . . . . . . . . . . . . . . . . . . . . 69Figure 4.2 XRD pattern, SEM and digital images for the MXenemembrane . . . . . . . . . . . . . . . . . . . . . . . . 70Figure 4.3 The schematic and the dimensions of the MXene an-tenna . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Figure 4.4 The simulation and measurement results of the struc-ture with and without Ti3C2Tx MXene membrane. . 73Figure 4.5 The S11 measurement results for the MXene antenna. 74Figure 4.6 The three-dimensional far-field gain simulation re-sults for the MXene antenna. . . . . . . . . . . . . . . 74Figure 4.7 Normalized gain of the antenna . . . . . . . . . . . . 75Figure 4.8 Gas sensing experiment setup. . . . . . . . . . . . . . 76Figure 4.9 The antenna sensor’s response to Acetone from 0 to440 seconds, showing the adsorption and desorptionof Acetone by the Ti3C2Tx MXene membrane. . . . . 77Figure 4.10 One cycle of gas sensing measurement for the Ace-tone, . . . . . . . . . . . . . . . . . . . . . . . . . . . 78Figure 4.11 The antenna sensor’s resonant frequency response to80 ppt concentration of Acetone gas for 8 cycles ofmeasurement. . . . . . . . . . . . . . . . . . . . . . . 78Figure 4.12 The antenna sensor’s resonant amplitude response to80 ppt concentration of Acetone gas for 8 cycles ofmeasurement. . . . . . . . . . . . . . . . . . . . . . . 79Figure 4.13 The antenna sensor response to different concentra-tions of Acetone gas ranging from 8 to 80 ppt. . . . . 80Figure 4.14 The antenna sensor’s frequency shift for different con-centrations of Acetone inside the gas sensing chamber. 80Figure 4.15 MXene antenna sensor’s response to different VOCs . 82Figure 4.16 MXene antenna sensor response to the presence ofhumidity, performed in a custom-made gas chamberwith 24 hours of time difference between the two ex-periments . . . . . . . . . . . . . . . . . . . . . . . . . 83Figure 4.17 Error bars for MXene antenna sensor’s humidity sens-ing response for the first set and second set of exper-iments. . . . . . . . . . . . . . . . . . . . . . . . . . . 83Figure 4.18 The sensing response of the MXene antenna sensor forIPA and the curve fitting over the measured responses 86xiiiLIST OF FIGURESFigure 4.19 The sensing response of the MXene antenna sensorfor Methanol. and the curve fitting over the measuredresponses . . . . . . . . . . . . . . . . . . . . . . . . . 86Figure 4.20 The sensing response of the MXene antenna sensorfor Ethanol and the curve fitting over the measuredresponses. . . . . . . . . . . . . . . . . . . . . . . . . . 87Figure 4.21 The sensing response of the MXene antenna sensorfor Acetone and the curve fitting over the measuredresponses. . . . . . . . . . . . . . . . . . . . . . . . . . 87Figure 4.22 The sensing response of the MXene antenna sensorfor humidity and the curve fitting over the measuredresponses. . . . . . . . . . . . . . . . . . . . . . . . . . 88Figure 4.23 The sensing response of the MXene antenna sensorand the curve fittings over the measured response fordifferent concentrations of Acetone. . . . . . . . . . . 88xivAcknowledgementsFirst and foremost, I would like to thank my supervisor, Prof. Mo-hammad Hossein Zarifi. Prof. Zarifi not only has been a supervisor butalso a mentor for me. His assistance has not been limited to my researchactivities. Ever since I arrived from Iran, he has been there to help me men-tally and emotionally. His presence was always a motivation throughout mystudy. Prof. Zarifi has taught me how to read, write, and think scientifically.Looking at the past two years, I see that we have achieved so many amazingthings together, in research, publications and the awards. He has given methe confidence to think scientifically and tackle new challenges.I would also like to thank the members of my committee, Prof. KevinGolovin, Prof. Shahriar Mirabbasi, for their outstanding guidance through-out the master’s program. We have achieved many things with Prof. Golovinduring my master’s study.Working on several projects with him was an hon-our for me. He has always been kind and helpful. I must admit that I havemissed him since he left for the University of Toronto. Professor Mirabbasi’spresence as a committee member has been a great honour for me. However,I wished I could have seen him in person, which I think would happen soon.I would like to extend my regards to prof.Pourziad and Prof. Mozaffariat the university of Tabriz. They were the ones to introduce me to research,and without their help, I wouldn’t have been in UBC for my master’s studies.A special thanks to Ali Maleki, who has been my best friend during the pasttwo years. I know that I am going to miss Ali’s presence and his challengingquestions.I would also like to thank the OMEGA lab team for shaping me intothe researcher I am today. I would like to extend my thanks to members ofProf. Johnson’s lab members with whom we shared lab space. I also wouldlike to acknowledge the support of the Natural Sciences and EngineeringResearch Council of Canada (NSERC) Discovery Grant Program, CanadaFoundation for Innovation program, Mitacs and Patriot One technologiesInc. for financial assistance, and CMC Microsystems for the use of theirsoftware and equipment. I would like to thank the Syilx Okanagan Nationfor the use of their unceded territory, the land on which the research wasxvAcknowledgementsconducted.I would like to thank those closest to me. I sincerely thank my parents,Nayer and Ahad, who were there when I needed help. My beloved brother,Kiarash, whom I miss his presence. By finishing this level of study, I try toset the bar high so he can set it even higher. Last but not least, I would liketo finish this acknowledgment with a poem from Saadi Shirazi, the masterof speech:xvi یناج نایم رد وت و دننابز رس رب ھمھ متفگ وت زج ثیدح ھک مدرک دھع فلاخ ھنDedicationTo my Parents Nayer and AhadxviiChapter 1IntroductionThe introduction will aim to describe the motivation and the objectivesthat will be achieved throughout this thesis. The task and the thesis outlinewill then be provided to deliver an overview of the content to come.1.1 MotivationSensors are increasingly becoming prevalent in our everyday life. Dif-ferent devices utilize sensors to support and enhance their performance inresponse to the changes in their surrounding. The growing technologicaldemands in various sectors, ranging from home appliances to industrial anddefence, have led to studying various sensing techniques. Previously, severaldifferent configurations of sensors were introduced, including piezoelectric,Micro-electromechanical systems (MEMS), optical, and microwave sensors,where they have been used for sensing physical parameters and chemicals[1–4]. Microwave sensing technology has shown supremacy over the othersensing methods due to its non-contact, real-time sensing response and com-patibility with CMOS devices [5, 6]. Microwave sensors exploit electromag-netic waves to interact with the materials in their environment; thus, theycan be designed to sense and transmit the sensing signals wirelessly to adistanced reader. Furthermore, the wireless feature of microwave sensingtechnology demonstrates that these sensors would not necessarily requirecabled connections to a readout circuit, reducing the mechanical drift, noise,vibration and spread of contamination through cables.Wireless microwave sensors have attracted significant attention, particu-larly for applications in hazardous and high-risk environments, where humanintervention is limited [7]. For instance, the integrity of oil/gas pipelines hasan essential role in oil/gas production and distribution. These structures aregenerally located in hard-to-access locations. Therefore, wireless microwavesensors could be utilized to sense cracks and defects in oil gas pipelines froma safe distance [8, 9]. Moreover, wireless microwave sensors embedded in thepersonal protective equipment (PPE) can benefit people working in high-risk environments, e.g. industrial areas and mines, by detecting hazards11.2. Thesis Objectives and Aimsand alerting users or the nearby supervisory units. Wireless microwave sen-sors have also been designed for wearable on-body sensing applications tomonitor physical and physiological signals such as pulse and motion. Pre-vious studies have introduced skin-mimicking and fabric embedded sensorsto track patients’ health conditions and transmit the sensing data wire-lessly in short distances [10, 11]. Despite the progress in developing wirelessmicrowave sensors, there are limited evidences on developing wearable haz-ardous liquid and gas detectors using microwave structures.Wearable microwave sensors also require flexibility, stretchability, andlightweight, where these requirements have shifted the focus towards study-ing the implementation of microwave and electronic devices using flexiblematerials such as polymers and nanomaterials [12, 13]. Among various flexi-ble and stretchable polymers/nanomaterials, conductive polymers and nano-materials have a promising potential to be used as an alternative for the con-ventional metals in wearable microwave and electronic sensors. Several stud-ies have investigated the flexibility and conductivity features of conductivepolymer/nanomaterials for sensing applications, such as strain (i.e., defor-mation) and gas sensing [14, 15]. Among conductive polymer/nanomaterialmaterials, MXene, as a new class of nanomaterials, has drawn the interestsof a broad spectrum of researchers due to as excellent conductivity and supe-rior mechanical properties [16]. Accordingly, The conductivity feature of theMXene has promoted utilizing for various applications such as developing5G communication devices and gas sensors [17–23]. The high conductivityof MXene has enabled the implementation of 5G antennas with a radia-tion efficiency of 99% [24]. Furthermore, the adsorption of volatile organiccompounds (VOC) by MXene could change its conductivity, enabling thedevelopment of VOC sensing MXene sensors [20]. Based on the reviewed lit-erature, there has been limited previous evidence on integrating the sensingand communication features to develop a MXene antenna sensor for wirelessgas and VOC sensing.1.2 Thesis Objectives and AimsThe primary goal of this thesis is to design, optimize and fabricate an-tenna sensor solutions for long-range wireless detection of hazards, includingVOCs and hazardous aqueous liquids in unsafe environments where humanintervention is restricted. To fulfill the goal, the following sub-objectiveswill be accomplished:1. Developing a textile-embedded antenna sensor with a compact size of21.3. Taskssmaller than 100cm2 to detect hazardous aqueous liquids on a super-hydrophobic fabric, which is utilized in developing PPE, and transmit-ting the sensing data to a distanced receiver within a range of 2 meters.Hypothesis: For this objective, an investigation will be performedto understand whether antenna structures with modified structurescan sense the presence of aqueous liquids on their surface in the pres-ence of a superhydrophobic fabric. Also, the study will evaluate if theantenna sensor can transmit the sensing data to a distanced receiverwhile there is no contact between the antenna surface and the sensedliquid.2. Investigating the feasibility of implementating a MXene membrane asan alternative for metal traces in the conventional antennas’ design todevelop a wireless VOC-sensing structure with a compact size below100 cm2 and wireless transmission range of 1.5 meters.Hypothesis: The investigation will be performed on a MXene membrane-based antenna to evaluate whether or not its feasible to utilize it fordetecting different VOCs, e.g., Acetone, Ethanol, Methanol, and IPA.additionally an investigation will be performed to find out if the MX-ene membrane-based antenna is able to transmit the sensing data toa distanced transceiver. This investigation will perform as a proof ofconcept.1.3 Tasks− Designing and simulating the proposed microwave structures(e.g., antennas and antenna sensors): Design, simulate and op-timize textile-embedded patch antenna sensor and MXene-based an-tenna sensor, using ANSYS high frequency structure simulator (HFSS),to implement the proposed microwave wireless sensing applications(e.g., hazardous aqueous liquid sensing and VOC sensing ). This taskis performed to evaluate the proposed structures’ wireless communi-cation and sensing capabilities prior to fabrication..− Fabricating the designed microwave structures: Fabricating thedesigned patch antenna sensor and a feedline for MXene-based antenna31.4. Thesis outlinesensor using Teflon substrates. Also, a superhydrophobic nanoparticle-treated fabric with properties similar to a PPE will be prepared tocover the antenna sensor in the first objective ( i.e., hazardous aqueousliquid sensing). For the second objective, a MXene membrane willbe synthesized and implemented as a radiating/resonating element todevelop the MXene-based antenna sensor for sensing VOCs.− Preparing the experimental setup: A Keysight N5222B VectorNetwork Analyzer (VNA), AN-79 absorbers, and an RFSPACE TSA900-12000 ultra-wideband PCB tapered slot antenna will be used toprepare the experimental setup for the performance evaluation of thedeveloped antenna sensors.− Evaluating and analyzing the proposed antenna and antennasensor based on measured results: Measurements will be per-formed using a wireless readout, and the measurement results will becompared with the simulation results to validate the structure’s per-formance. The results will be analyzed to understand the antenna sen-sors’ sensitivity and wireless communication for their pertinent sensingapplications. The evaluation will be performed for the initial objec-tive by detecting water droplets resembling hazardous aqueous liquidson the superhydrophobic fabric. Accordingly, the VOC sensing andwireless communication of the MXene-based antenna sensor would beevaluated using standard liquids vapors (e.g., IPA, Acetone, Ethanoland Methanol).1.4 Thesis outlineThis thesis comprises five chapters to fulfill the mentioned tasks andobjectives. Chapter 1 introduces the whole aim and a brief backgroundof this research. Chapter 2 presents a comprehensive review of the priorart of various wireless microwave sensors and antenna sensor technologies.In Chapter 3, a novel superhydrophobic fabric integrated antenna sensor isintroduced and evaluated for detecting hazardous aqueous liquids on PPE.Chapter 4 presents an investigation of a MXene-based antenna sensor forwireless detection of VOCs. The MXene-based antenna sensor was designed,fabricated using a readily interchangeable membrane to configure antennasfor wireless sensing of VOCs. Finally, chapter 5 is devoted to the thesisconclusion that summarizes the achievements, contributions of this thesis,and some directions for future works.4Chapter 2Literature Review: WirelessMicrowave Structure forSensing ApplicationsThe literature review will present two groups of the most promising wire-less microwave sensing methods, namely chipless passive wireless resonantsensors and antenna sensors, and their technologies that have been widelyinvestigated in the literature. These sensors’ operation principles, varioussensing scenarios, strengths and limitations will be thoroughly examined infour sections. Section 2.1 will present the operating principle of microwaveresonant structures. Section 2.2 and section 2.3 will investigate wireless mi-crowave sensors and antenna sensors, respectively, indicating their varioussensing applications ranging from dielectric sensing to strain, crack and soforth. Finally, section 2.4 and 2.5 present the recent studies on implementingflexible wearable antenna sensor and the implementation of conductive poly-mers and nanomaterials in fabricating microwave structures (e.g., antennasand antenna sensor) for sensing and communication applications.2.1 Dielectric properties and the Microwavesensing principleIn order to have an understanding of the implementation of microwavefrequencies for sensing, we need to provide a brief background on some elec-tromagnetic principles such as permittivity and loss tangent. Understandingthe sensing principle could be achieved by knowing the dielectric propertiesof a material interacting with varying external electromagnetic waves. Whena varying external electric field is applied to a material, the electrical chargeswithin the material (e.g., Protons and Electrons) behave as electric dipolesby aligning (i.e., polarizing) in the direction of the varying external electricfield, as seen in Figure 2.1 (a). The permittivity of a material (e.g. di-electric permittivity) is a parameter that defines the polarization ability of52.1. Dielectric properties and the Microwave sensing principlethe dipoles in the direction of an externally applied electric field, where thedipole polarization is higher for a material with high permittivity [25, 26].The permittivity of materials is generally expressed using the relative per-mittivity. The relative permittivity defines the dielectric properties of amaterial relative to that of free-space (8.85×10−12F.m−1), and it is writtenas the equation below:r = /0 (2.1)Here,  is the permittivity of material and is the free space permittiv-ity. The permittivity is a complex parameter where the real and imaginaryparts represent the stored energy and dissipated energy in the material,respectively. The complex permittivity can be presented as: = ′ − j.′′ (2.2)The real ( ′) and the imaginary ( ′′) parts are known as dielectricconstant and dielectric loss factor, respectively, where the imaginary partmust be negative to satisfy the energy conservation. The real part of thecomplex permittivity shows how the varying external electric field can alignthe dipoles and cause energy to be stored in the material. But just as theenergy can be stored in the material, it can also be dissipated. The energydissipation in a material is expressed by its loss tangent.The energy loss in a material can be attributed to its imaginary permit-tivity, also known as dielectric loss factor ( ′′), and conductivity (σ). Thedielectric loss factor accounts for the damping (i.e., dielectric damping) ofthe vibrating dipoles in the material dipoles and the conductivity loss is theresult of free charge [27]. To quantify the energy dissipation in a material,the loss tangent can be calculated. The loss tangent can be expressed byequation 2.3.tanδ =ω′′+ σω′(2.3)In this equation, tanδ is the loss tangent, and ′and ′′stand for the realand imaginary parts of the permittivity, σ is the material’s conductivity, andω is the angular frequency. The angular frequency dependency of the losstangent is due to the dependence of complex permittivity to the frequencychange, as illustrated in Figure 2.1 (b), showing that the dipoles’ interactionwith the varying external electric field can change for different frequencies,i.e., the stored and dissipated energy of a material is dependent on theapplied electric fields frequency [29].62.1. Dielectric properties and the Microwave sensing principleFigure 2.1: (a) The polarization of material dipoles in the presence ofexternally applied electric field (b) The polarization of the atoms for differ-ent frequency bands. It is observed that the permittivity of a material isfrequency-dependent. Reprinted with permission of [28]Based on what was mentioned about permittivity and loss tangent tothis point, we have seen that each material has a unique permittivity andloss tangent, distinguishing it from other materials. This difference in thematerials’ permittivity and loss tangent is implemented to characterize themin the microwave frequency domain leading to developing microwave sensors.Using the microwave sensing principle, various microwave structures, includ-ing dielectric probes [30, 31], waveguide-based structures [32], and planarresonant methods [33, 34], have been investigated to characterize materialsin different frequency bands. Among these structures, this literature reviewwill focus on passive wireless resonant sensors and antenna sensors.72.2. Passive wireless resonant sensors2.2 Passive wireless resonant sensorsConventional microwave sensors require connectivity through microwavecables and transmission lines to a readout device to visualize their response.However, in some environments, such as hazardous or isolated locations,the cabled connections could limit the access of the conventional microwavesensors and introduce mechanical drift, noise, or vibration in the measure-ments. To address these issues, researchers investigated microwave sensorsthat operated wirelessly. The observations showed that microwave technol-ogy enables a wireless readout (e.g., an antenna, an NFC tag or a coil) tointeract with a passive resonant structure (e.g., sensing tag or sensor) forinterpreting the variations in the dielectric properties or the geometry aselectrical signals on the reader side [35–37]. Investigation on wireless mi-crowave sensors led to fabricating low-cost and power-efficient structureswith no active circuit element in various geometries, namely passive wirelessresonant sensors, where their sensing applications utilize the permittivityand geometry change of the structure [38], [39]. These features promotedusing passive wireless resonant sensors in hazardous environments with re-stricted access, including but not limited to high temperature, high-pressureenvironments [40], [41].2.2.1 Wireless communication between the passive wirelessresonant sensor and readerFor the passive resonant sensor to communicate with the reader, it re-quires to be located in a specific range called radiative near-field. At thisparticular region, the reader can communicate with the sensor in the form ofelectromagnetic coupling, enabling it to interpret the changes on the sensor(Figure 2.2) [42], [7]. The radiative near-field (i.e., Fresnel region) of thereader is expressed by the equation below [43] :0.62×√D3λ6 radiativenear − field 6 2D2λ(2.4)Where λ is the wavelength of the reader at the operation frequency, and Dis the largest dimension of the reader structure (e.g., antenna). The passiveresonant sensor, located in the radiative near-field, reflects the interrogatingwaves of the reader in all frequencies due to their metallic-based structure(e.g., copper) [44]. However, unlike the other frequencies, the passive res-onant structure absorbs the interrogating waves at its resonant frequencyand generates a resonant profile in the interrogating reader’s frequency re-82.2. Passive wireless resonant sensorssponse, where this resonant profile depends on its geometry and effectivepermittivity [42, 45, 46].Figure 2.2: A passive wireless sensor and an interrogator antenna.Reprinted with permission of [42]Various sensing applications for wireless microwave sensors have beenstudied in the literature; yet, in the following part, we have investigated twogroups of passive wireless microwave sensors: The first group includes thesensors that operate based on characterizing the permittivity in the sensor’sclose environment, and the second group comprises the structures that sensephysical parameter changes in the geometry of the structure due to strainor rotation.2.2.2 Dielectric sensingThe first group are the wireless sensors developed to characterize materi-als (e.g., solids and liquid), on their surface or close proximity, based on thepermittivity of the material. Positioning the materials on the sensor’s sur-face (i.e., passive resonant structure’s surface) or close proximity changesthe sensor’s effective permittivity, causing a shift in the sensor’s resonantprofile [5]. The resonant profile change can be monitored wirelessly by areadout device, e.g., an antenna or a coil. Accordingly, several studies have92.2. Passive wireless resonant sensorsutilized novel structures to improve the wireless microwave sensor’s mate-rial characterization performance and their readout distance, where some ofthese studies and their outcome are presented in the following.One of the outstanding investigations proposed by Maleki et al. was aHigh Impedance Surface (HIS) in the form of 10× 10 unit cells of a passivemushroom structure, shown in Figure 2.3 (a), for wireless material detec-tion [47]. The passive structure was designed to operate at the frequencyof 4.455 GHz, and an interrogator antenna was located 430 mm above itto communicate with it. The structure’s sensing performance was evalu-ated by positioning solid samples of Teflon laminates and monitoring thestructure’s resonant profile. Introducing the sample materials changed theeffective permittivity of the structure, resulting in a frequency shift. Amongthe tested materials, the maximum resonant frequency change was for thesample with r = 9.2, where a 1.233 GHz shift in resonant frequency wasobserved, demonstrating the high sensitivity of the designed structure. Thehigh sensitivity was due to the high electromagnetic energy absorption in thepassive structure’s resonant frequency, which enabled a good material andwaves interaction. Despite the good sensing performance of the structure,its large footprint made it undesirable for the applications where a wirelessmicrowave sensor with a low profile was required.Figure 2.3: (a) wireless sensing platform composed of mushroom structureand an inductive matching layer. Reprinted with permission of [47]. (b) Liq-uid sensing measurement using passive resonator array tag and interrogatorantenna. Reprinted with permission of [48]102.2. Passive wireless resonant sensorsStudies were also performed to develop sensing tags with a small foot-print. A wireless sensing tag composed of a flexible SRR array was developedby Wiltshire et al., presented in Figure 2.3 (b), for permittivity characteri-zation of liquids [48]. The fabricated sensing tag had a footprint of 9 cm2,and a corresponding notch frequency was observed for the array at the fre-quency range of 2.6 GHz to 2.9 GHz. A Horn antenna was located 15-24cm above the array as an interrogator to monitor the permittivity changeson the tag. The sensing tag was aligned with the E-field direction of theHorn antenna for maximum field concentration. Furthermore, a microflu-idic channel was embedded in the array’s sensitive area to sense liquids thatflowed through it. Standard liquids (e.g., Water, Methanol, Ethanol, andIPA) were used to evaluate the sensing performance of the tag, and the liquidmeasurement analysis demonstrated a sensitivity of 1-2 MHz/unit r for theproposed wireless sensing system. The liquid measurements demonstratedfrequency shift and amplitude change for liquids of different permittivity.Furthermore, water demonstrated more significant frequency shift and am-plitude change among the liquids due to its higher dielectric permittivityand dielectric loss. The proposed wireless sensor could potentially be usedto fabricate low profile sensing tags on flexible substrates for wireless sens-ing, where they could be used for applications requiring more conformalityand flexibility (e.g. pipeline integrity monitoring).2.2.3 Strain and rotation sensingThe second group of wireless microwave sensors comprises the passivestructures which are used to sense physical parameters such as strain androtation [49–52]. These sensors operate through a change in the passivestructure’s position or its geometry. Variation in the passive structure’sgeometry causes a shift in its resonant profile where this could be observedas a shift in the structure’s resonant frequency. Investigation on the wirelessstrain and rotation sensing has led to designing numerous novel structures.The following presents number of recent studies on the wireless strain androtation sensors.We have demonstrated a strain sensing wireless microwave sensor byimplementing an array of 16 SRRs on a kirigami patterned sheet [50]. Aspresented in Figure 2.4 (a-c), the array was designed and fabricated on aRogers Ultralam 3850 flexible substrate to operate at the frequency of 2.79GHz. The fabricated array was then implemented on the kirigami patternedsheet, positioned below a Horn antenna and fixed at a distance of 21.5 cmto communicate with it. The experimental measurements demonstrated a112.2. Passive wireless resonant sensorsresonant frequency at 2.79 GHz with a resonant magnitude of −30.47 dB.Then, the kirigami sheet was stretched from one end while the other end wasfixed. The deformation of the kirigami sheet was expressed as strain, whichis related to fractional length changes. The utilized Horn antenna monitoredthe strain changes on the array embedded kirigami sheet. The measurementsdemonstrated that increasing strain value of the array embedded kirigamicaused a frequency upshift and amplitude change due to rotation of theSRRs and less interaction between array and antenna (Figure 2.4 (b)). Wewere able to demonstrate a significant range of strain sensing from 0.6 to21.3% implementing the array embedded kirigami sheet.Figure 2.4: (a) The schematic of the strain sensing structure, (b) Theresonators on a kirigami sheet, (c) The measurement setup. Reprinted withpermission of [50].Furthermore, a study by Maleki et al. proposed microwave structuresfor sensing rotation wirelessly [52]. A double parallel-plate comb-like Nested122.2. Passive wireless resonant sensorsSRR structure was developed for this investigation where, as shown in Figure2.5 (a,b), only the top plate rotated. Experimental measurements demon-strated that rotating the top plate of the Nested SRR caused the resonantgap between the top plate and bottom plate to decrease, resulting in a reso-nant frequency shift for the structure. The experimentally measured resultsdemonstrated a large rotation resolution of 20 µ− rad and an excellent sen-sitivity of 28 MHz/deg. Furthermore, the measured results showed a signif-icant dynamic range of approximately 40° for the proposed rotation sensingstructure. The proposed structure has the potential to be used for industrialapplications where monitoring small rotation variations is required.Figure 2.5: (a) The designed wireless rotation sensing structure with twoparallel comb-like nested SRR structures, (b) The fabricated rotation sensingpassive structure. Reprinted with permission of [52].Despite the high sensitivity and suitable wireless performance for variousapplications, the passive resonant sensing structure of wireless sensors is lim-ited to the radiative near-field region of the interrogator antenna. Addition-ally, their performance is dependent on their alignment with the interrogatorantenna. Therefore, misalignment between the passive wireless sensor andthe interrogator antenna would deteriorate the sensing response of the sen-sor. These drawbacks encouraged developing structures that could sense andtransmit the sensing data to a distanced receiver simultaneously. The follow-ing section investigates these sensing/transmitting structures, e.g., antennasensors.132.3. Antenna sensors2.3 Antenna sensorsAntennas are prominent structures used to transmit and receive signalswirelessly in different frequency spectrums ranging from microwave to mm-wave and terahertz, where they have been investigated in different planarand non-planar antenna configurations[43, 53–55]. Among them, planar an-tennas have demonstrated a wide range of applications due to their lowprofile, small footprint, and lightweight. These antennas are generally com-posed of a transmission line, a radiating element, and a ground plane wherethe transmission line supplies the radiation element by providing the inci-dent power.Consequently, the antenna’s reflection magnitude (i.e., reflection coeffi-cient) could be determined by a reflected and incident power ratio. The re-flection coefficient is used to determine the bandwidth of an antenna, whichis the range of frequencies at which the reflection coefficient is minimum,i.e., most of the incident power is radiated by the antenna, and a negligiblemagnitude is reflected. Reflection coefficient magnitude below −10 dB isconsidered appropriate for defining the antenna’s bandwidth since a smallportion of the incident power is reflected back [56]. Furthermore, severalexperiments have demonstrated that radiation parameters could be utilizedto convert materials’ physical (e.g., strain, temperature) or dielectric prop-erties to measurable radiation parameters where these could be monitoredin the antenna’s frequency response (e.g., bandwidth) [57–61]. For example,Ebrahimi Ghiri et al. developed a non-contact dielectric spectroscopy sys-tem, where two Vivaldi antennas, coupled to each other in their near-field,were used to construct a sensing unit. The antennas were directed towardseach other at their line of sight, and the sample material was located be-tween them [62]. The presence of sample materials changed the antenna’sbandwidth response, and this change was used to characterize different ma-terials.Recent evidence shows a growing interest in antenna sensors due to theirsimple configuration, passive operation, longer transmission range, and lowcost, enabling a low-profile sensing device and transmitting devices in a sin-gle antenna structure [56]. Patch antennas have been widely implemented asantenna sensors due to the resonant feature in their operation frequency, lowfabrication cost, lightweight, small size, and physical structure for sampleplacement [56], [63]. The antenna sensor structures have not been limited topatch antennas as various antenna structures, including printed monopole,dipole and patch antennas, have been investigated for antenna sensor ap-plications [63–65]. These applications could be categorized into three main142.3. Antenna sensorsgroups: dielectric, temperature, and strain/crack sensing antenna sensors.2.3.1 Dielectric sensingPositioning a material on the antenna sensor’s surface or its close prox-imity causes a change in the antenna’s effective permittivity and resultsin a resonant profile change. This feature has been investigated in severalantenna sensor structures to characterize the materials based on their di-electric permittivity [66–71]. Accordingly, several interesting studies havetried to enhance the sensing performance of the antenna sensors by creatingsensing regions with high electromagnetic energy concentration [55], [72, 73].For instance, implementing radiating slots, resonators, and electromagneticband gaps (EBG) have caused a high increase in the electromagnetic energyconcentration on the antenna sensor’s surface or close proximity. The highelectromagnetics energy concentration causes higher material-sensor interac-tion and better sensing performance. The followings present some recentlydeveloped antenna sensors that have been designed for dielectric sensingapplications.Yeo et al. designed and fabricated an antenna sensor by slot-loading amicrostrip patch antenna to sense the bulk-sized material samples, presentedin Figure 2.6 (a-c), [66, 67]. They investigated the slot-loaded antenna sen-sors for two configurations: (i) by extracting a rectangular slot from thepatch surface, close to the antenna’s radiating edge, and (ii) by extract-ing a meander-shaped slot close to the radiating edge of the antenna [66].Compared to the conventional patch antenna and antenna with the rect-angular slot, the antenna with a meander-shaped slot had a higher E-field(e.g. energy) concentration in its slot, enabling better sensing performance.The antennas were optimized and fabricated to operate at the frequency of2.5 GHz. The measurements were performed for the conventional, rectan-gular slot-loaded and meander-shaped slot-loaded antennas using materialswith permittivities ranging from 1 to 10.2. Comparing the antenna sen-sor’s measurement results showed that the antenna with a meander-shapedslot corresponded to a higher shift in the resonant frequency for changes inthe effective permittivity. For instance, RF-10 material (r = 10.2) demon-strated resonant frequency at 2.312, 1.854, and 1.577 GHz for conventionalpatch antenna, patch antenna loaded with rectangular slot, and patch an-tenna loaded with meander-shaped slot, respectively, confirming the betterperformance of the antenna sensor with the meander-shaped slot. However,despite the good sensitivity to bulk-sized solids, the study did not reportany results for small-sized materials such as solids and liquids.152.3. Antenna sensorsFigure 2.6: Designed antenna sensor structure by Yeo et al. (a) bare an-tenna, (b) antenna with rectangular slot, (c) antenna with meander slot(d) simulation results for bare antenna (e) simulation results for the an-tenna with single radiating slot (f) simulation results for the antenna withmeander-shaped slot. Reprinted with permission of [66]Jun et al. integrated a Coplanar Waveguide (CPW) antenna and anEBG structure, as presented in Figure 2.7 (a), to develop a sensor for liq-uid permittivity characterization [69]. The EBG structure consisted of arectangular patch array with empty trenches between the patches where thetrenches were implemented to be filled with sample liquids. Initially, thepatch array with empty trenches demonstrated a resonant frequency of 2.53GHz. In contrast, the trenches filled with liquids of different dielectric per-162.3. Antenna sensorsmittivity (e.g., Butan-1-ol, propan-2-ol, ethanol, and methanol) changed theresonant profile of the patch array, where it was observed as a shift in theresonant frequency of the sensor. The reflection coefficient of the antennawas monitored to evaluate the sensing response, and the achieved results,presented in Figure 2.7 (b), demonstrated the resonant frequency of 2.53,2.40, 2.26, and 2.16 GHz for butan-1-ol, propan-2-ol, ethanol, and methanol,respectively. Accordingly, the developed structure showed the potential ofbeing used for sensing applications in laboratories and clean rooms. De-spite the positive results obtained in this study, the fabricated structurewas bulky and would not be preferred in applications requiring a low-profilesensor structure, such as wearable sensors.Figure 2.7: (a) The photograph of the designed EBG based sensor (b) Theachieved results for different liquids. Reprinted with permission of [69].2.3.2 Temperature sensingTemperature is essential for controlling the manufacturing process, foodproduction, and human health monitoring [74]. Recent studies on antennasensors have led to designing for monitoring the temperature sensing in anenvironment [75, 76]. The antenna temperature sensors operate by the varia-tion in the thermal coefficient of dielectric constant (TCD) of the antenna’ssubstrate. Therefore, the change in this parameter affects the dielectric172.3. Antenna sensorsconstant of the antenna’s substrate, which would also impact the operationfrequency of the antenna.2.3.3 Strain and Crack sensingAntenna sensors can sense deformations caused by strain or crack sincethese phenomena cause change in the geometry of the antenna. Strain isa well-known parameter for quantifying the deformations in a structure’sgeometry, while crack is a serious threat to engineered components (e.g.,pipelines) [77]. Monitoring the crack and strain is essential for checkingthe structural integrity of the engineered components. In addition, evidenceshowed antenna sensors’ capability in monitoring strain and crack by con-verting the deformation in their geometry to a change in operation frequency[60, 78, 79]. Figure 2.8 (a-c), 2.9 (a-c) present the operation principle of astrain sensing and a crack sensing antenna, respectively, where introducingdeformation to the antenna’s structure results in a change of geometry anda shift in the operation frequency. Several studies have investigated sensingof different strain and crack scenarios using various configurations of patchantennas.Figure 2.8: (a) Patch antenna sensor with no deformation, (b) Patch an-tenna sensor with deformation, (c) change in the resonant frequency of theantenna due to the deformation. Reprinted with permission of [80].Omer et al. implemented an antenna strain sensor operating at 1.8 GHzand 2.4 GHz [78]. The antenna was fabricated on a flexible Felt substrate182.3. Antenna sensorswith a height of 3 mm and permittivity of 1.44. The designed antennawas bent in five different angles (e.g., 0°, 20°, 30°, 40°, and 55°), and acorrelation was observed between the antenna’s operation frequency and thebending angles as the operation frequency increased linearly with increasingthe bending angle (e.g., strain). The measurement results of the proposedstructure showed the high potential of antenna strain sensors to be used formonitoring deformation in structural health monitoring applications.Ke et al. presented an antenna sensor for identifying notch-shaped crackson a metal structure [81]. The antenna sensor was fabricated on an FR-4substrate with first and second resonant frequencies (e.g., resonant modes)at 1.8 GHz and 2.5 GHz. Initial simulations were performed for differentcrack properties (e.g., length and orientation) by monitoring the first andsecond resonant frequencies of the antenna. Studies demonstrated that acrack on its ground plane could affect the antenna sensor’s first and secondresonant frequencies, resulting in frequency shift. This shift in the resonantfrequency enables extracting the crack information. Validation experimentswere performed to understand the response of the antenna sensor to var-ious crack configurations. The measurements were performed for differentlengths and orientations of the crack where, as presented in Figure 2.10,each combination of orientation and length showed a specific resonant fre-quency (e.g. first and second resonant frequency). It was observed thatthe crack parameters affect the structure’s resonant frequency relative tothe change of current flow for different cracks. Further studies resulted inderiving relationships between the resonant frequency of the antenna sen-sor and notch-shaped cracks, which enabled characterizing the cracks forvarious parameters, including the length, the orientation and the crack’scentre location. The proposed work showed a high potential for structuralhealth monitoring applications, where monitoring crack size, orientation,and placement is required.Yi et al. developed a passive Radio frequency identification (RFID) an-tenna sensor for strain and crack sensing [82]. The proposed antenna sensorwas fabricated on a RT/duroid Rogers 5880 substrate, and the operationfrequency was observed at 913.623 MHz. Electromagnetic simulations wereperformed to evaluate the strain sensing performance of the antenna sensor,where a linear relationship between the resonant frequency of the antennasensor and strain was observed. Validation experiments were then performedto evaluate the strain sensing range and the resolution for the antenna sen-sor. A Yagi antenna was positioned at a distance of 2.1 m from the antennasensor to operate as a reader. The measurements demonstrated the capabil-ity of measuring strains between 20 µ and 1000 µ for the antenna sensor.192.3. Antenna sensorsFigure 2.9: (a) Configuration of a conventional patch antenna (b) TheS11 frequency response of a conventional patch antenna (c) Patch antennasensor embedded on a metal structure for crack sensing (d) The impact ofdifferent cracks for the current flow. Reprinted with permission of [81]Figure 2.10: The impact of crack on the antenna sensor’s (a) first and (b)second resonant frequency for different orientations and lengths of crack.Reprinted with permission of [81]202.3. Antenna sensorsFigure 2.11: Fabricated antenna by Yi et al. for three different crack open-ings of (a) 0.13 mm (b) 0.53 mm (c) 0.81 mm. Reprinted with permissionof [82]Furthermore, in another set of experiments, the crack sensing performanceof the antenna sensor was evaluated by measuring two different scenarios ofemulated crack sensing and fatigue crack sensing. The emulated tests forthe crack and fatigue crack were performed (Figure 2.11) in the range of 0to 53 mm and 0 to 24.3 mm, respectively. It was observed that for bothcases antenna sensor demonstrated a downward resonant frequency shift.The experiments showed that the antenna sensor could sense cracks in thesub-millimetre range and track the crack propagation.Mohammad et al. proposed an antenna sensor for monitoring fatiguecrack growth and opening [83]. Previous studies have demonstrated thesignificance of fatigue crack detection as this phenomenon causes 50%-90%of mechanical failures in the structures. The proposed antenna sensor wasfabricated on a Kapton HN substrate with a thickness of 50 µm, making theantenna structure flexible. The antenna sensor demonstrated the first andsecond resonant frequencies at 6.1 and 8.6 GHz. The fabricated antennasensor was bonded on a pre-cracked specimen, and 100 to 600 lb cyclic load-ing was applied to the structure to evaluate its frequency response (e.g.,first and second resonant frequencies). The fatigue crack sensing measure-ments were performed for a crack along the patch length, as presented inFigure 2.12. The measurement results demonstrated that growth in thecrack length caused the second resonant frequency of the antenna sensor212.3. Antenna sensorsto decrease, corresponding to a crack growth sensitivity of 22.1 MHz/mm.However, unlike the second resonant frequency, the first resonant frequencyof the antenna sensor did not represent a linear relation for crack growth.This phenomenon is due to the difference in the current density for the firstresonant frequency of antenna sensor compared to the current density of thesecond resonant frequency,resulting in a different interaction with the crack.Figure 2.12: Experimental setup for fatigue crack sensing by Mohammadet al.. Reprinted with permission of [83]Xue et al. proposed a novel antenna sensor structure for crack widthsensing [84]. Their proposed antenna sensor was composed of a patch an-tenna with an overlapping sub-patch on its surface. As observed in Figure2.13, the patch antenna was located on one side of the crack while the sub-patch was connected to the other side of the crack. Therefore, the growthin the crack width caused the sub-patch to move, increasing the patch an-tenna’s electrical length, e.g., generating a resonant frequency shift. Thepatch antenna was designed and fabricated on a Rogers 5880 substrate forthe operation frequency of 2.4 GHz.Crack sensing experiments were per-formed for the antenna sensor where the operation frequency of the antennasensor was interrogated wirelessly by a wide-band antenna. The measure-ment results showed a sensitivity of 120.24 MHz within 1.5 mm of crackwidth measuring range, demonstrating the potential of sensing crack widthby the proposed novel antenna sensor.Based on the investigated literature, antenna sensors have represented222.4. Wearable antenna sensorsFigure 2.13: The antenna crack sensor, proposed by Xue et al.. Reprintedwith permission of [84].suitable sensing and transmitting performance for various material, tempera-ture, crack, and strain sensing applications. Compared to wireless microwavesensors, they can transmit the sensing signal to a readout that is locatedin a more distant range. Furthermore, combining sensing and communicat-ing elements in a single device makes antenna sensors compact compared toconventional wireless sensors. Nevertheless, these structures require moreflexibility to be used in sensing applications which require conformality andflexibility. These led to designing and fabricating wireless microwave sensorsand antenna sensors with higher flexibility and conformality by implement-ing novel conductive 1D/2D material and polymers instead of copper andnew fabrication techniques.2.4 Wearable antenna sensorsRecent advances in designing antennas and microwave devices have fo-cused on developing wearable structures which are lightweight and flexiblefor on-body applications, replacing the metal-based sensor and communica-tion interfaces [85, 86]. Studies have shown that wearable and flexible anten-nas could be developed using different techniques including, Metal–Serpentine,3D assembly, liquid metal, and textiles [86]. Among these structures, liquidmetal-based antennas and textile-based antennas have promissing potentialto be used as wearable and flexible antenna sensors for different applications,such as strain, temperature, and liquids sensing.232.4. Wearable antenna sensorsLiquid metal has been utilized on patterning thin-film structures or in-side microfluidic channels to develop different kinds of antennas including,NFC antennas, monopole antennas, dipole antennas and patch antennas [87–89]. The advantage of liquid metal to Metal–Serpentine structures and 3Dassembly structure is its capability to support nearly unlimited strain levelsdue to its liquid state [90]. Additionally, the antennas fabricated using liq-uid metal would have mechanical deformation capability (stretchability andflexibility). Generally, this mechanical deformation capability would changethe antenna’s shape resulting in resonant frequency variation. Studies haveemployed the mechanical deformation feature of the liquid metal to developstrain monitoring antenna sensors.In this regard, So et al. developed a mechanically tunable fluidic metalalloy dipole antenna, as shown in Figure 2.14 (a), which operated in the1600 to 1900 MHz frequency range with a radiation efficiency of ≈ 90% [91].The developed antenna could withstand different mechanical deformationssuch as stretching, bending, rolling, and twisting (Figure 2.14 (b,c)). Addi-tionally, the antenna’s response showed no hysteresis as after each bending,rolling and stretching, the response returned to the initial value. The an-tenna demonstrated a free-space operation frequency of 1962 MHz, and by12 mm of increase in the length, the resonant frequency shifted to ≈ 1600MHz (Figure 2.14 (d)). The frequency tunning response of the antennasensor could be achieved by applying a ≈ 20% uniaxial strain. The devel-oped antenna demonstrated to be a good candidate to sense different strainvalues.In addition to liquid metal antennas, textile-based antennas have shownto be good candidates for antenna sensor applications. Textile-based anten-nas have represented an interesting and compatible alternative for wearableantenna applications where a device is required to be mounted on the user’sarms and legs. The antenna’s radiation properties are directly influenced bythe fabric properties, which have been used as a substrate for the antenna.Textile antennas can be manufactured by replacing the dielectric substrateof the antenna with a textile substrate and keeping the metallic componentsof the microstrip structure. Another design alternative could be developinga textile-based antenna where the dielectric substrate and metallic traceshave been replaced by textile substrate and conductive fabric. Differentfabrication methods have been proposed for fabricating textiles-based anten-nas, including screen printing, Inkjet printing and Embroidery [56]. Screenprinting is considered the simplest and most economical approach to manu-facture flexible antenna sensors; nonetheless, the screen printing techniquehas a low resolution, and the conductive layer’s thickness could not be con-242.4. Wearable antenna sensorsFigure 2.14: (a) The developed liquid metal dipole antenna where the liquidmetal was injected into two adjacent microfluidic channels, fabricated fromPolydimethylsiloxane (PDMS) (b) The rolled dipole antenna structure (c)The antenna’s self-healing response to the sharp cuts, such as ones made byrazor, (d) antenna’s sensing response to increasing the length, as shown, 12mm of increase in the antennas length shifted the resonant frequency to ≈1600 MHz. Reprinted with permission of [91].trolled [56]. The drawbacks of the screenprinting method were solved usinginkjet printing since this technique can produce a very high precision patternfor printing the antenna. The inkjet printing method utilizes ink dropletsof size up to few picometers, but in contrast, the inkjet printing techniqueis incompatible with particular sizes and types of ink, where they causeclogging of nozzles [92]. Contrary to the mentioned fabrication techniques,Embroidery was introduced where it uses specific conductive threads fromwhich the antenna sensor can be embroidered on a textile-based substrate.Embroidery is more advantageous to the other two fabrication techniquessince Embroidery machines are more preferred in the industry. The Em-broidery method does not require glue to connect the textile layers enablingreproducible geometries. Additionally, using this technique can improve thewashability of the antenna sensor integrated clothing [56]. Different con-figurations of antenna sensors have been developed using these fabricationtechniques [56].Labiano and Alomainy proposed a textile-based patch antenna for tem-252.4. Wearable antenna sensorsperature monitoring where the proposed antenna operated at a frequencyof 2.45 GHz, e.g., ISM band [93]. The conductive parts of the antenna werefabricated using copper tape, attached to a cotton-based substrate with r= 1.6 (Figure 2.15). The device’s temperature was increased from 20 to60 oC where this change caused variation in the dielectric constant of thesubstrate. The substrate’s dielectric constant change resulted in a resonantfrequency and resonant amplitude shift for the antenna. It was seen that a10 oC change in the temperature resulted in a 10 MHz of frequency shift forthe antenna. The developed antenna sensor showed the potential of beingused as a passive temperature sensor.Eldamak and Fear developed a liquid detection antenna sensor on a flex-ible filter paper for liquid sensing application [4]. The antenna was designedfor the frequency of 2-4 GHz, and the changes in the antenna’s radiationproperties were used for sensing purposes. The utilized cellulose filter papersubstrate could absorb liquids such as sweat through two slots incorporatedwithin the antennas structures, as shown in Figure 2.16 (a). Accordingly, ab-sorption of liquids by the substrate caused a change of the antenna sensor’seffective permittivity resulting in a shift in the resonant frequency (Figure2.16 (b,c)).The structure’s sensing performance was evaluated using an artificialsweat and salt solution where the NaCl concentration ranged between 8.5-200 mmol/L. The antenna sensor demonstrated resonant frequency and reso-nant amplitude shifts for various droplet volumes and NaCl concentrations,showing a promising potential of being used for hydration monitoring insweat while directly placed on the skin. The proposed sensor required noproximity to the human body, was low-cost, easy to fabricate and highlysensitive. Despite all the great achievements in the wearable antenna sensorsector, no previous work has demonstrated their application in detectinghazardous liquids and solutions where they can be effective in protectiveequipment.Additionally, a recent research trend has been investigated in developingantenna and wireless sensors with outstanding features (high flexibility andconformality) using conductive nanomaterials and polymers used for sensingapplications. The following section will investigate these structures in detail.262.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorsFigure 2.15: Fabric-based antenna sensor, developed by Labiano and Alo-maniy, for temperature sensing application. Reprinted with permission of[93].2.5 Conductive nanomaterials/ polymers-basedantennas and antenna sensorsRecent studies on conductive nanomaterials, composites and polymershave opened new doors to fabricating microwave structures since they pro-vide the required flexibility and conformality while adding more versatilityto microwave structures’ applications and features, including flexibility andwearability [94–96]. Based on the investigated literature, a wide range ofwireless communication, RFID, and wireless sensing applications have beenproposed for microwave structures, implementing conductive nanomaterialsand polymers. The following subsections investigate some pioneering stud-ies performed to design and fabricate antennas, antenna sensors, conductivenanomaterials, polymers and composites.272.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorsFigure 2.16: (a) Front view of antenna sensor developed by Eldamak andFear, (b) antenna sensor’s photo after absorbing 6 drops of saline solution,(c) measured frequency shift for the first and second resonant frequenciesof the antenna sensor for different water quantities at 2% NaCl. Reprintedwith permission of [4].2.5.1 Conductive polymers and nanomaterials forfabricating antennasRecent studies on conductive nanomaterials, composites and polymershave opened new doors to fabricating microwave structures since they pro-vide the required flexibility and conformality while adding more versatilityto microwave structures’ applications and features, including flexibility andwearability [94–96]. Based on the investigated literature, a wide range ofwireless communication, RFID, and wireless sensing applications have beenproposed for microwave structures, implementing conductive nanomaterialsand polymers. The following subsections investigate some pioneering stud-ies performed to design and fabricate antennas, antenna sensors, conductive282.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorsnanomaterials, polymers and composites.Fabrication of various antennas and wireless microwave structures havebeen reported implementing conductive nanomaterials and polymers, suchas Carbon Nanotubes (CNT), graphene and PEDOT:PSS. A number of therecently investigated structures are provided in the followingFor instance, Amram Bengio et al. used Carbon Nanotubes CNT tofabricate patch antennas, where the antenna demonstrated radiation effi-ciency of 94% at 10 GHz and 14 GHz [97]. Armam Bengio et al. showed thehigh potential of CNT to be used as an alternative for metals while havinga lower thickness. Their proposed antenna demonstrated the capability ofbeing used in the aerospace industry, where lightweight and mechanicallydurable structures are required.Several studies have investigated using graphene for fabricating microwavestructures [12], [98–100]. Song et al. designed a novel antenna using graphenecustomized to operate in the 5G regime. A graphene assembled film (σ =1.12×106S/m) was utilized to develop an array of patch antennas operatingat the frequency of 26 GHz [101]. The proposed array of patch antennaswas fabricated on Rogers 5880 substrate with a thickness of 0.508 mm. Theradiation measurements of the array demonstrated a similar radiation pat-tern compared to the copper array antenna. However, it had six times wideroperating bandwidth compared to the copper antenna array, making it moresuitable for the 5G applications.Sa’don et al. designed an array of a graphene-based patch antenna onthree different substrates of Kapton (r = 3.5), A4 paper (r3.2), and RTDuroid (r = 2.2) with thicknesses of 0.075 mm, 0.05 mm, and 0.254 mm,respectively [102]. The array antennas were composed of 6 graphene-basedpatches, where the utilized graphene sheet had a conductivity of 1.4286×106S/m. The arrays were proposed for 5G applications since they were designedto operate at 15 GHz. The simulations for these antennas demonstrated an-tenna gains of 7.62 dB, 7.09 dB and 7.19 dB for antennas fabricated on Kap-ton, A4 paper and RT Duroid substrates, respectively showing the potentialof using graphene for developing high gain antennas for 5G applications.In a recent investigation, Scida` et al. fabricated a Near-field communica-tion (NFC) device using graphene. One study demonstrated the capability ofmanufacturing G-shaped tag antennas for NFC applications using graphenematerial with a conductivity of 4.20× 105S/m [103]. The fabricated devicewas successfully evaluated by exchanging information with a smartphoneand an NFC reader, where its performance was comparable to a standard,commercial NFC metallic tag. Numerous works have investigated the im-plementation of graphene for microwave application; nonetheless, studies292.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorswere not limited to conductive nanomaterials as conductive polymers suchas PEDOT:PSS were also used for microwave antenna fabrication.Guerchouche et al. proposed a conductive polymer (CP) based antenna(e.g., PEDOT:PSS based antenna) on a recyclable and transparent glasssubstrate [104]. The proposed structure operated at the RFID UHF band(860-960 MHz) for “Green” electronics and biosensor application. Despitehaving a low gain (− 8.6 dB), the antenna showed a maximum read distanceof up to 75 cm while it had 5.5 µm thickness compared to the same coppergeometry with 18 µm thickness. The proposed antenna could potentially beused for wifi access points on glass windows or green and recyclable wirelesssensor structures.2.5.2 Conductive polymers and nanomaterials for wirelesssensor and antenna sensor applicationsInvestigation on fabricating microwave devices using conductive nano-materials and polymers has resulted in developing a number of wireless sen-sors and antenna sensors for different sensing applications such as bacteria,humidity and ammonia and strain sensing [15, 95, 105].Mannoor et al. fabricated a graphene-based wireless microwave sensor,presented in Figure 2.17(a,b), which was employed to detect bacteria insaliva and on the surface of medical equipment [95]. The graphene-basedsensor was printed on water-soluble silk to be positioned on a tooth. A two-turn coil antenna, connected to a frequency response analyzer, was usedto communicate with the sensor and interpret the changes wirelessly. Themeasurement showed that the graphene-based structure was sensitive to thebreath and Helicobacter pylori bacteria in a user’s saliva. Analyzing the sen-sor’s response indicated that exposure to breath changed its conductance,while the bacteria growth changed its resistance. The developed sensordemonstrated sensitivity to different concentrations of Staphylococcus au-reus bacteria, a bacterial strain that has been reported to survive up to 9weeks in the dry hospital environment. The sensitivity performance wasevaluated by monitoring the Staphylococcus aureus growth on the surfaceof an IV bag. The findings of this work showed the potential and the versa-tility of application in which the nanomaterial-based (e.g., graphene-based)microwave structure could be employed.Further, studies demonstrated the potential of some conductive nano-materials and polymers, such as graphene, in fabricating antenna sensorsand wireless microwave sensors for gas sensing. This investigation revealedthese nanomaterials’ ability to improve the antenna sensor’s sensitivity to302.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorsFigure 2.17: (a) Wireless sensor for detecting bacteria on IV bag, (b) Wire-less sensor on a tooth. Reprinted with permission of [95].various compounds such as humidity, CO2, and NH3 gases [14, 106]. Therehave been reports of integrating antenna sensors and conductive nanoma-terials for ammonia sensing by Lee et al. [14]. Their design was composedof an ammonia sensitive poly (m-aminobenzene sulfonic acid) single-walledcarbon nanotubes (PABS-SWNTs), which connected a stub to a patch an-tenna’s feed structure [14]. PABS-SWNT exposure to ammonia gas changedthe equivalent capacitance assigned to it, causing a change in the antenna’sresonant frequency. This enabled sensing different concentrations of ammo-nia gas; Therefore, the designed structure was exposed to 50, 75, and 100ppm concentrations of ammonia, causing 43.125, 46,875, and 48.750 MHzof a shift in the patch antenna sensor’s operation frequency. The proposedammonia sensor can be integrated with RFID tags for low-cost wireless am-monia gas sensing applications.Furthermore, reports have been on fabricating antenna strain sensorsusing conductive nanomaterials, including graphene, since their implemen-tation for manufacturing antenna sensors provides the requisite flexibility[24, 107]. One of the pioneering works in this domain was performed byTang et al., where they fabricated an antenna strain sensor, illustrated inFigure 2.18, by coating graphene on aluminum/cellulose paper [108]. Theantenna was manufactured for the frequency of 1.63 GHz using graphenewith a conductivity of 106 S/m. The proposed antenna sensor structuredemonstrated flexibility, stability, and reversible deformability in differentangles and bending scenarios. The proposed structure showed a high po-312.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorstential to be used for wearable strain sensing applications. Silver nanowires(AgNW) were also studied by Rai et al., and Song et al. in the form ofAgNW/PDMS antennas since the combination of AgNW and PDMS canprovide conductivity and flexibility, simultaneously to be used as flexibleantenna sensor [15, 105] . The proposed antennas’ radiation parameter wascharacterized under tensile strain. It was shown that the antennas weremechanically tunable, resulting in a frequency change for the proposed an-tennas. The AgNW/PDMS antennas showed the potential of being used aswireless strain sensors in applications such as wearable sensors.Figure 2.18: Graphene-based antenna sensor presented by Tang et al. forwireless strain sensing. The Figure shows that the fabricated antenna wasable to bend at different angles. Reprinted with permission of [108]2.5.3 Newly emerging MXene nanomaterial for antennaapplicationsAmong the investigated conductive materials in literature, MXenes, asnovel 2D nanomaterials, have shown the required flexibility, higher conduc-tivity, and versatility in the applications to which they have been applied.MXenes were introduced circa 2011 and they have been investigated forvarious sensing applications including pressure, strain, gas and humidity322.5. Conductive nanomaterials/ polymers-based antennas and antenna sensorssensing applications. This sensing applications are based on the mechanicalproperties of the MXene, such as flexibility and the high conductivity whichit posses. Furthemore, The high conductivity of up to 15000 S/cm makesMXene suitable for electronic and microwave applications [24]. Neverthe-less, few studies have been published on utilizing MXene for fabricatingantennas. Sarycheva et al. were the first to develop a MXene-based mi-crowave transmission line, dipole antenna and RFID tag for various MXenethicknesses using a one-step spray coating fabrication process [109]. Trans-mission lines with thickness ranging from 62 nm to 8 µm were studied, andthe results showed that thicker MXene film would have less attenuation as amicrowave transmission line. Furthermore, the MXene dipole antenna struc-ture studied in this demonstrated resonant frequency, resonant amplitude,and gain of 2.4 GHz, −65 dB, and 1.7 dB, respectively. Further measure-ments were performed for dipole antenna with thicknesses ranging from 114nm to 8 µm, and it was observed that decreasing the thickness of the MXenecaused an increase in the dipole antenna’s resonant amplitude. The depen-dency of the structures’ performance to the MXene’s thickness is relativeto the skin depth of the MXene, where thicker MXene fully satisfies theskin depth requirements. Based on the measured results, compensation isrequired between the performance of MXene-based microwave structure, us-ing thicker MXene, and the material amount used for the fabrication. Thisstudy showed the potential of MXene for fabricating microwave structures.Han et al. used a spray coating technique to fabricate three kinds offlexible MXene-based patch antennas for different thicknesses to be used forthe 5G applications. Initially, they developed a MXene-based microstriptransmission line to study the MXene’s efficiency in power delivery overseveral frequency bands. The MXene transmission line demonstrated an at-tenuation constant of 0.02 – 0.06 dB/mm in the 1–10 GHz frequency range,offering an insight into the electromagnetic energy transmission through theMXene. Furthermore, three MXene patch antennas were presented in Figure2.19, where they demonstrated up to 99% radiation efficiency compared to acopper-based patch antenna [24]. The fabricated antennas demonstrated an-tenna gains of 5.74, 7.14, and 5.48 dB at 5.6, 10,9, and 16.4 GHz frequencies.This work showed the potential of MXene-based antennas to be used for ap-plications where flexibility and conformality are required. Furthermore, theradiation properties of the antenna at frequencies of 5.6, 10,9, and 16.4 GHzsatisfied the frequency requirements in the emerging 5G communications.The high conductivity of a MXene film makes it a leading alternativefor metal traces in microstrip printed microwave structures. In addition toits exemplary microwave performance, several works have shown the sens-332.6. Summary of chapterFigure 2.19: (a) MXene based transmission line [109], (c) MXene baseddipole antenna [109], (c) MXene based patch antenna [24]. Reprinted withpermission of [24, 109]ing performance of MXene for strain/stress, humidity, VOC, ammonia, andUltra Violet (UV) sensing [110–113]. However, to the best of our knowl-edge, there is no study on developing a MXene-based antenna structure forsensing application.2.6 Summary of chapterWithin this chapter, an investigation was performed to explore the ap-plications of various wireless microwave sensors and antenna sensor configu-rations for various sensing applications. Furthermore, the investigated liter-ature showed an emerging trend in fabricating antennas and antenna sensorsusing novel conductive nanomaterials and polymers, where they have beenused for different sensing applications, including strain and gas.34Chapter 3Smart superhydrophobictextiles utilizing long-rangeantenna sensor for hazardousaqueous droplet detectionplus preventionThis chapter presents a novel modified patch antenna sensor to develop afabric-based sensor-embedded smart PPE. The antenna sensor was embed-ded inside a superhydrophobic fabric, used as a liquid-repellent surface forthe PPE, to sense the presence of hazardous aqueous liquids on the fabric’ssurface. This chapter begins with an introduction to the inset-fed rectan-gular patch antennas and their operation principle. The design, simulationand optimization of the antenna sensor are then explained in detail. Sub-sequently, the fabrication of antenna sensor and PPE-resembling fabrics,experimental setup and measurement results for the fabric-covered antennasensor is illustrated. The final part of this chapter presents the analyzedmeasurement results to evaluate the antenna sensor’s performance. The de-tails of this chapter are presented thoroughly in a collaborative jorunal paperbetween Okanagan Microelectronics and Gigahertz Applications (OMEGA)Lab and Okaanagan Polymer Engineering Research and Applications (OPERA)Lab, entitled “Smart superhydrophobic textiles utilizing long-range antennasensor for hazardous aqueous droplet detection plus prevention”. It is worthacknowledging that the fabrics, utilized in this work, were studies and fab-ricated in OPERA lab.3.1 BackgroundThe necessity for protecting the professionals across various hazardouswork fields and environments resulted in the creation of protective and353.1. Backgroundshielding equipment, namely personal protective equipment (PPE). PPEwould guarantee workers’ and professionals’ safety, especially when dealingwith toxic agents and radioactive substances [114, 115]. Studies have beenmade to enhance PPEs for pilots [116], firefighters [117], police officers [118],and military personnel [119], where breathability, flexibility, and comforta-bility are known to be a necessity. Efforts have been made for increasingthe safety PPEs towards hazardous chemicals by integrating and fabricatingthem using special chemicals, methods, and materials [120, 121]. Function-alized textiles or films that act as chemical barriers are a critical componentof next-generation PPE. Steinberg et al. demonstrated a breathable vapourtoxicant barrier based on graphene oxide, which allowed the selective trans-fer of water vapour while trapping toxic chemicals such as trichloroethyleneand benzene [120]. Although this technique provided a barrier against selectorganic agents, applications in the biomedical field require the resistance ofwaterborne pathogens to mitigate infection transfer. Studies have recentlydemonstrated the role of aqueous droplets and aerosols in infection trans-mission via talking, coughing, and sneezing [122–124]. This is especiallyimportant considering the recent and rampant spread of the SARS-CoV-2virus, where virus carrier particle sizes < 5µm can remain airborne indefi-nitely, and particles sizes of 100 µm can travel more than 6 m, serving as apath towards viral infection transfer for healthcare professionals or scientistsworking with hazardous pathogens [125]. PPEs have demonstrated a crucialrole in controlling and preventing airborne transmission of viruses, such asSARS-CoV-2 [126].Nevertheless, enhanced PPEs or new fabrication methods and materialsmay not provide enough safety against hazardous chemicals. For instance,some protective materials have limited time to operate, and their protectiveoperation will expire after a certain time. In addition to modified PPEs, pro-tective textiles are candidates to be integrated with sensor structures andcommunication devices to achieve an enhanced level of safety and awarenessthrough sensing and reacting to threats. Recently, the have been reports ofcombining PPE and different types of sensor [127–129] for various purposessuch as: monitoring the heart rate [130, 131], respiratory rate and skin tem-perature [131], detecting liquid or material residues, and chemical threats onPPEs [127, 129, 132]. However, these sensors are complex to fabricate andimplement, too expensive and space-consuming. Antenna sensors could bea candidate to be used for sensing applications on PPEs. These structurescould be embedded in PPE and optimized to sense specific materials, basedon the PPE’s application and the desired material to be sensed.363.2. Inset-fed rectangular patch antenna design and operation principle3.2 Inset-fed rectangular patch antenna designand operation principlePatch antennas have shown to be an ideal choice for operating as anantenna sensor on planar surfaces. Ever since their introduction, variousgeometries of patch antennas (e.g. Figure 3.1 ) have been investigated fordifferent RF/ microwave applications. This section focuses on the designand operation principles of the rectangular inset-fed patch antennas. Theseantennas are substrate printed structures comprising a thin layer of a copper-based radiating patch placed above a ground plane and supplied through amicrostrip feedline [43]. The radiating patch is normally placed 0.003λ0 <h < 0.05λ0 above the ground plane while a substrate is sandwiched betweenthe patch and the ground.Figure 3.1: Various shapes of patch elements (a) Square, (b) Rectangular,(c) Dipole, (d) Circular, (e) Elliptical, (f) Triangular, (g) Disc sector, (h)Circular ring, (i) Ring element. Reprinted with permission of [43]The utilized substrates for patch antenna fabrication vary between 2.2 <r < 12. Low dielectric constant provides better efficiency, wider bandwidthwhile they would cause the structure to consume larger space. On the otherhand, a higher dielectric constant is more preferred for microwave circuits asthey cause the antenna to have a smaller footprint; nevertheless, substrateswith a high dielectric constant have a more significant loss, lower efficiency,and smaller bandwidth [133]. As a result, the substrate choice is dependent373.2. Inset-fed rectangular patch antenna design and operation principleon the patch antenna application. Furthermore, the substrate’s dielectricconstant also affects the fields confined between the top microstrip struc-ture (e.g., the feedline and the radiating patch) and the ground plane. Thesubstrate with a higher dielectric constant has more field concentration in-side the substrate, while the substrate with a less dielectric constant wouldhave more fringing fields on the periphery of the patch (Figure 3.2 (a)).The fringing fields happen due to the finite dimensions of the patchstructure, and they are relative to the length-to-height (i.e., L to h) ratio,which is a small number; however, its effect on the antenna operation cannotbe neglected. These fringing fields happen along the width and the lengthof the patch antenna, where, as presented in Figure 3.2 (b), they are moresignificant along the width of the patch antenna. The fringing fields alongthe patch width are responsible for the antenna’s radiation.Figure 3.2: (a) Fringing effect in a microstrip structure (b) Microstrip fedpatch antenna. The fringing fields of the antenna are observed on the twoends of the patch. Reprinted with permission of [43]3.2.1 Rectangular patch antenna dimensionsIn order to analyze the effect of the fringing fields, the nonhomogeneousdielectric around the patch, consisting of substrate and air, is replaced with ahomogenous effective dielectric constant (Figure 3.3). The effective dielectricconstant is described by the following equation:383.2. Inset-fed rectangular patch antenna design and operation principleeff =r + 12+r − 12(1√1 + 12 hW) (3.1)where h,W, and r epresent the substrate height, the patch width, andthe dielectric constant of the substrate, respectively. The fringing fields ofthe patch cause the patch to be electrically larger compared to its physi-cal dimensions. Therefore, a relation has been introduced to express theextended length of the patch on each end [43]:∆L = ((eff + 0.3)(hW + 0.264)(eff − 0.258)( hW + 0.8)) (3.2)The extended dimensions are only considered for the patch length sincethe fringing fields of a patch are more significant in its width compared tothe length. Accordingly, an effective patch length is introduced, taking intoaccount the extended length of the patch:Leff= L+ 2∆L (3.3)Figure 3.3: Effective permittivity around a microstrip structure . Reprintedwith permission of [43]Consequently, considering h, r of substrate and fr ( e.g., the opera-tion frequency of the antenna) as specified constants, the patch’s width andlength (e.g., physical length) can be calculated by the following relations[43]:W =c2fr√eff− 2∆L (3.4)L =c2fr√eff+12(3.5)393.3. Design, Methodology and SimulationsAs seen in the equation, W presents the patch width, and L presentsthe patch length. In general, the physical patch length has a value between0.47λr and 0.49λr where λr is the wavelength in the substrate λd =λ0√r.Accordingly, due to the half-wavelength structure of the patch antennas,they offer a resonant operation frequency where this operation frequencycan be written as follows [43]:fr =12Leff√eff√µ00(3.6)In this equation Leff , eff , µ0, and 0 represent the effective length ofthe patch (e.g., the sum of physical and extended patch length), effectivepermittivity, free space permeability, and free space permittivity, respec-tively. Equation 3.6 shows that a change in the effective permittivity or thelength of the patch causes a change in the resonant frequency of the patchantenna. This equation confirms that the rectangular patch antenna couldbe used as an antenna sensor to characterize materials or sense deformationsin its structure where these changes would be reflected in the antenna’s effand Leff , respectively, causing a shift in the resonant frequency.3.3 Design, Methodology and Simulations3.3.1 Antenna design and the principle of operationA microstrip patch antenna was designed in the Ansys High FrequencyStructure Simulator (HFSS) to perform long-range sensing. Two meanderlines were placed inside a carved rectangle in the center of the patch antennato create a sensitive region for the antenna sensor when detecting materialsover that region. Two meander-shaped microstrip lines were shorted to theground through a via on one end while they were attached to the antennaon the other end. Based on the quarter-wavelength matching theory [134], amicrostrip line with a length of λ/4 shorted to the ground would operate asa resonator at the associated frequency. The meander lines were designed toresonate at 3.12 GHz. The simulated antenna’s shape and dimensions arepresented in Figure 3.4, where the simulation results (Figure 3.5) showedresonant frequency, resonant amplitude, and far-field radiation gain of 3.11GHz, −27.74 dB , and 5.6 dB, respectively. The antenna was fabricated on aRogers 5880 substrate with permittivity of 2.2 and height of 1.56 mm. Basedon what was discussed in the theory section, utilizing a substrate with lowpermittivity would increase the radiation efficiency of the designed antenna.403.3. Design, Methodology and SimulationsFigure 3.4: Designed antenna sensor structure, composed of the rectangularpatch antenna and meander-shaped resonant structure inside a rectangularslot. The antenna was implemented on a Rogers RT/duroid 5880 substrate.The meander line’s width was set to 0.43 mm, and they merged at theirconnection point to the via.Figure 3.5: Simulation results for the designed antenna sensor structure.413.3. Design, Methodology and SimulationsIt worths mentioning that, the dimensions and structure were achieved af-ter numerous simulations and investigations for different patch widths andlengths. The following sub-section describes the performed study, simula-tions and optimizations for the patch antenna sensor.3.3.2 Patch antenna sensor simulation and optimizationThe initial values for patch width and length were achieved from equa-tions 3.1-3.5. The inset feedline of the patch was modified for a good returnloss acquisition, which minutely altered the resonant frequency of the res-onator. Finite element simulations were performed for three different patchwidths (e.g., 30.5, 36.5, and 40.5 mm) to evaluate the width change impacton the antenna sensor’s performance. As presented in Figure 3.6, widths of30.5, 36.5, and 40.5 mm generated resonant profiles at 3.14 GHz (−27.87dB), 3.11 GHz (−27.78 dB), and 3.07 GHz (−36.30 dB), respectively.The simulation results showed that increasing the patch width decreasedthe operation frequency of the patch antenna. Far-field gain simulationswere also performed for the three mentioned patch widths, and the resultsare presented in Figure 3.7. As observed in the far-field gain simulationresults, the antenna gain for 30.5, 36.5, and 40 mm was 5.20, 5.45, 5.80 dB,respectively, showing that increasing the patch width increased the antennagain. Evaluating the simulation results, we chose a patch width of 36.5 tohave a smaller patch and keep the resonant frequency of the patch antennacloser to the meander-shaped microstrip line’s resonant frequency (e.g., 3.12GHz).Further simulations were performed to investigate the impact of changingpatch length in the resonant frequency of the patch antenna. The simulationresults for 21, 22, 23, 24 mm of length, presented in Figure 3.8, showedresonant frequencies of 3.21 GHz (−12.04 dB), 3.11 GHz (−27.78 dB), 3.01GHz (−1.28 dB), 2.92 GHz (−0.91 dB), respectively. The results showthat decrease in the length of the antenna cause decrease in the resonantfrequency of the antenna. The simulations showed that resonant frequencyshift for change in the length of the patch was ∆f = 96 MHz/mm while∆f= 5.9 MHz/mm for the change in the width of the antenna, confirmingthe higher impact of patch length change in the resonant frequency. Theseresults are also confirmed by observing equations 3.4-3.6. Simulations werealso performed to investigate the far-field radiation pattern of the patchantenna sensor for different lengths, and the results are provided in Figure3.9. The observed gain values for 21 mm and 22 mm were 4.6 dB and 5.3dB, respectively, implying that a small variation in patch length causes a423.3. Design, Methodology and SimulationsFigure 3.6: Simulation results for different widths of the patch antennaFigure 3.7: Radiation pattern simulations for the patch with differentwidths (a) E-planed (b) H-plane.433.3. Design, Methodology and Simulationshuge impact on the patch gain. The gain simulations were not performed for23 and 24 mm lengths as their S11 showed a high return loss, making theminappropriate for radiating power.The simulations showed that the patchlength of 22 mm was the best candidate among various patch lengths due tothe lower reflection magnitude and suitable antenna gain compared to theother lengths.Figure 3.8: Simulation results for different lengths of patch antennaFigure 3.9: Gain pattern simulation results for the patch with differentlengths (a) E-planed (b) H-plane.Further simulations were conducted to investigate the sensitivity of thedesigned antenna sensor, using a Rogers RT/duroid 6006 substrate sample.443.3. Design, Methodology and SimulationsSimulations were conducted by locating the samples in different locations ofthe antenna sensor. As illustrated in Figure 3.10(a), the Rogers 6006 samplewas placed on the patch surface (position A), transmission line (positionB), patch width (position C), and meander lines (position D). Simulationresults (Figure 3.10 (b)) showed that positioning the sample on the patchsurface (Position A) only increased the resonant amplitude of the antennasensor to −26.37 dB compared to the bare antenna sensor (3.11 GHz and−27.94). The minimal change in the resonant profile was due to the smallsize of the sample, which resulted in a negligible impact on the patch’seffective permittivity. The sample was then positioned on the antenna’stransmission line (Position B), where it caused a change in the resonantfrequency (resonant amplitude) to 3.10 GHz (−16.66 dB).Figure 3.10: (a) Sample location for simulations and (b) simulation resultsfor different sample locationsThe resonant profile change was due to the sample’s impact on the an-tenna sensor’s matching circuit. The sample was also positioned along thepatch width (Position C), where it generated resonant frequency and res-onant amplitude of 3.09 GHz and −29.31 dB, respectively. The samplelocated along the patch width (position C) caused more frequency shiftcompared to the patch surface (Position A) and the transmission line (Posi-tion B). This was due to the fringing fields along the patch width (Position453.3. Design, Methodology and SimulationsFigure 3.11: E-field concentration on the meander lines, without material(Left) and with material (Right).C), which caused a good material-wave interaction. Thus, more frequencyshift was observed for position C. The meander-shaped structure (PositionD) was the final location for sensing evaluation. The simulation revealeda higher frequency shift for the meander structure compared to the othersections of the antenna sensor. The observed resonant frequency and reso-nant amplitude were 3.05 GHz and −16.89 dB, respectively. This showedthe better sensing performance of the meander lines compared to the otherlocations on the antenna sensor’s structure. E-field concentration of the me-ander lines was also simulated for two scenarios, with and without sample.As shown in the results for these simulations (Figure 3.11), the meander-shaped structure on the antenna had a high E-field concentration comparedto the other regions of the antenna sensor. Moreover, the E-field concentra-tion on the periphery of the meander lines was higher for the antenna witha sample on its surface as compared to the bare antenna, showing a highmaterial-sensor interaction.According to simulation results observed in Figure 3.10 and Figure 3.11,the meander microstrip line region was chosen for sensitivity investigation.Positioning the sample on the meander line caused a change in the effectivepermittivity, resulting in a resonant frequency shift in the antenna sensor’sresponse, as shown in Figure 3.12. As dictated by equation 3.6, a changein the effective permittivity causes a change in the antenna’s resonant fre-quency. The simulation results (Figure 3.12) show that the operationalfrequency was 3.049 GHz for the antenna with the sample on its meanderline, which resulted in a 62 MHz frequency shift compared to the initial 3.11GHz operating frequency.463.3. Design, Methodology and SimulationsFigure 3.12: Simulation results for the designed antenna, with and withoutsample on its surface.Figure 3.13: Antenna gain in (a) E-plane and (b) in H-plane.So far, the meander-shaped structure was confirmed as the sensitiveregion of the antenna sensor for positioning material samples. The samplematerial was positioned on this region, and a significant frequency shift wasobserved. Still, further simulations were required to investigate the impactof a Rogers 6006 sample on the far-field antenna gain. Therefore, simulationswere performed for the antenna sensor’s far-field gain to observe the gainchange in the presence of the sample, e.g., Rogers 6006, on the meanderlines. Sample placement on the meander microstrip lines of the antenna473.3. Design, Methodology and Simulationssensor caused a gain reduction to 3.7 dB (Figure 3.13). The simulationresults indicated that the designed antenna sensor would be able to sensethe samples by a change in its S11 parameter and transmit the alterations toa distanced receiver. The simulations also showed that the antenna sensormaintained a good gain profile and radiation pattern even with a sample onits surface, as shown by the relatively high gain. The presence of the sampleon the antenna sensor did not cause significant attenuation in the antenna’sperformance.Simulations were also performed to evaluate the sensitivity of the sensorto water with different conductivity values. A water sample with a dielectricconstant of r = 81 was used in these simulations while the sample material’sconductivity was altered between 0 and 4 S/m. The conductivity of σ =0 S/m represented pure water, and a sample with σ = 4 S/m was used torepresent seawater. The achieved results are presented in Figure 3.14. Thesample with σ = 0 S/m had a resonant frequency and resonant amplitudeof 3.03 GHz and −13.7 dB, respectively. The results show that increasingthe conductivity would cause the sensor’s resonant frequency to shift toa higher frequency, while the resonant amplitude would also increase. Forexample, a sample with a conductivity of σ = 1 S/m would shift the resonantfrequency and resonant amplitude to 3.045 GHz and−12.79 dB, respectively.Furthermore, a sample with σ = 2 S/m and σ = 4 S/m demonstrated aresonant frequency of 3.048 GHz and 3.052 GHz, respectively, while theachieved resonant amplitude for the samples was −12.07 dB and −10.79dB, respectively. These results indicate that the meander-shaped structuredemonstrated good sensing performance for materials with low conductivity.Moreover, the simulations show that the designed antenna sensor structureis able to sense water samples with different conductivity. The simulationswere followed by the fabrication of the antenna sensor and the measurementof its performance.3.3.3 MaterialsTetraethyl orthosilicate (TEOS, 98%) and ammonium hydroxide (28.0 –30.0%) were purchased from Aldrich and used without further purification.(Heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethoxysilane (95 – 100%) wassupplied from Gelest. Ammonium persulfate (410) was purchased from MGChemicals. Ethanol (≥ 96%) and methanol (≥ 99.8%) were purchased fromVWR International (Radnor, PA). A Rogers RT/duroid 5880 was used tofabricate the microwave structures and a Keysight N5222B Vector NetworkAnalyzer (VNA) was used for the microwave spectrum measurements. Fi-483.3. Design, Methodology and SimulationsFigure 3.14: Simulation results for water with conductivity values rangingbetween 0 to 4 S/mnally, nylon/polyurethane fabrics were used as the protective textiles suitedfor PPE with a 91/9 nylon to polyurethane ratio.3.3.4 Preparation of superhydrophobic fabricsSuperhydrophobic fabrics were prepared by applying a two-step formu-lation via dip-coating. The surface roughness of the fabrics was increasedby the in-situ growth of silica nanoparticles on the surface. The nano-scalerough surface was then covered by low energy surface fluorosilane (as shownin Figure 3.15). For the first step, a methanol/deionized water mixture witha weight ratio of 70/30 % was prepared. 6 vol % of TEOS was added tothis and magnetically stirred for 30 min to achieve a homogeneous solution.The fabric was then immersed in the TEOS solution with L:R (liquor togoods ratio) of ∼ 17.5 w/w while mixing the solution at 1200 – 1400 rpm.The temperature was then elevated to 40 oC. 3.4 vol % of ammonium hy-droxide was added dropwise to the mixture, and the reaction was continuedfor 40 min. The nanoparticles were formed, and the solution changed to anopaque colloidal suspension. The fabric was then removed from the nanopar-ticle mixture and dried for 2 h at room temperature. In the second step,four different concentrations of the perfluorinated silane in methanol (0.5,1, 2, and 4 wt %) were prepared to find the optimum silane concentration.493.3. Design, Methodology and SimulationsThe silica-treated fabrics were dipped in the fluorosilane solutions with theL:R of ∼ 17.5 w/w at 25 oC for 10 min and then cured at 180 oC in an ovenfor 10 min.Figure 3.15: Schematic of superhydrophobic fabric preparation usingthe dip-coating of silica nanoparticles followed by (Heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethoxysilane3.3.5 Wettability analysis, surface characterization, anddurability evaluationStatic water contact angle measurements were performed using the ses-sile drop technique. 15 µL droplets of distilled water were deposited on thesurface utilizing a 2 mL syringe (Gilmont), and the water contact angle wasmeasured using a Rame´-hart goniometer (Model 100-25-A). Surface mor-phology and elemental analysis of the fabrics were obtained from a scanningelectron microscopy (Tescan Mira 3 XMU SEM) equipped with an energydispersive X-ray analysis (EDS) module. A thin layer of platinum (∼ 20 nm)was coated onto the textile surface to improve its electrical conductivity.The elemental analysis was performed using EDS in point analysis mode503.4. Antenna Sensor Fabrication and Experimental Setupat 1.0 kX magnification and the weight percentages of carbon (C), oxygen(O), silicon (Si), and fluorine (F) were scanned. The durability of the fab-rics was studied in terms of washing fastness and abrasion resistance. Thewashing fastness test was carried out according to ISO 105 C06. Briefly, themodified textiles with 100× 40 mm (L×W ) dimensions were immersed in4% w/v AATCC 1993 standard reference detergent without optical bright-ener and stirred mechanically at 40 oC. The textiles were removed after30-min stirring cycles with 40 rpm, rinsed twice with warm water (40 oC)and dried at ambient temperature. The abrasion resistance of the treatedfabrics was investigated using Martindale abrasion according to ISO 12947.The modified textiles were mounted to the instrument and abraded with afelt abrader in a standard Lissajous pattern. The water contact angle ofwashed or abraded textiles was measured after each washing or abradingcycle.3.4 Antenna Sensor Fabrication andExperimental SetupThe simulated planar patch antenna sensor was fabricated on a Rogers5880 substrate with a dielectric thickness of 1.56 mm, a relative permittivityof 2.2, and a loss tangent of 0.0009, with copper cladding on both sides. Aconventional chemical etching method for printed circuit board fabricationwas utilized to manufacture the antenna with a fabrication error better than+/− 0.1 mm. The sensor pattern was laser printed on glossy paper andheat-transferred on the substrate to preserve the copper from the etchant.The substrate was then placed inside a solution of ammonium persulfateetchant for 30 minutes to implement the antenna sensor structure on thesubstrate surface. After 30 minutes, the patch structure was washed withacetone to remove the ink, and a Subminiature version A (SMA) connectorwas soldered to the edge of the microstrip feed structure.Two fabric swatches with a size of 10×10 cm and a thickness of 0.6 ± 0.1mm were prepared to study the performance of the antenna sensor for liquiddetection in the presence of the treated and untreated fabrics. Either treatedor untreated fabrics were placed over the antenna to cover the antenna sen-sor’s top surface. The fabrics were pinned to Styrofoam to avoid mechanicalmovement and electrical instability during the experiments. Prior to theexperimental testing of the sensor and based on the fabric used in this work,the antenna sensor had to be calibrated indoors and outdoors in order toavoid the effects of humidity and temperature in the sensing performance of513.4. Antenna Sensor Fabrication and Experimental Setupthe antenna sensor. This calibration allowed the sensor to operate reliablyat a greater range of temperatures and humidity.Further baseline measurements and calibration processes were performedbefore running the experiments to suppress the impact of initial values suchas the fabric thickness and the coating on the final measurements of theliquid samples. Due to the incompatibility of the coating material on thetreated fabric with alcohols and acetone, different volumes of water wereused to validate the performance of the textile-covered antenna sensor. Thedeposited water droplet volume was 4, 8, 12, 16, and 20 µL. Initially, threedifferent regions were chosen on the antenna sensor’s surface to investigatethe sensitivity of the device and compare the results to the simulated values.The transmission line, antenna surface and meander shaped structurewere used for a sensitivity investigation. Furthermore, two sets of experi-ments were performed to evaluate the antenna sensor’s performance. In thefirst experiment, the reflection coefficient (S11 parameter) of the antennasensor was used to distinguish between different liquid samples on the sensor.A measurement setup was prepared in which a VNA measured the reflec-tion coefficient. The VNA was calibrated for 2001 points, 5 dBm microwavepower, and an IF bandwidth of 300 Hz. Broadband microwave absorbers(600 MHz - 40 GHz) from ECCOSORB, AN-79, sized 24” × 24” × 4.49”were utilized under the antenna which surrounded the measurement setup,in order to reduce the impact of reflection and interfering signals on themeasured profile of the antenna sensor.In the second test, the long-range performance of the antenna sensorwas studied by measuring a transmitted signal (S21 parameter) from theantenna sensor to an ultra-wideband antenna (UWB). The measurementsetup consisted of a RFspace TSA 900-12000 ultra-wideband PCB taperedslot antenna with an operational frequency range of 900 MHz to 12 GHz, thesame Keysight N5222B VNA, and the antenna sensor. The distance betweenthe antenna sensor and the interrogator antenna was fixed at 2.3 m. Here,the transmitted signal of the patch antenna sensor would be absorbed bythe UWB slot antenna (the readout device), where it would be displayed inthe readout as a transmission (S21) parameter. The VNA was calibrated for3601 points, 5 dBm power, and an IF bandwidth of 300 Hz. Both the sensorand interrogator antennas were aligned to minimize the Polarization LossFactor (PLF) as both exhibited linear polarization. The same microwaveabsorbers described above were used to minimize any interference signalsduring the experiment.523.5. Results and Discussion3.5 Results and Discussion3.5.1 Performance of Superhydrophobic FabricsThe superhydrophobic fabrics formed a robust Cassie-Baxter state on thesurface in which air pockets were trapped in the surface’s porosity and thecapillary resistance prevented the transition from the non-wetting Cassie-Baxter state to the wetted Wenzel state. Silica nanoparticles were used toincrease the nano-roughness on the fabric’s surface, and the surface energywas reduced by applying fluorosilane on the nano-roughened surface. Uponpreparing the superhydrophobic fabric, water contact angles were measuredfor each concentration of fluorosilane used in the solution. The water contactangle showed a significant increase from 152° ± 7° to 164° ± 7° as the fluo-rosilane concentration increased from 0.5 wt % to 1 wt %, and then slightlydecreased to 156° ± 3° and 160° ± 3° by further increasing the fluorosilaneconcentration to 2 wt % and 4 wt %, respectively (see Table 3.1).These results indicated that grafting the fluorosilane to the hydroxy-rich silica nanoparticle surfaces was sufficient with 1 wt % of fluorosilane.Increasing the fluorosilane concentration further likely produced unwantedside reactions, e.g., self-condensation of the tri-functional molecules. There-fore, the solution of 1 wt % fluorosilane in methanol was chosen as theoptimum concentration as it displayed the highest contact angle and a goodcombination of surface nano-roughness and low surface energy (see Figure3.16). This sample was chosen for further surface morphology and durabilitycharacterization.The surface morphology of the untreated fabrics and optimized treatedfabrics are presented in Figure 3.16 (a) and Figure 3.16 (b), respectively.The fibers of untreated textiles exhibited a smooth surface with minimalroughness in the nanometer scale. The elemental analysis of the surfacerevealed 56.8 wt % C, 43.2 wt % O, 0 wt % Si, and 0 wt % F on the surface.After treating the surface of the fabric, the spherical nanoparticles weredetectable on the surface with an average particle size of 2128 nm. Theelemental analysis of the surface showed 51.4 wt % C, 38.0 wt % O, 9.2 wt% Si, and 1.5 wt % F, indicating successful deposition of silica nanoparticlesand fluorosilane on the fabric surface.The washing fastness of the treated superhydrophobic fabrics is pre-sented in Figure 3.17a. The water contact angle of the treated fabrics with1 wt % fluorosilane was 164° ± 7° before washing and was later reducedto 145° ± 2° after 15 washing cycles, therefore still exhibiting water repel-lency. The water contact angle of treated fabrics was also measured after533.5. Results and DiscussionTable 3.1: Static water contact angle of treated fabrics with various con-centrations of (Heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethoxysilane inmethanol.Formulation Fluorosilane percentage (wt %) Water contact angle (°)F-0.5 0.5 152 ± 7F-1 1 164 ± 7F-2 2 156 ± 3F-4 4 160 ± 3Figure 3.16: SEM images of (a) untreated fabrics, and (b) fabrics treatedwith the silica nanoparticles with the optimized fluorosilane solution (themagnified view is presented in the lower right panel), (c) the abraded super-hydrophobic fabrics after 1000 Martindal.100, 250, 500, and 1000 Martindale abrasion cycles according to ISO 12947.The water contact angle of the treated fabric decreased from 164° ± 7° to145° ± 4° after 100 abrasion cycles. It then further reduced to 136° ± 3°after 500 cycles, and it finally plateaued at approximately 1313, as shown inFigure 3.17 (b). The SEM image of the superhydrophobic fabric after 1000Martindale abrasion cycles is presented in Figure 3.16 (c).Although the fabric exhibited a significant decrease in the number of543.5. Results and DiscussionFigure 3.17: The washing fastness of the superhydrophobic fabrics accord-ing to ISO 106 C06. (b) The abrasion resistance of the superhydrophobicfabrics, according to ISO 12947.adhered nanoparticles, the ones still adhered to the surface of the fabricsresulted in a water contact angle greater than 130°. These experimentsdemonstrated the durability of the treated fabric and indicated that thefabric finish was suitable for long-term and reusable PPE applications.3.5.2 Antenna PerformanceFigure 3.18 presents the fabricated antenna sensor, the dimensions ofmatching lines, and the size of the patch and meander lines. Two col-ored water droplets were deposited on the treated fabric to demonstratethe enhanced hydrophobicity compared to the untreated one (Figure 3.19(b,c)). Consider that these fabrics were used to cover the antenna sensors553.5. Results and DiscussionFigure 3.18: Fabricated antenna sensor with a patch length (L) of 36.5mm, patch width (W) of 22 mm, meander microstrip line length of 42 mmand width of 0.5 mm.Figure 3.19: (a) The antenna sensor covered with untreated fabric (wateris shown absorbed into the fabric) and (b) The antenna sensor covered withthe treated fabric (dyed water is shown beading up on the fabric).563.5. Results and DiscussionFigure 3.20: Measured S11 profile of the bare antenna sensor, comparedwith treated and untreated fabrics.surface and resemble a PPE in our experimnets. The measured scatter-ing parameter (S11) of the fabricated antenna sensor was compared againstthe antenna sensor’s response when covered with the treated and untreatedfabrics (Figure 3.20). Covering the antenna with the fabrics affected theresonant frequency and the resonant amplitude of the S11 profile by shiftingthe resonant frequency to a lower value, from 3.22 GHz for the bare res-onator to 3.206 GHz and 3.201 GHz for the untreated and treated fabrics,respectively. Similarly, the resonant amplitude changed to −25.0 dB and−22.4 dB for the untreated and treated fabrics, whereas the bare antennasensor demonstrated a resonant amplitude of −23.3 dB.The change in the S11 response of the antenna sensor could be related tothe permittivity change in the antenna sensor’s proximity, where in this case,the covering fabric affects the permittivity of the antenna sensor, eff . Whenrelating to equation 3.6, the resonant profile, namely the resonant frequency(f0), was affected when the effective length retains its value while the per-mittivity is changed. The S11 response of the antenna can be explained byS11 = (Z−1)/(Z+1), Z = Zin/Z0, where Z0 is the characteristic impedanceof the structure, which equals 50 ohms as a standard number for microwavesystems.In this case, the input impedance (Zin) of the antenna sensors is reliant573.5. Results and Discussionon structure size and design, substrate material, and the dielectric constantof the materials in the antenna sensor’s environment. Covering the surfaceof the antenna sensor with fabrics and later adding liquids changes the per-mittivity of the antenna sensor’s environment, which would cause changes inthe S11 response of the antenna sensor. The down shift in frequency agreeswith the simulation analysis, although the resonant amplitude differs due tothe local permittivity being vastly different in the simulation environment(RT/duroid 6006) as opposed to an experimental trial incorporating treatedor untreated fabrics.3.5.3 Liquid sensing with the antenna sensorA measurement setup was prepared for a single port (S11) test to performliquid sensing, as depicted in Figure 3.21. Three different regions on theantenna sensor were investigated to confirm the antenna’s most sensitiveregion for material sensing purposes, as compared to the simulation. Theresults of these experiments are compared in Figure 3.22, where the designedmeander shape microstrip line region demonstrated the highest sensitivityto an 8 µL droplet of water. The bare antenna displayed a return lossof −23.4 dB at a resonant frequency of 3.22 GHz. By drop-casting the 8µL droplet of water on the sensitive region of the antenna, the impedancechanged between the antenna and VNA’s power source. This changed thepower transmitted to the antenna, and the resonant frequency subsequentlyshifted to 3.17 GHz, a 50 MHz reduction. The other regions of the antennaand transmission line displayed no sensitivity to the water droplet as nofrequency shift was displayed in the measurement results. However, it shouldbe noted that although an 8 µL water droplet exhibits a diameter > 1 mm,large droplets from sneezes, coughing, or splashes from industrial chemicalsin hazardous workplaces can still exceed this droplet size after accumulationand coalescence. Moreover, some studies show that the droplet sizes insneezes or coughs are not limited to a specific value and that there is acontinuum of droplet size in a cough or sneeze [135, 136].The antenna sensor’s performance was validated by detecting 4 to 20µL water droplets deposited over the sensitive region of the antenna sensor,both while the sensor was bare and while it was covered by the treated oruntreated fabrics. The results in Figure 3.23 show a significant change in theresonant frequency for different volumes of water. The measured resonantfrequencies were 3.21, 3.19, 3.18, and 3.17 GHz for 4, 8, 12, 16, and 20µL droplets, respectively. The results validate the antenna performance fordetecting small volumes of water from 4 to 20 µL. Moreover, the measured583.5. Results and DiscussionFigure 3.21: Antenna sensor’s one port measurement setup.Figure 3.22: sensitivity comparison on the different regions over the antennasensor.593.5. Results and Discussionresults for treated and untreated fabrics are compared in Figure 3.24 (a) andFigure 3.24 (b), where the sensitivity of the sensor covered with the treatedfabric to the water droplet was found to be 350 kHz/µL (R2 = 0.942). Incontrast, the sensor’s sensitivity was found to be 1.1 MHz/µL (R2 = 0.866)when it was covered with the untreated fabric. The sensitivity differencebetween the coated and uncoated fabric was expected due to the coatedfabric’s tendency to repel the water droplet, keeping it further from thesensitive region of the sensor. Therefore, the water droplet had less impacton the effective dielectric properties in the sensor’s approximate environmentfor this condition.The resonant amplitude of the antenna sensor also demonstrated goodsensitivity to the presence and volume of the water droplets deposited on thetreated and untreated fabrics. However, the resonant amplitude should notbe used as the sensor’s response since the amplitude in an antenna sensorvaries with the distance between the antenna sensor and the interrogatorantenna, multipath signal reflection, and unknown obstacles. It should benoted that a slight deviation from linearity was experienced with the treatedfabric due to the exact placement of the droplet on the fabric. However,these results are still valid due to the high goodness of fit, R2 = 0.942, andthe distance between the droplets was within the micrometre range. Thisdeviation can be seen to a greater extent with the untreated fabric (R2 =0.866). Here, the water was able to spread randomly in many directionsand wet the fabric surface, which caused a more significant deviation, as theeffective permittivity changed over a larger area above the resonator. Thisexperiment showed that using superhydrophobic textiles in PPE providesgreater protection for professionals that require safety against hazardousliquids soaking into the fabric. Although the sensitivity of the microwaveresonator was compromised to a value of 750 MHz/µL, both sensing con-ditions achieved a high standard for sensing potentially hazardous aqueousliquids.3.5.4 Long-range liquid detection on the treated anduntreated fabricA measurement setup was prepared to validate the antenna sensor’s per-formance by measuring the transmission signal (S21-parameters) betweenthe bare antenna sensor and an interrogator antenna located 2.3 m away.Figure 3.25 shows the experimental setup for the long-range measurement ofthe antenna. Transmission measurements between the antennas were com-pared for the bare antenna, the antenna covered by the treated fabric, and603.5. Results and DiscussionFigure 3.23: Comparison of S11 parameters for a bare sensor with differentvolumes of water.Figure 3.24: Comparison of the resonant notch frequency and the resonantamplitude of the antenna sensor for (a) untreated fabric and (b) treatedfabric.613.5. Results and Discussionthe antenna covered by the untreated fabric (Figure 3.26).Figure 3.25: Setup for long-range liquid detection on fabric.The results showed that the resonant frequency and the resonant ampli-tude changed from 3.231 GHz and −34.14 dB for the bare antenna sensor to3.194 GHz and −42.47 dB, and 3.191 GHz and −34 dB for the sensor cov-ered by the treated and untreated fabrics, respectively. These values for theresonant frequency and resonant amplitude were used as the baseline for theantenna sensor’s response for the rest of the experiments with water droplets.Droplets with volumes of 4-20 µL were placed over the sensitive region of theantenna to demonstrate the efficacy of the antenna sensor in liquid dropletdetection for the treated and untreated fabrics. Figure 3.27(a) comparesthe resonant frequency and the resonant amplitude for the untreated fabric,where the 4-20 µL water droplets were deposited on the sensing region. Dueto the wetting and spreading of the droplet into the fabric, the resonant623.5. Results and DiscussionFigure 3.26: Transmission coefficient (S21) measured results for bare an-tenna, antenna covered with treated fabric and covered with untreated fab-ric.Figure 3.27: The resonant frequency and resonant amplitude response ofthe antenna sensor covered by (c) untreated and (d) treated fabrics for watervolumes deposited over the sensitive region, ranging from 4 to 20 µL.633.5. Results and Discussionfrequency was downshifted by 90 MHz and the resonant amplitude dropped4 dB for one droplet of 20 µL. As shown in Figure 3.27(a), increasing thewater droplet volume on the untreated fabric not only created a large down-ward frequency shift but also degraded the quality factor from 67 with nodroplet to 40 with a 20 µL droplet. Figure 3.27(b) presents an experimentwith the same volumes on the same spot of the antenna sensor but coveredwith the treated fabric. The presence of a 20 µL water droplet shifted theresonant frequency for 12 MHz, which was around 8 times smaller than thesensor’s response with untreated fabric.As observed in the measurement results, positioning water droplets onthe fabric caused changes in the effective permittivity of the antenna sensor’ssensing region, where it subsequently caused a shift in the resonant frequencyaccording to equations 3.1-3.5. A smaller shift in the resonant frequency wasobserved for the treated fabric as compared to the untreated fabric. Thiswas due to the hydrophobicity of the treated fabric, which prevented thewater sample from soaking into the textile, causing a smaller change in theeffective permittivity of the sensing region (Figure 3.25 coloured drop-inset).Comparing the −3 dB quality factor of the measured S21 profile revealedthat the quality factor of the treated fabric was slightly larger than thequality factor of the untreated fabric when the water droplets were on thefabric. This difference could be utilized to identify treated or untreatedfabrics and monitor the life-time of the treated fabric.Table 3.2 summarizes the resonant parameters of the antenna sensor withtreated and untreated fabrics for 4 and 20 µL droplets of water. The resultsshow that the designed antenna sensor was capable of sensing the presenceof water on its surface and transmitting the sensing data as a signal to adistanced receiver for both treated and untreated fabrics.Although the antenna sensor had a greater sensitivity to liquids while theuntreated fabric was placed on its surface, a greater deviation from linearitywas observed. This was due to the water that had soaked into the untreatedfabric, as opposed to the water beading up on top, which was observed withthe treated fabric. On the other hand, the treated fabric caused an airgap between the antenna sensor and the sample, which reduced the sensor’ssensitivity. Regardless, the antenna sensor showed a 12 MHz shift for 20µL, which is sufficient for detecting hazardous aqueous liquids.3.5.5 Chapter summaryIn this chapter, a textile-embedded antenna sensor was developed tosense hazardous aqueous liquids on textiles, e.g., PPE, which demonstrated643.5. Results and DiscussionTable 3.2: Impact of Water Droplets on the Antenna Sensor’s ResponseAntenna SensorResonantFrequency(GHz)ResonantAmplitude(dB)Quality FactorUntreated FabricNo Drop 3.192 -33.88 674 µL Drop 3.169 -34.87 5420 µL Drop 3.102 -38.25 40Treated FabricNo Drop 3.196 -42.7 704 µL Drop 3.193 -42.8 6920 µL Drop 3.189 -43.1 66high sensitivity to detect small volumes of liquids on treated and un-treatedfabrics ranging between 4-20 µL. The developed antenna sensed the waterdroplets on both fabrics (treated superhydrophobic and untreated fabric) forthe single port (S11) and transmission (S21) measurements. The presence ofwater droplets caused the resonant frequency and resonant amplitude shifts,where these parameters were monitored for using a VNA. This sensing re-sponse certifies the antenna sensor’s application to detect hazardous aqueousliquids on the outer layer of PPE fabrics.65Chapter 4MXene-based planar antennafor VOC sensing applicationThis chapter reports on designing and fabricating a MXene antenna sen-sor for VOC and humidity sensing. The utilized MXene membrane wasa Ti3C2 MXene that previously showed electrical conductivity as high as6.5 × 106 S m–1 [137]. The proposed structures are used for long-rangesensing applications where a wideband antenna was used to interpret thechanges in the MXene antenna wirelessly. The chapter begins with a dis-cussion on the fabrication of the Ti3C2Tx MXene membrane. The design,optimization and performance evaluation of the MXene antenna will then beinvestigated by simulating the structures in Ansys HFSS software, fabricat-ing the structures, testing and analyzing their performance. Later, the VOCsensing performance of the MXene antenna sensor is investigated and dis-cussed in detail. The final MXene antenna sensor design and the evaluationwere achieved after weeks of numerous design, simulations, optimizations,testing. The details regarding some part of this chapter are presented thor-oughly in a papers entitled “MXene Membrane in Planar Microwave Reso-nant Structures for 5G”. It is worth mentioning that the MXene membrane,utilized in this work was received from UBC’s Nanomaterials and PolymerNanocomposites Laboratory (NPNL), and Center of Materials Science andEngineering at Beijing Jiaotong University4.1 BackgroundMXene, a growing family of two-dimensional (2D) materials, has been in-tensively studied due to their potential applications in energy storage [138],electromagnetic-interference shielding [139], catalysis [140] and biosensor[141]. Among several MXenes reported to date [142, 143], titanium carbide(Ti3C2Tx) has received the most significant attention from the microwavecommunity owing to its high conductivity (up to 10,000 S/cm), excellentflexibility [144], and superior mechanical strength [145]. Ti3C2Tx MXene,664.2. Experimental section for MXene fabricationwhich can be synthesized by selectively etching aluminum (Al) layers fromthe metallically conductive bulk MAX phase [146], also offers the flexibilityto terminate its surface with functional groups, such as oxygen (-O), hy-droxyl (-OH), and fluorine (-F) [147]. Exploiting this feature, Lee et al. havedemonstrated the potential of an interdigital capacitor consisting of Ti3C2TxMXene as interface material to detect VOC gases (Ethanol, Methanol, Ace-tone, and ammonia) at room temperature [23]. Ti3C2Tx MXene was alsoemployed by Kim et al. as a metallic channel in a chemiresistive sensor thatcould detect VOC gases as low as 50 parts per billion [19]. Thus, MXenehas demonstrated its outstanding potential to implement low-profile high-efficiency microwave antennas and high-performance VOC gas sensors asan interface material. Despite the mentioned potentials, to the best of ourknowledge, no previous work has reported a MXene resonator-based multi-functional microwave platform, serving as both a gas sensor and an antennato transmit the sensed data wirelessly.4.2 Experimental section for MXene fabrication4.2.1 MaterialsSynthesis process of the initial Ti3AlC2 powder (200 mesh) used in thiswork can be found in our previous work [32]. Lithium fluoride (LiF, 99.995%)was purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloricacid (HCl, 36-38 wt.%) was purchased from Beijing Tongguang Fine Chemi-cals Company. Mixed cellulose membranes (pore size 0.22 µm, 50 mm) werepurchased from Shanghai Xin Ya Purification Equipment Co., Ltd.4.2.2 Preparation of multilayer Ti3C2TxThe multilayer Ti3C2Tx (m-Ti3C2Tx) powder was obtained using a typ-ical selective etching method. In general, 2 g of LiF and 40 ml of 9M HClsolution were mixed in a Teflon conical, stirred for 15 min at room tem-perature (RT) to create a homogeneous etching solution. Then, Ti3AlC2powder was carefully added to the LiF-HCl etching solution with a ratioof 1 g Ti3AlC2: 20 ml etchant and left at 35oC for 24 h while magneticstirring the mixing solution. After that, the obtained Ti3C2Tx suspensionwas washed several times using deionized (DI) water until the pH of the su-pernatant reached neutral. Finally, the black Ti3C2Tx sediment was driedin a vacuum oven at 60 oC for 48 h.674.2. Experimental section for MXene fabrication4.2.3 Assembly of Ti3C2Tx membrane1 g of m-Ti3C2Tx powder was redispersed in 200 ml of DI water and ultra-sonically delamination for 1 h. Then, the delaminated Ti3C2Tx (d-Ti3C2Tx)supernatant was collected after being centrifuged for 1 h at 4000 rpm. Theconcentration of d-Ti3C2Tx in the resulting supernatant is approximately4 mg/ml. Ti3C2Tx MXene membranes were assembled by vacuum-assistedfiltration (VAF) using a hydrophilic mixed cellulose membrane. The free-standing Ti3C2Tx membranes were obtained after drying at RT for 1 h anddetached from the cellulose membrane.4.2.4 CharacterizationThe phase composition of the samples was carried out by a RigakuUltima IV X-ray diffractometer (XRD) using Cu Kα radiation. The mi-crostructures and morphologies of initial Ti3AlC2, Ti3C2Tx powder andmembranes were studied using a ZEISS Gemini SEM 500 field emissionscanning electron microscopy (FE-SEM).4.2.5 Characterization of Ti3C2Tx powder and membraneThe assembly strategy of the free-standing Ti3C2Tx membrane is il-lustrated in Figure 4.1. The m-Ti3C2Tx was firstly prepared by etchingTi3AlC2 in the LiF-HCl etching solution. This is confirmed by the completedisappearance of the Ti3AlC2 characteristic peaks in the etched sample (Fig-ure 4.2 (a)). Note that the shift of (002) peaks from 9.52° to 6.66° indicatesits corresponding interplanar spacing has expanded from 9.28 A˚ to 13.26 A˚.The morphological transition from initial Ti3AlC2 to m-Ti3C2Tx was shownin the SEM micrographs (Figure 4.2 (b, c)). It can be seen that the initialTi3AlC2 presents a typical MAX structure with densely layered morphol-ogy (Figure 4.2 (b)), but a separated accordion-like structure was observedin m-Ti3C2Tx (Figure 4.2 (c)). This is ascribed to the selective removal ofAl atoms from Ti3AlC2. After further ultrasonication treatment, the m-Ti3C2Tx powder was delaminated and formed a stable d-Ti3C2Tx colloidalsolution, as evidenced by its strong Tyndall effect (Figure 4.1), indicatingthe excellent hydrophilicity and dispersity of d-Ti3C2Tx in water. As shownin the bottom right in Figure 4.1, the well-delaminated d-Ti3C2Tx colloidalsolution is the prerequisite to assemble Ti3C2Tx membrane by VAF. Theoptical photos of the Ti3C2Tx membrane are shown in the bottom left ofFigure 4.1. It is obvious that the Ti3C2Tx membrane can be easily detached684.3. Fabrication of the antenna sensor and experiment setupfrom the filter membrane without any cracks, which presents excellent flexi-bility and strong resistance to bending. The SEM observations from the topview showed that the surface of such Ti3C2Tx membrane was smooth withthin ripples on it. The fractured edge of the Ti3C2Tx membrane (≈ 3 µmin thickness) exhibited with a well-aligned lamellar nanostructure throughalmost the entire cross-section, confirming the d-Ti3C2Tx can be easily as-sembled to a free-standing membrane using the VAF technique.Figure 4.1: Schematic illustration of the assembly process of Ti3C2Txmembrane.Following the Ti3C2Tx MXene fabrication, a MXene antenna sensor wasimplemented to investigate its microwave performance as an antenna, byusing the prepared Ti3C2Tx MXene.4.3 Fabrication of the antenna sensor andexperiment setupTo investigate the microwave capabilities of the Ti3C2Tx MXene mem-brane, a microstrip feed structure was fabricated on a Rogers 6010 substratewith a thickness of 1.91 mm, permittivity of 10.2, and dielectric loss tangentof 0.0023, using a conventional chemical etching method [148]. The scatter-ing parameters (S -parameters) of the fabricated Ti3C2Tx MXene membranewere measured by a Keysight N5222B Vector Network Analyzer (VNA) to694.3. Fabrication of the antenna sensor and experiment setupFigure 4.2: a) XRD patterns of initial Ti3AlC2 and m-Ti3C2Tx. SEM im-ages of (b) Ti3AlC2, (c) m-Ti3C2Tx, (d, e) surface and (f) cross-section ofTi3C2Tx membrane.investigate the microwave performance of the fabricated structures. TheVNA was calibrated for the frequency range of 1 to 5 GHz with 4001 pointsand an IF bandwidth of 300 Hz. Sub Miniature version A (SMA) connectorswere used to connect the microstrip structures to the VNA.An additional experiment setup was prepared to study the long-rangegas sensing capability of the MXene antenna sensor. The MXene antennasensor was located in a custom-made gas sensing chamber positioned be-neath a receiver antenna. The gas chamber included a liquid container anda fan, where the former was used for deposition of the VOC liquids, andthe latter was used to improve the airflow within the chamber, acceleratingthe evaporation and recovery sequences of the VOCs during the experiment.The chamber was insulted to prevent gas leakage, maintaining a constantVOC concentration during the evaporation/adsorption sequence. The VOCmeasurements were performed using standard liquids (Acetone, Isopropylalcohol, Ethanol, and Methanol) to evaluate the MXene antenna sensor’s re-sponse in the presence of different VOCs. The transmission coefficient (S21)between the antenna sensor and the receiver antenna was monitored by theVNA to track the changes in the resonant frequency and resonant amplitudein the presence of different VOC gases. An RF-Space TSA 900-12000 ultra-wideband PCB Tapered slot antenna, with an operating frequency range of704.4. MXene antenna sensor900 MHz to 12 GHz and linear polarization was used as receiver antennato interpret the changes in the MXene antenna’s response. Accordingly,broadband microwave absorbers (600 MHz - 40 GHz, ECCOSORB, AN-79)surrounded the fabricated microwave structures and the setup to minimizethe interfering signals and reflections.4.4 MXene antenna sensorA C-shaped microstrip feedline was designed and fabricated on a Rogers6010 substrate, with permittivity of r = 10.2 and thickness of 1.91 mm,to investigate the Ti3C2Tx MXene circular membrane’s capability to act asan antenna and radiate signals in long-range (Figure 4.3). The C-shapedfeedline was utilized to deliver power efficiently from a 50-ohm connectorthrough the quarter-wavelength matching lines to the Ti3C2Tx MXene cir-cular membrane. Figure 4.3 shows the final design of the MXene antennaand its dimensions normalized by the effective wavelength of the substrate.The MXene antenna measurements and simulations were performed in twosteps: (i) without the Ti3C2Tx MXene circular membrane on the C-shapedfeed structure and (ii) the flattened Ti3C2Tx MXene circular membrane lo-cated on the C-shaped feedline structure. The simulation and measurementresults, presented in Figure 4.4, show that the simulated case (i) structurefeatured a reflection coefficient above – 0.16 dB, where most of the incidentpower reflected to the source.In contrast, the simulation for case (ii) showed a resonant profile at a fre-quency of 1.84 GHz, with a resonant amplitude of –25.066 dB, which demon-strated that the MXene integrated structure was matched to the powersource enabling it to radiate power more efficiently as compared to case (i).Furthermore, the single-port measurements of case (i) demonstrated that thereflection coefficient (S11) remained above –0.5 dB, indicating that 98.5%of the incident wave was reflected to the port. Therefore, positioning theTi3C2Tx MXene membrane on the microstrip feed structure demonstrated aresonant profile at the frequency of 1.84 GHz, with a reflection (S11) ampli-tude of – 38.895 dB, demonstrating that the incident power was efficientlydelivered to the MXene for radiation.The sharp changes in the phase response of the MXene antenna at 1.84GHz frequency also corresponded to the resonance in this frequency, i.e., val-idating the resonant frequency response of the antenna (Figure 4.5). Higherresonant frequencies were not the points of interest for the MXene antenna’soperation, as the higher orders of the resonant frequencies created unfavored714.4. MXene antenna sensorFigure 4.3: The schematic and the dimensions of the MXene antennanormalized by the effective wavelength of the structure.radiation patterns [149]. Moreover, a negligible difference was observed be-tween the simulation and measurement results. This can be attributed tothe ideal conditions under which the simulations were performed, where aflat Ti3C2Tx circular membrane was attached to the surface of the substrate.However, in a real setting, the Ti3C2Tx MXene circular membrane wasnot perfectly flat. This was evident through the minimal wrinkles that wereproduced on each membrane in the fabrication process and any warping thatmight have occurred during the thermal annealing steps during the fabri-cation. To investigate the effect of these wrinkles on the MXene antenna’sperformance, a set of 6 measurements were performed where the Ti3C2TxMXene membrane was rotated radially ∼ 60o for each measurement. Asobserved in table 4.1, the rotation of the Ti3C2Tx MXene membrane causedthe wrinkles’ position to change, resulting in a minimal change for the MX-ene antenna’s response during the measurements. The calculated standarddeviation for the 6 cycles of the experiment demonstrated standard devia-tions of σf = 0.0082 GHz and σA = 1.0198 dB for the resonant frequencyand resonant amplitude, respectively. Based on the calculated standard de-viations, the resonant frequency shift was minor for the rotated Ti3C2TxMXene membrane. Accordingly, the changes for the resonant amplitudewould also be negligible since the resonant amplitude of the MXene antennawas below – 30 dB for all the measurements, satisfying the bandwidth re-724.4. MXene antenna sensorFigure 4.4: The simulation and measurement results of the structure withand without Ti3C2Tx MXene membrane.quirements for the antenna.Table 4.1: S11 results and resonant frequency/amplitude error bars for 6cycles of measurement performed for the MXene antenna.MeasurementnumberResonantfrequency (GHz)Resonantamplitude (dB)First 1.84 - 39.63Second 1846 - 35.54Third 1.832 - 35.58Fourth 1.841 - 38.89Fifth 1.836 - 35.68Sixth 1.833 - 35.39In addition to the S-parameters measurements, which provided an over-all perspective of the designed antenna performance, far-field measurementswere required to evaluate the transmission capabilities of the antenna. There-fore, the far-field gain and the antenna’s radiation pattern were simulatedand measured. The antenna gain simulations demonstrated a 1.9 dB gainfor the antenna, as shown in Figure 4.6; accordingly, the gain measurementscorresponded to a 2 dB gain for the antenna. As presented in Figure 4.7,radiation patterns of the antenna were also simulated and measured where a734.4. MXene antenna sensorFigure 4.5: The S11 measurement results for the MXene antenna.Figure 4.6: The three-dimensional far-field gain simulation results for theMXene antenna.good correlation was observed between them. Based on the observed simu-lation and measurement results, it is indicated that a good correlation existsbetween the simulation and measurement results, confirming the measuredresults’ accuracy and promoting the use of MXene antenna for long-rangeVOC sensing.744.5. Gas sensing response of the MXene antenna sensorFigure 4.7: Normalized gain of the antenna in (a) E-plane (b) H-plane.4.5 Gas sensing response of the MXene antennasensorThe fabricated MXene antenna sensor was positioned inside a custom-made gas sensing chamber, while a distanced wideband transceiver was usedto interpret the changes in the MXene antenna sensor’s response. The stan-dard liquids were drop-casted into a liquid container inside the custom-madechamber, to be evaporated and adsorbed by the Ti3C2Tx MXene membrane.A LabVIEW software was used to record the transmission (S21) responsebetween the MXene antenna sensor and the receiver antenna. The transmis-sion (S21) response between the MXene antenna sensor and the transceiverantenna was monitored to evaluate the potential effect of VOC vapours’presence on the response of the antenna sensor by tracking the correlationof the transient and steady-state responses with the evaporation-adsorptionand recovery processes, respectively.4.5.1 MXene antenna sensor for Acetone vapour sensingThe MXene antenna sensor was positioned inside the enclosed custom-made chamber, shown in Figure 4.8, for preliminary VOC sensing measure-ments, where Acetone was used as the test sample during these experi-ments. The measurements were performed at room temperature and RH%of 25 ± 1 oC and 55 ± 3%, respectively. Initially, the bare MXene antenna754.5. Gas sensing response of the MXene antenna sensorsensor’s response was recorded for 260 seconds to specify the transmission(S21) measurement’s baseline response between the antenna sensor and thetransceiver. Then the VOC sensing measurement was conducted for a cycleof 440 seconds, including (i) 260 seconds of evaporation/adsorption for (80parts per thousand) ppt of Acetone and (ii) 180 seconds of desorption andrecovery.Figure 4.8: Gas sensing experiment setup.As shown in Figure 4.9, the evaporation of Acetone and its adsorption bythe Ti3C2Tx MXene membrane demonstrated a resonant frequency shift inthe transmission (S21) response between the MXene antenna sensor and thetransceiver antenna. As observed in the measurement results, the adsorp-tion of Acetone gases caused a 0.1231 GHz resonant frequency shift for theantenna, i.e., the resonant frequency changed to 1.9571 GHz. Accordingly,pumping synthetic air to the custom-made chamber during the recovery se-quence restored the resonant frequency response to the initial value of 1.84GHz. The frequency shift during each evaporation/adsorption sequence isrelative to the sensing mechanism of the MXene, which involves the adsorp-tion of gas molecules by the MXene’s functional groups and the defects inthe MXene’s structure [23]. Based on the MXene’s sensing mechanism, theadsorption of the VOCs cause an increase in the resistivity of the Ti3C2TxMXene membrane, where it was translated to a resonant frequency shiftin the transmission response (S21) of the sensing system. Additionally, the764.5. Gas sensing response of the MXene antenna sensortime-based measurement of frequency response, presented in Figure 4.10,showed a frequency shift trend and return to the initial frequency for ad-sorption and recovery sequences.Figure 4.9: The antenna sensor’s response to Acetone from 0 to 300 seconds,showing the adsorption and desorption of Acetone by the Ti3C2Tx MXenemembrane.Furthermore, the Acetone sensing measurement was performed for 8 con-secutive cycles to evaluate the repeatability of the measurement results. Asobserved in Figure 4.11, the evaporation of Acetone and its adsorption bythe Ti3C2Tx MXene membrane caused the resonant frequency of the an-tenna sensor to have a 0.123 GHz frequency with a standard deviation ofσ = 0.0012 GHz. Accordingly, it was observed that during the recovery se-quence for all 8 cycles, the resonant frequency response returned to the initialvalue of 1.84 GHz. The small standard deviation of the measured resultsconfirmed their repeatability and reproducibility for the MXene antenna gassensor. The error bar figure for the resonant frequency measurements of 8consecutive sensing cycles is provided in Figure 4.11 (b).The resonant amplitude of the transmission response (S21) was also mon-itored during these experiments for additional evaluation of the measuredresults. Adsorption of Acetone by the Ti3C2Tx MXene membrane demon-strated an increase in the resonant amplitude of the antenna sensor. How-ever, the results revealed an inconsistency in the resonant amplitude shiftduring the consecutive cycles. This inconsistency for the resonant amplitudewas due to the environmental effects, including noise and interfering signals,making tracking the resonant amplitude challenging in the real-case wireless774.5. Gas sensing response of the MXene antenna sensorFigure 4.10: One cycle of gas sensing measurement for the Acetone, includ-ing bare sensor measurement, acetone evaporation/adsorption sequence andrecovery sequence.Figure 4.11: (a) The antenna sensor’s resonant frequency response to 80 pptconcentration of Acetone gas for 8 cycles of measurement and (b) resonantfrequency shift error bar, for 8 cycles of measurement.gas sensing scenarios. The results for the resonant amplitude of the MXeneantenna sensor are provided in Figure 4.12.Further experiments were also performed to monitor the response of the784.5. Gas sensing response of the MXene antenna sensorFigure 4.12: The antenna sensor’s resonant amplitude response to 80 pptconcentration of Acetone gas for 8 cycles of measurement.MXene antenna sensor to different concentrations of Acetone. Each concen-tration of Acetone was introduced for 240 seconds, during which evapora-tion/adsorption happened, and then the Acetone was purged by pumpingsynthetic air into the costum-made custom-made chamber for 260 secondsto recover the initial response. The experiments were performed in temper-ature and humidity of 24 ± 1 oC and 58 ± 3 %, respectively. As presentedin Figure 4.13, the investigation was conducted for 8, 16, 24 and 80 pptconcentrations of Acetone, where they generated 70, 74, 80.6, 124 MHz offrequency shift, respectively, showing the capability of the MXene antennain sensing different concentrations of the Acetone gas.Analyzing the recorded results for 8, 16, and 24 ppt, the frequency shifttrend can be modelled using y = 0.0007x + 0.0644 equation with R2 =0.9905. The equation presented in Figure 4.14 shows that the MXene an-tenna sensor shows a frequency shift of 0.7 MHz/1ppt of Acetone. Thischange in the frequency of the MXene antenna sensor for different Acetoneconcentrations is due to the rise in the number of Acetone molecules insidethe gas sensing chamber, which results in more Acetone molecules adsorp-tion by the Ti3C2Tx MXene membrane, and consequently, more frequencyshift in the antenna sensor’s response [150].Furthermore, the experiment was repeated for 5 times to evaluate therepeatability of the measurement results and indication of the measurementerror bars, shown in Figure 4.13 (b). The measurement results indicated a794.5. Gas sensing response of the MXene antenna sensorFigure 4.13: The antenna sensor response to different concentrations ofAcetone gas ranging from 8 to 80 ppt.Figure 4.14: The antenna sensor’s frequency shift for different concentrationsof Acetone inside the gas sensing chamber.standard deviation of σ8ppt= 0.0020, σ16ppt = 0.00055, and σ24ppt = 0.0022for 8 ppt, 16 ppt and 24 ppt concentration, respectively. The standarddeviation values for different concentrations validated the measured resultsand their repeatability during consecutive measurements.804.5. Gas sensing response of the MXene antenna sensor4.5.2 Response of the sensor to different VOC vapoursAdditional standard liquids, including Methanol, Ethanol and IPA, wereused in a subsequent set of experiments to evaluate the response of theMXene antenna sensor to different VOCs inside the chamber. Four separateexperiments were conducted using these liquids, plus Acetone, where theMXene antenna sensor was exposed to 80 ppt concentrations of the liquidsone at a time. The measurements were performed at room temperature andRH% of 25 ± 1 oC, and 55 ± 3%, respectively. The measurement for eachliquid took 700 seconds, which included (i) 260 seconds for bare MXeneantenna sensor’s measurement for defining the measurement baseline, (ii)300 seconds of evaporation/adsorption of the liquids, and (iii) 180 seconds ofde-gassing and recovery sequence. The evaporation/adsorption and recoverysequences were performed, respectively, by pumping standard liquids to thecustom-made chamber and purging the VOCs by adding synthetic air to thecustom-made gas sensing chamber.As presented in Figure 4.15, the measurement results showed a resonantfrequency shift of 124, 93, 76, and 20 MHz for Acetone, Methanol, Ethanoland IPA during evaporation/adsorption sequence, respectively. The ob-served resonant frequency shift is due to the adsorption of gas moleculesby the MXene molecules resulting in a resistivity increase. Accordingly,the difference in the resonant frequency shift for the MXene antenna sen-sor demonstrates that the interaction of VOC molecules with the Ti3C2TxMXene membrane can differ for various VOC vapours [23, 110, 150, 151].The measured transmission (S21) response demonstrated frequency shifts fordifferent VOC gases, confirming the MXene antenna sensor’s capability indetecting different VOCs. Changes in the resonant frequency of the antennasensor in the presence of various liquids and their distinguishability demon-strated the high potential of the MXene antenna sensor in sensing variousVOC gas vapours in closed environments. Still, in addition to sensing dif-ferent VOCs, evaluating the influence of humidity on the MXene antennasensor’s response is essential for the sensor’s performance.4.5.3 Sensor’s response to different Humidity levelsFurther experiments were performed to evaluate the response of the MX-ene antenna sensor to various levels of humidity ranging from 55 ± 1% to94 ± 1%, while the room temperature was set to be 23 ± 0.2 oC. Theantenna sensor was positioned inside the sealed custom-made gas sensingchamber, and humidity was pumped into the chamber. Each experiment814.5. Gas sensing response of the MXene antenna sensorFigure 4.15: MXene antenna sensor’s response to the presence of Acetone,Ethanol, Methanol and IPA in a time frame of 700 seconds.cycle consisted of two sequences of water molecules’ adsorption by Ti3C2TxMXene membrane and desorption, where the first one happened as the hu-midity was pumped into the chamber for 300 seconds, and then the secondoccurred as synthetic air was pumped into the chamber for 300 seconds. Theresults for these experiments, provided in Figure 4.16 (a,b), show that forhumidity levels below 77.8 ± 2%, the MXene antenna sensor is insensitiveto variation in the ambient humidity. However, a significant frequency shiftwas observed in the response of the MXene antenna sensor for humiditylevels above 77.8 ± 2%. Increasing the humidity level from 77.8 ± 2% to 94± 1% resulted in a 213.1 MHz frequency shift in the system’s transmissionresponse (S21). The change in the resonant frequency of the antenna sensorcan be attributed to the penetration of water molecules between the MXeneflakes, which increases the interlayer spacing and, consequently, increasesthe resistivity between the Ti3C2Tx MXene nanosheets, i.e., increasing theresistivity of the Ti3C2Tx MXene membrane [152].The humidity measurement was proceeded for 6 consecutive cycles toevaluate the repeatability of the experiment results, where the results demon-strated 213.1 MHz of frequency shift (i.e., changing the resonant frequencyfrom 1.84 GHz to 2.026 GHz) with a standard deviation of σ = 0.0051MHz. The minimal standard deviation of the measured results showed therepeatability of the humidity sensing results for the MXene antenna sensor.However, in order to evaluate the reproducibility of the sensor’s response,824.5. Gas sensing response of the MXene antenna sensorFigure 4.16: MXene antenna sensor response to the presence of humidity,performed in a custom-made gas chamber with 24 hours of time differencebetween the two experiments (a) humidity sensing response (b) humiditysensing response, performed 24 hours later.Figure 4.17: Error bars for MXene antenna sensor’s humidity sensing re-sponse for the first set and second set of experiments.a complementary humidity measurement was performed 24 hours after theinitial measurements, using the MXene antenna sensor and the same ex-perimental setup. This experiment was performed for five cycles where thesimilar adsorption/desorption timing of 300 seconds humidity and 300 s re-covery sequences was dedicated for each cycle. As presented in Figure 4.16834.6. Discussion(b), the measurements for these experiments demonstrated 215.1 MHz of fre-quency shift with a standard deviation of σ = 0.0025 MHz. The measuredresults of the second set of humidity sensing experiments concurred withthe first set approving the reproducibility of the experiments and the resultsfor the humidity sensing measurements. Additionally, the error bars of thefirst and second set of humidity experiments are provided in Figure 4.17,confirming the reproducibility and repeatability of humidity experiments.4.6 DiscussionThe observed changes in the frequency response of the MXene antennasensor demonstrate the capability of the device in monitoring VOCs andtheir concentration changes by the Ti3C2Tx MXene membrane-base antennasensor, which has occurred due to the interaction of VOC molecules withthe Ti3C2Tx MXene membrane in the form of adsorption of these moleculesby active defects of the MXene’s surface. Accordingly, a clear understandingof the Ti3C2Tx MXene’s sensing performance can be achieved by fitting anexponential curve to the MXene antenna sensor’s response VOC sensingresponse.As observed in Figures 4.18-4.22, the evaporation/adsorption rate andthe recovery rate of the cycles for different samples can be modelled using anexponential equation and time constants. As shown in Table 4.2, the transi-tion time for the VOC exposed Ti3C2Tx MXene membrane remained below26 seconds which showed a prompt response to different VOCs. Neverthe-less, the transition time for the humidity was 31.60 s. The slower transitiontime of the humidity could be used as an exclusion criterion to differentiatebetween the humidity and VOCs. In addition to the transition time, the re-covery time constant was checked where it remained below 6 seconds for IPA,Ethanol and Methanol; however, the recovery time constant was observedto be 30.84 and 21.91 seconds for the Acetone and humidity, which mightbe due to the different interactions of Acetone and water molecules with theTi3C2Tx MXene membrane [110]. Still, the observed response is consideredto be rapid compared to the similar microwave VOC sensing investigation[153–155].Additionally, the analysis performed on different concentrations of Ace-tone, using the fitting curves (Figure 4.23), demonstrated that the adsorp-tion and recovery rates are nearly the same numbers for different concen-trations. As presented in Table 4.3, the rise and fall time constants for thedifferent Acetone concentrations are below 26 and 36 seconds, respectively,844.6. DiscussionTable 4.2: Fitting curve data for different VOC samplesFitting Curve (A.e−(t−t1)/τ + y0), R2(%)Sensingsamples (80 ppt)Sensing sequence Recovery sequenceτ(s) R2 τ(s) R2Acetone 22.41 96.47 30.84 98.41IPA 25.92 99.92 4.28 99.96Ethanol 21.58 95.15 5.05 99.22Methanol 20.83 97.21 4.84 99.61Humidity 31.60 97.24 21.91 94.05Table 4.3: Fitting curve data for different concentrations of AcetoneFitting Curve (A.e−(t−t1)/τ + y0), R2(%)AcetoneConcentration (ppt)Sensing sequence Recovery sequenceτ(s) R2 τ(s) R28 24.32 92.69 33.74 92.1216 25.67 89.45 31.54 98.8224 19.56 99.95 35.67 94.8080 22.41 96.47 30.84 98.41showing that the MXene antenna sensor has a rapid sensing response andrecovery even for different Acetone concentrations. As mentioned before, theincrease in the concentration of Acetone results in more change in the resis-tivity of the MXene membrane. Thus, higher concentrations cause higherfrequency shifts in the sensor’s response.Furthermore, as mentioned in Table 4.3, the smallest measured concen-tration for Acetone was 8 ppt; nevertheless, the detection limit of the pro-posed long-range VOC sensing structure can be calculated using equation4.1 [155, 156]:detectionlimit =3.3SDb=3.3× 7× 10−46.96× 10−4 = 3.31ppt (4.1)Where b is the device’s resonant frequency sensitivity (GHz/ppt) andSD is the average standard deviation of the recorded baseline measurements( 6.96 × 10–4 ). Based on the simulated, measured, and evaluated results,the developed MXene antenna sensor has a high potential of being used forapplications where wireless rapid detection of high concentration VOCs isrequired.854.6. DiscussionFigure 4.18: The sensing response of the MXene antenna sensor for IPAand the curve fitting over the measured responsesFigure 4.19: The sensing response of the MXene antenna sensor forMethanol. and the curve fitting over the measured responses864.6. DiscussionFigure 4.20: The sensing response of the MXene antenna sensor for Ethanoland the curve fitting over the measured responses.Figure 4.21: The sensing response of the MXene antenna sensor for Acetoneand the curve fitting over the measured responses.874.6. DiscussionFigure 4.22: The sensing response of the MXene antenna sensor for humid-ity and the curve fitting over the measured responses.Figure 4.23: The sensing response of the MXene antenna sensor and thecurve fittings over the measured response for different concentrations of Ace-tone.884.7. Chapter summary4.7 Chapter summaryChapter 4 of this thesis proposed a novel method to fabricate VOC sens-ing antenna sensors using Ti3C2 MXene membranes. The antennas sensorstructure was composed of a 20 µm-thick lightweight MXene membrane,which had 26.66 wt.% of a 35 µm copper in the same size. The MXene actedas a radiating and sensing element, which was simply added to or removedfrom the circuits. The developed antenna sensor demonstrated the abilityto sense different VOCs, including Acetone, IPA, Ethanol, and Methanol,with 80 ppt concentration. Further experiments showed MXene’s abilityto sense different Acetone concentrations between 8 to 80 ppt, demonstrat-ing the antenna sensor’s capability to distinguish different concentrationsof VOCs. The antenna sensor also showed the ability to detect differenthumidity values inside the gas sensing chamber. The MXene antenna sensorwas able to sense humidity values above 77.8 ± 2% by demonstrating a shiftin the resonant frequency; nonetheless, it was insensitive to humidity valuesbelow 77.8 ± 2%. The developed MXene-based antenna sensor operated fortemperatures between 23 to 25 oC. Overall, the developed MXene based an-tenna sensor performed as a proof of concept for investigating MXene-basedVOC antenna sensor, where they could have a great potential of being usedin industrial areas with high VOC concentrations and chemical labs . Ad-ditionally, the MXene membrane, which acted as a sensing and radiatingelement, could be simply removed and replaced with another membrane if itbecame VOC molecule-saturated to ensure an efficient sensing performancefor the device.89Chapter 5Conclusion5.1 Conclusion and contributionsThis thesis successfully demonstrated and validated the development ofantenna sensor solutions for long-range wireless sensing of hazardous aque-ous liquids on PPEs and VOCs in an environment. For the first objective, anantenna sensor was designed and embedded inside superhydrophobic textileto sense hazardous aqueous liquid on PPE. The developed textile-embeddedantenna sensor was tested by sensing water droplets resembling hazardousaqueous liquids. Accordingly, a wireless readout antenna was used to com-municate with the developed textile-embedded antenna sensor, interpretingthe changes in the antenna sensor’s response.The second objective of this thesis was successfully demonstrated bydeveloping a MXene antenna sensor where A MXene membrane was imple-mented as a radiating/sensing element to sense the presence of the VOCsand transmit the sensing data to a distanced receiver. A secondary antennawas used to communicate with the MXene antenna and receive the sensingdata. The developed MXene antenna sensor performance was evaluated bysensing IPA, Ethanol, Methanol and Acetone in the custom-made chamber.Following the conclusion, the contributions of this thesis are summarized asfollows:5.2 Contributions− A textile-embedded antenna sensor was developed where the antennasensor was able to sense the presence of hazardous aqueous liquids bya shift in its resonant frequency.− The textile-embedded antenna sensor transmitted the sensing datawirelessly to a transceiver antenna at a distance of 2 meters while itwas able to sense the presence of aqueous liquids in the µL range.− A MXene membrane was utilized to develop a planar antenna struc-905.3. Future worksture where the antenna was successfully able to communicate with asecondary transceiver antenna.− The MXene antenna was employed as an antenna sensor to detect thepresence of various VOCs in a custom-made chamber, by a shift in theresonant frequency and transmit the sensing data to a receiver at adistance of 1.4 meters.5.3 Future worksWhile this thesis successfully demonstrated the objective aimed for, thereremains several factors to be studied further to develop the solution for fu-ture commercialization and improvement. The following presents the futurework for the solution presented in this thesis:− Future work could investigate integration of an array structure of an-tenna sensors to enhance the sensing performance of the devices indetecting hazardous aqueous liquids and VOCs.− Future work could also study developing the proposed antenna sensorson the flexible substrates to ensure more flexibility and stretchabilityfor the antenna sensors.− Various other hazardous solutions and VOCs with different volumesand concentrations could be be tested in the future works.− Future work is still required to explore the optimization of the MXeneparameters, such as conductivity, for wireless microwave VOC sensing,enabling the detection of low concentrations of the VOC.− Studies could also be performed for fabricating circularly polarizedantenna sensors with different MXene shapes and different feed struc-tures in future iterations of these works, enabling more efficient trans-mission of the sensing data to a receiving antenna sensor.− Future works could investigate developing selective microwave sensorstructure for aqueous liquid and VOC sensing using post processing ofdata and machine learning algorithms− Future works could study the effect of temperature in the response ofMXene-membrane based antenna sensor91Bibliography[1] M. 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Thesis/Dissertation
eng
Electrical Engineering
Vancouver : University of British Columbia Library
An error occurred getting the license - uri.
Graduate
Zarifi, Mohammad Hossein
An investigation on copper-based and MXene-based planar antenna sensors for hazardous substance detection
Text