Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Dordanihaghighi, Sara
Master of Applied Science - MASc
University of British Columbia
Rapid growth in industry and the improper disposal of pollutants have led to the contamination of the environment and humans’ food chains by heavy metal ions. The accumulation of heavy metal ions in living organisms results in lethal diseases such as cardiovascular diseases, heart failure, and cancer. Therefore, sensing and eliminating heavy metal ions have attracted researchers’ attention in recent years. Graphene Quantum Dots (GQDs) have emerged as one of the unique materials in fluorescence sensing of heavy metal ions due to their unique properties such as tunable bandgap, high solubility, large surface area, and absorptivity. In this work, novel amine-functionalized Graphene Quantum Dots (FGQDs) and nitrogen-doped Graphene Quantum Dots (DGQDs) were developed via a bottom-up chemical synthesis procedure. FGQDs and DGQDs were synthesized using citric acid as the carbon precursor and 3-aminobenzylamine as the surface modifier and dopant material. FGQDs and DGQDs were studied using different characterization techniques, including PL, FTIR, XPS, XRD, and TEM, to determine the fluorescence quenching of GQDs-based materials in the presence of heavy metal ions. The prepared nanostructures showed a uniform size of 5-6 nm, and a high content of amine groups and nitrogen doping. The FGQDs and DGQDs illustrate a quantum yield of 13.78% and 14.01%, respectively. The FGQDs showed an effective fluorescence quenching towards Fe³⁺ ions with a detection limit of 2.43 ppm with high selectivity and sensitivity among other heavy metal ions. On the other hand, the DGQDs samples showed effective fluorescence quenching towards Hg²⁺ ions with a detection limit of 1.99 ppm.
i Synthesis of Amine-functionalized and Nitrogen-doped Graphene Quantum Dots and Their Applications in Heavy Metal Ions Sensing by Sara Dordanihaghighi B.Sc. in Materials Science and Engineering, Sharif University of Technology, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) July 2021 © Sara Dordanihaghighi, 2021 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, the dissertation entitled: Synthesis of Amine-functionalized and Nitrogen-doped Graphene Quantum Dots and Their Applications in Heavy Metal Ions Sensing submitted by Sara Dordanihaghighi in partial fulfillment of the requirements for the degree of Master of Applied Science in Mechanical Engineering Examining Committee: Dr. Mohammad Arjmand (School of Engineering) Supervisor Dr. Sepideh Pakpour (School of Engineering) Supervisory Committee Member Dr. Alexander Uhl (School of Engineering) Supervisory Committee Member Dr. Feng Jiang (Faculty of Forestry) University Examiner iii Abstract Rapid growth in industry and the improper disposal of pollutants have led to the contamination of the environment and humans’ food chains by heavy metal ions. The accumulation of heavy metal ions in living organisms results in lethal diseases such as cardiovascular diseases, heart failure, and cancer. Therefore, sensing and eliminating heavy metal ions have attracted researchers’ attention in recent years. Graphene Quantum Dots (GQDs) have emerged as one of the unique materials in fluorescence sensing of heavy metal ions due to their unique properties such as tunable bandgap, high solubility, large surface area, and absorptivity. In this work, novel amine-functionalized Graphene Quantum Dots (FGQDs) and nitrogen-doped Graphene Quantum Dots (DGQDs) were developed via a bottom-up chemical synthesis procedure. FGQDs and DGQDs were synthesized using citric acid as the carbon precursor and 3-aminobenzylamine as the surface modifier and dopant material. FGQDs and DGQDs were studied using different characterization techniques, including PL, FTIR, XPS, XRD, and TEM, to determine the fluorescence quenching of GQDs-based materials in the presence of heavy metal ions. The prepared nanostructures showed a uniform size of 5-6 nm, and a high content of amine groups and nitrogen doping. The FGQDs and DGQDs illustrate a quantum yield of 13.78% and 14.01%, respectively. The FGQDs showed an effective fluorescence quenching towards Fe3+ ions with a detection limit of 2.43 ppm with high selectivity and sensitivity among other heavy metal ions. On the other hand, the DGQDs samples showed effective fluorescence quenching towards Hg2+ ions with a detection limit of 1.99 ppm. iv Lay Summary Heavy metal ions contaminations have been increased drastically due to the fast-growing technology in recent years. The introduction of a small amount of these ions to soils, rivers, and lakes, and humans’ food chains results in cardiovascular disease, liver damage, cancer, reproductive disease, and nervous system dysfunction in human bodies. In recent years, the detection and elimination of heavy metal ions in humans’ food chains became one of most important concerns in different countries such as Canada. One of the most straight-forward techniques for the detection of heavy metal ions is fluorescence detection techniques using quantum dots materials. In this work, Graphene Quantum Dots architected with amine and nitrogen-containing groups are synthesized through a simple chemical synthesis method and their effect on heavy metal ions detection was investigated. The results demonstrate that these novel quantum dots can detect iron (III) and mercury (II) ions in an aqueous environment due to their promising fluorescence properties. v Preface XPS characterizations were performed by Rana Sodhi (Surface Interface Ontario, Department of Chemical Engineering and Applied Chemistry, University of Toronto). XRD characterizations were developed by the manager of analytical service, Charles Dal Castel (Department of Chemical Engineering, University of Waterloo). TEM analysis was conducted by Natalie Hamada (Canadian Center for Electron Microscopy, McMaster University). All other works presented in this thesis, such as synthesis method, heavy metal ions detection techniques, UV-Vis, PL, and FTIR characterizations were designed, implemented, and analyzed by Sara Dordanihaghighi affiliated to Nanomaterials and Polymer Nanocomposites Laboratory (NPNL) at the University of British Columbia’s Okanagan campus, under the supervision of Prof. Mohammad Arjmand. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Tables ................................................................................................................................ ix List of Figures .................................................................................................................................x List of Abbreviations ...................................................................................................................xv Glossary ...................................................................................................................................... xvi Acknowledgements ................................................................................................................... xvii Dedication ................................................................................................................................. xviii Introduction ................................................................................................................1 1.1 General Background ....................................................................................................... 1 1.2 State-of-the-Art ............................................................................................................... 1 1.3 Thesis Chapters ............................................................................................................... 2 Literature Review .......................................................................................................4 2.1 Adverse Effects of Heavy Metal Ions ............................................................................. 4 2.1.1 Heavy Metal Ions Detection Methods ........................................................................ 7 2.2 Carbon-based Nanomaterials ........................................................................................ 12 2.2.1 Graphene Quantum Dots........................................................................................... 14 2.2.2 Graphene Quantum Dots Structure and Properties ................................................... 15 2.3 Graphene Quantum Dots Synthesis .............................................................................. 19 2.3.1 Top-Down Method.................................................................................................... 20 vii 2.3.2 Bottom-up Methods .................................................................................................. 24 2.4 Applications of GQDs-based Nanomaterials ................................................................ 28 Experimental Section ...............................................................................................34 3.1 Materials and Setups ..................................................................................................... 34 3.2 Synthesis Methods ........................................................................................................ 34 3.2.1 GQDs Synthesis ........................................................................................................ 35 3.2.2 FGQDs Synthesis ...................................................................................................... 37 3.2.3 DGQDs Synthesis ..................................................................................................... 37 3.3 Characterizations........................................................................................................... 38 3.3.1 Photoluminescent Spectroscopy ............................................................................... 38 3.3.2 Ultraviolet-Visible Spectroscopy (UV-Vis) ............................................................. 39 3.3.3 X-ray Photoelectron Spectroscopy (XPS) ................................................................ 40 3.3.4 Fourier-Transform Infrared Spectroscopy (FTIR) .................................................... 40 3.3.5 Transmission Electron Microscopy (TEM) .............................................................. 40 3.3.6 X-ray Diffraction Analysis (XRD) ........................................................................... 41 3.4 Heavy Metal Ions Sensing ............................................................................................ 41 3.5 Samples Irradiated by UV-light .................................................................................... 42 Results and Discussion .............................................................................................44 4.1 Synthesis ....................................................................................................................... 44 4.1.1 GQDs Synthesis ........................................................................................................ 44 4.1.2 FGQDs Synthesis ...................................................................................................... 45 4.1.3 DGQDs Synthesis ..................................................................................................... 47 4.2 Quantum Dots Structure ............................................................................................... 49 viii 4.2.1 Fluorescence Properties ............................................................................................ 49 4.2.2 FTIR Characterization .............................................................................................. 50 4.2.3 XPS Characterization ................................................................................................ 52 4.2.4 XRD Characterization ............................................................................................... 57 4.2.5 TEM Characterization ............................................................................................... 58 4.3 Heavy Metal Ions Sensing ............................................................................................ 59 4.3.1 Sensors Selectivity .................................................................................................... 59 4.3.2 FGQDs Sensors ......................................................................................................... 61 4.3.3 DGQDs Sensors ........................................................................................................ 64 Summary and Conclusions ......................................................................................71 5.1 Results Summary .......................................................................................................... 71 5.2 Future Works ................................................................................................................ 72 Bibliography .................................................................................................................................74 ix List of Tables Table 2.1 Classic quantum dots applications in heavy metal sensing. ......................................... 12 Table 2.2 Advantages and disadvantages of GQDs synthesis methods........................................ 28 Table 2.3 Some of the applications of GQDs. .............................................................................. 30 Table 2.4 Detection of heavy metal ions by GQDs-based nanostructures. .................................. 33 Table 3.1 The synthesis conditions and denoted names of different samples in FGQDs. ............ 38 Table 4.1 Quantum yields of synthesized samples. ...................................................................... 49 Table 4.2 Detailed FTIR characterization results of the samples. ................................................ 52 Table 4.3 Elemental atomic percentages of GQDs, FGQDs, and DGQDs obtained from XPS analysis. ......................................................................................................................................... 53 x List of Figures Figure 1.1 Structure of 3-aminobenzylamine. ................................................................................ 2 Figure 2.1 Flowchart of Chapter 2. ................................................................................................. 4 Figure 2.2 (Left) Addition of heavy metal ions to CdSe quantum dots and (Right) change in the fluorescence intensities of quantum dots in the presence of heavy metal ions. Reproduced with permission [35], copyright 2019 RSC Advances. ........................................................................... 9 Figure 2.3 Schematics of heavy metal ions sensing setups a) AAS [43], b) Electrochemical sensors, reproduced with permission [27], copyright 2018, Elsevier, c) ICP-MS [44], and d) fluorescence sensing........................................................................................................................................... 10 Figure 2.4 Schematics of quantum dots energy bonds. ................................................................ 11 Figure 2.5 Structure of a) graphite, reproduced with permission [63], copyright 1999, IEEE, b) single-walled carbon nanotube, reproduced with permission [64], copyright 2005, IEEE, c) multi-walled carbon nanotube, reproduced with permission [65], copyright 2020, MDPI, d) Graphene, reproduced with permission [66], copyright 2018, Nature Publishing Group e) graphene quantum dots, reproduced with permission [67], copyright 2016, Springer, f) fullerene, reproduced with permission [68], copyright 2015, Taylor and Francis group. ....................................................... 13 Figure 2.6 A schematics of graphite (reproduced with permission [78], copyright 2016, Scholar Commons), graphene (reproduced with permission [79], copyright 2015, Advanced Healthcare Materials), and GQDs (reproduced with permission [78], copyright 2016, Scholar Commons). 14 Figure 2.7 Effect of size on GQDs’ PL properties. Reproduced with permission [84], copyright 2014, Royal Society of Chemistry. ............................................................................................... 17 Figure 2.8 Decreasing the bandgap of GQDs by enlarging the π system or addition of n-orbitals. Reproduced with permission [118], copyright 2018, ACS Publications. ..................................... 18 xi Figure 2.9 Top-down and bottom-up routes for the GQDs synthesis. Reproduced with permission [129], copyright 2017, Royal Society of Chemistry. .................................................................... 20 Figure 2.10 Synthesis of GQDs by coal. Reproduced with permission [136], copyright 2013, Nature Communication. ................................................................................................................ 21 Figure 2.11 Synthesis process of nitrogen- and sulfur-doped GQDs. Reproduced with permission [144], copyright 2014, Royal Society of Chemistry. .................................................................... 24 Figure 2.12 Microwave-assisted synthesis of self-passivated GQDs from glucose. Reproduced with permission [185], copyright 2012, ACS Publications. ......................................................... 25 Figure 2.13 Cage opening mechanism of the GQDs production. Reproduced with permission [187], copyright 2015, ACS Publications. .................................................................................... 26 Figure 2.14 Possible mechanism for nitrogen-doped GQDs. Reproduced with permission [193], copyright 2016, RSC Advances. ................................................................................................... 28 Figure 2.15 GQDs properties and applications. ............................................................................ 29 Figure 2.16 A perovskite solar cell with phenylenediamine-functionalized N-GQDs. Reproduced with permission [209], copyright 2016, Wiley Online Library. ................................................... 31 Figure 3.1 Flowchart of Chapter 3. ............................................................................................... 34 Figure 3.2 Applied heating pattern of CA solution....................................................................... 35 Figure 3.3 CA solution heating process. a) room temperature, b) 150°C, c) 200°C, d) 250°C, e) 300°C, f) DI water addition. ......................................................................................................... 36 Figure 3.4 Purification process of GQDs. a) as-synthesized suspension, b) as-synthesized GQDS suspension after adding ethanol, c) sample b after centrifugation to settle the GQDs. ................ 36 Figure 3.5 Synthesized samples of a) GQDs, b) FGQDs (100-48), and c) DGQDs. ................... 38 xii Figure 3.6 Quartz cuvette of PL and UV-Vis setup containing: a) quantum dots samples, b) reference sample. .......................................................................................................................... 42 Figure 3.7 Samples irradiated with UV light. a) DGQDs, b) FGQDs (100-48), c) GQDs. .......... 43 Figure 4.1 Flowchart of Chapter 4. ............................................................................................... 44 Figure 4.2 Carbonization of CA. Reproduced with permission [181], copyright 2012, Elsevier. 45 Figure 4.3 PL characterization of GQDs, FGQDs (75mg-48h), FGQDs (100m-48h) and FGQDs (125mg-48h) samples.................................................................................................................... 46 Figure 4.4 PL characterization of FGQDs (100mg-24h), FGQDs (100mg-48h) and FGQDs (100mg-68h) samples.................................................................................................................... 47 Figure 4.5 PL characterization of GQDs, FGQDs, and DGQDs sample. ..................................... 48 Figure 4.6 PL characterization of a) GQDs, b) FGQDs, and c) DGQDs under different excitation wavelength. ................................................................................................................................... 50 Figure 4.7 FTIR characterization spectra of the GQDs, FGQDs, and DGQDs samples. ............. 51 Figure 4.8 Full scan XPS spectra of GQDs, FGQDs, and DGQDs. ............................................. 53 Figure 4.9 XPS spectra of a) C1s and b) O1s of GQDs. ............................................................... 54 Figure 4.10 XPS spectra of a) C1s, b) O1s, and c) N1s of FGQDs. ............................................. 55 Figure 4.11 XPS spectra of a) C1s, b) O1s, and c) N1s of DGQD. .............................................. 56 Figure 4.12 a) N-bond configuration in GQDs plane and b) GQDs structure showing oxygen- and hydrogen-containing functional groups. ....................................................................................... 57 Figure 4.13 XRD characterization of a) GQDs, b) FGQDs, and c) DGQDs. ............................... 58 Figure 4.14 TEM characterization and size distribution of a) GQDs ( in 40 counts), b) FGQDs (in 40 counts), and c) DGQDs (in 15 counts) (the insets of TEM characterizations are the magnified images of selected area). ............................................................................................................... 59 xiii Figure 4.15 Relative fluorescence intensity of GQDs, FGQDs, and DGQDs samples in the presence of different metal ions ([Mn+]) = 50 ppm. ...................................................................... 61 Figure 4.16 PL characterization of FGQDs sample in the presence of Fe3+ ions. ........................ 62 Figure 4.17 UV-Vis characterization of FGQDs sample in the presence of Fe3+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. ............................ 62 Figure 4.18 Stern-Volmer plot of FGQDs sample at different concentrations of Fe3+ ions. ........ 63 Figure 4.19 PL characterization of DGQDs sample in the presence of Hg2+ ions. ...................... 65 Figure 4.20 UV-Vis characterization of DGQDs sample in the presence of Hg2+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. ............................ 65 Figure 4.21 Stern-Volmer plot of DGQDs sample at the different concentrations of Hg2+ ions. Inset: the linear relationship of relative fluorescence intensity in the presence of Hg2+ ions. ...... 66 Figure 4.22 PL characterization of DGQDs sample in the presence of Fe3+ ions. ....................... 67 Figure 4.23 UV-Vis characterization of DGQDs sample in the presence of Fe3+ ions. The characterization was performed in a wavelength range of 240 to 600 nm. .................................. 68 Figure 4.24 Effect of co-existing Hg2+ and Fe3+ ions on the DGQDs sample in the PL characterization. ............................................................................................................................ 69 Figure 4.25 UV-Vis characterization of DGQDs in the presence of Hg2+ and Fe3+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. ............................ 69 xiv List of Symbols 𝜋 bond Covalent bond 𝜎 bond van der Waals bond 𝜋 state Ground state 𝜋∗ Excitation state 𝜑 Quantum yield I Intensity of integrated emission 𝜂 Solvent refractive index A Optical density R Reference T Transmission A Absorbance n Integer 𝜆 X-ray wavelength 𝜃 Incident angle F Fluorescence intensity in the presence of heavy metal ions 𝐹0 Fluorescence intensity in the absence of heavy metal ions 𝜎 Standard deviation m Linear plot slope xv List of Abbreviations TSDR Toxic Substance and Disease Registry ECCC Environment and Climate Change Canada CCAA Canadian Clear Air Act GLWQA Great Lakes Water Quality Agreement CFIA Canadian Food Inspection Agency ICP-MS Inductively Coupled Plasma Mass Spectroscopy AAS Atomic Absorption Spectroscopy UV Ultraviolet GQDs Graphene Quantum Dots CNT Carbon Nanotube SWCNT Single-wall Carbon Nanotube MWCNT Multi-wall Carbon Nanotube GO Graphene Oxide FGQDs Functionalized-Graphene Quantum Dots DGQDs Doped-Graphene Quantum Dots PL Photoluminescence UV-Vis Ultraviolet-Visible Spectroscopy XPS X-ray Photoelectron Spectroscopy FTIR Fourier-Transform Infrared Spectroscopy TEM Transmission Electron Microscopy XRD X-ray Diffraction ppm Part Per Million CA Citric Acid LOD Limit of Detection xvi Glossary Photoluminescence Emission of light from substance caused by the irradiation of other light Fluorescence Ability of a substance emitting visible light triggered by non-visible light Quantum confinement Change of electronic and optical properties of the material when the material’s size is comparable to the wavelength of the electron Surface/edge state Electronic state found at the edge and surface of the material Quantum yield Ratio of the photons emitted to the number of photons absorbed Fluorescence quenching Decrease in fluorescence intensity of a sample xvii Acknowledgements I would like to thank my supervisor, Professor Mohammad Arjmand, for his continuous support and guidance. He provided me the freedom to explore my projects to encourage my growth as a researcher while guiding me with ideas and proper feedback at the same time when required. My wholehearted thanks to him for multiplying my motivations for research. I would like to express gratitude to Syilx Okanagan Nation for their support in research in their beautiful land. I would also like to thank Mitacs Accelerate program for providing support and training as part of my studies. I am also grateful to ZEN Graphene Solutions, Ltd (Ontario) and especially Dr. Colin van der Kuur, for their grant support and guidance in part of my work. I am thankful to Prof. Pakpour and Prof. Uhl for their guidance and valuable inputs as part of my thesis advisory committee. Their feedbacks during the annual progress meeting in my first year were effective in getting this shape of my work. I would also like to truly thank all my labmates in NPNL. Thank you all for your guidance and motivations. I would like to especially thank Drs. Ali Akbarisehat, Haddadi, and Shiraz for their motivations and guidance. Also, special thanks for my family who appreciated my motivation for learning and research. Above all, I would like to thank my husband, Dr. Sepehr Zarif Mansour, who has always been standing next to me and encouraging me during the challenges of graduate school and life. I am truly thankful for having you in my life. xviii Dedication …To all international graduate students who worked so hard in global pandemic… 1 Introduction 1.1 General Background Industrial and biotechnological revolution in the world has resulted in environmental contaminations such as release of toxic heavy metal into human’s food chain. Adverse effect of heavy metal ions on human health have increased concerns over last decade in the world. The presence of heavy metal at high concentrations leads to many disorders in neurological, reproductive, and cardiovascular systems in human’s bodies. Hence, real time monitoring of heavy metal ions presence is required. One of the most common and efficient technique is the fluorescent sensors by semiconductor quantum dots due to their selectivity, sensitivity, and fast detection. Among all semiconductor quantum dots, Graphene Quantum Dots (GQDs) have attracted considerable attention for detection of heavy metal ions due to their photostability, low toxicity, and tunable fluorescence properties. In recent years, functionalized-GQDs (FGQDs) and doped-GQDs (DGQDs) have emerged as one of the best candidates for fluorescence sensing of heavy metal ions due to their ease of structure manipulation and optimization for this application. Among all new studies in this field, amine-functionalized and nitrogen-doped GQDs have been studied the most in fluorescence heavy metal ions sensing due to their unique and different properties. It is observed that based on synthesis technique of functionalized- and doped-GQDs, the detection of heavy metal ions may vary. 1.2 State-of-the-Art In this M.Sc. thesis, using FGQDs and DGQDs, among the best candidates compared to conventional quantum dots, the heavy metal ions sensing will be performed using their fluorescence properties. As amine-functionalized and nitrogen-doped GQDs proved to be useful for heavy metal ion sensing due to their high affinity to metal ions [1, 2], we developed new amine-functionalized and nitrogen-doped GQDs using 3-aminobenzylamine and determine their accuracy toward heavy metal ions sensing. The innovation in our study includes: Utilizing new functionalization and dopant material for samples synthesis • Comparing chemical structure of DGQDs and FGQDs in interaction with heavy metal ions 2 • Sensing two different heavy metal ions by FGQDs and DGQDs To the best of our knowledge, there is no study on the comparison of FQGDs and DGQDs using the same functionalization and dopant material to determine the effect of nitrogen-containing groups on the heavy metal ions sensing. Most of studies on FGQDs and DGQDs were developed using ammonia, phenyldiamine, phenylalanine, and dimethylamine as the functionalization and dopant material, proved to be useful for the determination of Cu2+ [3, 4], Fe3+ [5, 6], and Hg2+ [1, 2]. Hence, one of the literature gaps is the effect of PL behaviour of nitrogen-containing group positions in the GQDs structure, whether in the GQDs plane or on their edges, and their applications in heavy metal ion sensing. Based on our idea, we chose 3-aminobenzylamine as the functionalization agent due to having highly active groups of NH2 dangling from an aromatic group (Figure 1.1). Figure 1.1 Structure of 3-aminobenzylamine. 1.3 Thesis Chapters Chapter 1 is a brief introduction to the subject of study, objectives, focus, innovations, and structure of this M.Sc. thesis. Chapter 2 is devoted to the literature review of the subject, introducing the environmental pollution and effect of heavy metal ions, previous common techniques for ions detections, and GQDs emergence, structure, and applications, especially in heavy metal ions sensing. Chapter 3 of this study is devoted to our particular synthesis and characterization techniques explained in detail. The synthesis procedures are inspired from most common synthesis techniques such as chemical synthesis. However, to optimize the synthesis methods based on the materials concentrations and available facilities, the design of synthesis procedures is explained in detail in Chapter 3. Moreover, Chapter 3 contains the characterization techniques and conditions of the 3 synthesized materials. The last section of Chapter 3 shows the heavy metal ions sensing platform that we chose for the detection. Chapter 4 contains the results and discussion parts of the study, in which the quantum dots synthesis, structure, properties, and their effects on heavy metal ions sensing are clarified. First, the synthesis process and developed materials are explained to clarify our synthesis method and conditions. Then, the structure and properties of the quantum dots are determined based on common characterization techniques. Finally, the effect of these quantum dots on the heavy metal ions sensing is performed. Lastly, Chapter 5 is devoted to conclusions, our challenges along the way, gaps in our study, and our suggestions toward possible future studies. 4 Literature Review To introduce the subject, a literature review in Chapter 2 is outlined. The flowchart of the Chapter is shown in Figure 2.1. Figure 2.1 Flowchart of Chapter 2. 2.1 Adverse Effects of Heavy Metal Ions Metal elements with high atomic weights between 63.5 to 200.6 g.mol-1 and high specific gravity (higher than 5.0 g/cm3) are considered heavy metals. The vast development of industrial and agricultural activities in the world, such as metal plating, mining, fertilizers, pesticides production, and batteries disposal, has increased the discharge of pollutants into the environment, negatively affecting human health [7]. As heavy metals accumulate in living organisms and are not biodegradable, they are highly toxic and carcinogenic. A high concentration of heavy metal, such as mercury (Hg), cadmium (Cd), lead (Pb) , arsenic (As), cobalt (Co), and chromium (Cr), is a significant concern to the environmental ecosystem [8]. The introduction of a very low amount of Cd, Hg, and Pb (~0.02 mg/L) to the human body resulted in cardiovascular disease, liver damage, cancer, reproductive disease, and nervous system dysfunction. Furthermore, heavy metal contamination of rivers, lakes, and soil results in crop growth inhabitant, a decrease in quality and 5 yields of agricultural products, and an adverse effect on mammals’ health and their reproductivity and immune systems [9]. Mercury (Hg) has been known as the most toxic element on the planet based on the US Government Agency for Toxic Substances and Disease Registry (TSDR) [10]. Industrial activities have resulted in the spill of mercury in the soil, water, and eventually the planet's food chain. The sources of human mercury exposure are dental amalgam, incandescent lights, thermometers, barometers, fossil fuels emissions, batteries, and incineration of medical wastes. A high mercury concentration results in death, mental retardation, dysarthria, blindness, neurological deficits, loss of hearing, developmental defects, and abnormal muscle tone [11, 12]. On the other hand, although Iron (Fe) is an essential element in biological systems, the accumulation of an abnormal amount of this ion results in heart failure, diabetes, and cancer [13]. The harmful effects of heavy metals in Canada have also been recognized. Environment and Climate Change Canada (ECCC) established the adverse health effects of heavy metals on residence-consuming fish caught in the vicinity of mercury chlor-alkali plants in the mid-1970s. Therefore, the new promulgation of the chlor-alkali mercury release regulations was initiated under the Canadian Clean Air Act (CCAA). In 1978, the Canada-U.S. Great Lakes Water Quality Agreement (GLWQA) identified ten heavy metals as the most toxic substances in the Great Lakes, such as As, Cd, Cr, Cu, Fe, Pb, Hg, Ni, and Zn. Hence, the studies over the heavy metal causes and detection received attention over subsequent years. Currently, mercury is the first candidate on the list of heavy metals [14]. In 1970, Ontario Minamata disease, which is a neurological syndrome triggered by severe mercury poisoning, affected two First Nation communities of northwestern and southern Ontario by consumption of fish contaminated by mercury and illegal disposal of industrial waste, respectively [15, 16]. The symptoms included narrowing of the visual field and impaired hearing, abnormal eye movements, tremors, ataxia, and dysarthria in the residences. The concerns over the mercury contamination resulted in the Grassy Narrows and Islington Indian Bands Mercury Pollution Claims Settlement Act and the Government of Ontario's English and Wabigoon River Systems Mercury Contamination Settlement Agreement Act establishment, which led to the creation of the Mercury Disability Fund (MDF) in a total of $16.67 million [17, 18]. Another example of heavy metal contamination in Canada and new regulations establishment is lead contamination in houses. The government of Canada restricted lead-containing products, such as pipes, paints, car parts, electronics, and 6 batteries, in the 1980s. Houses built after 1990 must be painted with lead-free paints in both interior and exterior spaces. Before this time, all houses were painted by lead-containing paints, which resulted in severe health issues, especially in children under six years old. Moreover, the National Plumbing Code allowed lead as a material in pipes until 1975. It allowed the use of lead-based solder in plumbing until 1986, resulting in toxic drinking water for residents. Currently, the lead pipes are still available in some parts of Canada; therefore, the government is taking the initiative to expand residents’ knowledge of the hazard and replace some of the old pipelines [19]. A recent study of Toxic Metals in Selected Food from April 1, 2018 to Mach 31, 2019 was published by the Canadian Food Inspection Agency (CFIA). They have investigated the concentration of heavy metals in different products, such as barn products, baby formula, meal replacement, protein powders, and rice products in four Canadian regions, including Atlantic (Halifax), Quebec (Montreal), Ontario (Toronto, Ottawa) and the West (Vancouver, Calgary). The results showed that the lead, arsenic, and cadmium concentrations in baby formulas had been increased by 300%, 23%, and 120%, respectively, while the mercury concentration decreased by 70% from 2012 to 2019. An increase in lead, arsenic and cadmium in meal replacement has also been reported from 2012 to 2019. On the other hand, in the rice products, the lead, arsenic, and cadmium concentration decreased by 18%, 50%, and 40%, respectively, while the concentration of mercury did not change from 2009 to 2019. Moreover, the study reported a decrease in the mentioned heavy metal concentration in protein powder from 2009 to 2019 [20]. Although heavy metals are in different forms and complexes in nature such as, ions, mercury chloride, cupper fumes, ferric oxide, niccolite, etc., studies have shown that the uptake of heavy metals by organisms correlates with free heavy metal ions, rather than the total metal concentration in solution. Therefore, free metal ion concentration determines the bioavailability and toxicity of heavy metals to the organisms [21-23]. Accordingly, the detection of heavy metal ions in the environment, especially drinking water and human’s food chain, has drawn the attention of researchers [24]. The current sensing technologies for heavy metal ion sensing are described in the following subsections. 7 2.1.1 Heavy Metal Ions Detection Methods Several detection methods for heavy metal ions have been proposed in recent years. The most common procedures, such as Atomic Absorption Spectroscopy (AAS), Electrochemical Sensors, Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Fluorescent Sensors, are explained in the next sections. Atomic Absorption Spectroscopy: AAS is an analytical technique to calculate the quantities and concentration of chemical elements by absorbing electromagnetic radiation applied in a specific wavelength. A schematic of the AAS setup is shown in Figure 2.3 a. As the sample is irradiated by ultraviolet and visible light, part of the light is absorbed by the sample based on the type of atoms, and the rest is transmitted. Although narrow-line source provides high intensity and accuracy toward atoms recognition, multielement lamps could also be used. AAS measures the wavelength of transmitted light and compares them with the original incident light, and the detector provides energy absorption at a different wavelength. The absorption is directly proportional to the concentration of atoms in the sample [25]. Although AAS is one of the most accurate and sensitive techniques for heavy metal ion detection in different samples, it requires an expensive setup and standard solutions, which makes this method unsuitable for on-site detections [26]. Electrochemical Sensors: An electrochemical system consists of three sensing electrodes (working, reference, and counter electrodes) immersing in a test solution as the electrolyte. A schematic of the electrochemical sensor is shown in Figure 2.3 b. Stripping voltammetry, as an electrochemical method, includes two steps: pre-concentration and dissolution. Firstly, the metal cations accumulate on the working electrode, simultaneously reducing to zero-valent metal under the negative potential. Next, the dissolution of produced complexes on the electrode in the anodic direction, which is also known as oxidation, results in zero-valent metal cations. As a result, a high dissolution current will be achieved during the oxidation process and based on different kinds of heavy metal ions, the current peak potential changes [27]. For example, Cheng et al. [28] reported exceptionally low detection limits in Cu2+, Cd2+, and Hg2+ determination using reduced graphene oxide and silver nanoparticles composites electrode with values of 10-15 M, 10-21 M, and 10-29 M, respectively. Electrochemical sensors are simple, sensitive, and appropriate for on-site detection. However, the electrode preparation process is time-consuming, and the electrodes' physical properties have to be completely controlled to achieve accurate results [26]. 8 Inductively Coupled Plasma Mass Spectroscopy: ICP-MS is an analytical technique to determine the trace amounts of elements in various samples. A schematic of the ICP-MS setup is shown in Figure 2.3 c. In this setup, the liquid samples are converted to aerosol and transferred to high-temperature argon plasma to be atomized and ionized. Next, the ions are guided to the mass analyzer by electrostatic lenses of the setup. The mass analyzer divides the ions based on their mass-charge ratio, and the detector measures the ions [29, 30]. Schramel et al. [31] had a comparative study on the concentration of Pb, Cd, Hg, antimony (Sb), bismuth (Bi), tellurium (Te), thallium (Tl), and palladium (Pd) in urine samples by ICP-MS and AAS techniques. They have reported that by the ICP-MS technique, all the elements could be detected, while using the AAS method, tellurium (Te), thallium (Tl), and bismuth (Bi) could not be detected in samples. They have concluded that ICP-MS is superior compared to AAS for physiologically and environmentally caused metal levels in urine samples. Furthermore, they have reported that the ICP-MS could be a reference method for the AAS technique due to the detection capability of lower concentrations of elements. One of the most critical advantages of ICP-MS over AAS is multi-element detection capability, which is useful the for simultaneous detection of different elements. Furthermore, this method requires simple sample preparation and can detect the elements in a broad analytical range and low detection limits. On the other hand, the setup has some disadvantages, including the high cost of equipment, maintenance, and complex analysis [30] Fluorescence Sensing: Quantum dots have been used for fluorescence sensing due to their proper surface state, improving dispersibility and fluorescence properties. Introducing analytes, such as heavy metal ions, to quantum dots results in the charge configuration on the quantum dots surface and enhance or quench the light-emitting intensity. Therefore, different analytes can be detected in the presence of quantum dots by their fluorescence activities. Scientists have been working on different selective fluorescent probes for heavy metal ions detection by the modification of quantum dots' surfaces with different functional groups based on applications. Classic quantum dots, such as zinc sulphide (ZnS) [32, 33], cadmium selenide (CdSe), cadmium sulfide (CdS) [34, 35], and cadmium telluride (CdTe) QDs [36, 37], have been reported as great candidates for heavy metal ion detection. Introduction of different thiol surfactants or functional groups such as L-cystine [38], rhodamine [39], thioglycolic acid [40], sulfur calixarene [41], and mercaptoacetic 9 acid [42] on the surface of quantum dots can enhance the selectivity of the sensors. For instance, N-acetyl cysteine capped cadmium selenide quantum dots have been used for Hg2+ detection by Subramaniyan et al. [35]. The addition of heavy metals to CdSe quantum dots rapidly quenched the fluorescence and resulted in the aggregation of quantum dots and formed a large cluster; however, in the presence of other ions, the fluorescence did not change (Figure 2.2). Optic methods via fluorescence properties of quantum dots have been evolved as one of the most effective techniques in heavy metal ion detection. Although the beforementioned methods are useful in some applications and proved to be accurate and sensitive, they have some drawbacks, including expensive setup equipment, complex analysis of the results, extensive sample preparation, and inflexibility for on-site sample detection. Hence, the introduction of fluorescence sensing using quantum dots gained attention as an alternative due to its low cost, being fast, sensitive and selective, and compatibility in aqueous environment [26]. A schematic of the fluorescence sensing setup is shown in Figure 2.3 d. Figure 2.2 (Left) Addition of heavy metal ions to CdSe quantum dots and (Right) change in the fluorescence intensities of quantum dots in the presence of heavy metal ions. Reproduced with permission [35], copyright 2019 RSC Advances. 10 Figure 2.3 Schematics of heavy metal ions sensing setups a) AAS [43], b) Electrochemical sensors, reproduced with permission [27], copyright 2018, Elsevier, c) ICP-MS [44], and d) fluorescence sensing. The luminescence properties of quantum dots are sensitive to their surface states, including chemical and physical interactions, which lead to the change of radiative recombination efficiency. Therefore, quantum dots have photoluminescence (PL) or quenching effect [45]. As the light beam is irradiated to quantum dots, the valence band's electron absorbs the photon and travels to the conduction band. Afterward, the electrons may travel to the valence band and emit photons or fall into electron traps between valence and conduction band, resulting in radiative waves that could be seen. When the electrons fall into these traps, most of the other electrons are quenched in a non-radiation form, resulting into non-radiative waves. The quantum dot surface defects result in trapping the electrons; hence, direct recombination decreases while exciton light emission is low, as it emits solely from the surface defect state [46]. Figure 2.4 shows the interaction of UV light with quantum dots. 11 Figure 2.4 Schematics of quantum dots energy bonds. The mechanisms behind the quenching effects of heavy metal ions on quantum dots can be related to the inner filter effect, non-radiative recombination, binding reaction, and electronic transmission [47]. On the other hand, fluorescence enhancement of quantum dots by heavy metal ions is rare due to the increase of trap state or defect passivation on the quantum dots’ surface [48]. Studies on heavy metal ion detection by quantum dots revealed that the metal ion concentration and the fluorescence enhancement or quenching have a linear relationship. Therefore, metal ions concentration can be easily determined by fluorescence alterations. Fluorescence detection of the heavy metal ion by quantum dots requires ligands or functionalization groups that participate in the reactions to increase the dispersibility of quantum dots in the environment and improve the affinity of heavy metal ions to quantum dots [49]. Some examples of classic quantum dots for heavy metal ions detection are outlined in Table 2.1. 12 Table 2.1 Classic quantum dots applications in heavy metal sensing. Quantum Dots Modifying Group Detected Heavy Metal Ion Limit of Detection Ref. CdS L-cysteine Hg2+ 2.4 nM [47] CdSe ZnS Cu2+ 0.13 M [50] CdTe Mercaptopropionic Acid Ag+ 5 nM [26] CdSe/ZnS Core-shell L-carnitine Cd2+ 0.15 μM [51] CdTe Dithizone Pb2+ 0.006 nM [52] ZnS N-acetyl-L-cysteine Hg2+ 5 nM [53] ZnS Mercaptopropionic Acid Hg2+ 2.5 nM [54] Although classic quantum dots for heavy metal detection proved to be accurate and selective, they have shown some drawbacks, such as poor photostability, short lifetime, high toxicity, and high cost [55]. Hence, the introduction of Graphene Quantum Dots (GQDs) with their significant optical properties, over classic quantum dots results in their application in heavy metal ion sensing [56]. 2.2 Carbon-based Nanomaterials Nanoscale materials exhibit novel and unpredictable characteristics that could be beneficial in different applications. For example, nanoscale materials show extraordinary strength, chemical reactivity, electrical conductivity, superparamagnetic behaviour, and others compared to micro- or macroscale materials. Therefore, nanomaterials studies have been widely popular in recent decades. The usage of nanomaterials in different applications leads to the development of science of nanotechnology [57]. Graphite is one of the most used materials in the world. Due to the favorable physical properties and molecular structure, the usage of graphite is widely known in the industry. With appreciation of nanotechnologies, graphite has also been used to develop carbon-based nanomaterials, such as single- and multi-wall carbon nanotubes (SWCNT and MWCNT), graphene, GQDs, fullerene, and so forth. Some of the carbon-based nanomaterials structures are shown in Figure 2.5 [58]. These materials have been widely used in different applications, such as 13 electronics [59], biomedical [60], environment [61], and energy-related applications [62], and proved to have high mechanical, thermal, and optical properties. Figure 2.5 Structure of a) graphite, reproduced with permission [63], copyright 1999, IEEE, b) single-walled carbon nanotube, reproduced with permission [64], copyright 2005, IEEE, c) multi-walled carbon nanotube, reproduced with permission [65], copyright 2020, MDPI, d) Graphene, reproduced with permission [66], copyright 2018, Nature Publishing Group e) graphene quantum dots, reproduced with permission [67], copyright 2016, Springer, f) fullerene, reproduced with permission [68], copyright 2015, Taylor and Francis group. Among all carbon-based nanomaterials, graphene received particular attention in many applications [69]. Graphene is a 2D hexagonal or honeycomb structure of sp2 hybridized carbon atoms with the thickness of one atom layer [70]. The carbon atoms in a layer are covalently bonded (σ bond), while between the layers are van der Waals forces due to π bond structure. The σ bonds are heavily overlapped, stable, and not reactive. On the other hand, π bonds are slightly overlapped, unstable, and reactive. In graphene, three sp2 electrons can form three 𝜎 bonds with neighboring atoms in a basal plane [71]. These bonds are identical, and their angles are 120°. It is worth mentioning that the sp2 hybridized electrons are responsible for the structural stability of graphene. The fourth orbital of the carbon atoms is oriented vertically to the sp2 hybridized carbon atoms plane and creates π bonds between other carbon atoms in the upper/lower layers [72]. These quasi-free π electrons that are shared among all the atoms in a layer are responsible for graphene’s high electrical conductivity [73]. Although graphene is a plane of carbon atoms, their edge's chemical 14 properties differ from the plane. Graphene edges exhibit higher electron transfer rate and catalytic power compared to the plane. Hence, the different parts of the graphene structure have triggered the researchers to introduce functionalized graphene materials [74-76]. Moreover, graphene derivatives, including graphene oxide, reduced graphene oxide, graphene quantum dots, doped and functionalized-graphene have attracted numerous researchers due to their outstanding properties resulting from the presence of an aromatic ring, free π-π electron, size, edge defects, and reactivity of their functional groups [77]. An illustration of graphite, graphene, and GQDs structures, is shown in Figure 2.6. Figure 2.6 A schematics of graphite (reproduced with permission [78], copyright 2016, Scholar Commons), graphene (reproduced with permission [79], copyright 2015, Advanced Healthcare Materials), and GQDs (reproduced with permission [78], copyright 2016, Scholar Commons). 2.2.1 Graphene Quantum Dots GQDs are fragments of graphene below 20 nm in size and feature quantum confinement and size effect [80]. GQDs could be generated from exfoliation and cutting of graphite or graphene nanosheet. In recent years, GQDs have been greatly under consideration due to their distinctive characteristics like large surface area, low toxicity, and superior optical properties [81]. Due to these excellent properties, GQDs distinguish themselves from conventionally researched fluorescent compounds [82]. Although the 2-dimensional graphene plane has shown desirable properties, they have some shortcomings, including aggregation and undesirable dispersion in liquid media. Furthermore, since graphene is a semiconductor with zero bandgaps, it cannot be implemented in PL applications such as fluorescence sensing, solar cells, etc. [83]. 15 The emergence and synthesis of GQDs and graphene were introduced almost at the same time [80]. However, due to the unique characteristic and synthesis methods of GQDs, they have been researched less than graphene. GQDs not only exhibit graphene’s fantastic properties, but also show PL properties [84], high dispersibility[85], larger surface area [143, 144], high absorptivity [86], adequate biocompatibility [87], and low toxicity [88]. In addition, the ability to tune the GQDs’ bandgaps makes them an excellent candidate to improve PL properties and applications [83]. 2.2.2 Graphene Quantum Dots Structure and Properties Structure: Graphene quantum dots (GQDs) are zero-dimensional materials resulting from “cutting” the graphene plane in a size range of 2 to 20 nm and variable thicknesses but less than ten layers [89]. GQDs consist of sp2 and sp3 hybridized carbon. The ideal GQDs consist of only sp2 clusters of carbon atoms; however, through the synthesis methods, there are always some oxygen-containing groups such as hydroxyl, epoxide, carboxyl, and carbonyl groups bonded to sp2 clusters and create sp3 clusters [90-93]. GQDs’ edges and surface change their electronic band structure compared with the bulk material. In the potentially defective part of the surface and edges of GQDs, a new electronic energy level can be formed through the introduction of other atoms bonded with covalent and non-covalent forces [94]. Therefore, these materials exhibit bandgaps with different energies based on the types of atoms and chemical groups connected with them on the surface or edges. GQDs show a quantum size effect and bandgap in contrast to graphene due to their small size; hence, they display unique optoelectronic properties compared to graphene. GQDs with specific properties intrigue scientists to synthesize functionalized-GQDs (FGQDs) and heteroatom doped-GQDs (DGQDs). The functionalization and doping of GQDs can improve their thermoluminescence, mechanical, thermal, electronic, and optical properties [95]. The edge termination of GQDs and potential to be functionalized make them a great candidate to optimize their properties for different applications. In principle, as the GQDs contain carboxylic acid moieties, one can replace these groups with organic, inorganic, or biological functional groups, including amide, cyano, nitro, aldehyde, and fluorine by covalent or non-covalent bonds [96]. The introduction of heteroatoms in the GQDs lattice, such as nitrogen, boron, sulfur, and phosphorous results in different properties of GQDs due to the disorientation of electron distribution and 16 changing the spin of dopant zones [97, 98]. As a result, DGQDs have been introduced as one of the most effective fluorescence probes with other advantages, including low cytotoxicity, broadband optical absorption, and high chemical stability against photobleaching, to be used in different applications [99, 100]. Photoluminescence properties: The photoluminescence (PL) ability is described as the capability of a molecule absorbing an incident photon that results in the excitation and transformation of an electron in the structure to the higher energy level and the emission of a photon as the electrons reach a stable energy level [101]. As graphene exhibits zero-bandgap, the PL activity cannot be seen in the graphene structure. On the other hand, GQDs exhibit a non-zero bandgap and PL activity, which can be related to adding functional groups on the edges and surface, introducing dopant elements, or altering the size of the GQDs dimension [102]. Although the PL mechanism of GQDs has been under debate and the primary mechanism has not been yet defined, the aforementioned parameters and alterations in the GQDs structure can control the two main PL mechanisms. Firstly, an intrinsic quantum confinement effect is based on the GQDs dimensions and the number of aromatic rings in the structure. Secondly, an extrinsic effect of the electron energy levels of 𝑠𝑝2 carbons in GQDs lead to altering the bandgap, which is related to the surface functional groups, edge effect, and dopants in the structure [103, 104]. The PL emissions of GQDs could be changed by the beforementioned parameters that result in different colour emissions, including ultraviolet [105], blue [106], green [107], yellow [108], red [109], and near-infrared radiation [110]. Hence, by manipulation of physical and chemical features of GQDs, they reveal tunable optical properties. As their size can control the bandgap of GQDs, studies have shown that the size of GQDs can be influenced by parameters related to the synthesis process, including reaction time, concentration, and temperature [111, 112]. Sk et al. [84] revealed that the PL of GQDs could be adjusted by their size, shape, edge state, attached chemical moieties, heteroatom doping, and surface defects by numerical studies (Figure 2.7). 17 Figure 2.7 Effect of size on GQDs’ PL properties. Reproduced with permission [84], copyright 2014, Royal Society of Chemistry. Furthermore, the synthesis method is also an important factor in controlling GQDs sizes. For example, the top-down synthesis methods exhibit a wider range and inhomogeneity of size distribution in GQDs. Hence, the PL emission would be dependent on the excitation wavelength [113]. However, the excitation-independent behaviour of PL emission has also been reported, which can be seen in higher uniformity and lower number of defected sites in GQDs structure. Studies have shown that an increase in GQDs diameter leads to a smaller bandgap and a redshift in PL emission wavelength [102]. The theoretical investigations showed that the grafting of functional groups on GQDs surface and edges results in a decrease in symmetry of GQDs structure, leading to the delocalization of the electrons and a higher concentration of holes on the edges. This phenomenon results in the redshift of PL and narrowing the bandgap [114]. As the functional groups have more ability to donate an electron to GQDs, the bandgap would be smaller. In addition to the electron-donating ability of functional groups, if the functional groups increase the sp2 domain of π-conjugated system in GQDs, it will exhibit a narrow bandgap and a redshift in PL emission wavelength [115]. On the other hand, by decreasing the functional groups such as oxygen-containing groups, an intermediate n-orbital is removed between π and π∗ orbitals 18 resulting in a blueshift in PL emissions [116]. In addition, the introduction of amine groups on the GQDs edges leads to the formation of an interbond in the bandgap region due to the strong interactions of nitrogen atoms with GQDs. Hence, the bandgap decreases and consequently leads to the simple transformation of the proton from ammonium moiety to the core graphene, and increase the PL ability [117]. The degree of functionalization was proved to influence the PL ability of GQDs. For example, the functionalization of GQDs with a higher degree of aniline derivatives resulted in the improvement of extrinsic emission wavelength [116]. Yan et al. [118] decreased the GQDs bandgap by increasing the π system or addition of n-orbitals to the structure of GQDs (Figure 2.8). Increment of the π system has been applied by conjugation of GQDs with various polyaromatic compounds, such as o-phenylenediamine (OPD), 2,3- diaminonaphthalene (DNPT23), 1,8-diaminonaphthalene (DNPT18), and 1,1′-bi(2-naphthylamine) (BNPTL), by a condensation reaction between C=O and -NH2 groups to form C=N bonds under a particular solvothermal condition (180℃, 12 hours). On the other hand, the formation of N-C=O amide bonds resulted in the introduction of the n-orbitals between π and π* orbitals by the addition of electron-donating functional groups such as panisidine (Anln-OCH3), 4-(trifluoromethoxy)aniline (AnlnOCF3), or 4-(trichloromethoxy)aniline (Anln-OMLl3), to GQDs during the condensation reaction between -COOH and -NH2 under solvothermal condition (180 °C , 12 hours). Figure 2.8 Decreasing the bandgap of GQDs by enlarging the π system or addition of n-orbitals. Reproduced with permission [118], copyright 2018, ACS Publications. 19 Electronic properties: In the planar geometry of GQDs, each carbon atom has three sp2-hybridized bonding orbitals connected to the neighboring carbon atoms through stable covalent bonding. The fourth bonding stays unpaired on the planar lattice in a π-orbital. This structure is responsible for creating a unique electronic structure. In an ideal 2D carbon lattice, the bandgap could be zero; however, due to the lateral dimensions, the presence of defects, functional groups, and physical conformation, GQDs exhibit a non-zero bandgap. The presence of the edge passivation by various functional groups results in a sp3-hybridized cluster on the GQDs edges and surface. As the sp2 clusters present more energy than sp3 clusters, they can be quickly excited through absorbing light and show PL activities [119]. The bandgap can be modified by size and functional groups on the GQDs surface in the range of 0 eV to more than 6 eV [120]. 2.3 Graphene Quantum Dots Synthesis Numerous synthesis approaches have been developed for the synthesis of GQDs [80, 121-127]. Over time, the synthesis routes have become easier, more efficient, size controllable, and led to higher yield of GQDs. Generally, the synthesis of GQDs can be divided into two categories of top-down and bottom-up methods. The top-down approach is based on obtaining nanoscale GQDs from the bulk carbon materials using cleavage methods. These methods consist of chemical, electrochemical, and physical techniques. On the other hand, bottom-up approaches lead to nanoscale GQDs using molecular and atomic precursors to grow into nanosized GQDs [67, 128]. A conceptual illustration of the GQDs synthesis by the top-down and bottom-up method is shown in Figure 2.9. 20 Figure 2.9 Top-down and bottom-up routes for the GQDs synthesis. Reproduced with permission [129], copyright 2017, Royal Society of Chemistry. 2.3.1 Top-Down Method It is well known that top-down approaches are more comfortable and faster than bottom-up approaches and a wide variety of carbon-based materials, including graphene oxide (GO) [130], CNTs [131], carbon black [132], fullerene [133], graphite [134], carbon fiber [90], coke [135], coal [136], and carbon fibers [137] can be used to synthesize GQDs. However, the top-down method results in a low-yield product, and GQDs prepared by this method often have functional groups on their edges that might not be desirable. Therefore, the nanomaterials made by top-down method require post-modification. More details on common outlines of the top-down approach are given below. Acidic and oxidative cleavage method: Oxidative cleavage, also known as acidic exfoliation, is based on the usage of strong acids and oxidizers to exfoliate the carbon-carbon bonds of carbon-21 based materials to create nanoscale GQDs. This approach is fast, simple, and is the best approach to produce GQDs on a large scale [124]. In this method, the idea of unzipping or exfoliating the carbon-based bulk material such as carbon fibers, MWCNT, coals, etc., consists of the carbon-carbon bonds breaking by strong acids or oxidative agents such as H2SO4 [138], HNO3 [139], and KMNO4 [140]. The introduction of acids or oxidative agents results in the intercalation of negative ions between different layers of precursors and the de-bonding of the carbon bonds, which leads to the creation of oxygen-containing functional groups on GQDs’ edges. These GQDs might be hydrophobic or hydrophilic based on their functional groups [80]. It is worth mentioning that the addition of oxidative chemicals can increase the functional groups, resulting in the increment of d-spacing between different layers of carbon precursors [141]. For instance, coal contains graphitized carbon domains that are weakly linked by amorphous carbon bridges, can be easily cut by strong oxidative agents, as shown in Figure 2.10 [136]. Figure 2.10 Synthesis of GQDs by coal. Reproduced with permission [136], copyright 2013, Nature Communication. Although oxidative cleavage showed some advantages, it has some shortcomings. Based on this method, strong acids may cause “burning” the structures of GQDs and need complex post-processing treatments [142]. Therefore, a new oxidative cleavage method introduced by Lu et al. [143] used H2O2 and resulted in the desired product in less than 90 minutes. Hydrothermal and solvothermal method: In this method, the carbon materials are cut into GQDs under high temperatures and pressures. The hydrothermal treatment involves preparing GQDs in an aqueous suspension of carbon materials, while in the solvothermal treatment, organic solvents are the main media of the process [144]. Generally, the carbon precursors should be treated by 22 strong oxidation procedures before hydrothermal or solvothermal reactions. It is reported that oxidation treatment creates oxidized functional groups, such as hydroxyl, and carboxyl groups, resulting in structural defects in carbon materials [145]. Therefore, they are potentially active, and have a strong affinity to react with other chemicals [146]. Microwave/ultrasound-assisted method: As the hydrothermal/solvothermal and oxidative cleavage methods are time-consuming processes, the introduction of the microwave-assisted method showed a faster procedure with a higher yield. The first microwave-assisted synthesis of GQDs was reported in 2012 in the acidic environment using GO as the primary precursor [89]. The microwave radiation acts as a heat source in GO, leading to the fracture of carbon bonds to form GQDs. On the other hand, the ultrasound-assisted technique was used to provide GQDs at room temperature. Based on the proposed mechanism, the ultrasound waves result in bubbles in the medium and produce high energy. The obtained energy can simply debond the carbon bonds in the precursors, resulting in the GQDs formation. It is believed that the size and thickness of the obtained GQDs can be tuned based on the ultrasonic waves’ energy [147]. In the ultrasonic-assisted method, the cleaving and reduction steps to produce GQDs can be obtained simultaneously by ultrasonic waves. Therefore, the microwave-assisted method can create GQDs in a short time with a high yield and efficiency [148]. Other top-down methods: Apart from the abovementioned methods, other methods were developed by researchers to synthesize GQDs. However, these methods are complex, expensive, and are not used frequently. Some of the reported methods are electrochemical oxidation [149, 150], magnetron sputtering [151], synthesis under UV irradiation [152], nanolithography [140], chemical vapor deposition (CVD) [153], pulsed laser ablation [154], and electron beam irradiation [155]. These methods have some disadvantages, including low yield, expensive process, and time-consuming reactions [124]. FGQDs and DGQDs synthesis: The first stage of FGQDs synthesis is to create initial suspensions such as graphene oxide, GQDs, and graphite oxide suspension [156]. In this stage, the precursor is mixed with harsh acids under a high temperature and vigorous stirring [157], or ultrasonication [158]. By forming these suspensions, the carbon-containing honeycomb sheets are parted from each other and then make a more convenient functionalization step. Furthermore, the oxidation of GQDs sheets and GO sheets provides hydroxyl and carboxyl groups on their edges, making them 23 a proper candidate for the replacement of some functional groups with some other functionalization groups [159]. In top-down chemical synthesis, by the addition of the reactant chemicals to GQDs, the COOH groups on the edges are converted to highly activated COO- ions. Furthermore, OH groups of GQDs can engage in a chemical reaction with other materials, resulting in free bonds of the carbon atoms on edges and creating an active site for covalent bonding with functionalized materials. These radicals and active edges are highly prone to react with positive radicals of functionalization materials to neutralize the GQDs. Several studies have used this method for FGQDs synthesis. This method can be conducted over refluxing or simple stirring for a couple of hours. For the purification process, dialysis or vacuum distillation can be used [160-173]. On the other hand, some reports reported a more complex procedure for FGQDs. In this method, in order to functionalize the target material to GQDs, firstly, GQDs should be functionalized and turned to an intermediate material acting as a bridge. Afterward, the intermediate material interacts with the target functionalized material and result in the desired FGQDs [174]. DGQDs have also been synthesized by the oxidative cleavage synthesis method. In this method, the oxidative/reductive agents act as scissors to cut the bulk graphene-based materials and provide smaller planes of GQDs while providing the opportunity for attacking the dopant elements into the GQDs structure [175]. For instance, Lee et al. [176] synthesized nitrogen-doped GQDs through a solvothermal process of poly-acrylonitrile-based carbon fibers in the presence of H2SO4 and HNO3 at 90 °C for 24 hours, followed by filtration and dialysis to remove the excessive chemicals. Double doped-GQDs have also been prepared by a top-down method. Zhang et al. [144] synthesized nitrogen and sulfur-doped GQDs by the hydrothermal reaction of graphene oxide, ammonia, and sulfur at 180 °C for 12 hours (Figure 2.11). 24 Figure 2.11 Synthesis process of nitrogen- and sulfur-doped GQDs. Reproduced with permission [144], copyright 2014, Royal Society of Chemistry. The microwave-assisted method has also been performed for top-down GQDs synthesis. In this method, the electromagnetic microwaves provide concentrated intensive energy to break the chemical bonds of the precursor and create the opportunity for diffusion of dopant elements. Moreover, this method’s homogeneous heating results in uniform size in produced nanostructures [177]. For instance, nitrogen-doped GQDs have been synthesized by 1,3,6-trinitropyrene and hydrazine hydrate addition to a glass reactor under microwave irradiation, followed by filtration and dialysis for purification by Li et al. [178]. 2.3.2 Bottom-up Methods Unlike top-down methods, few bottom-up techniques have been developed for the synthesis of GQDs. The bottom-up approaches consist of precise control over the synthesis mechanism. Therefore, it might be possible to synthesize GQDs with high purity, well-controlled particle size, and lower structural defects [70, 179]. The most common bottom-up procedures are outlined below. Carbonization method: This method is used to synthesize GQDs and FGQDs [180-183]. Regarding the synthesis of GQDs, the carbonization process exhibited lower cost and more environmentally friendly ability among all bottom-up approaches [183]. One of the most amazing synthesis routes based on bottom-up approaches of GQDs was reported by Bayat et al. [183]. They 25 have reported a synthesis route of GQDs under the hydrothermal condition of a glucose aqueous solution. Under high temperature and pressure during the hydrothermal condition, the hydrogen atoms join other hydroxyl groups of glucose molecules and generated water molecules as byproducts. Simultaneously, the carbon atoms network together, resulting in GQDs nanosheets. Furthermore, Dong et al. [184] reported a simple procedure to synthesize GQDs by controlling the carbonization degree of citric acid in an alkaline solution. They reported that by tuning the carbonization degree through controlling the reaction time of the heated solution, GQDs or GO could be synthesized. Based on the results, only after 30 minutes of reaction, GQDs were produced. In contrast, the production of GO needed further reactions over 2 hours. Usage of microwave radiation in this method has also been proved to accelerate the synthesis reactions by shortening the fusion duration of the process. This method provides simultaneous heating that might result in uniform particle sizes. For example, Tang et al. [185] reported a synthesis method for GQDs using an aqueous glucose solution using the microwave-assisted method (Figure 2.12). It was found that the variation of the microwave power, heating time, source concentration, and suspension volume directly affects the size and fluorescence properties of synthesized GQDs. Figure 2.12 Microwave-assisted synthesis of self-passivated GQDs from glucose. Reproduced with permission [185], copyright 2012, ACS Publications. Cage-opening of fullerene: Several studies have been reported on the production of GQDs via cage-opening of Fullerene [186, 187]. It has been reported that the synthesis of GQDs via a cage-opening approach results in well-defined GQDs with desirable size and shape. These studies 26 showed that fullerene could be fragmented at high temperatures and forms carbon clusters. Afterward, these carbon clusters are combined and lead to the formation of GQDs. By optimizing the temperature and cluster density in the process, the desirable size of GQDs is achieved. The mechanism of cage opening of Fullerene has been shown in Figure 2.13. Figure 2.13 Cage opening mechanism of the GQDs production. Reproduced with permission [187], copyright 2015, ACS Publications. FGQDs and DGQDs synthesis: Reports have shown that the most promising method for FGQDs synthesis is the carbonizations of citric acid and functionalization materials. In general, the citric acid and functionalization materials were mixed and heated. In this stage, the carbonization of citric acid results in GQDs formation, the surface modifier molecules create in-situ covalent bonding on GQDs edges [188]. Based on some reports, during heating of a mixture of citric acid and surface modifier agents in an open lid container (air contact), a polymer-like aromatic intermediate can link to the double bond of the functionalization materials, resulting in the polymerization of citric acid. Air is crucial for this method since oxygen is the initiator for free-radical polymerization [189]. Although the high number of functional groups results in better functionalization, the perfect structure of GQDs can be achieved through less functional groups on the edges. However, the smaller number of functional groups results in incomplete functionalization. Hence, the concentration of modifier materials and citric acid should be optimized. Furthermore, the excess amount of functional groups leads to smaller GQDs and might destroy the fluorescence quenching results [188]. In order to remove the extra materials generated 27 during the synthesis process, generally, the final suspension is dialyzed and freeze-dried to collect the FGQDs [188-190]. In another research study, Liu et al. used ethylene glycol as the carbon source and synthesized their FGQDs by centrifuging [191]. In the hydrothermal synthesis process for FGQDs synthesis, the carbon source was carbonized to create graphite dots at a high temperature and pressure. During the hydrothermal process, through the diffusion and condensation of graphite dots, the GQDs can be formed with different functional groups, including carboxylic, hydroxyl, epoxide, etc. On the other hand, the presence of modifier agents in the synthesis media helps GQDs have covalent bonding with other functional groups. Hence, the functional groups are anchored to GQDs edges and form stable FGQDs. One of the most famous bottom-up synthesis methods of DGQDs is the carbonization of the precursor in the presence of dopant material. In this process, as the GQDs’ plane is formed, the dopant can create covalent bonds with carbons and sit in the planar structure of GQDs, resulting in DGQDs formation. Afterward, the reaction could be carried out through a chemical reaction at high temperatures and pressures, such as hydrothermal and solvothermal processes [192]. Besides, the carbonization could be performed by the electrochemical method as well. The applied voltage in the cells could provide the required energy for the carbonization and polymerization process. For example, Tian et al. [193] synthesized nitrogen-doped GQDs via a green and large-scale method by an electrochemical process using platinum plates as anode and cathode electrodes and o-phenylenediamine in water as the electrolyte in voltage mode of 5V for 1 hour. In this method, the polymerization process is attributed to phenazine structure formation by coupling o-phenylenediamine and amino groups. Eventually, the two-dimensional nitrogen-doped GQDs with 2-6 nm lateral size were formed (Figure 2.14). 28 Figure 2.14 Possible mechanism for nitrogen-doped GQDs. Reproduced with permission [193], copyright 2016, RSC Advances. To better understand the top-down and bottom-up methods, some of the advantages and disadvantages of these methods are compiled in Table 2.2. Table 2.2 Advantages and disadvantages of GQDs synthesis methods. Methods Advantages Disadvantages Ref. Top-down ✓ Lower cost of carbon precursors materials ✓ Simple modification process of GQDs ✓ Narrow size distribution in hydrothermal and microwave-assisted methods ✓ Simple equipment needed for electrochemical synthesis ✓ High yield through microwave-assisted and electrochemical methods ✓ Post-fabrication modification needed ✓ Sophisticated equipment for microwave-assisted method ✓ Lack of control on GQDs sizes ✓ Wide size distribution of oxidative cleavage [132, 137, 194-196] Bottom-up ✓ Eco-friendly and green methods ✓ Cost-effective ✓ Good control over size distribution ✓ Synthesizing desirable doped and functionalized GQDs ✓ Needs precise investigation during the synthesis process ✓ Processes include complex chemical reactions ✓ Low yield ✓ Time-consuming [184, 186, 197-199] 2.4 Applications of GQDs-based Nanomaterials GQDs applications: Usage of GQDs has been reported in numerous applications, as shown in Figure 2.15. In this section, some of the interesting applications of GQDs are presented in Table 2.3. 29 Figure 2.15 GQDs properties and applications. 30 Table 2.3 Some of the applications of GQDs. Area Applications Example Ref. Fluorescence Sensing Detection of non-metallic ions, inorganic materials, and biomaterials Dopamine detection with the limit of 5 nM in the presence of ascorbic acid, uric acid, glutamic acid, lysine, Ca2+, and Zn2+. [200] Detection of cholesterol with the limit of 0.006 mM in the presence of glucose, uric Acids, urea, cysteine, ascorbic acids. [201] Biomedical Cellular imaging Imaging of breast cancer cell MCF-7 membrane, nucleus, and cytoplasm for the first time in 2012 using GQDs. [132] Cell imaging in Cervical Carcinoma HeLa Cell. Showed higher PL stability compared to conventional dyes and CdTe QDs. [202] Photovoltaics Solar cells Facilitating the photon-generated electron-holes production in heterojunction solar cells, which improved efficiency to 12.3%. [203] Usage of GQDs/poly (3,4-ethylene dioxythiophene) core-shell in organic photovoltaic solar cells increased efficiency up to 26%. [204] Supercapacitors Wearable devices Exhibiting superior rate capability with a high scan rate (1000 V/s), excellent power response, and a small RC time constant (103.6 μs). [205] Improving the migration rate of electrolyte ions in supercapacitors and excellent long-term stability even at 10000 charge-discharge cycles. [206] Anticancer therapy Drug delivery GQDs showed high intercalating ability toward DNA molecules. [152] The combination of GQDs and cisplatin increased the cytotoxicity of the drug in different cancer cells. [207] FGQDs and DGQDs applications: FGQDs and DGQDs have received massive interest in energy-related and electronic applications such as solar cells sensors, supercapacitors, batteries, and biomedical applications such as cancer therapy, drug delivery, and bioimaging [142]. In this section, some of the most advanced applications of FGQDs and DGQDs will be outlined briefly. Solar cells have received massive interest in the industry to provide a clean and renewable energy 31 source. The charge carriers of the solar cells, electrons, and holes are generated in a light-absorbing semiconductor, which creates electricity through a conductive material. FGQDs and DGQDs could be great candidates for these applications due to their tunable bandgap, strong PL properties, and absorption in the ultraviolet range. For example, Hasan et al. [208] reported that the introduction of nitrogen-doped GQDs into the titanium dioxide thin film of solar cells would result in the charge transfer enhancement in the active layer of solar cells, as well as limit the charge recombination. Tetsuka et al. [209] inserted an electron-donating phenylenediamine-functionalized N-GQDs with a narrow bandgap between the CH3NH3PbI3 and TiO2 layers of a perovskite solar cell to improve its charge collecting properties. They proved that nitrogen-doped GQDs enhanced the transportation of generated electrons to the absorbing layer by up to six-fold (Figure 2.16). Figure 2.16 A perovskite solar cell with phenylenediamine-functionalized N-GQDs. Reproduced with permission [209], copyright 2016, Wiley Online Library. Research have proved that nitrogen-doping in graphitic structure could modulate the electronic properties of GQDs due to the high affinity of nitrogen atoms. Moreover, nitrogen-doping and amine-functionalization proved to enhance the optical properties of GQDs such as PL and catalytic activity [176]. FGQDs and DGQDs could also be used in supercapacitors due to their transparency, high electrical conductivity, large surface area, tunable bandgap, and high dispersibility in liquid media. GQDs-based micro-supercapacitors proved to have a high-power density and cycling stability. For example, the incorporation of nitrogen and oxygen-doped GQDs into the gold electrode of a micro-supercapacitor has shown to enhance the loading density and volumetric 32 capacitance [210]. Furthermore, highly flexible supercapacitors have been developed by introducing FGQDs on carbon fibers in their structures. Hence, the usage of FGQDs proved to be a great candidate for designing wearable devices [211]. Due to the large surface area of DGQDs and FGQDs, which can provide active sites for electrolyte reactions, they could also be great candidates for Li and Na batteries. It was reported that oxygen-enriched-functionalized GQDs increase the Li+ ion diffusion and cycling performance and minimize active materials loss. Another example of ultra-long cycling performance in Li batteries has been reported by fluoride-functionalized GQDs [212]. PL sensors based on FGQDs and DGQDs have been mostly developed for heavy metal ions and alkali/alkaline-earth metal ions sensing and monitoring. Over the last decade, due to the severe contamination of the environment by heavy metal ions, many attempts have been conducted to develop the application of DGQDs and FGQDs in heavy metal ions sensing through their significant PL activities and quenching. A decrease in the fluorescence intensity of a sample is fluorescence quenching. The quenching might be arise from various molecular interactions, rearrangement, energy transfer, and collision [213]. The precise mechanism of GQDs-based nanomaterials quenching is still under debate. However, most reports have suggested a quenching mechanism for GQDs-based nanomaterials. They have reported that the primary mechanism relies on creating electron traps through the chemical interaction of GQDs-based materials and heavy metal ions. After light emission to DGQDs and FGQDs, electrons of the valence band may jump into the conduction band. These electrons may retransmit to the valence band and emit light or fall into an electron trap, resulting in a non-radiative transition and quenching. The metal ion concentration has a linear relationship with changes in the intensity of fluorescence quenching, providing a sensing platform for heavy metal ions. The heavy metal ion detection by DGQDs and FGQDs is affected by various parameters such as synthesis route, types of functionalization/dopant groups, concentration, nanostructure size, chemical reaction types, heavy metal ion concentrations, quantum yields, and affinity of heavy metal ion to GQDs [56]. Different examples of DGQDs and FGQDs applications in heavy metal ion PL sensing, synthesis routes, and quantum yield, which is the ratio of the photons emitted to the number of photons absorbed by a material, are outlined in Table 2.4.33 Table 2.4 Detection of heavy metal ions by GQDs-based nanostructures. GQDs-based Nanomaterial Synthesis Route Quantum Yield Average Size (nm) Heavy Metal Ion Limit of Detection Ref. Valine-FGQDs Bottom-up chemical 0.28 3.0 Hg2+ 0.4 nM [188] Aptamer-FGQDs Top-down chemical 0.90 4.0 Pb2+ 0.6 nM [55] Ethylenediamine-FGQDs Top-down hydrothermal 0.80 4.2 Ni2+ 30 nM [214] Thymine-FGQDs Top-down hydrothermal 0.42 6.5 Zn2+ 0.05 nM [215] Rhodamine B-FGQDs Bottom-up hydrothermal 0.41 4.5 Hg2+ 0.16 [216] D-penicillamine-FGQDs Bottom-up chemical 0.99 4 Fe3+ 1.2 μM [217] Amino-FGQDs Bottom-up hydrothermal 0.32 3.5 Cu2+ 5.6 nM [218] Boron and nitrogen co-DGQDs Bottom-up chemical 0.17 2 Hg2+ 4.3 nM [219] Boron-DGQDs Bottom-up hydrothermal 0.16 - Fe3+ 31 nM [220] Nitrogen-DGQDs Top-down hydrothermal 0.08 5 Hg2+ 23 nM [1] Nitrogen-DGQDs Bottom-up hydrothermal 0.64 1.9 Cr6+ 91 nM [221] Sulfur-DGQDs Top-down electrochemical 0.10 3 Fe3+ 4.2 nM [13] 34 Experimental Section In this chapter, the synthesis methods of GQDs, FGQDs and DGQDs, characterization techniques, and heavy metal ion sensing assessment will be explained in detail. A flowchart of the information presented in Chapter 3 is shown in Figure 3.1. Figure 3.1 Flowchart of Chapter 3. 3.1 Materials and Setups Citric acid powder (C6H8O7, 99%), 3-aminobenzylamine powder (C6H8N2, 99%), oxalic acid powder (C2H2O4, 99%), nitric acid (HNO3, 70% solution), and ethanol (C2H5OH, 95%) were purchased from Millipore Sigma. The ions standard solution, including calcium (II), chromium (III), magnesium (II), zinc (II), mercury (II), nickel (II), sodium (I), potassium (I), manganese (II), lead (II), copper (II), iron (III), arsenic (III), and cadmium (II), were purchased from Supelco Co. (US) in a concentration of 1000 mg/L in nitric acid. All materials were used without further purification. DI water was prepared by ELGA LabWater system and used for all experimental steps. The stirrers used in this study were heating magnetic stirrers (AREC digital ceramic, VELP, USA), and the centrifugation was performed by centrifuge 5805 (Eppendorf, Germany). 3.2 Synthesis Methods The GQDs, FGQDs, and DGQDs samples were synthesized by a bottom-up chemical method. This section is divided into three sub-sections of GQDs synthesis, FGQDs synthesis and DGQDs synthesis, respectively. 35 3.2.1 GQDs Synthesis Synthesis of GQDs using citric acid (CA) as the precursor has been reported in several studies [126, 184]. First, 20 mL solution of CA in water (2 mM) was heated in a heating mantle according to Figure 3.2 while stirring. Figure 3.2 Applied heating pattern of CA solution. The colour of the CA solution started to change from colourless (Figure 3.3 a) to pale yellow at 150°C (Figure 3.3 b) during the vigorous stirring and bubbling. By increasing the temperature, the colour changed to dark yellow at 200°C and 250°C, respectively (Figure 3.3 c and d), and eventually, to dark orange at 300°C (Figure 3.3 e). An increase in the temperature evaporated most of the water in the suspension and resulted in a more viscous suspension. Next, the temperature was declined by turning off the mantle, followed by adding 5 mL deionized water to the dark orange suspension to terminate the reaction (Figure 3.3 f). Afterward, 35 mL DI water was added to the suspension under stirring to reach a homogeneous suspension with a pH of 3.42. 36 Figure 3.3 CA solution heating process. a) room temperature, b) 150°C, c) 200°C, d) 250°C, e) 300°C, f) DI water addition. For the purification method, 300 mL ethanol was added to the mixture to reach the translucent suspension (Figure 3.4 a and b). Finally, the suspension was centrifuged for 30 minutes at 11000 rpm. After centrifugation, the sediments were reserved (Figure 3.4 c) and heated at 50°C to reach a brown powder (GQDs). A suspension of GQDs (8 mg/mL) in water will be provided as GQDs suspension. Figure 3.4 Purification process of GQDs. a) as-synthesized suspension, b) as-synthesized GQDS suspension after adding ethanol, c) sample b after centrifugation to settle the GQDs. 37 3.2.2 FGQDs Synthesis The functionalization process was performed through a post-treatment method inspired by Wang et al. [222]. Based on this study, the reaction time for GQDs and the functionalization materials was 48 hours. However, we have experimented other timeframes such as 24 and 68 hours. For the FGQDs synthesis, 60 mL GQDs suspension, 25 mL oxalic acid solution (0.5 M), 10 mL deionized water and specific amount of 3-aminobenzylamine were mixed for 5 minutes at 25°C (Figure 3.5). Next, the suspension was transferred to a sealed Erlenmeyer flask and stirred at 50°C with a speed of 500 rpm. The obtained mixture was denoted as FGQDs (8mg/mL) sample. To determine the most proper concentration of 3-aminobenzylamine and reaction time as a function of PL performance, three samples containing 0.75 g, 1.00 g and 1.25 g of 3-aminobenzylamine were prepared and the post-treatment processes were set as 24, 48, and 68 h, respectively. The name of the samples was denoted as FGQDs (Xmg-Yh) (3-aminobenzylamine mass×100 - time of chemical post-treatment in hours), provided in Table 3.1. 3.2.3 DGQDs Synthesis DGQDs samples were prepared via a bottom-up chemical synthesis method. The synthesis method of DGQDs was similar to the GQDs synthesis method but in the presence of 3-aminobenzylamine. First, 1 g of 3-aminobenzylamine was added to 20 mL aqueous solution of CA (2 mM) and 25 mL of Oxalic acid (0.5 M). Next, the mixture was heated with a heating mantle under stirring for 40 minutes, similar to GQDs synthesis process. The same purification process as GQDs was used for the DGQDs samples. Finally, the suspension stirred at 50°C for 48 h in a sealed Erlenmeyer flask, resulted to DGQDs (8mg/mL) sample. Final prepared GQDs, FGQDs, and DGQDs suspensions are shown in Figure 3.5. 38 Figure 3.5 Synthesized samples of a) GQDs, b) FGQDs (100-48), and c) DGQDs. Table 3.1 The synthesis conditions and denoted names of different samples in FGQDs. 3.3 Characterizations To determine the samples properties and characteristics, different characterization techniques were performed. In this section, the performed characterization techniques are explained in detail. 3.3.1 Photoluminescent Spectroscopy PL spectroscopy is a contactless and non-destructive analysis method of the electronic structure of materials. The sample absorbs the emitted light in a Quartz cuvette holder and a photo-excitation happens in the sample. Hence, the electrons move to permissible excited states and energy bands. Returning the electrons to their original state or alternative state will lead to a radiative or non-radiative process. All fluorescence setups include a source of light, a sample holder and a detector. Firstly, the incident light with a specific wavelength is emitted to the sample in the sample holder. Sample Name 3-aminobenzylamine Mass (g) Chemical Reaction Time (h) FGQDs (75mg-48h) 0.75 48 FGQDs (100mg-48h) 1.00 48 FGQDs (125mg-48h) 1.25 48 FGQDs (100mg-24h) 1.00 24 FGQDs (100mg-68h) 1.00 68 39 After excitation, the detector, positioned at 90° to the incident light to minimize the scattered light intensity, will detect the emitted lights and transfer them to a computer for further analysis [223]. Herein, a PL setup (AvaLight-LED, Avantes, USA) was used by an LED light source (AvaLight-DHc, Avantes, USA) for PL measurements of different samples. The incident light wavelength was 365 nm and the emitted lights were recorded within the wavelength range of 300-800 nm. All detected curves were collected and analyzed by AvaSoft 8.1 software, provided by the Avantes Co. A nitric acid solution (3 mL, 2 mM) was selected as the reference sample for PL measurement in pH 2.1. The PL Quantum Yield (QY) values were measured by equation (2.1), where ϕ is quantum yield, 𝐼 is the intensity of integrated emission, 𝜂 is the solvent refractive index, 𝐴 is the optical density, and R refers to the reference standard, fluoresceine sodium (0.1 M in NaOH) with ϕ = 0.72 [4]. ϕ = ϕR ×IIR×ARA×η2ηR2 Equation (2.1) 3.3.2 Ultraviolet-Visible Spectroscopy (UV-Vis) Ultraviolet-visible spectroscopy measures the absorbance spectra of a compound in a solution or solid. UV-Vis setup includes a source light, two transparent sample holders for the analyzed sample, a reference sample, and a detector. In a wavelength range of 200-800 nm, the incident beam, which excites electrons from the ground state to the first state of the compound, will be emitted to the samples in a Quartz cuvette. Afterward, the detector measures the radiation’s intensities that pass through the analyzed sample and compares it with the reference sample (nitric acid solution with pH of 2.1). The intensity of the analyzed sample beam (I) and the reference sample beam (I0) will be recorded. If the analyzed sample does not absorb light, the intensities will be equal (I=I0). However, if the sample compound absorbs the light, the intensity of the analyzed sample will be lower than the reference sample (I<I0), and the difference will be plotted on a graph versus wavelength. Absorption may be presented as transmittance (T = I/I0) or absorbance (A=log I/I0) [224]. Herein, a UV-2700i (Shimadzu, Japan) setup was used for UV-Vis characterizations of the suspensions. The incident light wavelength was applied in the range of 200-800 nm and the absorbance was recorded by LabSolutions UV-Vis software. 40 3.3.3 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy is a surface analysis technique that can identify the elements, chemical state, electronic state, and the bonded elements within the sample. The solid surface of the sample will be irradiated by X-ray beam, and the detector will measure the kinetic energy of the emitted electrons from 1-10 nm of the sample surface. The spectrum will be plotted based on the scattered electron counts and electron kinetic energy. As each element has a specific characteristic energy, the spectrum peaks provide information regarding the type of surface elements, except for hydrogen [225]. Herein, Thermo Fisher Scientific Escalab 250Xi (Thermo Fisher Scientific, E. Grinstead (UK)) with Aluminum K-alpha X-ray source gun with a nominal spot size of 900 μm was used for XPS characterizations. 3.3.4 Fourier-Transform Infrared Spectroscopy (FTIR) Fourier-transform infrared spectroscopy can define the type of molecules in a sample. This technology can only identify the chemicals that absorb the infrared light and this setup measures the absorption. After exposure of the sample to infrared light, the computer records the absorption data and generates the absorbance spectra through the Fourier transform process. Afterward, the absorbances will be compared with the available database and the chemicals and molecules within the sample will be defined [226]. Herein, the samples were provided in a powder form by drying at 25ºC overnight. All FTIR characterizations were performed by a Thermo Scientific Nicolet iS20 FTIR Spectrometer (USA) within a wavelength of 400-4000 cm-1 using Ge crystal. 3.3.5 Transmission Electron Microscopy (TEM) Transmission electron microscopy is a technique providing high-resolution images of the samples. Interaction of electrons with the samples results in the images for nano-scale morphology. The electron source emits the electrons through a high vacuum chamber to the electromagnetic lenses and sample at the bottom from the top part of the setup. Some of the electrons will pass through the sample and are collected by the detector creating the light areas. Some of the electrons will be scattered due to the crystal orientation or composition of the samples, and will disappear from the beam, providing dark shadows in the image. By adjusting the speed of electrons and electromagnetic wavelength via lenses, the quality of the picture could be adjusted. Faster electrons 41 with shorter wavelengths increase the image quality and details of the pictures. The sample’s image is shown through a computer providing information on the sample’s structure, morphology, and texture. To perform a TEM characterization, samples need to be thin enough for electrons to pass through them (electron transparency) and withstand the vacuum conditions. Hence, the samples require a special preparation process for this characterization. The preparation process includes dehydration, sputter coating, sectioning, etc. [227]. Herein, the images were taken using a Titan 80-300 TEM (FEI, US) microscope equipped with a CEOS image corrector operated at an acceleration voltage of 300 kV. The distribution graphs were reported based on the average diameter of the best spherical shaped quantum dots, without any overlap to other quantum dots, in the TEM images. The number of counts is reported in the corresponding result section. 3.3.6 X-ray Diffraction Analysis (XRD) X-ray diffraction analysis is a non-destructive technique to determine the structure of the crystals in a material, and the type of crystals in samples. Moreover, XRD could define the different phases in the sample. Since X-rays have the same magnitude wavelength as the space between crystal planes, they have been selected for this characterization. The d-spacing of the lattice in the structure can be determined by the Bragg’s law (Equation 2.2). 𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃 Equation (2.2) In this equation, n is an integer, 𝜆 is the wavelength of the X-ray, d is the spacing of the crystal layer, and θ is the angle between the incident ray and scatter plane [228]. In this setup, the incident X-ray radiation is emitted to the sample. The crystal atoms scatter the X-ray through interaction with electrons of the atoms, and produced scattered waves are sensed by the detector. Herein, the synthesized quantum dots were analyzed by Rigaku Miniflex II X-ray diffractometer (Japan) with a Cu K𝛼 anode (𝜆 = 1.54 Å) operating at step size 0.05° and speed of 1.5 s/step. 3.4 Heavy Metal Ions Sensing Heavy metal ions sensing was determined by two important characterization setups, i.e., PL and UV-Vis. In PL, characterizations of the FGQDs and DGQDs were performed on 3 mL of a 13.33 42 mM suspension in water. For interaction with heavy metal ions solution, 40 μL of the GQDs, FGQDs, and DGQDs samples were added to 3 mL of heavy metal ions solutions. The solutions of heavy metal ions in water were prepared in a range of 10 to 100 ppm (mg/L) for sensing characterizations by PL and UV-Vis setups. The names of the heavy metal ions solutions were denoted as the ion name and the concentration in ppm: ion-ppm. For example, Fe-40 is a solution of Fe (III) ions at a concentration of 40 ppm in DI water. The Quartz cuvette containing the samples is shown in Figure 3.6. Each PL characterization of the samples with and without heavy metal ions have been repeated 3 times to have more accurate results. Figure 3.6 Quartz cuvette of PL and UV-Vis setup containing: a) quantum dots samples, b) reference sample. 3.5 Samples Irradiated by UV-light The samples were irradiated by UV flashlight at 365 nm (Turbo LIGHTFE Black light, UV301D-Plus, Canada) in the dark to determine the colour of the samples. The pictures of DGQDs, FGQDs (100-48) and GQDs samples exposed to UV light are shown in Figure 3.7. 43 Figure 3.7 Samples irradiated with UV light. a) DGQDs, b) FGQDs (100-48), c) GQDs. As GQDs consist of light atoms, they have a small dielectric constant and weak spin-orbit coupling. Therefore, GQDs have stronger carrier-carrier interaction, leading to having a much larger energy band than other inorganic semiconductors. Hence, GQDs show PL colour at lower wavelength and higher energy, such as blue colour [229]. Functionalization of GQDs by 3-aminobenzylamine resulted in the introduction of amine-containing groups on the FGQDs edges. The presence of these functional groups resulted in the formation of additional interbond and resulted in the smaller bandgap of structure, which emits lower-energy light, such as orange-red colour, upon UV light excitation [117]. The introduction of nitrogen atoms to DGQDs structure resulted in the green-fluorescent colour in UV light. This phenomenon is attributed to the smaller size of nitrogen atoms compared with carbon atoms. The presence of small nitrogen atoms in the graphitic structure resulted in an increase in the size of sp2 clusters, as they stand out of the graphitic plane and smaller bandgap of the structure. Therefore, the DGQDs showed a green colour in UV light [230]. 44 Results and Discussion 4.1 Synthesis In this section, the synthesis procedures for GQDs, FGQDs and DGQDs are explained in detail. A flowchart of Chapter 4 is shown in Figure 4.1. Figure 4.1 Flowchart of Chapter 4. 4.1.1 GQDs Synthesis In this process, the GQDs were synthesized by the incomplete carbonization of CA, followed by eliminating water from the system. The incomplete carbonization of CA results in GQDs preparation through CA reactions and elimination of water (Figure 4.2). Among all bottom-up synthesis procedure, carbonization is a cost-effective, eco-friendly, and facile method. Therefore, in this thesis, the bottom-up synthesis procedure was performed [184]. The purification process was performed in the presence of ethanol. The presence of ethanol in the system results in the sedimentation of GQDs, and it is an effective way for the purification of GQDs. As the carbonization step might result in the creation of unwanted ions and complexes, ethanol results in the salting-out of GQDs. As the ions have a higher affinity to water than ethanol, they dissolve in water. After the addition of ethanol, ethanol replaces the GQDs in water due to the higher affinity of ethanol to water than GQDs. Hence, the GQDs are settled in the bottom of the container, while the ions are dissolved in the mixture of ethanol and water. The centrifugation step was applied to accelerate the speed of GQDs settlement [231]. The low-cost, fast, and facile procedure of CA carbonization for GQDs synthesis is an advantage over other procedures. 45 Figure 4.2 Carbonization of CA. Reproduced with permission [181], copyright 2012, Elsevier. 4.1.2 FGQDs Synthesis FGQDs samples were synthesized by a post-treatment process of the GQDs sample by controlling 2 parameters of chemical reactions, e.g., 3-aminobenzylamine concentration and time of reaction. The names of the samples are denoted as X-Y, X is concentration of 3-aminobenzylamine in grams, and Y is the reaction time in hours. For the concentration optimization, different samples containing 0.75 g FGQDs (75mg-48h), 1.00 g FGQDs (100mg-48h) and 1.25 g FGQDs (125 mg-48h) of 3-aminobenmzylamine were prepared within 48 hours of chemical reaction of post-treatment process at 50 °C. The results of PL characterizations are shown in Figure 4.3. In this figure, the peak summit for GQDs, FGQDs (75mg-48h), FGQDs (100mg-48h) and FGQDs (125mg-48h) samples are at 457 nm, 477 nm, 487 nm, and 487 nm, respectively. The introduction of 3-aminobenzylamine resulted in the higher emission intensity of GQDs PL characterizations. By increasing the concentration of 3-aminobenzylamine from 0.75 g to 1.00 g, the PL intensity started to grow, resulting in a higher emission. However, by adding 1.25 g of 3-aminobenzylamine, the growth was not substantial and resulted in the appearance of a second peak at 570 nm, attributed to the creation of the larger quantum dots [232]. An increase of 3-aminobenzylamine concentration provides resources to have extra chemical reactions and creates nanostructure with larger sizes or agglomeration of particles, which is not desirable in this study. Differences in the structures and sizes of FGQDs might result in different properties and make the analysis complex. Hence, considering the sufficient 46 concentration of 3-aminobenzylamine and more uniform quantum dots sizes, the most optimum features were offered by FGQDs (100mg-48h) sample. A small shift of PL peaks after 3-aminobenzylamine is attributed to the amino functionalization of edges. Primarily, 𝜋 - 𝜋∗ transitions in GQDs, attributed to the graphitic plane, dominated at lower wavelength regimes (457 nm). As excitation wavelength shifts to higher amounts (487 nm), n–𝜋∗ transitions, which is attributed to the functionalization groups, dominated and 𝜋 - 𝜋∗ transitions are repressed [146, 233]. An increase in the intensity of PL peak after functionalization of GQDs (2.33 times) could be attributed to the replacement of non-radiative carboxylic and epoxide groups with amine groups, resulting in the creation of n orbitals between the conduction and valence bands of GQDs and thus shrinking the bandgap [5, 146, 218]. Figure 4.3 PL characterization of GQDs, FGQDs (75mg-48h), FGQDs (100m-48h) and FGQDs (125mg-48h) samples. In the next step, the chemical reaction time of the post-treatment was changed. In this step, the concentration of 3-aminobenzylamine was kept at 1.00 g and different post-treatment times of 24, 48, and 68 hours were investigated at 50°C and the results of PL characterization are provided in Figure 4.4. As can be seen, by increasing the reaction time from 24 to 48 hours of the post-treatment, the PL intensities increased substantially. However, for the sample of 68 hours post-treatment time, the increase in the peak intensity is not remarkable. Thus, it can be said that after 48 hours, the functionalization of 3-aminobenzylamine GQDs edges was relatively completed. 47 With the concentration of 1.00 g of 3-aminobenzylamine in this sample, after 48 hours of post-treatment, most of the functionalization groups were bonded with GQDs planes and prolonging the reaction time did not have significant effects on creating more functionalization bonding. Therefore, the most suitable sample for FGQDs is the FGQDs (100mg-48h) sample, denoted as FGQDs in the rest of the study. Although the peak of FGQDs (100mg-48h) in PL characterization (Figure 4.4) correspond to the blue region, the sample exposed to UV-light has an orange-red colour in Figure 3.7. When the sample is exposed to UV-light, the chemical reactions and other electrical transition may result in an orange-red colour that can be seen by naked eyes, while the PL characterization cannot detect all of them [117]. Figure 4.4 PL characterization of FGQDs (100mg-24h), FGQDs (100mg-48h) and FGQDs (100mg-68h) samples. 4.1.3 DGQDs Synthesis The DGQDs were synthesized with a mixture of 20 mL solution of CA in water (2 mM) and 1 g of 3-aminobenzylamine within 48 hours of stirring on the heating mantle. The PL characterization of GQDs, FGQDs, and DGQDs is shown in Figure 4.5 with the peak of DGQDs at 522 nm. Comparison of PL characterization of FGQDs and DGQDs samples revealed that the nitrogen-containing groups in the DGQDs structure resulted in the larger size of quantum dots; hence, a small red shift in PL spectrum was observed. The red shift on DGQDs PL spectrum could be explained by the lower energy of LUMO (lowest unoccupied molecular orbital) in DGQDs. Doping nitrogen atoms in GQDs result in graphitic N, leading to the injection of one excess 48 electron into the unoccupied π* orbitals (LUMO) of the sp2 lattice of GQDs. Hence, the LUMO energy level is decreased due to the increase in electron density. This phenomenon leads to a narrower bandgap and a red shift [100]. In addition, nitrogen doping introduces a new surface state (N-state) in the DGQDs, resulting in the alteration of the electronic structure and the distribution of molecular orbitals in GQDs. Electrons trapped in the N-state suppress the non-radiative recombination and create a high yield of radiative recombination [192]. Electronegativity of nitrogen atoms could explain the increase in the peak intensity of PL spectra in DGQDs. In GQDs, carbon atoms near the H-terminated end display a weak negative charge due to hydrogen bonds. Carbon atoms, which are not close to the H-terminated end, become neutral with the delocalized electrons distributed at each carbon atom within the aromatic ring. This phenomenon creates a uniform π-electron cloud density distributed at each carbon atom. In DGQDs, the N atom has higher electronegativity (3.04) and strong electron-accepting ability than carbon atoms with 2.55 electronegativity. Hence, the nitrogen atoms have more tendency to attract electrons, which results in creation of a non-uniform π-electron cloud density in the structure [100]. Figure 4.5 PL characterization of GQDs, FGQDs, and DGQDs sample. The quantum yields of samples, which present the ratio of the photons emitted to the number of photons absorbed by the samples, are reported in Table 4.1 based on Equation (2.1). The post-49 treatment of FGQDs resulted in a 44.90% increase in quantum yield of GQDs, while the DGQDs showed a 47.31% increase. Table 4.1 Quantum yields of synthesized samples. Samples Quantum Yield (%) GQDs 9.51 FGQDs (100mg-48h) 13.78 DGQDs 14.01 4.2 Quantum Dots Structure In this section, the fluorescence properties and chemical structure and properties of GQDs, FGQDs (100-48) and DGQDs are investigated. 4.2.1 Fluorescence Properties To explore the different fluorescence properties of GQDs, FGQDs, and DGQDs samples, a PL characterization was performed at different wavelengths of 320 nm, 340 nm, 360 nm, 380 nm, and 400 nm (Figure 4.6). Although GQDs display exclusive fluorescence properties due to the quantum confinement effect, they contain different nanostructure sizes, resulting in different fluorescence behaviour under different excitation wavelengths [234]. Moreover, it is believed that a higher incident energy induces the 𝜋 → 𝜋∗ of sp2 domains in GQDs structures, while in a larger wavelength and lower energy, the n→ 𝜋∗ are stimulated and dominate the PL behaviour. The GQDs behaviour in Figure 4.6 a shows that the intensity decreased by increasing the incident wavelength and it shows a redshift. In larger wavelengths, the structures with larger bandgaps are less induced due to the lower energy of the radiations. Therefore, fewer structures will be stimulated, showing less intensity and a redshift in the peaks. The FGQDs behaviour shown in Figure 4.6 b exhibits lower bandgap differences than GQDs, based on the intensity of the peaks. The functionalization of the GQDs provides more uniform energy in the structures. On the other hand, DGQDs in Figure 4.6 c shows lower intensity in smaller wavelengths. This phenomenon could correspond to the higher oscillation of the quantum dots orbital structure in higher incident energy, which increases the non-radiative radiations and lowers the intensity in PL behaviour 50 [235]. Moreover, by increasing the wavelength of incident radiation, the PL intensity increased, which could be related to the smaller bandgap of structures with larger sizes due to the existence of amine functional groups on the edges of DGQDs. Figure 4.6 PL characterization of a) GQDs, b) FGQDs, and c) DGQDs under different excitation wavelength. 4.2.2 FTIR Characterization FTIR was used to assess the surface chemistry of GQDs, FGQDs, and DGQDs. Figure 4.7 shows the FTIR spectra of different samples. The exact wavelength of FTIR characteristic is outlined in Table 4.2. Based on Figure 4.7 and Table 4.2, for all samples, characteristic peaks at 1564.46, 1593.87 and 1564.46 cm-1 in GQDs, FGQDs and DGQDs, respectively, are assigned to the C=C stretching, which is attributed to the GQDs structure and carbon atoms bonding in their aromatic structure. The bonding stretch of C-O and O-H (Table 4.2) provide evidence of carboxylic and hydroxylic group in the samples. In the FGQDs sample, the additional peaks at 1392 - 1500 cm-1 show the presence of nitrogen-containing bonding in the structure. For example, the peak at 1277 51 cm-1 for GQDs is attributed to C-O stretching, while in the FGQDs, this peak vanished and new peaks at 1392 cm-1 and 1464 cm-1, corresponding to -NO2, N-H, and N=N, were observed. Therefore, the post-treatment functionalization of GQDs resulted in the introduction of amine-functionalization groups in GQDs structure and removal of some oxygen-containing groups. As shown in Table 4.2, the general structure of DGQDs is a mixture of GQDs and FGQDs, having oxygen- and nitrogen-containing chemical groups. The N=N stretching band could not be found in DGQDs, while it appeared in FGQDs at 1384.63 cm-1, indicating the successful grafting of 3-aminobenzylamine on the edges of GQDs nanosheets. Furthermore, although C=N stretching can be seen in the FGQDs sample, the peak in DGQDs shows a sharper intensity, revealing a higher amount of C=N structures in DGQDs at 1564.46 cm-1. Based on the data in Table 4.2, FTIR results indicate the successful formation of amino group on the FGQDs sample and nitrogen bonding in the structure of DGQDs. Figure 4.7 FTIR characterization spectra of the GQDs, FGQDs, and DGQDs samples. 52 Table 4.2 Detailed FTIR characterization results of the samples. Samples Wavelength Peak (cm-1) Bonding Stretch or Vibration GQDs 1277 C-O 1390 C-H 1565 C-H / C=C 3373 O-H FGQDs (100-48) 1392 C-H / -NO2 1464 N-H / N=N 1596 C=N / C=C 2660 O-H 2921 O-H 3369 -NH2 DGQD 1277 C-O 1386 C-O / C-H 1565 C-O / C-H / C=N / C=C 3373 O-H / -NH2 4.2.3 XPS Characterization The XPS characterization was performed to determine the functional groups and chemical bonds of GQDs, FGQDs, and DGQDs. The full-scan spectra of the samples are shown in Figure 4.8 and are individually explained in detail in the following parts. Moreover, the atomic percentages of carbon, hydrogen and nitrogen atoms in different samples are shown in Table 4.3. The atomic percentages of carbon are 76.8%, 66.6% and 66.6% in GQDs, FGQDs and DGQDs, respectively. The oxygen atomic percentage is the highest in DGQDs as 32%, while it is 22.3% in FGQDs and 21.4% in GQDs. Regarding the bond of nitrogen atoms, after functionalization and doping of 3-aminobenzylamine, the FGQDs sample contains 11.2% nitrogen atoms, and DGQDs sample holds 1.5%, while GQDs does not contain any nitrogen atoms. 53 Figure 4.8 Full scan XPS spectra of GQDs, FGQDs, and DGQDs. Table 4.3 Elemental atomic percentages of GQDs, FGQDs, and DGQDs obtained from XPS analysis. Samples C1s (%) O1s (%) N1s (%) GQDs 78.6 21.4 0 FGQDs 66.6 22.3 11.2 DGQDs 66.6 32.0 1.5 Figure 4.9 shows the XPS spectra of the prepared GQDs. As can be seen, two intense peaks at 284 and 529 eV correspond to C1s and O1s of GQDs, respectively. The measured spectrum of C1s (Figure 4.9 a) shows the existence of sp2 C (C=C) at a binding energy of 283.7 eV, sp3 C (C–C, and C–OH) at 284.2 eV, and C–O–C of epoxide and carbonyl group (C=O) at 287.2 eV. The formation of GQDs planes is confirmed by the creation of C=C bonding. The existence of sp3 C peak in the XPS spectra suggests that oxygen-containing groups were grafted on the edges of GQDs planes. The XPS spectrum of O1s (Figure 4.9 b) shows peaks at 529 eV (corresponding to quinone structure), 531.3 eV, and 532.8 eV (attributed to C=O of carbonyl groups), confirming the enrichment of GQDs with oxygen-containing functional groups (Figure 4.12 b). 54 Figure 4.9 XPS spectra of a) C1s and b) O1s of GQDs. For the XPS survey spectrum analysis of FGQDs in Figure 4.10, three total peaks were detected at 282.9 eV for C1s, 529.5 eV for O1s, and 397.4 and 399.2 eV for N1s in FGQDs. The high-resolution spectrum of the C1s signal shows three peaks at 282.4, 283.8, and 286.5 eV (Figure 4.10 a), representing the sp2 and sp3 carbon structures of C=C, C-C, and C-N/C-O-C groups. The O1s spectrum (Figure 4.10 b) shows fitting peaks at 529.9, 531.3, and 532.6 eV belonging to C=O groups of carbonyl and quinine. The N1s peak (Figure 4.10 c) shows characteristic peaks at 397.4, 399.2, and 400.2 eV, which can be attributed to pyridinic N, pyrrolic N, and quaternary N, respectively, indicating successful nitrogen functionalization of FGQDs [236]. 55 Figure 4.10 XPS spectra of a) C1s, b) O1s, and c) N1s of FGQDs. The XPS spectra of DGQDs in Figure 4.11 shows a similar pattern to FGQDs for C1s and O1, while the N1s corresponded to the formation of only pyridinic N in the structure. The high-resolution spectrum of C1s signal shows peaks at 283.9 eV, which is assigned to the graphitic sp2 carbon structure (C=C), 285.2 eV, which represented sp3 carbon structure of (C-C, C-O, C-N) and 287.1 eV, which is related to the carbonyl groups (C=O). The O1s spectrum reveals peaks at 529.6 eV, attributed to the C=O group of quinone, and 531.3 and 532.3 eV, assigned to the C=O groups [91, 192]. 56 Figure 4.11 XPS spectra of a) C1s, b) O1s, and c) N1s of DGQD. Based on the provided data, the FGQDs showed quaternary, pyrrolic and pyridinic N, while DGQDs show a considerably higher possibility of pyridinic N compared to pyrrolic and quaternary N. The three typical N-bond configurations, i.e., quaternary N, pyrrolic N, and pyridinic N, are shown in Figure 4.12. The quaternary N is covalently bonded with 3 neighboring carbon atoms, pyrrolic N is formed by the integration of N atoms into a five-membered heterocyclic ring, and pyridinic N originates from the sp2 hybridized N atom with two sp2 hybridized adjacent carbon atoms [237, 238]. It can be concluded that based on the synthesis procedure and conditions, the FGQDs contain more nitrogen-containing functional groups on the edges than DGQDs. Moreover, 57 DGQDs show a high number of sp2 hybridized C and N bonding in pyridinic form while showing less amine-functionalized groups. Figure 4.12 a) N-bond configuration in GQDs plane and b) GQDs structure showing oxygen- and hydrogen-containing functional groups. 4.2.4 XRD Characterization The crystalline structures of the synthesized GQDs before and after functionalization and doping with amine groups were assessed using XRD. Figure 4.13 a, b, and c illustrate the XRD patterns of GQDs, FGQDs, and DGQDs, respectively. As shown in Figure 4.13 a, the XRD pattern of GQDs showed a broad characteristic peak centered at 26.8°, corresponding to the lattice spacing (002) of the graphene structure in GQDs [239]. The broad characteristic peak of the lattice spacing graphene-like structure of GQDs is also obvious for FGQDs located at around 26°. Some new sharp peaks appeared in the XRD pattern of FGQDs at 2θ range of 20-50° after functionalization with 3–aminobenzylamine, which can be related to the different crystalline structures of grafted 3–aminobenzylamine molecules on the surface of GQDs [91]. The XRD pattern of DGQDs shown in Figure 4.13 c illustrates a peak at 28.2º and a different pattern compared to the GQDs and FGQDs samples. The shift of 26.8º to 28.2º may be attributed to the nitrogen atom doping in the graphene-like planes. As the nitrogen atoms are smaller than carbon atoms, the lattice shrinks, resulting in the peak shifting to higher angles. 58 The disappearance of the main characteristic peak of GQDs after doping can be linked to the surface coverage of DGQDs with 3–aminobenzylamine groups [91, 240], decreasing the X-ray diffraction intensity of the lattice spacing of graphene-like structure compared to the FGQDs sample. The d-spacing of GQDs, FGQDs, and DGQDs were calculated as 0.33 nm, 0.33 nm, and 0.34 nm, respectively, based on the Bragg’s law (Equation 2.2). Figure 4.13 XRD characterization of a) GQDs, b) FGQDs, and c) DGQDs. 4.2.5 TEM Characterization The morphology and particle size distribution of GQDs, FGQDs, and DGQDs were studied using TEM. Figure 4.14 shows TEM images and particle size distribution histograms of GQDs, FGQDs, and DGQDs. TEM results of GQDs (Figure 4.14 a) and FGQDs (Figure 4.14 b) showed a spherical structure and a uniform particle size distribution in the range of 1.5-2.7 nm and 2.1-4.5 nm with average particle sizes of 1.85 and 3.85 nm, respectively. Based on the TEM images, GQDs and FGQDs' d-spacings are 0.19 and 0.21 nm, respectively. Contrary to GQDs and FGQDs, DGQDs 59 demonstrated an aggregated structure, evident in Figure 4.14 c. The size distribution of DGQDs is in the range of 4.3-6.7 nm with 2 peaks of average particle sizes at 4.65 and 6.35 nm and d-spacing at 0.22 nm. The [1120] lattice fringes of graphene in DGQDs are not clear enough due to the aggregation of nanoparticles and the coverage of DGQDs nanosheets’ surface with 3–aminobenzylamine. TEM observations are in line with the XRD results, except for small difference in d-spacings. Figure 4.14 TEM characterization and size distribution of a) GQDs ( in 40 counts), b) FGQDs (in 40 counts), and c) DGQDs (in 15 counts) (the insets of TEM characterizations are the magnified images of selected area). 4.3 Heavy Metal Ions Sensing In this section, first, the effect of fluorescent quenching of different heavy metal ions on GQDs, FGQDs, and DGQDs, is investigated. The results are then analyzed by PL and UV-Vis characterizations to confirm the findings. 4.3.1 Sensors Selectivity Based on section 3.4, the heavy metal ion sensing was performed on the synthesized GQDs, FGQDs and DGQDs. The metal elements with high atomic weights between 63.5 to 200.6 g.mol-60 1 and high specific gravity (higher than 5.0 g/cm3), including antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc are defined as heavy metal ions. The effect of some of the heavy metal ions, and other toxic metal ions, such as Li+, Na+, Ba2+, Ca2+, Mg2+, and Al3+ was evaluated. First, the effects of the presence of Al3+, Fe3+, Zn2+, Cd2+, Mg2+, Ni2+, Pb2+, Hg2+, Ca2+, Co2+, Mn2+, Cr2+, Cu2+, As2+, Li+, Na+, and Ba2+ with a concentration of 50 ppm of each solution on the fluorescence intensity of different samples are shown in Figure 4.15. Herein, the F and F0 are the fluorescence intensities of the synthesized samples in the presence and absence of heavy metal ions. In GQDs suspension, none of the ions showed a significant quenching in the fluorescence intensities. On the other hand, in FGQDs, the fluorescence quenching by Fe3+ (F/F0 = 0.46) is higher than Hg2+ (F/F0 = 0.80) and the other ions. For DGQDs, the quenching effects by Hg2+ (F/F0 = 0.33) and Fe3+ (F/F0 = 0.76) ions are significant compared to the other ions. The F/F0 measurements have been repeated 3 times for each sample in different heavy metal ions solutions, specified by error-bar in Figure 4.15 and the average amount has been determined for the F/F0 ratio. Heavy metal ions are unstable in the solutions and could easily react with ligands as electron acceptors. The selectivity of FGQDs and DGQDs toward Fe3+ and Hg2+ ions, respectively, could be due to these ions' higher affinity and chelating speed than other heavy metal ions. Hence, Hg2+ and Fe3+ ions may increase the collision and subsequent formation of a transient complex, resulting in the PL quenching [241, 242]. It is expected that FGQDs is a selective sensor of Fe3+ ions, and DGQDs are more selective toward Hg2+ ions. However, further characterizations must be performed at the different ions’ concentrations. 61 Figure 4.15 Relative fluorescence intensity of GQDs, FGQDs, and DGQDs samples in the presence of different metal ions ([Mn+]) = 50 ppm. 4.3.2 FGQDs Sensors Figure 4.16 shows the fluorescence intensity of the FGQDs sample in the presence of Fe3+ ions in a concentration range of 10 to 100 ppm. It can be seen that by increasing the Fe3+ concentration from 10 to 100 ppm, the fluorescence intensity showed a significant decrease, indicating that Fe3+ can effectively quench the fluorescence of FGQDs. Figure 4.17 shows the UV-Vis spectra of GQDs and FGQDs in the presence of Fe3+ ions at different concentrations of 10 to 100 ppm. The FGQDs sample showed high adsorption in wavelengths lower than 450 nm, indicating lower energy to encourage more electron transitions. The peak in GQDs sample at 248 nm is attributed to the electron transition of 𝜋 → 𝜋∗ of aromatic sp2 domain. The UV-Vis spectrum of FGQDs exhibit an absorption peak at 248 and a shoulder at 286 nm, which are attributed to 𝜋 → 𝜋∗ of graphene-like plane and n → 𝜋 of amino groups on the edges, respectively. By addition of Fe3+ ions, a redshift was observed, which is attributed to the introduction of Fe3+ ions to the FGQDs structure. Therefore, by entering the Fe3+ ions in the FGQDs structure, the bandgap decreases, inducing the electrons with a lower energy and a larger wavelength. Moreover, by adding Fe3+ ions, the absorption peaks show a significant increase at 286 nm, indicating an increase in the bandgap of the structure, which results in the easier transition of electrons and higher absorption. Furthermore, the substantial increase in the 286 nm peak of Figure 4.17 indicates the higher accuracy of the FGQDs sensors. Hence, the FGQDs sample is a more selective and sensitive sensor for Fe3+ ions. 62 Figure 4.16 PL characterization of FGQDs sample in the presence of Fe3+ ions. Figure 4.17 UV-Vis characterization of FGQDs sample in the presence of Fe3+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. The Stern-Volmer relationship defines the kinetics of a photophysical intermolecular deactivation such as fluorescence quenching, where the presence of another chemical species can accelerate the 63 decay rate of a chemical in excite-state [243]. By fitting the fluorescence intensity changes in different heavy metal ions concentrations to the Stern-Volmer equation, the sensors' quenching constant and detection limits could be determined. The relationship of fluorescence changes F0/F (F0 and F are the intensities of fluorescence peaks in the absence and presence of Fe3+ ions, respectively) at different concentrations of Fe3+ ions is shown in Figure 4.18. It is worth mentioning that the F0/F values were measured 3 times for each sample and the average value were reported in the Stern-Volmer graph. The Stern-Volmer equation (4.1) was used to determine the linear relationship [218]. Here Ksv is the Stern-Volmer quenching constant, and C is the concentration of Fe3+ ions. F0/F = Ksv [C] + 1 Equation (4.1) The F0/F shows a good linear relationship with Fe3+ ions at different concentrations of 10 ppm to 100 ppm, and the linear regression equation is F0/F = 0.35C + 1 with R2 = 0.99. The limit of detection (LOD) is obtained based on 3𝜎/m (σ is the standard deviation of the regression of the blank signal and m is the slope of the linear plot), respectively. Therefore, LOD of FGQDs is 2.43 ppm. Figure 4.18 Stern-Volmer plot of FGQDs sample at different concentrations of Fe3+ ions. 64 These results showed that FGQDs exhibit a more selective and sensitive detection towards Fe3+ ion and might be very useful in environmental applications. This selectivity and sensitivity could be attributed to the high affinity and recognition of amino groups on the edges of FGQDs toward Fe3+ ions [5, 244]. FGQDs contain pyrrolic N bonding on its GQDs edges, which could react with Fe3+ ions and result in the fluorescence quenching of FGQDs. 4.3.3 DGQDs Sensors From the results in Figure 4.15, the effect of Fe3+ and Hg2+ ions should be investigated. The effects of Hg2+ ions in DGQDs are shown in Figure 4.19. It is evident that by increasing the concentrations of Hg2+ ions from 10 to 90 ppm, the PL intensity decreased significantly. This indicates that DGQDs samples have a high sensitivity toward Hg2+ ions. It is reported that the relative sensitivity and selectivity of Hg2+ ions towards the DGQDs fluorescence quenching could be due to several reasons, such as the high affinity of Hg2+ ions towards nitrogen in the DGQDs structure and higher ability of creating a stable non-radiative complex with the DGQDs samples [245, 246]. Figure 4.20 shows the UV-Vis characterization of DGQDs in the presence of Hg2+ ions at different concentrations of 10 to 50 ppm. The DGQDs exhibit a high absorption in the ultraviolet and visible light regions, with a peak at 246 nm. By the addition of Hg2+ ions, the adsorption intensity increased gradually. At higher concentrations of Hg2+ ions, 20 to 50 ppm, a new peak appeared at 302 nm, which is attributed to the creation of a new energy level with a small bandgap in the DGQDs structure, resulting in inducing the electrons transitions by lower energy in larger wavelengths [235]. Therefore, the DGQDs samples exhibited high sensitivity and accuracy towards Hg2+ions. 65 Figure 4.19 PL characterization of DGQDs sample in the presence of Hg2+ ions. Figure 4.20 UV-Vis characterization of DGQDs sample in the presence of Hg2+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. To determine the LOD of DGQDs samples, the data were fitted to the Stern-Volmer plot shown in Figure 4.21. The F0/F values were measured 3 times for each sample and the average value is reported in the graph. The pattern of relative fluorescence intensity in the absence and presence of Hg2+ ions is linear in the range of 0 ppm up to 30 ppm Hg2+ (No-Hg to Hg-30). After adding Hg2+ 66 ions at higher concentrations, the pattern became non-linear. Therefore, three linear regions of 0 to Hg-30, Hg-40 to Hg-70, and Hg-80 to Hg-100 were selected to calculate the LOD. As can be seen, Figure 4.21 depicts three different linear regions, which correspond to different quenching mechanisms in different concentration ranges for DGQDs [247]. To determine the LOD of DGQDs sensors, the stern-Volmer equation were fitted to different regions and the R2 values were compared. The R2 values in the Hg-0 to Hg-30, Hg-40 to Hg70, and Hg-80 to Hg-100 range were 0.98, 0.99, and 0.98, respectively. Therefore, the most accurate LOD was calculated in the Hg-40 to Hg-70 range. The calculated Stern-Volmer equation is F0/F =1.20C + 1 with R2 = 0.99, where c is the concentration of Hg2+ ions. The LOD (3𝜎/m, where σ is the standard deviation of the regression of the blank signal and m is the slope of the linear plot) is 1.99 ppm. Figure 4.21 Stern-Volmer plot of DGQDs sample at the different concentrations of Hg2+ ions. Inset: the linear relationship of relative fluorescence intensity in the presence of Hg2+ ions. The DGQDs exhibit a relative sensitivity toward Hg2+ ions, which resulted in the fluorescence quenching of DGQDs samples. However, based on Figure 4.15, the DGQDs samples showed sensitivity toward Fe3+ ions too. These selectivity and sensitivity must be explored further. Therefore, the fluorescence intensity of the DGQDs sample in the presence of different concentrations of Fe3+ ions was observed (Figure 4.22). It is shown that by increasing the Fe3+ ions concentration from 10 ppm to 30 ppm, the fluorescence intensity decreased to 75% of the original 67 DGQDs sample. A higher concentration in Fe3+ ions from 30 to 100 ppm did not exhibit a drastic decrease (for a clear figure in PL characterization, samples with 60 to 100 are not included in Figure 4.22). Therefore, the DGQDs samples exhibited some limitations towards the detection of Fe3+ ions. The UV-Vis characterization was also performed for the DGQDs sample in the presence of Fe3+ ions (Figure 4.23). UV-Vis spectra showed that DGQDs exhibit more adsorption in the ultraviolet (peak in 242 nm) than in the visible region. The addition of Fe3+ ions from 10 to 40 ppm into the DGQDs sample resulted in high absorption in the visible light region and showed a peak at 340 nm. The increase in the intensity of 340 nm peak indicates that the introduction of Fe3+ ions leads to creating a new low energy level of DGQDs; thus, an electron transition in lower energy and larger wavelength occurred. However, as shown in Figure 4.22, in the Fe-40 sample, the pattern of the new peak is very similar to Fe-30, which is attributed to the limitation of the DGQDs sample for Fe3+ ions detection. Figure 4.22 PL characterization of DGQDs sample in the presence of Fe3+ ions. 68 Figure 4.23 UV-Vis characterization of DGQDs sample in the presence of Fe3+ ions. The characterization was performed in a wavelength range of 240 to 600 nm. Based on the provided data and discussion, we hypothesized that the DGQDs samples could be a combination of GQDs structure and FGQDs structure due to their synthesis procedures. Hence, the possibility of introducing amino-containing groups to the edges of the DGQDs sample is high, but lower than FGQDs samples. These results also were confirmed by the XPS characterization, which showed low nitrogen content in the structure of DGQDs and limited functional groups. This phenomenon is evident in the detection of Fe3+ ions by DGQDs in a lower concentration range than FGQDs. To investigate further, a new set of examinations must be performed to determine the fluorescence behaviour of DGQDs in the presence of both Fe3+ and Hg2+ ions. Figure 4.24 shows the PL characterization of DGQDs samples in the presence of Fe3+ and Hg2+ ions at 40 ppm. As can be seen, the PL behaviour of DGQDs in the presence of both ions is highly similar to the Hg-40 condition, indicating that the effect of Hg2+ ions could overcome the effects of Fe3+ ions. Therefore, although the nitrogen-containing structures in the DGQDs have a lower quantity compared with the FGQDs sample, the DGQDs are stronger in the quenching Hg2+ ions. To support the PL characterization, the UV-Vis characterization was also performed for DGQDs in the presence of both ions at 40 ppm (Figure 4.25). As can be seen, the UV-Vis pattern of DGQDs in contact with both ions is highly close to the Hg-40 condition with the elimination of 340 nm peak of Fe-40 and creation of a new peak at 302 nm similar to Hg-40. Hence, we conclude that the 69 DGQDs sample is relatively more selective and sensitive toward Hg2+ ions detection than Fe3+ ions. It can be concluded that DGQDs containing pyridinic N bonding in the GQDs planes that might react with Hg2+ ions, resulting in the fluorescence quenching of DGQDs. Figure 4.24 Effect of co-existing Hg2+ and Fe3+ ions on the DGQDs sample in the PL characterization. Figure 4.25 UV-Vis characterization of DGQDs in the presence of Hg2+ and Fe3+ ions. The characterization was performed in a wavelength range of 240 nm to 600 nm. 70 Based on the achieved results, the FGQDs sensors could be a great candidate to detect Fe3+ ions with an LOD of 2.43 ppm. This is attributed to amine groups, specifically pyridinic N on the edges of the graphene plane. The DGQDs sensor provides detection of Fe3+ and Hg2+ ions in a liquid environment. However, detection of Fe3+ ions was limited due to the dominance of Hg2+ ions detection mechanism over the Fe3+ ions. This phenomenon is explained based on the synthesis procedure of DGQDs. The detection of Hg2+ ions by DGQDs samples is attributed to the presence of nitrogen atoms in the structure, creating pyrrolic N bonds in the plane. Therefore, the DGQDs sensors could be a great candidate for Hg2+ ions with an LOD of 1.99 ppm. 71 Summary and Conclusions 5.1 Results Summary In this study, GQDs were synthesized by a chemical bottom-up procedure using CA as the main precursor. The FGQDs were synthesized by a post-treatment process using GQDs as the precursor and 3-aminobenzylamine as the functionalization material, while the DGQDs were synthesized through a bottom-up procedure using CA and 3-aminobenzylamine as the precursors. The GQDs, FGQDs, and DGQDs showed excellent PL properties. The PL properties of FGQDs and DGQDs increased significantly compared to GQDs, making them great candidates for different applications such as heavy metal ions sensing. Although the PL mechanism of GQDs is still under debate, the results suggested that the functionalization of GQDs with 3-aminobenzylamine led to smaller bandgap, resulting in the creation of a more uniform energy structure compared to GQDs. The GQDs PL behaviour showed that the intensity of the PL peaks decreased by an increase in the incident wavelength demonstrating a redshift. This is attributed to the lower energy of the incident radiations activating structures with smaller bandgaps. On the other hand, the DGQDs show an increase in PL intensity by increasing the incident wavelength. This phenomenon could be attributed to larger bandgap of DGQDs structure compared with GQDs. The FTIR characterizations of GQDs, FGQDs, and DGQDs showed evidence of C=C bonding in their structures, attributed to the graphitic structure. Moreover, the footprint of carboxyl and hydroxyl groups are observable in the FTIR pattern of GQDs pattern, while the amine-containing groups were observed in the FTIR patterns of FGQDs and DGQDs samples. The XPS characterization showed that FGQDs and DGQDs contain carboxylic, hydroxyl, pyrrolic N, pyridinic N, and quaternary N bonds in their nanostructure, while GQDs contain only oxygen functional groups. The XRD characterization of GQDs showed a wide peak at 26.8º, attributed to the lattice spacing of graphene structure. The FGQDs sample showed a more crystalline structure and new peaks due to the functionalization of the structure with 3-aminobenzylamine. On the other hand, the DGQDs pattern showed a shift in the peak due to the doping process. The TEM characterization provided images of GQDs and FGQDs with average sizes of 1.85 nm and 3.85 72 nm, respectively. The DGQDs images showed two average sizes of 4.65 nm and 6.35 nm and the agglomeration of nanostructures due to the high affinity of amine-containing groups to each other. The FGQDs and DGQDs proved to be good candidates for the heavy metal ions sensing platform based on their PL properties. FGQDs were used to identify Fe3+ ions in the presence of other heavy metal ions with an LOD of 2.43 ppm. We attributed the sensitivity of FGQDs toward Fe3+ ions to the presence of pyrrolic N bonding in the structure. The introduction of Fe3+ ions to the FGQDs structure resulted in the quenching of PL properties. On the other hand, DGQDs were used for sensing of Hg2+ ions with an LOD of 1.99 ppm. The detection of Hg2+ ions could be mainly related to the pyridinic N bonding in the DGQDs structure. 5.2 Future Works PL and chemical properties of GQDs-based nanomaterials are highly sensitive to their synthesis methods. Therefore, planning the design and fabrication of GQDs-based nanomaterials require high precision and repeated measurements during the synthesis process. There is a high potential to synthesize a broad spectrum of GQDs-based nanomaterials toward heavy metal ions sensing using different precursors and methods. The synthesis procedure proposed in this study could be a great method for large-scale production in heavy metal ions sensing applications; however, the reproducibility of the synthesis methods and sensors performance should be further investigated. It is postulated that a more precise synthesis procedure results in a higher quantum yield and sensitive platform. For example, the optimization of the functionalization concentration, reaction time, and temperature in the synthesis methods could affect the size and fluorescence properties of the GQDs and thus their heavy metal ions sensing. There is potential for new studies on the synthesis of FGQDs and DGQDs with 3-aminobenzylamine using different precursors and synthesis routes. For example, it is predicted that in a top-down synthesis method of GQDs, using GO, and a post-treatment with 3-aminobenzylamine, the sensitivity toward the heavy metal ions can be enhanced [248]. This phenomenon could result in a top-down synthesis of GQDs and a more defective structure. Hence, the nitrogen-containing group might react with GQDs edges more easily, leading to a higher functionalization degree. In this case, although the heavy metal ion 73 sensor’s sensitivity could increase, the high number of defects may dominate the photon traps in the GQDs structure and result in the lower quantum yield of the produced material. As the stability of synthesized materials has a direct effect on PL quenching in the presence of heavy metal ions, the stability of the quantum dots is another potential study. The study of the zeta potential and dynamic light scattering (DLS) characterization of synthesized GQDs, FGQDs, and DGQDs could determine the stability of the quantum dots in the suspension. It is worth mentioning that the dispersion of the quantum dots in the suspension could be more explored through the DLS characterization than TEM characterization. Furthermore, as the pH of the environment affects the PL properties of quantum dots [249], a new study can be conducted on the relationship between the acidity of the synthesized quantum dots and the PL activities in heavy metal ions solution. Another potential area is to study the quenching mechanism of FGQDs and DGQDs. As the quenching mechanism is divided into two categories of static and dynamic, a kinetic study on the reaction of prepared nanostructures with the heavy metal ions could define the dominant mechanism. For example, in the case of static quenching, the contact of the heavy metal ions and GQDs-based materials form complex chemicals; therefore, extra characterizations of the mixture could identify the complex structures and chemical reactions. On the other hand, if the dynamic quenching mechanism dominates, the heavy metal ion can interact with GQDs-based materials, leading to energy or charge transfer that would decline over the course of reactions. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Arjmand, Mohammad
Synthesis of amine-functionalized and nitrogen-doped Graphene Quantum Dots and their applications in heavy metal ions sensing
Text