{"http:\/\/dx.doi.org\/10.14288\/1.0388295":{"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool":[{"value":"Applied Science, Faculty of","type":"literal","lang":"en"},{"value":"Non UBC","type":"literal","lang":"en"},{"value":"Materials Engineering, Department of","type":"literal","lang":"en"}],"http:\/\/www.europeana.eu\/schemas\/edm\/dataProvider":[{"value":"DSpace","type":"literal","lang":"en"}],"https:\/\/open.library.ubc.ca\/terms#identifierCitation":[{"value":"Nanoscale Research Letters. 2020 Jan 13;15(1):7","type":"literal","lang":"en"}],"https:\/\/open.library.ubc.ca\/terms#rightsCopyright":[{"value":"The Author(s).","type":"literal","lang":"en"}],"http:\/\/purl.org\/dc\/terms\/creator":[{"value":"Ruan, Dongliang","type":"literal","lang":"en"},{"value":"Qin, Liming","type":"literal","lang":"en"},{"value":"Chen, Rouxi","type":"literal","lang":"en"},{"value":"Xu, Guojie","type":"literal","lang":"en"},{"value":"Su, Zhibo","type":"literal","lang":"en"},{"value":"Cheng, Jianhua","type":"literal","lang":"en"},{"value":"Xie, Shilei","type":"literal","lang":"en"},{"value":"Cheng, Faliang","type":"literal","lang":"en"},{"value":"Ko, Frank K.","type":"literal","lang":"en"}],"http:\/\/purl.org\/dc\/terms\/issued":[{"value":"2020-01-13T17:13:43Z","type":"literal","lang":"en"},{"value":"2020-01-13","type":"literal","lang":"en"}],"http:\/\/purl.org\/dc\/terms\/description":[{"value":"Particulate matter is one of the main pollutants, causing hazy days, and it has been serious concern for public health worldwide, particularly in China recently. Quality of outdoor atmosphere with a pollutant emission of PM2.5 is hard to be controlled; but the quality of indoor air could be achieved by using fibrous membrane-based air-filtering devices. Herein, we introduce nanofiber membranes for both indoor and outdoor air protection by electrospun synthesized polyacrylonitrile:TiO\u2082 and developed polyacrylonitrile-co-polyacrylate:TiO\u2082 composite nanofiber membranes. In this study, we design both polyacrylonitrile:TiO\u2082 and polyacrylonitrile-co-polyacrylate:TiO\u2082 nanofiber membranes with controlling the nanofiber diameter and membrane thickness and enable strong particulate matter adhesion to increase the absorptive performance and by synthesizing the specific microstructure of different layers of nanofiber membranes. Our study shows that the developed polyacrylonitrile-co-polyacrylate:TiO\u2082 nanofiber membrane achieves highly effective (99.95% removal of PM2.5) under extreme hazy air-quality conditions (PM2.5 mass concentration 1 mg\/m\u00b3). Moreover, the experimental simulation of the test in 1 cm\u00b3 air storehouse shows that the polyacrylonitrile-co-polyacrylate:TiO\u2082 nanofiber membrane (1 g\/m\u00b2) has the excellent PM 2.5 removal efficiency of 99.99% in 30 min.","type":"literal","lang":"en"}],"http:\/\/www.europeana.eu\/schemas\/edm\/aggregatedCHO":[{"value":"https:\/\/circle.library.ubc.ca\/rest\/handle\/2429\/73317?expand=metadata","type":"literal","lang":"en"}],"http:\/\/www.w3.org\/2009\/08\/skos-reference\/skos.html#note":[{"value":"NANO EXPRESS Open AccessTransparent PAN:TiO2 and PAN-co-PMA:TiO2 Nanofiber Composite Membraneswith High Efficiency in Particulate MatterPollutants FiltrationDongliang Ruan1*\u2020 , Liming Qin2\u2020, Rouxi Chen3,4*\u2020, Guojie Xu5, Zhibo Su2, Jianhua Cheng3, Shilei Xie1,Faliang Cheng1 and Frank Ko6*AbstractParticulate matter is one of the main pollutants, causing hazy days, and it has been serious concern for publichealth worldwide, particularly in China recently. Quality of outdoor atmosphere with a pollutant emission of PM2.5is hard to be controlled; but the quality of indoor air could be achieved by using fibrous membrane-based air-filtering devices. Herein, we introduce nanofiber membranes for both indoor and outdoor air protection byelectrospun synthesized polyacrylonitrile:TiO2 and developed polyacrylonitrile-co-polyacrylate:TiO2 compositenanofiber membranes. In this study, we design both polyacrylonitrile:TiO2 and polyacrylonitrile-co-polyacrylate:TiO2nanofiber membranes with controlling the nanofiber diameter and membrane thickness and enable strongparticulate matter adhesion to increase the absorptive performance and by synthesizing the specific microstructureof different layers of nanofiber membranes. Our study shows that the developed polyacrylonitrile-co-polyacrylate:TiO2 nanofiber membrane achieves highly effective (99.95% removal of PM2.5) under extreme hazy air-qualityconditions (PM2.5 mass concentration 1 mg\/m3). Moreover, the experimental simulation of the test in 1 cm3 airstorehouse shows that the polyacrylonitrile-co-polyacrylate:TiO2 nanofiber membrane (1 g\/m2) has the excellent PM2.5 removal efficiency of 99.99% in 30 min.Keywords: Particulate matter (PM) pollution, Aerosol filtration, Electrospinning, Nanofiber membraneHighlights\u0001 Development of transparent PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranes\u0001 Synthesis and controlling of the properties ofnanofiber membranes by electrospinning\u0001 Strong PM adhesion and absorptive performancewith the specific microstructure\u0001 Nanofiber membrane shows excellent PM2.5removal efficiency (99.99%) in 30 minIntroductionThe particulate matter (PM) pollution issues are mainlycaused by the high pollution manufacturing industry andare serious concerns worldwide, especially in China re-cently [1, 2]. Due to the severe environmental issues,people wear masks to filter pollute air outdoors in pol-luted weather conditions, and further equipment for airfiltration becomes popular to clean indoor air quality inmetropolises [3]. Right now, non-woven fibrous mediahave been used in different air filtration applications,from indoor air filter to personal protective equipment,such as N95 respirator. High-filtration efficiency or low-pressure drop is conducive to improve the quality of airfiltration [4\u20137]. Non-woven microfibers with smaller\u00a9 The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http:\/\/creativecommons.org\/licenses\/by\/4.0\/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.* Correspondence: ruan.dongliang@dgut.edu.cn;chenrx3@mail.sustech.edu.cn; frank.ko@ubc.ca\u2020Dongliang Ruan, Liming Qin and Rouxi Chen contributed equally to thiswork.1Guangdong Engineering and Technology Research Centre of Advanced andNanomaterials, Dongguan University of Technology, Dongguan 523808,China3South China Institute of Collaborative Innovation, Dongguan 523808, China6Department of Material Engineering, University of British Colombia,Vancouver V6T1W9, CanadaFull list of author information is available at the end of the articleRuan et al. Nanoscale Research Letters            (2020) 15:7 https:\/\/doi.org\/10.1186\/s11671-019-3225-2diameter leads to not only greater filtration efficiency, butalso larger pressure drop. For example, nanofiber-basedair filters with a diameter smaller than 500 nm have high-filtration efficiency and low air permeability [8]. Therefore,the development of a high-performance nanofiber air filtermembrane garners enormous interests from both researchand applications worldwide, since nanofibers are rapidlybecoming a feasible material alternative.Among many approaches such as molecular technol-ogy, biological preparation, and spinning technique, elec-trospinning is a relatively simple and effective method,and also suitable and compatible with the preparation ofnanofiber membranes [9\u201312]. Recently, nanofiber mem-branes have been successfully produced using differentpolymers by electrospinning for indoor air protection[13, 14]. Compared to other polymer materials, as PVA(polyvinyl alcohol), PS (polystyrene) and PVP (polyvinyl-pyrrolidone), the studies indicate that PAN (polyacrylo-nitrile) is a preferred material for particle filtration [15].Moreover, some additional materials are easily coated onelectrospun nanofibers, such as ZnO, TiO2, carbonnanotubes, silica, and silver. The artificial functional ma-terials have been modified on different surfaces to in-crease the roughness and micro-nano structure [16, 17].Among various coating materials, nanostructured TiO2has received considerable interest, due to its remarkableUV-ray catalysis and shielding property [18\u201320]. Theaim of the study is to develop electrospun nanofiberswith rough surface, low-filtration pressure and resist-ance, which can actively capture PM2.5 based on themulti-stage structure of nanofiber membranes.Therefore, we present an approach for the fabrica-tion of polyacrylonitrile (PAN):TiO2 and developedpolyacrylonitrile-co-polyacrylate (PAN-co-PMA):TiO2 nano-fiber membrane by electrospinning (as shown in Suppl.Scheme 1.). The hierarchical PAN:TiO2 and particularly,PAN-co-PMA:TiO2 nanofiber membrane exhibited excel-lent filtration efficiency and good permeability, which ispromising for air filter applications.MethodsMaterialsPolyacrylonitrile (PAN, MW: 100000) and polyacrylonitrile-co-polymethyl acrylate (PAN-co-PMA, MW: 150000) werepurchased from Scientific Polymer; Polyvinylpyrrolidone(PVP, mw=55000) was purchased from Sigma; N,N-dimethylformamide (DMF) was purchased from Anachemia; Nano-meter titanium dioxide (TiO2, Anatase, D < 25 nm) was pur-chased from Aldrich. All raw materials were used as receivedwithout further purification.Electrospinning for Nanofiber MembraneThe PAN:TiO2 nanofiber membrane was fabricated byelectrospinning. In the procedure, nanometer TiO2 andPVP (1:1, w\/w) were added to DMF, and then PAN andPAN-co-PMA was added with final concentration of10% (w\/w). The mixture was heated and stirred to forma milk-white viscous solution for 24 h at 90\u00b0. The vis-cous solution was loaded into a plastic syringe equippedwith an 18-gauge stainless steel needle. During electro-spinning, the needle was supplied with a high positiveelectrostatic voltage. The ground collector was coveredby PP nonwovens at a distance of 20 cm to the spin-neret. The PAN:TiO2 and PAN-co-PMA:TiO2 nanofibermembranes were fabricated in a relative humidity of45% at 25\u00b0. After electrospinning, the PAN:TiO2 andPAN-co-PMA:TiO2 nanofiber membranes were coveredby another piece of nonwovens to protect the surfacefrom damage. This composite membrane was dried inan oven for 3 h at 90\u00b0.AnalysisScanning electron microscope (SEM) images were takenby a field emission SEM S3000N (Hitachi, Japan) andTransmission electron microscopy (TEM) images weretaken by Hitachi H7600 (Japan). The crystal structurewas characterized by X-ray diffraction (XRD) using aRigaku X-ray diffractometer with graphite monochroma-tized Cu K\u03b1 irradiation (MultiFlex XRD, Japan). Thediameter of nanofiber was measured using Image J soft-ware. The pore size of membranes was characterized by(Pore tester CFP-1100-AIP, MI). Fourier-transform in-frared spectroscopy (FTIR) is from PerkinElmer (Fron-tier, PE, USA). Air permeability was measured usingautomatic air permeability meter (NingFang YG461E-111, China). The pressure drop and PM concentrationwere measured using PM Concentration 2.5 Tester(DustTrack 8520 TSI). PM particle number concentra-tion was detected by laser particle counter (Purific Y09-301, China) and the removal efficiency was calculated bycomparing the concentration before and after filtration.The photograms were captured by a digital camera(Nikon, D90).Results and DiscussionStructure and Composition of Nanofiber MembraneThe typical nanofiber composite membranes of the op-tical images of 2 layers, 3 layers, and their SEM imageswere shown in Fig. 1a\u2013d, respectively. The nanofibermembrane and the PP non-woven fabric support waslayered, but the binding force was strong, because staticelectricity accumulates between the PP non-woven fabricand the nanofiber membrane during the electrospinningprocess. For example, we saw the layers of nanofiber andPP non-woven clearly in the 2-layer PAN:TiO2 nanofibermembrane (Fig. 1a), and top-view of the nanofibermembrane displayed PP microfiber and nanofibers struc-tures obviously as shown in Fig. 1b. The structure ofRuan et al. Nanoscale Research Letters            (2020) 15:7 Page 2 of 8fabrication for a 3-layer was similar. We observed 3layers\u2019 structure (PP non-woven, nanofiber, and PP non-woven) and the first nanofiber layer was entangled withthe non-woven fabric support in the SEM of the PAN:TiO2 nanofiber membrane, as shown in Fig. 1b, d.In order to synthesize the designed nanofiber mem-branes, we have developed and further optimized the ap-proach by tuning the electrospinning parameters, suchas spinning time, the receiving distance, temperatureand humidity, voltage, traverse speed and rotation speedof the receiving roller. In the synthesizing process, wefound that spinning time was controlling the thicknessof nanofiber membranes, if we kept other electrospin-ning parameters unchanged. The shorter spinning timeproduced thinner nanofiber membranes. We produced adifferent thickness of nanofiber membranes by using dif-ferent spinning time, as shown in Fig. 2. From the im-agines of short spinning times as 15, 30, and 45 min, theskeleton of PP nonwoven was observed clearly in thenanofiber membrane (Fig. 2a\u2013c). As the spinning time in-creasing to 1 and 2 h, the PP non-woven skeleton grad-ually became unclear and blurred, as shown in Fig. 2d, e,respectively. Finally, the visibility of the nonwoven fabricskeleton became hardly being observed, when the spin-ning time was as long as 4, 6, and 8 h (Fig. 2f\u2013h).In the SEM and TEM of PAN:TiO2 nanofiber mem-brane, the 3-layer one displayed the cross-sectionalstructure in the nanofiber membranes and nanofiberlayer bonded to the non-woven fabric support (Add-itional file 1: Figure S1 in supporting data). The nanofi-bers have prominent TiO2 nanoparticles on the surface,which can be clearly observed in the TEM imagine(Additional file 1: Figure S1C). EDS, XRD, and FTIRidentified that TiO2 nanoparticles were located on thesurface and inside of the nanofibers in the anatase forms(Additional file 1: Figure S2\u20134 in supporting data).In PAN membranes, the fiber diameter ranged from100 to 400 nm (average 237 nm) and the averagemolecular weight was around 100,000 Da. In PAN-Co-PMA membrane, the fiber diameter was 400~800 nm(average 678 nm) and an average molecular weight of150,000. Because of the difference in molecular weight,it was clearly observed that the average and ranges diam-eters between the PAN:TiO2 and PAN-Co-MA:TiO2Fig. 1 Morphology of PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membrane composited with PP non-woven air filter (layers): optical photographof nanofiber membranes of 2-layer (a) and 3-layer (c), and their enlarged top-views (c, d), respectivelyRuan et al. Nanoscale Research Letters            (2020) 15:7 Page 3 of 8nanofiber membranes are certainly different, as shownin Fig. 3a, b. The size of the fiber diameter influencesthe pore size and air permeability of the nanofiber mem-brane, in addition to the particle filtration efficiency andpressure drop of the nanofiber membrane, as shown inFig. 3c. Due to the smaller fiber diameter, the pore sizeof PAN:TiO2 nanofiber membranes were smaller thanPAN-co-PMA:TiO2 nanofiber membranes. Compared tothe thickness of membrane, the nanofiber diameter hada larger influence on membrane pore size. Althoughthickness had a strong effect for the pore size of thenanofiber membrane (spinning time in 1 h), it onlyslightly changed the pore diameter, after the thicknessreached a critical point (the spinning time longer than 2 h),as shown in Fig. 3c. It was similar to the air permeability ofthe nanofiber membrane, and the air permeability droppedwith longer spinning time (membrane thicker), and mem-branes reached a plateau, when spinning time of 2 h. Theair permeability of PAN:TiO2 nanofiber membranes wasmuch lower than that of PAN-co-PMA:TiO2 when elec-trospun for 2\u201310 h. However, the variance of air perme-ability of PAN-co-PMA:TiO2 nanofiber membranes (32\u201335 mm\/s) was higher than PAN:TiO2 nanofiber mem-branes (6\u201310 mm\/s). It was probably due to the PAN:TiO2 nanofiber membrane (smaller diameter) depositdensely under similar spinning durations compared to theFig. 2 Morphology of PAN:TiO2 nanofiber membranes with different spinning times (different thicknesses): a 15 min, b 30 min, c 45 min, d 1 h, e2 h, f 4 h, g 6 h and h 8 hRuan et al. Nanoscale Research Letters            (2020) 15:7 Page 4 of 8PAN-co-MA:TiO2 nanofibers. Therefore, the smallernanofiber diameter and pore size of the nanofiber mem-brane experienced decreased flux, causing low air perme-ability Additional file 1: Figure S5.Applications for Particles PurificationThe aerosol filtration efficiency and the pressure drop ofPAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranewere studied. For both of nanofiber membrane, as thespinning time increased from 15 min to 2 h, the aerosolfiltration efficiency increased sharply from as low as ~ 20to 97% of and 50% for PAN-co-PMA:TiO2 and ~ 50 to99% for PAN:TiO2, respectively (in Fig. 4a). The filtra-tion efficiency of both nanofiber membranes was closeto 100% if the spinning time was longer than 3 h. Mean-while, the pressure drop increased with longer spinningtime (thickness increasing). In the study, PAN:TiO2nanofiber membrane continuously increased quickly to600 Pa, when the spinning time was longer than 3 h,even reached 1000 Pa (spinning time longer than 8 h).However, the PAN-co-PMA:TiO2 nanofiber membraneincreased much slow and kept the pressure drop around200. Compared to the PAN-co-PMA:TiO2 nanofibermembrane, PAN:TiO2 membrane had smaller diameterFig. 3 Diameter distribution of different PAN type (3% TiO2) nanofibers: (a) PAN:TiO2, (b) PAN-co-PMA:TiO2, and (c) average pore size andpermeability of PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranesRuan et al. Nanoscale Research Letters            (2020) 15:7 Page 5 of 8and pore size and the membrane blocked the aerosolparticles. At the same time, the smaller pore size causedthe limited air permeability and higher pressure drop tomaintain gas flow.In the filtration efficiency study for different size parti-cles, we generated simulated polluted air in hazy days byburning cigarettes and it contained CO, CO2, NO2, andvolatile organic compounds, such as tar, nicotine, for-maldehyde, and benzene. In the studied model system,we found that the thickness (spinning time) of nanofibermembrane had a strong effect of the filtration efficiency.For example, the filtration efficiency of PAN:TiO2nanofiber membrane was higher than 90% if the spin-ning time was longer than 45 min, or close to 100%, ifthe spinning time was longer than 2 h) for the all testedparticles at diameter from 0.3 to 3 \u03bcm, as shown in theFig. 4b. Compared to PAN:TiO2 nanofiber membrane,the overall filtration efficiency of PAN-co-PMA:TiO2nanofiber membrane was lower if the spinning time wasshorter than 3 h. The filtration efficiency was also closeto 100% for all the tested particles, if the spinning timewas longer than 4 h in our study (Fig. 4c). The results ofthe filtration efficiency for both nanofiber membraneswere similar to aerosol results. The large fiber diameterFig. 4. PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranes\u2019 filtration efficiency with (a) pressure drop of aerosols (a) and particle size (b, c);and the removal capability of (d) PAN:TiO2 and (e) PAN-co-PMA:TiO2 nanofiber membranein simulated polluted air testRuan et al. Nanoscale Research Letters            (2020) 15:7 Page 6 of 8caused the big porosity between the fibers, increasingthe possibility of particles passing through. The filtrationefficiency on particulate matter reached a plateau, whenthe membrane thickness was to a certain level.Further, we studied PM2.5 removal process of PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranes for 2h, and the field tests were in a 1-m3 chamber of real pol-luted air environment. The model system of the air cham-ber was designed (shown in Additional file 1: Figure S6)and the initial PM2.5 concentration was 1 mg\/m3. Weused the circular nanofiber composite membranes forPM2.5 filtration and the PM2.5 particles in the air cham-ber were recorded every minute in total 120 min. The re-sult of two nanofiber membranes was shown in Fig. 4d, e.PAN-co-PMA:TiO2 nanofiber membranes removed allPM2.5 in 120 min, and thinner (spinning time \u2264 2 h) com-pletely reduced PM2.5 in 50 min, and membranes withelectrospinning time of 0.25 h and 0.5 h even filtered allPM2.5 in about 20 min. PAN:TiO2 nanofiber membraneshad better removal of PM2.5 in the tests, and the mem-branes (electrospinning time > 4 h) could not reduce thePM2.5 in 2 h, as shown in Fig. 4e. Generally, PAN-co-PMA:TiO2 nanofiber membrane had higher removal ofPM2.5 than that of PAN:TiO2 nanofiber membrane.ConclusionIn summary, we synthesized the PAN:TiO2 and PAN-co-PMA:TiO2 nanofiber membranes by using electro-spinning and the properties of nanofiber membranes, asair permeability, aerosol test, and PM trapping were sys-tematically evaluated. The microfiber non-woven, thenanofiber membrane, and the non-woven fabric bracketwere well composited into a multi-layer structure byelectrostatic force for two types of nanofiber mem-branes. The bonding structure of PAN-co-PMA:TiO2nanofiber membrane displayed excellent air permeability(284\u2013339 mm\/s) and removal of PM2.5. Moreover, thedeveloped nanofiber membranes were cost-effective andpractical PM2.5, which would be applicable as a com-mercial air purifier filter to prevent PMs in the future.Supplementary informationSupplementary information accompanies this paper at https:\/\/doi.org\/10.1186\/s11671-019-3225-2.Additional file 1: Figure S1. (a) SEM of cross-sectional PAN@TiO2 nano-fiber membrane (b) SEM at 10 \u03bcm and (c) TEM imagine at 500 nm ofPAN@TiO2 nanofiber membrane. (TiO2 content of 3%). Figure S2. EDSimage of PAN@TiO2 nanofiber membrane. Figure S3. EDS image of CK\u03b11 (a) and Ti K\u03b11 (b). Figure S4. XRD of PAN-TiO2 nanofiber membrane.Figure S5. FTIR of PAN:TiO2 and PAN-co-PMA:TiO2 NFM(Nanofiber Mem-brane). Figure S6. Simulated polluted air test device. Figure S7. SEM ofPAN nanofibers with(a) and without (b) the TiO2, PM2.5 filtration effi-ciency of PAN nanofibers &PAN:TiO2 nanofibers in Simulated polluted airtest device (120min).AbbreviationsDMF: N,N-dimethyl formamide; FTIR: Fourier-transform infrared spectroscopy;PAN: Polyacrylonitrile; PAN-co-PMA: Polyacrylonitrile-co-polyacrylate;PM: Particulate matter; PS: Polystyrene; PVP: Polyvinylpyrrolidone;SEM: Scanning electron microscope; TEM: Transmission electron microscopy;VA: Polyvinyl alcohol; XRD: X-ray diffractionAcknowledgementsThis work was supported by the Key Field R&D Program of GuangdongProvince (2019B010941001). Natural Science Foundation of GuangdongProvince China (grant no. 2018A0303100022), Dongguan Social Science andTechnology Development Project (grant no. 20185071631280), ChinaPostdoctoral Science Foundation Grant (grant no. 2018M630949),and theCentral University Scientific Research Project (grant no. 2017BQ051).Authors\u2019 ContributionsRC, LQ, and DR contributed equally to this work. All authors read andapprove the final manuscript.FundingThis work was supported by the Key Field R&D Program of GuangdongProvince (2019B010941001). Natural Science Foundation of GuangdongProvince China (grant no. 2018A0303100022), Dongguan Social Science andTechnology Development Project (grant no. 20185071631280), ChinaPostdoctoral Science Foundation Grant (grant no. 2018M630949),and theCentral University Scientific Research Project (grant no. 2017BQ051).Availability of Data and MaterialsPlease find the availability of data in supporting data.Competing InterestsThe authors declare that they have no competing interests.Author details1Guangdong Engineering and Technology Research Centre of Advanced andNanomaterials, Dongguan University of Technology, Dongguan 523808,China. 2Dongguan Beyclean Environmental Protection Science andTechnology Co. Ltd., Dongguan 523690, China. 3South China Institute ofCollaborative Innovation, Dongguan 523808, China. 4Department of MaterialsScience and Engineering, Southern University of Science and Technology,Shenzhen 518055, China. 5State Key Laboratory of Precision ElectronicManufacturing Technology and Equipment; Guangdong Provincial KeyLaboratory of Micro-nano Manufacturing Technology and Equipment,Guangdong University of Technology, Guangzhou 510006, China.6Department of Material Engineering, University of British Colombia,Vancouver V6T1W9, Canada.Received: 6 March 2019 Accepted: 5 December 2019References1. Mannucci PM, Franchini M (2017) Health Effects of Ambient Air Pollution inDeveloping Countries[J]. Inter J Environ Res Public Health 14(9):1048.https:\/\/doi.org\/10.3390\/ijerph140910482. Khwaja HA, Hussain MM, Naqvi I, Malik A, Siddiqui SA, Khan A (2016) TheState of Ambient Air Quality of a Mega City in Southeast Asia (Karachi,Pakistan). AGU Fall Meeting Abstracts3. 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Nanoscale Res Lett 13(1):48. https:\/\/doi.org\/10.1186\/s11671-018-2465-xPublisher\u2019s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.Ruan et al. 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