Electrospun Nanofibers for Sensors

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Yan Li1, Mohammed Awad Abedalwafa1,2, Liqin Tang1, De Li1, Lu Wang1 Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China1; Department of Technical Textile, Faculty of Industries Engineering and Technology, University of Gezira, Wad Madani, Sudan2

18.1 INTRODUCTION Electrospinning is one of the most important technologies for the preparation of micro/nanoscale continuous polymer fibers. In general, conventional textile fibers have a diameter of more than 5 mm, and the fiber diameter obtained by melt or solution spinning is usually in the range of 500 nm to 5 mm, with the assistance of mechanical stretching. Electrospun fibers, however, possess a diameter down to the submicrometer or even nanoscale, and the range is from 300 nm to 5 mm. Thus, it can be seen that electrospinning technology can significantly reduce the fiber diameter, into the micro/nanoscale, and enhance the specific surface area (SSA) to great advantage (Su et al., 2014, Long et al., 2012a,b). Furthermore, the raw materials can include a wide range of natural polymers, synthetic polymers, and inorganics. By adjusting the ratio of spinning solution, the spinning liquid composition, and the posttreatment process, and with the combination of multiple technologies, fibers can be prepared with diverse properties. Consequently, electrospun nanofibrous membranes (NMs) can be applied in filtration, catalysis, supercapacitors, lithium batteries, sensors, and tissue engineering, among others (Ding et al., 2015). Among all the applications of NMs, sensors have always been one of the hot research topics. A sensor is a device that can be perceived, measured, and converted to usable output signals in accordance with certain rules. In some disciplines, sensors are also known as sensitive components, detectors, converters, and so on. These different formulations reflect the use of different technical terms for the same type of device in different technical fields, depending only on device usage. In the field of electronic technology, electronic components that can feel signals are often referred to as sensitive components, such as a thermal component, magnetic component, photosensitive component, and gas-sensing component. In most instances, the signal output is in a form of electricity, which is easy to transfer, convert, process, display, and so on (Mondal and Sharma, 2016). There are other kinds of signals, such as voltage, current, capacitance, resistance, etc., and those output signals guiding and determining the sensor designing principles. Frequently, the sensor consists of a sensing component and a transition component. The sensing component is the unit that can directly respond to the external stimulus; and the transition component is responsible for converting the stimulus into a signal portion, Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00018-2 Copyright © 2019 Elsevier Inc. All rights reserved.

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then suitable for transmission/measurement. Sensing response materials are at the core of the sensing component, as well as being the accurate components of the sensor, and from the sensor type to the sensing response material structure. Since the 1990s, the rise of nanotechnology and nanomaterials has injected new vitality into the study of sensors. For the purpose of improving detection performance, some researchers attempted to introduce NMs into the configuration design of the sensing response material (Barkalina et al., 2014). It is noteworthy that sensing performance on one hand depends on the constituent parts of sensing component, and on the other hand depends on the material structure and geometric scale of the sensing component. When the scale of materials turns from the macro to the micro/nano level, consequential effects begin to significantly influence sensing performance, such as small-scale effects, surface and interface effects, and quantum size effects, showing a number of characteristics that macroscales do not have. With the decrease in sensor size, the surface energy can be increased, and at the same time, the proportion of atoms on the material’s surface increases accordingly. When the surface atomic ratio is raised to a certain extent, the characteristics of sensors will be determined more by the surface atoms than by the internal lattice atoms. Furthermore, the increase in SSA provides a large number of areas and channels that enhance the interaction between the determinant and the NMs, and thus sensitivity is further improved. In addition, the use of NMs can reduce the energy consumption and the overall size of the sensor, thereby extending their service scope. In this chapter, we will review recent progress in the development of NMs as a sensing response material and their applications in four predominant sensing schemes (electrochemical, optical, resistive, and mass-change-sensitive sensors), and illustrate them with examples showing how they have been applied and optimized. Moreover, we will also discuss their intrinsic fundamentals and optimal designs. Ultimately, we will highlight gaps requiring further research (Fig. 18.1).

18.2 HOW TO DESIGN ELECTROSPUN NMeBASED SENSING MATERIALS The aforementioned inherent structural advantages of NMs have given them unique potential in sensor application and attracted attention on a worldwide scale. However, most of the NMs have some chemical inertness themselves, which means they do not have some special response features. In view of this, researchers have designed many special ways to fabricate NMs via traditional electrospinning, so as to meet the demands of the sensor response unit (Rezaei et al., 2016). Some of these ways are, for instance, (1) physical blending of the polymeric materials or functional additives, (2) electrospinning setup regulation (jet, collector, nozzle, etc.), and (3) surface modification of the NM or immobilization of responding substances on the NM surface. These are the most typical methods to give NMs special response performance. Moreover, these convenient, mature, and effective methods can be applied either alone or in combination with others. Next, we will give some typical examples to illustrate how NMs are endowed with target sensing properties (Fig. 18.2).

18.2.1 REGULATING THE CONSTITUENT PARTS OF THE POLYMERIC SOLUTION At this writing, the introduction of functionalized active molecules or polymers into the polymeric solution is one of the most commonly used methods to optimize nanofiber-based sensors. Occasionally, it is impossible for some functional agents to be directly involved in the polymeric solution due to

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FIGURE 18.1 Overview of NMs applications in sensors. NP, nanoparticle; QCM, quartz crystal microbalance.

their poor solubility or low molecular weight. Blending polymers with different characteristics or doping the polymeric solution with functional agents can not only endow the end-product with sensing responding properties, but also is beneficial to improve the properties of NMs (such as wettability, bioactivity, mechanical property, etc.).

18.2.1.1 Solution: Polyaniline-Based Blending System Polyaniline (PANI) is one kind of conductive polymer, with high temperature resistance, good environmental stability, and excellent electrical conductivity. Unfortunately, the solubility of PANI in all kinds of solvent is very poor, thus, it is really hard to assemble the PANI into sensors (Sen et al., 2016). Referring to its chemical structure, many benzene ring structures exist in the molecular backbone, which induce adjacent molecules to form hydrogen bonds, resulting in a high rigidity. Therefore, even a low-concentration solution shows a strong gelation tendency (Mondal and Sharma, 2016). Although

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FIGURE 18.2 Schematic illustration of the measures to endow NMs with sensing properties throughout the electrospinning process.

PANI films can be fabricated via hot press molding, the poor mechanical properties of the films raise another obstacle restricting solid-phase PANI application (Baker et al., 2017). To explore an feasible method to fabricate solid-state PANI without losing its functionality, Wen et al. (2017) blended PANI with polyamide-66 (PA-66), and then dissolved them in formic acid to obtain a mixed solution for electrospinning. The proportions of the polymeric mixture also were carefully studied; when the proportion of PANI was excessive, the formation of NMs would be difficult because of the high solution conductivity. The PANI was evenly distributed in the resulting PANI/PA66 NMs; therefore, the final NMs not only possessed a self-standing property, but also had the distinctive characteristics of PANI, as shown in Fig. 18.3 (Wen et al., 2015). In addition to PA-66, researchers also doped PANI with polyacrylonitrile (PAN) and silver nanowires (Ag NWs) for further electrospinning. The strength of the resulting PANI/PAN/Ag NW nanomaterials was improved, and the excellent conductivity of PANI was not broken up during the process (Rezaei et al., 2016). In addition, the PANI/PAN/Ag NW NMs were also endowed with a new characteristic: they were antibacterial. Except PA-66 and PAN, PANI has also been blended with PA-6 (Ding et al., 2011), polyvinyl butyral (Si et al., 2014), polyethylene oxide (PEO) (Li et al., 2014a), single-walled carbon nanotubes (Ebrahim et al., 2014), polyethersulfone (Voinova et al., 2002), and so on. The stable PANIbased blended solution systems were fabricated into NMs via electrospinning, and thereafter the NMs were available in electrochemical, colorimetric, and resistive sensors.

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FIGURE 18.3 Fabrication of polyaniline/polyamide-66 (PANI/PA 66) composite NMs via electrospinning. FTIR, Fourier transform infrared.

18.2.1.2 Process: Formulated Functional Agents with the Bulk Phase of Nanofibers Directly dispersing the functional agents into the polymer solution for electrospinning is the main method for the preparation of composite sensing responding NMs. Despite its easy operation, the functional agents themselves, on occasion, have high surface free energy; therefore the possibility of aggregation is sufficiently high so as to restrict its development. In addition to treating the solution mixture with ultrasonic dispersion, or manifold repetition of blending, or modifying agents with a surface active agent, many efforts have been made to fabricate functional sensing NMs, such as multijet, side-by-side, coaxial, and emulsion electrospinning. A structure-tunable Janus fiber was fabricated by Chen et al. via side-by-side electrospinning. One side was made of Eudragit L100 (an anionic copolymer based on methacrylic acid and methylmethacrylate) and 2% (wt/vol) 8-anilino-1-naphthalenesulfonic acid ammonium salt (C16H16N2O3S). The other side was made of 10% (wt/vol) polyvinyl pyrrolidone (PVP) K60 and 1% (wt/vol) rhodamine B with red color. By manipulating the port angle, the width, interfacial area, and volume of each side of the resulting Janus nanofibers are tunable. These Janus nanofibers can serve as a new platform for different kinds of sensors (Wang et al., 2011a,b). Xue et al. designed a unique hierarchical coreeshell polydimethylsiloxane (PDMS)epolycaprolactone (PCL) nanofiber for oxygen detection (Xue et al., 2015). The O2-sensitive probe molecules were embedded in PDMS, since PDMS possesses an extraordinary gas permeability. The PCL layer played the role of biocompatibility promoter. Li et al. (2013) designed a composite NM consisting of PA-6 and nitrocellulose (NC), via multijet electrospinning (Fig. 18.4), which can perform as the immobilization platform for gold nanoparticles (Au NPs). Au NPs are very unstable when dispersed in solution, their aggregation can be easily induced by pH, temperature, storage light intensity, etc. If Au NPs were immobilized on a solid

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FIGURE 18.4 Fabrication of polyamide-6/nitrocellulose (PA 6/NC) composite nanofibrous membrane via multijet electrospinning and application as a gold nanoparticle (NP) immobilization platform.

platform, their stability could be improved by strong interactions between the Au NPs and the platform through covalent bonds to polymer functional groups. In this research, NC nanofibers provided a strong hydrophobic force to immobilize the Au NPs. In consideration of the poor water wettability and mechanical properties of NC nanofibers, the researchers added PA-6 into the NC structure construct. Benefiting from the advantages of PA-6, the wettability was improved and an extra interaction was formed between the Au NPs and the NH2 on the PA-6 surface. This monolithic structure consisted of Au NPs immobilized NMs (Au NP@NM) can be applied in surface-enhanced Raman scattering (SERS), colorimetric, and electrochemical sensors.

18.2.1.3 Posttreatment: NM Functionalization In general, the surface properties of nanofibers depend primarily on the chemical composition of the spinning feedstock and the surface structure of the fibers. A certain degree of chemical inertia has shown up and limits their sensing application. Apart from the aforementioned means, the surface modification of nanofibers refers to the use of physical or chemical methods to alter their surface, while maintaining the original inherent characteristics of the nanofibers under the premise of giving them new features. In this case, several methods have been proposed to functionalize NMs by introducing active sites for further target substance detection. Physical dip-coating as one of simplest methods to endow nanofibers with active sites for target interaction, and it is also a method that is easy to handle and fast. The interaction between nanofibers and sensitive probe molecules often involves van der Waals forces, electrostatic forces, hydrophobic interactions, and hydrogen bonding (Beachley and Wen, 2010). Sun et al. coated a polystyrene NM with 3-mercaptopropionic acid to detect Cu2þ (Sun et al., 2012). Wang et al. coated an ultrathin layer of polypyrrole (PPy) on the surface of titanium dioxide (TiO2)/ZnO NMs, and the thickness of the functional layer of PPy was c. 7 nm. The layer of PPy could minimize NH3 gas diffusion resistance, and the gas-sensing performance was thus improved (Wang et al., 2009). Ho et al. (2013) also designed

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FIGURE 18.5 The fabricating process of NM based sensor for HCHO detection, (A) fabricate the chitosan nanofibers via electrospinning, (B) Coating the chitosan NMs with polyethyleneimine (PEI), and (C) the interaction between PEI with HCHO on the gold electrode.

NH3-sensing materials by coating PPy on the surface of tungsten oxide nanofibers. At an operating temperature of 100 C, the PPy-coated NMs showed good sensitivity and a fast resistance-increasing response. Moreover, a formaldehyde detection system was built by Wang et al. (2014a,b). By utilizing polyethylenimine as a coating substance and chitosan NMs as a platform, they achieved a detection limit of 5 ppm. This NM-based sensor is shown in Fig. 18.5. Jia et al. reported a new NM-based architecture comprising electrospun PA-6/polyallylamine hydrochloride (PAH) nanofibers coated with multiwalled carbon nanotubes, used to detect the neurotransmitter dopamine (Jia et al., 2016a). Senthamizhan et al. directly decorated Au nanoclusters on a polysulfone (PSU) NM surface via dipcoating and then used them for H2O2 detection. The sulfur groups in PSU may provide a bridge for the Au nanoclusters and itself (Senthamizhan et al., 2015). NMs can also perform as a substrate for the in situ polymerization of AgNO3, HAuCl4, aniline, pyrrole, etc. Taking a conductive polymer monomer of PANI as the example, if successful polymerization happened on the NM surface, improvements in the adhesion and electrical contact of the resulting sensors would occur. An electrochemical sensor was prepared by in situ polymerization of aniline on the surface of a polyimide NM (Chen et al., 2013). The synthesized and homogeneously distributed PANI not only possessed excellent thermal properties, but also showed good electrical conductivity, pH sensitivity, and significantly improved electromagnetic impedance properties. There is another study that focused on constructing PANI-coated NMs via in situ polymerization. First, PA-6/TiO2 composite NMs were prepared utilizing an electrospinningeelectrospraying process in which TiO2 NPs were embedded in the PA-6 NM. Subsequently, the PANI coating layer was fabricated

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by a further in situ polymerization process. In these circumstances, a p type to n type (p-n) junction was formed at the interface between PANI and TiO2. This junction provided to the PA-6/TiO2/PANI composite NMs serious outstanding features such as good reproducibility, selectivity, and an apparent improvement in response compared with PA-6/PANI NMs. Another unique hierarchical coreeshell NM fabricated by Zhou et al. contained PCL as the core and in situepolymerized PANI as the shell layer (Zhou et al., 2017). Cho et al. grafted a pyrene derivative and a rhodamine B derivative onto crosslinked poly(2-hydroxyethylmethacrylate-co-N-methylolacrylamide) NMs through surfaceinitiated in situ polymerization to realize heavy metal ion and pH value detection (Cho et al., 2016). Another typical strategy is in situ synthesis of NPs on the NM, and the typical operation protocol used is to mix the metal ions with the polymer solution. Then, the metal ions are fixed by complexation between them and the polymer matrix or by the steric hindrance formed by the interactions between polymer-specific functional groups and the metal ions, followed by an in situ reduction reaction. Utilizing in situ synthesis, composite NMs with metal NPs on their surface are successfully fabricated. The advantages of this protocol include stable particle size of the NPs, uniform distribution in the nanofibers, and good combination of NPs and polymer matrix. Apart from blending metal ions with polymer solutions, Kundu et al. also fabricated Au NPecovered PAH NMs by a posttreatment method. The PAH NMs were directly immersed in HAuCl4 solution, and the in situ synthesis was initiated via UV photoirradiation. The Au@PAH composites, with a continuous metallic structure, showed ohmic behavior with low resistance (Kundu et al., 2011). The aforementioned methods can be classified as physical methods. In contrast to physical methods, chemical modification provides a more stable method for long-term preservation of sensing responding molecular functionality. To ensure productive immobilization of sensing target-active compounds, appropriate chemical modification should be undertaken to introduce reactive functional groups prior to chemical bonding. Carboxyl (Li et al., 2015a,b) and amine groups (El-Moghazy et al., 2016) are the most widely used functional groups for surface modification of the polymer nanofiber, and they can be introduced onto the surface of NMs by the hydrolysis reaction using mild solutions of bases and acids, respectively (Ma et al., 2017; Mercante et al., 2017). An example is aptamer-modified multichannel carbon nanofibers for bisphenol A detection. The multichannel carbon nanofibers were generated in two steps: electrospinning of a blended polymeric solution consisting of PAN and polymethylmethacrylate and then a thermal treatment at 400 C, followed by carbonization. Even though the NMs had undergone several treatments, the obtained carbon nanofibers still could not be used for aptamer modification. A carboxyl-functionalization process was necessary and performed by an oxidative treatment (Kim et al., 2016a). Ondigo et al. developed a sensing system based on polyvinylbenzyl chloride (PVBC) NMs. The probe was fabricated through postfunctionalization of the PVBC with the 2-(20 -pyridyl)imidazole ligand by a wet chemical method (Ondigo et al., 2013). Nowadays, the preparation of functional electrospinning composite nanofibers has become one of the most active research areas for polymer nanofibers. The strategies for NM-based sensing material construction are varied. In addition to dip-coating, in situ polymerization, layer by layer (LBL), and chemical modification, we still have solegel (Senthil and Anandhan, 2014), physical or chemical plasma (Zhang et al., 2010; Wei et al., 2005; Cho et al., 2012), gas- or liquid-phase deposition (Baranowska-Korczyc et al., 2015), atomic layer deposition (Won-Sik et al., 2010), and so on.

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18.3 ELECTROCHEMICAL SENSORS Electrochemical sensors are used as an analytical method for determining the concentration of an analyte based on its electrochemical properties and corresponding changes in the solution. Typically, in an electrochemical sensor, the electrocatalytic reaction produces a quantifiable current (amperometric sensing) (Zhao et al., 2017) or a measurable electric potential or charge buildup (potentiometric sensing) (Shaibani et al., 2016), or it detectably modifies the electrical conductivity of a medium (conductometric sensing) between electrodes (Li et al., 2017). There are other types of electrochemical detection systems, for example, impedimetric sensors, which deal with impedance for both reactance and resistance, and field-effect biosensors, which use an electronic transistor to detect current as a consequence of a potentiometric outcome at the surface of a gate electrode. A typical sensing system generally involves a reference electrode (made from Ag/AgCl or glassy carbon materials on most occasions), an auxiliary or counterelectrode, and a working electrode/redox electrode (sensing electrode). The working electrode transduces elements in the reaction, while the counter- and working electrodes fulfill the requirements of good conductivity and chemical stability. Of these three types of electrode, the working electrode plays a vital role in the sensing system. Owing to the ability to produce nanofiber structures from a variety of materials according to application, the relatively high SSA of NMs, and the tunability of their structure characteristics, NMs are sometimes applied to working electrode modification. In addition, the nanofibrous structure has been found that it can provide faster electron transfer than NP-based films. (Dubal et al., 2015; Ding et al., 2010). As of this writing, researchers have developed electrochemical sensors for the detection of heavy metals, hydrogen peroxide (H2O2), carbohydrates, nucleic acids, cells, proteins, etc., and the scope of detectable substances keeps expanding.

18.3.1 METAL, METAL OXIDE, AND CERAMIC NANOFIBER-BASED ELECTROCHEMICAL SENSING As we all know, electrospinning can fabricate polymeric nanofibers in a very simple process. This technology also can be applied to metal, metal oxide, and ceramic nanofiber fabrication. Most metal/ metal oxide/ceramic nanofibers are produced from the sintering of a precursor, which is presented in Fig. 18.6. In general, the fabrication undergoes three steps: preparation of a precursor solution containing polymer and metal salts/ceramic sources via solegel methods, followed by electrospinning of the precursor solution with appropriate process parameters to obtain precursor nanofibers, and then heat treatment of the precursor nanofibers in a gas atmosphere, in which temperature and heating rate are two decisive factors. The obtained electrospun metal/metal oxide/ceramic nanofibers are generally made of nanosized grains or crystals. The grains’ size would affect the SSA, thus affecting the sensing performance. Efforts have been made to use metal nanofibers directly to construct electrodes, such as platinum (Pt) (Vassilyev et al., 1985), Au (Adzic et al., 1989), and so on. Pt, as a noble metal, has excellent catalytic activity in a wide pH range (alkaline to neutral). Previous researchers have witnessed its outstanding electrocatalytic effect on glucose oxidation, and it is also the best catalyst in the dehydrogenation of anomeric carbon in the C1 position (Habrioux et al., 2007); however, it also suffers from low sensitivity and poor selectivity. Considering this, Liu et al. designed PteAu nanocorals, Pt

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FIGURE 18.6 A representative process for metal/metal oxide/ceramic nanofibers.

nanofibers, and Au microparticles by electrospinning. The spinnable precursor polymeric solution was prepared by dissolving PVP in a solvent (dimethylformamide mixed with water) and then blending it with Pt salt (H2PtCl6) and Au salt (HAuCl4). After electrospinning, the H2PtCl6eHAuCl4ePVP nanofibers were calcined in air at 500 C for 3 h, the PVP component was removed, and the PteAu nanocorals were formed. Pt nanofibers and Au microparticles were prepared in the same fashion. The glucose-detection performance of these three nanofibers was studied, and the results indicated that the PteAu nanocoralebased amperometric sensor showed higher sensitivity and improved selectivity (Liu et al., 2012). Compared with monometallic materials, bimetallic materials or alloys often exhibit better catalytic properties than their counterparts, which suggests that binary composition interfaces are expected to produce synergistic effects to achieve better electrocatalytic activity and selectivity toward the electrooxidation of glucose (Jiang and Zhang, 2010). Furthermore, it is necessary to point out that the stability may be decreased because of the chemisorbed intermediates and adsorbed chloride ions. Metal oxide and ceramic NMs have been projected for future applications in catalysis, supercapacitors, and Li ion batteries, owing to their interconnected porosity and enormous SSA and other useful properties. Especially, metal oxide and ceramic NMs present great potential in electrochemical sensing, not only because they possess the electrocatalytic activity for immobilizing sensing responding molecules, but also because they can boost the sensing characteristics. As the result of their

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cost effectiveness, specificity, mobility, and fast response with high sensitivity, electrospun metal oxide, ceramic, and composite NM-based electrochemical sensors are expected to play a crucial role in environmental monitoring and biomedical diagnostics. Cabrita et al. described a new titanite NMbased electrode for L-ascorbic acid, which was fabricated by modifying titanite NMs with nanocrystalline semiconducting bismuth sulfide (Bi2S3) NPs. Bi2S3 is an important member of the VeVI group and it is a direct bandgap material (Eg ¼ 1.2 eV). When it is used to modify titanite NMs, the complex material displays high electrochemical hydrogen storage and electrochemical interaction ability. Under the optimal pH condition (pH ¼ 7), a sensitivity of 38 mA/cm2 mM, along with high stability and a linear range from 1 to 10 mM, was realized (Cabrita et al., 2014). Liu et al. synthesized spinel compounds to improve conductivity and catalytic activity. Spinel compounds of MFe2O4 (M ¼ Co, Ni, Cu, etc.) are well known owing to their notable electrocatalytic properties. Herein, heterojunction NiFe2O4 NMs and CoFe2O4 NMs were prepared via electrospinning and subsequent thermal treatment processes, and PVP was selected as the platform polymer. Both as-prepared materials were capable of hydrazine detection with outstanding sensitivity. The MFe2O4 NMs possess abundant micro/meso/macro structures on their surface. These structural features could afford more accessible transport channels for the effective reduction of mass transport resistance and improvement of the density of exposed catalytic active sites. All these advantages endow the CoFe2O4 NMs with a high sensitivity of 1327 mA/cm2 mM in the linear range of 0.01e0.1 mM and a fast response (shorter than 3 s) (Liu et al., 2015a,b). Nanostructured metal oxides and ceramic NMs also open up the possibilities of immobilized biological elements (enzymes, antibodies, receptor proteins, cells, nucleic acids, etc.), as shown in Fig. 18.7. These biological elements play the role of sensing element to a

FIGURE 18.7 Electrochemical biosensing via enzyme immobilization onto an electrospun nanofiber surface.

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transducer material and transmit a signal. Ahmed et al. successfully designed a glucose biosensor, which was prepared from an electrospun ZnO nanofiber, and the precursor solution consisted of PVP and zinc acetate. After calcination, the ZnO NMs had an average diameter of 250 nm. Subsequently, glucose oxidase was immobilized on the ZnO NM surface through physical adsorption and superior enzymatic activity was maintained. Their electrochemical sensing measurements reported that the sensitivity was 70.2 mA/cm2 mM (Ahmad et al., 2010).

18.3.2 CONDUCTIVE POLYMERIC NANOFIBER-BASED ELECTROCHEMICAL SENSING Conducting polymers have shown great potential for their application as electrochemical electrodes. Poly(3,4-ethylenedioxythiophene) (PEDOT), a typical conducting polymer, has diverse structures and response mechanisms upon exposure to different substances. Fabricated nanostructured PEDOT (such as nanofibers and NWs) has become one of the latest trends. Ali et al. constructed a microfluidic impedimetric nitrate sensor by using graphene oxide (GO) nanosheets and PEDOT NMs to construct an electrochemical sensing interface. Furthermore, the interface was grafted with nitrate reductase enzyme molecules to realize nitrate sensing. The sensor has demonstrated the ability to accurately detect and quantify nitrate ions in real samples extracted from soil. The addition of GO further enhanced the charge transfer resistance of the electrode. Eventually, the sensor provided a sensitivity of 61.15 U/(mg/L)/cm2 with a wide concentration rage of 0.44e442 mg/L, and the detection limit was 0.135 mg/L (Ali et al., 2017). Zampetti et al. also designed a gas sensor based on PEDOT. The hybrid structure design in their work consisted of titania NMs and a coating layer of PEDOTepolystyrene sulfonate (PSS), which is suitable for NO2 detection, and the detection limit of the sensor was 1 ppb (Zampetti et al., 2013) (Fig. 18.8).

18.4 OPTICAL SENSORS 18.4.1 COLORIMETRIC SENSORS Environmental monitoring has received global attention in recent years due to environmental contamination. The harm of pollutants is often imperceptible and the damage induced by pollutants often irreversible. Therefore, the vital importance of early and accurate detection of pollutants has driven the development of colorimetric sensors, which are easy to use and inexpensive. Moreover, the detection results depend on the observation of a color change, and sometimes the color can be noted by the naked eye without the assistance of equipment. Compared with paper, film, glass slides, quartz, etc., NMs (Ahmad et al., 2010) have become some of the best candidates for colorimetric sensor substrates. In the past several decades, a variety of colorimetric agents, such as dyes, polymers, and metallic NPs, have been used for colorimetric detection of specific targets. To modify them with different forms of sensing responding molecules, several pre- and posttreatments have been used, which will be talked about in the following paragraphs.

18.4.1.1 Blending Blending colorimetric sensing responding molecules in a polymeric solution and then applying electrospinning to fabricate nanofibers carrying embedded colorimetric sensing agents is a typical way to design NM-based colorimetric sensors. The importance of the blending technique is that it is

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FIGURE 18.8 (A) Photo of the fabricated microfluidic sensor using a poly(3,4-ethylenedioxythiophene) (PEDOT) NMd graphene oxide (GO) composite to modify the working electrode (WE) for detection of nitrate ions. The channel was loaded with food dye for easy visualization. (B) Scanning electron microscopy image of the PEDOT NMeGO composite. (C) Geometry and layout of the WE of the sensor CE and RE refer to counterelectrode and reference electrode, respectively. (D) Schematic of the surface immobilization of PEDOT NMeGO with nitrate reductase enzyme to realize electrochemical nitrate detection by catalytic conversion of nitrate to nitrite. Reprinted with permission from Ali, M.A., Jiang, H., Mahal, N.K., Weber, R.J., Kumar, R., Castellano, M.J., Dong, L., 2017. Microfluidic impedimetric sensor for soil nitrate detection using graphene oxide and conductive nanofibers enabled sensing interface. Sensors and Actuators B: Chemical 239, 1289e1299. © 2017 Elsevier Ltd.

versatile, simple, and cost effective. However, the spinnability of the blended solution should not be affected. The agents often involve dyes, polymers, and monomers of polymers. Herein, the monomers of polydiacetylene (PDA) are introduced in detail. Kim et al. synthesized a series of PDA monomers (DAs) in view of the various sensing targets. Li et al. designed and synthesized a new DA, with pentaethylene glycol (5EG) as a side chain. The substrate used for 5EG embedment was PAN, because

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FIGURE 18.9 (A) Self-assembly and polymerization of 10,12-pentacosadiynoic acid (PCDA) and PCDAepentaethylene glycol (5EG). (B) Schematic representation of the preparation of the polydiacetylene (PDA)-embedded polyacrylonitrile (PAN) NMs. DA, diacetylene.

PAN NMs possess relatively high tensile strength and good tolerance to photo or heat treatment. As shown in Fig. 18.9, a photochemically initiated polymerization was used to convert DAs into PDAs. At the same time, the DA-embedded NMs changed from white to blue. Ultimately, the PDA-embedded NMs were applied for Pb2þ detection. With increasing Pb2þ concentration, the PDA-embedded NMs show a distinctive color change from blue to red (Li et al., 2014b). As a photometric reagent used in spectrophotometric determination of uranium dioxide (UO2), 2(5-bromo-2-pyridylazo)-5-(diethylamino)phenol was successfully blended with cellulose acetate (CA) followed by electrospinning to prepare a visual colorimetric strip. The new visual colorimetric strip exhibited high sensitivity for detecting UO2, with a yellow-to-purple color change signal (Hu et al., 2017). To overcome the limitation of the dye leaching out of dye-doped nanofibers in the presence of moisture, Schoolaert et al. functionalized a halochromic dye (methyl red or rose bengal) with chitosan, and then blended with poly(3-caprolactone) to fabricate fast-responding and user-friendly biocompatible, halochromic NM sensors. The result showed that efficient dye immobilization with minimal dye leaching was achieved within the biomedically relevant pH region, without significantly affecting the halochromic behavior of the dyes. In addition, the halochromic NM sensors showed high and reproducible pH sensitivity by providing an instantaneous color change in response to change in pH in aqueous medium and when exposed to acidic or basic gases (Schoolaert et al., 2016). Dimethylglyoxime (DMG) has been used as a reagent to detect nickel since L.A. Chugaev discovered their reaction in 1905. Najarzadekan and Sereshti applied DMG to the colorimetric detection of Ni2þ based on NMs. Typically, polycaprolactam (N6) blended with DMG was electrospun to NMs collected on a glass slide impregnated with a polyvinyl alcohol (PVA) solution to make it

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FIGURE 18.10 (A) Polarizing optical microscopy and (B) optical image of curcumin-blended cellulose acetate (CC-CA) NMs. (C) Fourier transform infrared spectra of nanofibers in the various fabrication stages of the strip. (Insets show the corresponding nanofiber images and curcumin powder.) (D) UVeVis absorption spectra for selectivity of Pb2þ in the presence of various metal ions. (E) Bar chart of CC-CA membrane for various heavy metal ions. (F) UVeVis for different concentrations of Pb2þ. Reprinted with permission from Raj, S., Shankaran, D.R., 2016. Curcumin based biocompatible nanofibers for lead ion detection. Sensors and Actuators B: Chemical 226, 318e325. © 2016 Elsevier Ltd.

transparent, to use as a colorimetric sensor to detect Ni2þ. As a result, the colorimetric sensor (N6eDMG/PVA@glass) showed a color change from transparent to red when exposed to Ni2þ, due to the formation of a Ni(DMG)2 complex (Najarzadekan and Sereshti, 2016). Furthermore, Raj and Shankaran developed a simple, biocompatible, and selective colorimetric sensor strip for Pb2þ detection based on curcumin by using electrospinning of curcumin-blended CA NMs (see Fig. 18.9). Their sensor showed good simplicity, low cost, efficiency for quantitative detection of Pb2þ with a detection limit of 20 mM, and excellent selectivity against various heavy metal ions (Raj and Shankaran, 2016) (Fig. 18.10).

18.4.1.2 Immobilization Immobilization of colorimetric agents on the polymeric NMs is a key step in colorimetric sensor development. A good immobilization technique should be simple, fast, and durable; moreover, the sensing activity of the colorimetric agent should not be affected by the immobilization process. There are many techniques for immobilizing: coating, adsorption, embedment, and covalent binding.

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Coating often involves adsorbing the colorimetric agents onto a polymeric NM. From the information collected earlier, we can find that coating is a simple and fast process. However, coating is also likely to face a problem in that the molecules would gradually leach from the NM, especially in a liquid-detection system, leading to unstable sensitivity and/or a short lifetime. If the NM surface can be first modified with complementary functional groups or selected carefully, the leaching problems raised by coating may be conquered. For example, Arslan et al. selected electrospun PA-6,6 NMs as a substrate for silicon quantum dot (Si QD) decoration. The visible-light-emitting Si QDs were decorated onto the PA-6,6 NMs by covering the surface as a “nanodress” by simple impregnation/dipcoating and heat-curing methods. The results suggested that the Si QDedecorated flexible polymeric NMs could be utilized for colorimetric detection of low concentrations of trinitrotoluene (Arslan et al., 2017). Functional groups in the colorimetric agents can be covalently bound to reactive groups on the surface of the polymeric NMs allowing robust immobilization. The NM surface can be chemically modified to bond the colorimetric agents. For example, Li et al. modified deacetylated CA (DCA) NMs with pyromellitic dianhydride (PMDA) (DCAePMDA) to use as sensing colorimetric agents for Pb(II) ions. The DCAePMDA was applied for the simultaneous naked-eye detection and removal of Pb(II) from water matrices by simple filtration followed by treatment with sodium sulfide solution. The color of the DCAePMDA changes from white to dark yellow-brown due to the formation of PbS, which can be useful for the colorimetric detection and adsorption of Pb(II), with a detection limit of 0.048 mM and maximum adsorption capacity of 326.80 mg/g (Li et al., 2015a,b). Pourjavaher et al. (2017) developed and characterized a smart label for pH monitoring based on bacterial cellulose (BC) NM-immobilized anthocyanins extracted from red cabbage (Brassica oleracea), as their vacuolar pigments depend on pH. The authors investigated the formation of new bonds between the glucose units of BC and the aromatic rings of the anthocyanins. Their sensor showed color variation within the pH range 2e10 (Pourjavaher et al., 2017).

18.5 RESISTIVE SENSORS Resistive sensors depend on changes in electrical resistance when the surface of the sensor is exposed to the target, due to the sensitive reaction between them. Resistive sensors based on NMs have been studied because of their high sensitivity and fast response and recovery time. There are three main strategies to fabricate resistive sensors based on NMs, using inorganic, organic, and hybrid nanofibers.

18.5.1 INORGANIC NANOFIBERS Sensing performance is based on the operating temperature, sensor structure, and material morphology. Semiconductor metal oxideebased sensors were constructed by grinding the metal oxide into a paste firstly, then coating the paste on the surface of ceramic tubes. Unfortunately, these processes would damage the morphology and structure of the oxides and subsequently decrease the performance of the sensor (Hu and Li, 2011). Electrospinning offers a simple and useful technique to fabricate nanocrystalline metal oxide membranes with a high porosity, high surface area, and small grain size. These fabricated nanocrystalline metal oxide membranes are profusely used as chemical sensors, because of their good performance, including high operating temperature, fast response and

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recovery time, and high sensitivity. The sensing response is known as the ratio between the initial resistance in air (Rair) and the target gas-sensing resistance (Rgas).

18.5.1.1 Pristine Metal Oxides Metal oxides such as TiO2, ZnO, CeO, WO3, SnO2, V2O3, and Ln2O3 have been extensively used in the fabrication of highly sensitive, simply designed, and fast response and recovery sensors. For example, Lim et al. fabricated mesoporous In2O3 nanofibers with high SSA by electrospinning a PVA/indium acetate precursor solution and subsequent calcining. The gas-sensing properties of the mesoporous In2O3 nanofiber-based sensors for CO in air were analyzed and compared with those of a commercial In2O3 powder. The authors found that the highly elevated response of In2O3 nanofibers could be attributed to the high surface area, which provides a large amount of surface sites for adsorption of and reaction with CO (Lim et al., 2010). On the other hand, Rojas et al. established a UV sensor based on SnO2 nanofibers, which were fabricated by electrospinning of a PEO/SnO2 solution. Their nanofibers showed a direct response to UV light with faster response times upon exposure to longer wavelength light. In the presence of UV, the nanofibers’ conductance and mobility increased (Rojas et al., 2010).

18.5.1.2 Surface Modification of Metal Oxides (Doping) Surface modification of metal oxides is an effective method to improve the sensing properties of the sensors. Various catalysts, such as Pd, Ni, Co, Au, Nd, La, Sm, ZnO, and LaOCl, have been applied to modify the surface of metal oxides. For example, Noon et al. enhanced the sensitivity of nanocrystalline TiO2 nanofiber-based NO2 sensors by using Pd-doped TiO2 nanofiber mats, which were prepared by electrospinning followed by calcination at 600 C. Pd-doped TiO2 nanofibers showed promising gas-sensing characteristics, such as low operation temperature (180 C) and sufficient gas response (R/R0 ¼ 38 to 2.1 ppm NO2) (Moon et al., 2010). On the other hand, Nikan et al. (2013) studied the effects of ZnO on the response and selectivity of SnO2-based sensors for selective detection of C2H5OH and CO in the presence of CH4. ZnO-doped SnO2 fibers were synthesized by electrospinning and calcination. ZnO-doped SnO2 showed good selectively to detect both CO and ethanol in the presence of methane, with the highest responses (Nikan et al., 2013). Yang et al. (2011) prepared a highly volatile organic compound (VOC) (such as n-butanol) sensor based on Au-functionalized WO3 composite nanofibers. Compared with WO3 nanofibers, and Au/WO3 nanofibers, the obtained Au/WO3 nanofiber-based sensor exhibited improved and excellent sensing properties (detection limit of 100 ppm, short response and recovery time of 5 and 10 s) (Yang et al., 2015). Moreover, Xiong et al. prepared LaOCl-doped SnO2 nanofibers by a simple one-step electrospinning technique for the detection of CO2 gas in different oxygen-containing backgrounds. The sensor, based on LaOCleSnO2 nanofibers, exhibited optimal response toward CO2 at 300 C with response and recovery times of 24 and 92 s (Xiong et al., 2017). With a different structure, Wang et al. (2015) successfully synthesized ruptured Nd-doped In2O3 porous nanotubes by single-nozzle electrospinning and calcination to detect formaldehyde. The results showed excellent gas-sensing capability with improved sensitivity, by 3.5 times, and detecting limit (100 ppb) (Wang et al., 2017).

18.5.2 ORGANIC NANOFIBERS Organic nanofibers have attracted more and more attention for resistive sensor application as alternatives to metal oxide nanofibers. Organic nanofibers have great potential in the fabrication of

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low-cost, large-scale, lightweight, flexible sensors, due to their attractive properties such as mechanical flexibility, conjugated backbones, ease of use, and adjustable electrical conductivity. Gao et al. reported a simple approach to fabricating scalable flexible tactile sensors using a nanofiber assembly of regioregular poly(3-hexylthiophene) (P3HT) using electrospinning. The P3HT nanofiber assemblies were applied to detect pressure changes and bending angles by observing the resistance changes, with good repeatable responses (Gao et al., 2012). In another work, Pinto et al. electrospun PANI doped with camphorsulfonic acid and PEDOTePSS separately to obtain individual nanofibers, which were captured on Si/SiO2 substrates to use as sensors in the presence of various alcohol vapors. The nanofiber sensor responses were observed to be faster or comparable to those of thin-film sensors and had the potential to be used in detecting small amounts of gases (Pinto, 2013). On the other hand, Sun et al. fabricated aligned microfibrous arrays of PEDOT:PSSePVP with curledarchitecture-based stretchable strain sensors, using a novel reciprocating-type electrospinning setup with a spinneret in straightforward simple harmonic motion. By benefit of the curled architecture of the as-spun fibrous polymer arrays, the sensors could be stretched reversibly, with a linear elastic response to strain up to 4%, as shown in Fig. 18.11, which is three times higher than that from electrospun nonwoven mats (Sun et al., 2013).

18.5.3 HYBRID NANOFIBERS To avoid high operating temperature of the inorganic nanofibers and low conductivity of the organic nanofibers, organic/inorganic composite (hybrid) nanofiber-based sensing devices have been established to improve the sensitivity along with decreasing the operating temperature. Substantial research has been advanced on hybrid nanofiber-based sensors. Pang and coworkers prepared PA-6/TiO2 composite nanofibers as templates via an electrospinningeelectrospraying process. Subsequently, they fabricated PA-6/TiO2/PANI composite nanofibers using in situ polymerization of aniline, which exhibited great ability to detect ammonia with good reproducibility, selectivity, and apparent improvement in response compared with PA-6/ PANI composite nanofiber (Pang et al., 2014). Continuing their work, they prepared cellulose/TiO2 composite nanofibers using electrospinning and immobilization of TiO2 NPs onto the surface. Then, cellulose/TiO2/PANI composite nanofibers were applied to detect ammonia vapor at room temperature, and they showed high response values and sensitivity compared with cellulose/PANI composite nanofibers (Pang et al., 2016). Sharma et al. reported the fabrication of SnO2/PANI composite nanofibers for hydrogen gas sensing at low temperature. The SnO2/PANI composite nanofibers showed improved hydrogen gas sensing at near to room temperature compared with pristine SnO2 nanofibers (Sharma et al., 2015) (Fig. 18.12).

18.6 MASS-CHANGE-SENSITIVE SENSORS (QUARTZ CRYSTAL MICROBALANCE SENSORS) 18.6.1 MECHANISM In 1959, Sauerbrey demonstrated the dependence of quartz oscillation frequency on the change in surface mass. He coined the term QCM (quartz crystal microbalance) in the late 1950s, and it was his work that led to the use of quartz plate resonators as sensitive microbalances for thin films. When

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FIGURE 18.11 (A) characteristics of a strain sensor device based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonatee polyvinyl pyrrolidone microfibrous arrays at different strains. (B) The change in resistance of this device under different strains. (C) Current response of this device to a continuous strain from 0% to 4%. (D) Current response of this device to different external strains. Reprinted with permission from Sun, B., Long, Y-Z., Liu, S-L., Huang, Y-Y., Ma, J., Zhang, H-D., Shen, G., Xu, S., 2013. Fabrication of curled conducting polymer microfibrous arrays via a novel electrospinning method for stretchable strain sensors. Nanoscale 5, 7041e7045. © 2013 Elsevier Ltd.

voltage is applied to a quartz crystal causing it to oscillate at a specific frequency, the change in mass on the quartz surface is directly related to the change in frequency of the oscillating crystal; the massefrequency shift relation for quartz crystal resonators is as follows (Voinova et al., 2002; Reyes et al., 2017): . Df ¼ 2f02 Dm AðmrÞ1=2 (18.1) where Df is the measured frequency shift, f0 is the fundamental frequency of a bare QCM chip, Dm is the mass change per unit area, A is the electrode area, r is the density of quartz, and m is the shear modulus of quartz crystal (Fig. 18.13).

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FIGURE 18.12 (A) Schematic of the preparation of cellulose/TiO2/polyaniline (PANI) composite nanofibers. (B) Dynamic response of cellulose/PANI and cellulose/TiO2/PANI composite nanofibers to 10e250 ppm ammonia. (C) Sensitivity of SnO2/PANI composite nanofibers for H2 gas at 1000 and 2000 ppm. (A and B) Reprinted with permission from Pang, Z., Yang, Z., Chen, Y., Zhang, J., Wang, Q., Huang, F., Wei, Q., 2016. A room temperature ammonia gas sensor based on cellulose/TiO2/PANI composite nanofibers. Colloids and Surfaces A: Physicochemical and Engineering Aspects 494, 248e255. © 2016 Elsevier Ltd.

For the QCM, the most unique feature is that the change in frequency can determine the mass of analyte adsorbed in nanograms per square centimeter. Consequently, enormous materials such as metals, ceramics, polymers, self-assembled monolayers, lipids, and waxes have been used as sensitive coatings on a QCM to improve the sensor sensitivity and selectivity for chemical analytes (Jia et al., 2016b). One of the major challenge, that the flat electrode surface limits the immobilization degree of the absorbing sites per unit area, has accompanied the rapid development of QCM sensors (Jia et al., 2014). Driven by the actual need, increasing attention has been paid to the development of nanostructured coatings on QCMs to improve the sensor sensitivity, for instance, nanofibers (Jia et al., 2015). By taking advantage of the large SSA of the nanostructured sensing materials, the performance of the QCM sensors has been greatly enhanced. In 2004, Ding et al. (2004) for the first time combined electrospinning and QCM technology to prepare a highly sensitive ammonia sensor. A series of polyacrylic acid (PAA)/PVA nanofibers with

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FIGURE 18.13 A scheme to illustrate the working principle of the quartz crystal microbalance (QCM).

diameters of 100e400 nm and different composition ratios were electroplated on the surface of the QCM electrode to form a fiber-sensing film with the thickness of a micrometer. PAA is a weak anionic polyelectrolyte and can react with ammonia, but a large number of carboxyls in its molecular chain can make strong hydrogen bonds with water molecules, and thus pure PAA aqueous solution is difficult to electrospun. The use of water-soluble PVA with this compound can greatly improve the spinnability. Subsequently, the heat resistance of the composite PAA/PVA nanofiber film can be enhanced by thermal crosslinking treatment. As the results, the nanofiber with 11%, 18%, 25%, and 33% of PAA content in the mixed spinning solution showed the average resonance frequency of 40, 150, 240, and 380 Hz, respectively. In addition, the concentration of ammonia and the relative humidity in the detection tank will affect the detection sensitivity of the QCM. Compared with a smooth membrane, a nanofiber sensing film has a higher detection sensitivity, the main reason being its high SSA. In addition to the traditional one-dimensional (1D) nanofiber structure, the structure of PAA electrospun materials has made new progress. A novel 2D nanomaterial, nanonets, was developed for use as QCM sensors. Owing to the existence of the cobweb structure, the average diameter of the fibers is reduced to 264 nm, which is smaller than that of PAA/PVA blended nanofibers (743 nm). Compared with the SSA and porosity of PAA/PVA blended nanofibers (0.26 m2/g and 0.000154 cm3/g,

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FIGURE 18.14 Several nanonet structures generated from different polymer systems. (A) Poly(m-phenylene isophthalamide) (PMIA). (B) Polyacrylic acid (PAA). (C) Chitosan (CS). (D) Gelatin. (E) Polyurethane (PU). (F) Polyamide-6 (PA6).

respectively), the nanonets reached 3.26 m2/g and 0.006815 cm3/g, respectively. The above mentioned super properties make these materials a promising to apply as a gas sensor (Wang et al., 2011a,b). Many polymers have been fabricated into nanonets, which are displayed in Fig. 18.14. PAA nanonet coatings for QCM electrodes are also used in the detection of trimethylamine (TMA) (Wang et al., 2011a,b). By adding different amounts of NaCl into the PAA solution, the conductivity of the spinning solution can be adjusted to achieve fine regulation of the cobweb structure in the fiber membrane. When the addition amount of NaCl in the spinning solution is 0.1%, the optimum structure of the nanofiber web can be obtained. The results of the sensing test are shown in Fig. 18.15. The QCM sensor prepared with PAA/NaCl (0.1 wt%) showed response to 1, 10, 20, 50, and 100 ppm trimethylamine gas at 30, 274, 507, and 726 Hz. The sensor also showed a faster response speed, which was 180 s, and the frequency response reached 90% of the maximum response value. The fiber-sensing membrane response curves of PAA/NaCl (0.1 wt%) had the highest diffusion constant (Dc), which was consistent with the highest sensitivity of the sensor. The fibrous membrane structure had a large number of internal connections of the cobweb structure, increasing the SSA of the film, thereby promoting the TMA gas diffusion in the film movement.

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FIGURE 18.15 (A) Response of sensors coated with polyacrylic acid (PAA)eNaCl NMs containing different concentrations of NaCl (0, 0.1, and 0.2 wt%) and exposed to increasing trimethylamine (TMA) concentrations ranging from 1 to 100 ppm. The NMs were obtained at a voltage of 30 kV and relative humidity of 25%. The coating load was 3500 Hz for each sensor. The inset is the amplified image in the range of 0e2000 s. (B) Frequency shift versus TMA concentration (1e100 ppm) for three NM-coated quartz crystal microbalance (QCM) sensors. (C) Frequency shift of the PAAeNaCl (0.1 wt%) composite nanofibrous membranes (CNMs)-coated sensor with a coating load of 3500 Hz versus 20 ppm of various volatile organic compounds. The inset shows the schematic of the gas-sensing mechanism between PAA nanonets and TMA. (D) Dft/DfN (Dft/DfN < 0.5) as a function of t1/2/h when exposed to 1 ppm of TMA vapors for three NM-coated QCM sensors. The lines are linear fits to Fickian diffusion.

18.7 SUMMARY AND PERSPECTIVES With the continuous development of electrospinning technology, the application of electrospun nanofiber materials with unique properties in the field of sensors has received more and more attention. Through the fine regulation of nanofiber sensing membrane structure, the performance of the sensor,

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including sensitivity, response speed, recovery, repeatability, and stability, has been greatly improved. The electrospun nanomaterials with good structure have proven to be the ideal choice for replacing the widely used solid film materials and improving the performance of sensors, and they are expected to open a new way to prepare high-performance sensors. It is foreseeable that in the near future, on the basis of the current research, more and more highly sensitive and practical applications of electrostatic spun nanofiber sensors will be developed for human healthy life and social economic development. Moving forward, it is reasonable to believe the electrospun nanofiber-based sensors may throw a light on the development of future sensors.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51603034), the China Postdoctoral Science Foundation (2016M601473), the Fundamental Research Funds for the Central Universities (2232017D03), the 111 Project “Biomedical Textile Materials Science and Technology, B07024,” and KLTST201622.

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