key: cord-0040560-00nzo5ml
authors: Wang, X.F.; Ding, B.; Yu, J.Y.
title: Functional nanofibers in sensor applications
date: 2014-03-27
journal: Functional Nanofibers and their Applications
DOI: 10.1533/9780857095640.2.209
sha: 19c5172129d6498dcd5155b24e9dceac4ccf256c
doc_id: 40560
cord_uid: 00nzo5ml

Over the last decade, the interest in electrospun nanomaterials and their applications has increased. The fascinating and unparalleled properties of electrospun nanomaterials, such as large surface-to-volume ratios and high open porosity, have opened new and unexpected fields of application, especially in ultrasensitive sensors. By exploiting the inherent physical, electrical and mechanical properties of nanomaterials, it is possible to improve the performance of conventional sensors by increasing their sensitivity, selectivity, portability and power efficiency. In this chapter, the recent progress in the development of electrospun nanomaterials is reviewed. In particular, applications in some predominant sensing approaches, such as acoustic wave, resistive, photoelectric, optical and biological, are discussed.

© Woodhead Publishing Limited, 2012 Teo and Ramakrishna, 2006; Greiner and Wendorff, 2007; Agarwal et al ., 2009 ). To date, over 100 synthetic and natural polymers have been successfully electrospun into fi bers with a broad range of applications owing to their low dimensions and large surface areas.

Generally, electrospun fi bers are collected as nonwoven membranes with randomly arranged structures, which have greatly limited their applications in electronic devices or biomedical applications Agbenyega, 2008; Zhang and Chang, 2008; Liu et al ., 2010) . In order to fully realize the potential of electrospun fi bers, it is important to fabricate fi brous assemblies with controllable microstructures. Recently, several groups have demonstrated that electrospun nanofi bers could be collected as uniaxially aligned arrays by using specially designed collectors. For example, Li and coworkers (2003) have demonstrated that nanofi bers can be uniaxially aligned by introducing insulating gaps into conductive collectors. Matthews et al. (2002) have demonstrated that aligned fi bers could be obtained by using a rotating collector at high speed; the mandrel rotation speed and fi ber orientation strongly infl uence the properties of the electrospun nanofi bers. Zussman et al. (2003) have demonstrated the use of a wheel-like bobbin as the collector to position and align individual polymer nanofi bers into parallel arrays. However, because the edge of such a bobbin has to be relatively sharp, this technique does not seem to be feasible for forming well-aligned nanofi bers over large areas. In addition, it was possible to obtain various patterned architectures of the electrospun nanofi bers by varying the design of electrode pattern (Li et al ., 2005) . One of the most interesting features associated with this approach is that this technique enables direct integration of nanofi bers with controllable confi gurations into an electrode system such that the nanofi bers can be fabricated and aligned simultaneously, which will signifi cantly simplify the production of nanofi ber-based devices.

Different nanofi ber morphologies can be obtained via control of the processing conditions, enabling one to produce smooth as well as beaded (Lin et al ., 2010) and helical (Kessick and Tepper, 2004) structures (see Fig. 11 .1 a-c). Recent demonstrations from a number of groups indicate that this technique is also capable of generating nanofi bers with ribbon (Koombhongse et al ., 2001) , necklace-like (Jin et al ., 2010) , porous (Kanehata et al ., 2007; Ding et al ., 2008a) , core-shell (Sun et al ., 2003) , hollow (Dror et al ., 2007) , multichannel tubular (Zhao et al ., 2007) , nanowire-in-microtube , multicore cable-like (Kokubo et al ., 2007) and tube-in-tube (Mou et al ., 2010) structures (see Fig. 11 .1 d-m).

Compared with inorganic nanorods and carbon nanotubes, the electrospun nanofi bers are continuous, which possess them with high axial strength combined with extreme fl exibility. Yet, these assemblies would possess excellent © Woodhead Publishing Limited, 2012

11.1 Different morphologies of electrospun fi bers: (a) beaded, (b) smooth (Reprinted with permission from Lin et al . (2010) . © 2010 American Chemical Society), (c) helical (Reprinted with permission from Kessick and Tepper (2004) . © 2004 American Institute of Physics), (d) ribbon (Reprinted with permission from Koombhongse et al . (2001) . © 2001 Wiley), (e) necklace-like (Reprinted with permission from (Jin et al ., (2010 structural mechanical properties. Additionally, a multitude of functions can be incorporated into the fi bers and an extremely broad range of potential applications exists in which electrospun nanofi bers can make major contributions. These include not only textile, ultrafi ltration, tissue engineering, catalysis and mechanical reinforcement applications, but also extend to the fabrication of sensors, batteries and other types of devices. It would be meaningful to combine the electrospinning technique with controllable nanofi bers assembly for sensors fabrication, as this would provide a simple, rapid and cheap way to construct ultrasensitive devices. Although the use of nanofi bers offers the prospect of high sensitivity and rapid detection, the ability to incorporate nanofi bers into device architectures is limited by the diffi culty in manipulating and locating the nanostructures with respect to the microelectrodes.

Electrospun fi brous membranes typically have fi ber diameters in the range 100-500 nm, but properties such as surface area and porosity become more signifi cant when the fi ber diameter falls below 20 nm . The major challenge for developers is to come up with robust methods for manufacturing extremely small nanofi bers in large quantities and with a uniform size. We have found a procedure for generating net-like structured nanowires with small diameters (~20 nm) in three-dimensional (3-D) fi brous mats by optimization of various processing parameters during electrospinning (Ding et al ., 2006b) . This novel process, termed 'electronetting', allows one-step fabrication of ultrathin 'nanonets' with large quantities and uniform size. The electrospun fi bers act as a support for the soap-bubble-like structured nanonets comprising interlinked one-dimensional (1-D) nanowires (see Fig. 11 .2 ). The major distribution region (over 70%) of nanowire diameters was in the range of 10-30 nm, which was one order of magnitude less than that of common electrospun nanofi bers. The region of pore-width distribution of nanonets ranged from 20 to 550 nm, which was much less than that of pores among electrospun nanofi bers. Therefore, nanonets possess great potential for application in fi ltration systems for the removal of particles or viruses with a size to nanometer ranges. To date, our group has successfully prepared various nanonets based on different polymer systems such as nylon-6 (Ding et al ., 2006b) , polyacrylic acid (PAA) (Ding et al ., 2006b; Wang et al ., 2010b) , poly(vinyl alcohol) (PVA) (Ding et al ., 2008b and polyurethane (PU) (see Fig. 11 .2 ). The formation of the nanonets is considered to be due to the electrically forced fast phase separation of the charged droplets which move at high speed between the capillary tip and the collector. The formation, morphology and area density of the nanonets in electrospun fi brous membranes are strongly affected by the solution properties and several parameters in the process of electrospinning.

Nanonets exhibit several fascinating characteristics, such as extremely small fi ber diameter and pore width, notable specifi c surface area and high porosity, superior mechanical performances, which make them attractive candidates for use in a range of applications including ultrasensitive sensors and ultrafi ltration. Furthermore, nanonets have the potential to be used as ultrafi ne fi lters to intercept viruses and bacteria, such as infl uenza A (H1N1) virus, severe acute respiratory syndrome (SARS) virus, Escherichia coli , etc. . Clearly, there is growing interest in the process, but the results reported to date are primarily focused on the empirical production and the proposed uses of polymer nanofi bers. At the same time, thorough understanding of the mechanisms of the breakup of liquid jet and subsequent nanonets formation is needed for the development of robust methods of process control.

Development of electrospun nanomaterials, such as nanofi bers and nanowebs, provided researchers with an opportunity to construct electronic interfaces with components whose sizes are comparable to the size of molecules, potentially leading to a much more effi cient interface. Nanometer cross-sections of nanomaterials gives them enhanced surface sensitivity and allows them to utilize the benefi ts of size effects, such as quantization and singlemolecule sensitivity. The comparatively large surface area and high porosity make electrospun nanomaterials highly attractive candidates for use in a range of devices, including ultrasensitive sensors. This is one of the most desirable properties for improving the sensitivity of sensors because a larger surface area will absorb more analytes and change the sensor's signal more signifi cantly. A survey of open publications related to 'electrospinning and sensors' over the past ten years (at the time of writing) is given in Fig. 11 .3 . The rapid rise in data clearly demonstrates that this subject has attracted increasing attention recently.

The performance of a sensor is highly dependent on the confi guration, morphology, thickness and the composite ratio of the sensing materials. There have been a vast number of methods developed to fabricate nanostructured sensing materials. Either single-component electrospinning or composite/ doped electrospinning can be used to produce nanofi ber functionalized sensing materials. Within each technique, the structure of the material may be random or ordered. However, up to now, preparation of electrospun sensing materials has always involved complicated polymer syntheses which make the fabrication time consuming; sometimes the leakage of sensing molecules affects the sensing performance. More available systems and new fabrication approaches need exploring. Recently, some research has focused on fi bers with secondary porous structures. Moreover, it is a great challenge to immobilize the sensing elements onto the surface of the electrospun porous fi bers. This section describes the design and fabrication processes that have been developed to realize electrospinning nanostructured sensing materials.

Single-component electrospinning can be used to produce nanofi bers functionalized sensing materials. An interesting feature of this technique is that more sensing elements can be established on the surface of nanofi bers, as well as more absorption sites exposed to analytes. To date, more than 100 types of natural and synthetic polymer have been electrospun into onecomponent nanofi bers, such as poly(ethylene oxide), PVA, polystyrene (PS), polyacrylonitrile (PAN), polylactide, poly(ɛ-caprolactone), PU, polyamide, cellulose acetate (CA), poly(vinyl acetate) (PVAc) and many more (Huang et al ., 2003; Lu et al ., 2009) ; however, only few of them were used as sensing elements (Ding et al ., 2005b) . Besides polymer-based sensing materials, nanostructured ceramics also have potential applications in nanoscale electronics, optical and sensing devices due to their notable electronic properties. The fabrication of sensing materials can be implemented over single-component electrospinning, but the remaining hindrance is that some polymers or ceramics cannot obtain nanostructured morphology via electrospinning directly (Wang et al ., 2010a) . A promising approach to solve this problem is to use composite or doped electrospinning and even assemble these materials on the surface of nanomaterials.

© Woodhead Publishing Limited, 2012

Early work on electrospinning mainly dealt with conventional polymers that could be synthesized with suffi ciently high molecular weights and could be dissolved in appropriate solvents. In an effort to greatly expand the functionality and thus the scope of applications associated with fi brous structures, composite and doped electrospinning have recently been developed to generate nanofi bers with a range of chemical compositions, and therefore various electronic, magnetic, optical and biological properties.

In the past several decades, there have been many reports on the fabrication of composite nanofi bers via electrospinning, including polymer/ polymer, polymer/inorganic and inorganic/inorganic composites (Wang and Pan, 2008; Lu et al ., 2009) . Recently, a large number of polymer/metal-oxide composite fi bers have been produced by electrospinning in combination with sol-gel processes. In many cases, the composite fi bers could be converted into metal-oxide fi bers by subsequent calcination. Such composite fi bers allow the fabrication of polymer fi bers with special functionalities or of precursor fi bers. For instance, Wang and coworkers (2006) obtained pure WO 3 nanofi bers with controllable diameters of around 100 nm by electrospinning PVAc/tungsten isopropoxide solutions and subsequent calcinations (see Fig. 11 .4 a, b). The prepared WO 3 ceramic nanofi bers have a quick response to ammonia in various concentrations, suggesting potential applications of the electrospun WO 3 nanofi bers as a sensor material for gas detection ( Fig. 11.4 c) .

Pure and doped metal-oxide semiconductors (MOS) nanostructured materials, such as ZnO, SnO 2 and other wide band gap metal oxides, have recently attracted considerable interest due to their unique physical properties and a wide range of possible applications (Andersson et al ., 1998; Li et al ., 2000; Schmidt-Mende and MacManus-Driscoll, 2007) . However, most pure MOS are not stable in air and its electrical properties are significantly affected by adsorption of O 2 , CO 2 , hydrocarbons, S-containing compounds and water (Jimenez-Gonzalez, 1997). Therefore, it is highly desired to prepare 1-D MOS nanostructures doped with a selection of elements, such as Li, Ga, In, N, Al, Sn and P, to enhance and control their mechanical, electrical and optical performance. Electrospinning proved to be a simple, low-cost and reliable technique for dopant incorporation to attain modifi ed structural, electrical and optical properties for these semiconducting nanofi bers. Lotus and coworkers (2010) have investigated the effect of dopant addition on morphological, electrical and optical properties of semiconducting oxide nanofi bers obtained by electrospinning. Pure SnO 2 and SnO 2 polycrystalline electrospun nanofi bers doped with multiwall carbon nanotubes (MWCNTS) are synthesized by Yang et al. (2007) . The SnO 2 / MWCNTS composite nanofi bers exhibited enhanced sensitivity compared to pure SnO 2 nanofi bers. Doping with materials in the semiconductors may affect the electron transfer, which is considered in their investigation.

Another area that has been explored for the parallel immobilization of sensing materials over large areas of electrospun nanomaterials is the use of surface assembly. With bifunctional molecules an effi cient surface modifi cation can be achieved. A further development of this idea resulted in self-assembled monolayers (SAMs). Between neighboring adsorbed molecules, both hydrogen bonding and weak van der Waals forces are active (Tsukruk et al ., 1997) . The result is a densely packed layer of sensing molecules standing steeply at the surface. Interfaces functionalized by SAMs can be useful for electron transfer in the direct oxidation of organic substances, and they may also act as ion-selective surfaces. Assembly of sensing materials can also be driven by electrostatic interactions, resulting in layer-by-layer assembly (LBL) (Decher et al ., 1992; Huang et al ., 2001; Mamedov et al ., 2002; Hammond, 2004) . The LBL technique involves the alternate adsorption of anionic and cationic polyelectrolytes on a charged substrate by sequential dipping of the substrate into the appropriate aqueous polyelectrolyte solutions. This yields a highly tailored polymer thin fi lm on the substrate. This technique effectively coated the individual electrospun fi bers, thus leaving the inherent high surface area of the electrospun membrane intact (Ding et al ., 2004a (Ding et al ., , 2005a Ogawa et al ., 2007) . The LBL assembly technique has been successfully applied to sensor fabrication (Nohria et al ., 2006) . One of the amazing advantages of this technique is the versatility, in that a vast range of functional groups can be incorporated within the structure of the fi lm. Furthermore, this technique is not limited to planar substrates and has been demonstrated in LBL assembly onto 2-and 3-D architectures (Kato et al ., 2002) . The combination of electrospinning and LBL assembly with different electrospun substrates and sensing layers or functionalities will afford new properties and tremendous fl exibility for fabricating sensors. Wang et al. (2004) reported a sensitive optical sensor by assembling fl uorescent probes onto electrospun CA nanofi bers, which showed fl uorescence quenching upon exposure to even an extremely low concentration (ppb) of methyl viologen cytochrome in aqueous solutions (see Fig. 11 .5 ). There are also examples of self-assembling noble metal nanoparticles onto electrospun fi bers (Formo et al ., 2008; Huang et al ., 2008) . Li et al. (2006) have prepared silver nanoparticles deposited onto the surface of PAN nanofi bers by photoreducing the desired metal salt directly. And, the solution route is also a generally simple and attractive approach for the fabrication of noble metal nanoparticles with a great level of control for both the size and dispersity onto the electrospun fi bers. 

Quartz crystal microbalances (QCMs), one of a broad class of acoustic wave techniques, whose history dates back to 1880, when Pierre and Jacques Curie discovered the piezoelectric effect, where pressure exerted on a small piece of quartz causes an electrical potential difference between the deformed surfaces (Marx, 2003) . The essence of QCM as an analytical tool lies in its ability to detect small mass changes of even smaller than nanogram order deposited on crystal surface and to act as a sensor. Interfacial mass changes can be related to changes in the QCM oscillation frequency by applying Sauerbrey's equation (Sauerbrey, 1959) :

where Δ f is the measured frequency shift, f o is the fundamental frequency of a bare QCM chip, Δ m is the mass change per unit area, a is the electrode area, ρ is the density of quartz and µ is the shear modulus of quartz crystal.

In the past decade, QCMs have been widely explored as sensors for the detection of biomarkers, drug targets, virus capsids, bacteria and living cells. Up to now, a variety of materials such as metals, ceramics, polymers, selfassembled monolayers, dendrimers, lipids and waxes , Grate, 2000 have been employed as sensitive coatings on QCM to improve the sensor sensitivity and selectivity for chemical analytes. It is widely accepted that the sensitivity of QCM sensors toward a specifi c analyte is enhanced by increasing the specifi c surface area of the sensing materials (Kolmakov and Moskovits, 2004) . Recent efforts have been focused on the development of nanostructured coatings on QCM to improve the sensor sensitivity (Ding et al ., 2004b; Chao et al ., 2007) . We have demonstrated the possibilities of fabricating novel QCM sensors by electrospinning deposition of nanofi brous membranes as sensitive coatings on the electrodes of QCM. Taking advantage of the large specifi c surface area, high porosity and good interconnectivity of these electrospun fi brous assemblies, we have successfully detected NH 3 (Ding et al ., 2004b (Ding et al ., , 2005b , H 2 S (Ding et al ., 2006a) , formaldehyde (Wang et al ., 2010a) , moisture (Wang et al ., 2010b) and trimethylamine (TMA) (Wang et al ., 2011) with new detection limits at room temperature. Sensing experiments were carried out at room temperature in a fl ow-type gas testing system (Ding et al ., 2005b ) and a static-type gas testing system ( Fig. 11.6 a) by measuring the resonance frequency shifts of QCM due to the additional mass loading. The experiments indicated that the sensitivity of the fi brous membrane-coated QCM sensors was several © Woodhead Publishing Limited, 2012 times higher than that of the casting fi lm-coated QCM sensors. Moreover, the fi ber-coated QCM sensors with small average fi ber diameter had a higher sensitivity. One exciting and challenging aspect of the work of our group lies in the fabrication of a novel TMA sensor by using an electronetting technique to deposit a PAA nanonets on a QCM (Wang et al ., 2011) . The versatile nanonets created enhanced interconnectivity and additional surface area, and facilitated the diffusion of analytes into the membranes, which signifi cantly boosted the gas diffusion coeffi cient and sensing properties ( Fig. 11.6 b) . These proof-of-principle experiments demonstrate the possibility of using electrospinning techniques to regulate the structure of membranes and hence obtain high surface activity and gas sensitivity; further experiments using more complex fi ber arrangements and structures should expand the capabilities of this platform.

Surface acoustic wave (SAW) devices for gas detection have also attracted growing attention (Janata et al ., 1994, Crooks and . The change in electrical conductivity or mass of the sensing layer perturbs the velocity of the propagating SAWs due to mechanical and piezoelectric effects. Concentrated efforts have been focused on depositing nanostructured coatings on SAWs via electrospray or electrospinning technique to improve the sensor sensitivity (Sarkar et al ., 2006; He et al ., 2010; Li et al ., 2010) . For example, He et al. (2010) demonstrated that a novel H 2 sensor could be prepared by electrospinning deposition of poly(vinylpyrrolidone) (PVP)/ LiTaO 3 composite nanofi ber on the electrodes of SAWs. Upon exposure to H 2 , the PVP fi bers adsorb H 2 molecules due to the hydrogen bond and the mass increase. Therefore, the frequency of the SAW device decreases as the adsorbed molecules reduce the acoustic wave velocity ( Fig. 11.7 ) . This indicates that the electrospinning technique can afford a simple approach for the fabrication of composite nanomaterials for sensing technology.

Commonly, resistance changes caused by the interaction between electrodes and analytical samples can be obtained by measuring the electrical conductivity (or resistivity) and evaluating to extract analytical results. Signifi cant progress has been achieved in developing highly sensitive MOS resistive sensors using novel 1-D nanostructured architectures such as TiO 2 , SnO 2 , ZnO, In 2 O 3 , WO 3 and other wide band gap metal oxides. The fundamental principle of these devices is associated primarily with the adsorption of the chemisorbed species on the surface of the MOS inducing electric charge transport between the two materials, which changes the resistance of the oxide. 1-D sensing architectures provide unparalleled advantages in terms of facilitating fast mass transfer of the analyte Injector PAA/NaCl nanonets Fan 11.6 (a) Schematic of a gas testing system for TMA detection. (b) Response of sensors coated with PAA-NaCl nanonets containing different concentrations of NaCl (0 (dotted line), 0.1 (black line), and 0.2 wt% (gray line) and exposed to increasing TMA concentrations ranging from 1 to 100 ppm. The inset shows the schematic of gas-sensing mechanism between PAA nanonets and TMA. (Reprinted with permission from Wang et al . (2011) . © 2011 Royal Society of Chemistry.) © Woodhead Publishing Limited, 2012 molecules to and from the interaction region, as well as requiring charge carriers to traverse any barriers introduced by molecular recognition events along the entire wire. Among the variety of methods for the preparation of such nanostructured architectures, electrospinning is one of the most simple, versatile and cost-effective approaches, offering the ability to produce long, continuous organic or inorganic nanofi bers with potential for alignment and spooling.

So far, many attempts are carried out to prepare ultrasensitive resistive sensors to detect NH 3 , H 2 S, CO, NO 2 , O 2 , CO 2 , moisture and volatile organic compound (CH 3 OH, C 2 H 5 OH, C 5 H 10 Cl 2 , C 6 H 5 CH 3 , C 4 H 8 O, CHCl 3 , C 2 H 2 Cl 2 , C 3 H 6 O, C 3 H 7 NO, C 2 HCl 3 , N 2 H 4 , (C 2 H 5 ) 3 N, C 6 H 14 , etc.) vapors with new detection limits using electrospun nanofi ber functionalized MOS such as TiO 2 , ZnO, WO 3 , MoO 3 , SnO 2 , In 2 O 3 and ITO (Choi et al ., 2009; Qi et al ., 2009; Wang et al ., 2009; Zhang et al ., 2009) . Wang et al. (2010c) studied the H 2 sensing properties of p-NiO/n-SnO 2 composite nanofi bers (NSNFs) synthesized through an electrospinning method ( Fig. 11.8 ) . The sensor response was measured between 260°C and 380°C by comparing the resistance of the sensor in dry synthetic air with that in target gases. Extremely fast response-recovery behavior (~3s) has been obtained at the operable temperature of 320°C, based on the prepared sensor, with the detection limit of approximate 5 ppm H 2 . Many applications of electrospun nanofi bers could be greatly improved by increasing the surface area and porosity of the fi bers. 

Recently, metallic nanoparticles encapsulated into dielectric matrices have been considered to have practical applications in photoelectric devices owing to their enhanced third-order nonlinear susceptibility (Haglund et al ., 1993) . The enhancement of photoresponse behaviors arising from the excitation of the surface plasmon is attributed to the generation of hot electrons. The generated hot electrons drift or diffuse to the oxide barrier and tunnel to the counterelectrode. Generation of hot electrons and electron tunneling through the oxide nanowires may contribute to the enhanced photoresponse behavior in the hybrid nanowire (Hu et al ., 2006) . Recently, Shi and coworkers (2009) have developed a method to fabricate Au nanoparticles (AuNPs) embedded in silica nanofi bers via electrospinning and thermal decomposition of hybrid nanofi bers, which exhibited an obvious photoelectric response under illumination ( Fig.  11.9 a) . Photoelectric response measurements were conducted under dark condition or under alternate light illuminations of different wavelengths ( k = 410, 550 or 680 nm). Data were obtained by an electrochemical analyzer with applied constant voltage of 10 V every 5 min. Upon light illumination, the hybrid peapod nanofi bers presented a wavelength-dependent photoelectric current response. Particularly, the current photoresponse reached a maximum value under illumination at a wavelength of 550 nm, which was close to the surface-plasmon-resonance absorption band of the AuNPs nanofi bers ( Fig. 11.9 b) . These results show the potential of using gold nanopeapodded silica nanowires as wavelength-controlled optical nanoswitches. The microreactor approach can be applied to the preparation of a range of hybrid metal-dielectric 1-D nanostructures that can be 

Optical sensor-based electrospun nanomaterials can be grouped into three general types: (a) quenching-based fl uorescent optical sensors; (b) colorimetric sensors; and (c) Fourier transform infrared (FTIR) spectroscopy optical sensors. For quenching-based fl uorescent optical sensors, utilizing the fl uorescence quenching of the sensing material against the targeted chemical molecules, the sensitivity of the device can be dramatically affected by the accessibility of the sensing elements to the quencher or analyte. Electron-defi cient metal cations such as Fe 3+ or Hg 2+ and nitro aromatic compounds such as 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene can serve as quenchers for a fl uorophore (Wang et al ., 2002b (Wang et al ., , 2002c . The pyrene methanol (PM) (Wang et al ., 2002a) and conjugated polymers (Long et al ., 2009) were chosen as fl uorescent indicators due to their large Stokes shift, high quantum yield, strong absorbance, excellent photostability and lifetime. Here the quenching of fl uorescence is due to the interactions of an electron-rich indicator and electron-defi cient quencher, and the degree of quenching depends on the amount of analyte. Electrospinning was used as a novel and simple method to fabricate fl uorescent optical sensors. Wang and coworkers (2002b) have successfully developed nanofi brous PAA-PM membrane optical sensors for metal ion (Fe 3+ or Hg 2+ ) and DNT detection using the electrospinning technique. Recently, electrospun nanofi brous PS fi lm doped with a fl uorescent conjugated polymer was developed as a sensory device for detection of the explosive DNT ( Fig. 11 .10 a, b) (Long et al ., 2009) . Polymer acting as a fl uorescence probe presented relatively large affi nity for the nitroaromatic compound DNT, which is mainly based on photoinduced charge transfer and ϖ-ϖ stacking interaction and could provide a strong driving force for fast fl uorescence quenching. Second, the electrospun nanofi brous sensing fi lm possessed remarkable specifi c surface area, in which more recognition elements were located on the surface. Lv et al. (2010) reported a method for the colorimetric visualization of trace amount of HCl gas using electrospun porphyrinated polyimide (PPI) nanofi brous membrane as schematically illustrated in Fig. 11 .10 c. The dual chromo-and fl uorogenic responses of the nanofi brous membrane upon exposure to HCl gas are interpreted in terms of the out-of-plane distortion of porphyrin macrocycle, which ultimately affects its optical properties. Exposing the PPI nanofi brous membrane to HCl gas for only 10 s generates a fast color change (from reddish russet to green) on the sensor surface, which can be ascribed to the protonation of the neutral porphyrin moieties © Woodhead Publishing Limited, 2012 in PPI . It is notable that the intensity of green color increases with the concentration of HCl gas (5-100 ppm). Additionally, the reusability of this PPI nanofi brous membrane sensor is proved by color recovery after puffi ng with N 2 for a certain time. FTIR spectroscopy is another widely used optical method to study the interaction of electromagnetic radiation in the infrared region with chemical compounds. The mechanism of FTIR sensor is based on the fact that different functional groups within a compound absorb radiation at different frequencies and yield unique infrared absorbance spectra. The large band width (400-4500 cm −1 ) and the distinct absorbance bands render FTIR technology suitable for sensor applications. Hahn et al. (Luoh and Hahn, 2006) fi rst utilized the electrospun nanocomposite PAN/oxide fi ber mats as optical sensors in conjunction with FTIR spectroscopy to detect CO 2 gas. The absorbance spectra showed a higher sensitivity with a fi ber mat, regardless of its type, than without, indicating gas adsorption on the fi ber mat.

Biosensors are becoming essential in the fi elds of health care, chemical and biological analysis, environmental monitoring and good processing industries (Ahmad et al ., 2010) . Electrospinning is a versatile method by which to fabricate biocompatible nanomaterials. The novel nanostructure increases the surface area and mass transfer rate, which signifi cantly improves the biochemical binding effect and sensor signal to noise ratio. The past several decades have witnessed big progress on fabricating biological devices for fast and reliable monitoring of biological targets.

Up to now, most biological sensors-based electrospinning techniques were used to detect glucose. Among those devices, amperometric glucose biosensors, with glucose oxidase (GO x ) and without GO x , have been an intensively investigated research area because a low detection limit can be achieved easily. Ren et al. (2006) have developed an amperometric biosensor by electrospinning deposition of nanofi brous GO x /PVA membranes as sensitive coatings on the surface of the Au electrode. Chronoamperometric measurements demonstrated that electrospun fi brous enzymatic electrodes exhibited a rapid response (1 s) and a higher response current (l µA level) to glucose in the normal and diabetic level. The linear response range (from 1 to 10 mM) and the lower detection limit (0.05 mM) of the sensor are satisfying. The electrospun method makes it convenient and effi cient to prepare the enzymatic electrode for biosensors.

Apart from glucose biosensors, Luo et al. (2010) utilized the surface functionalized electrospun nitrocellulose nanofi bers as a direct-charge-transfer biosensor to detect E. coli O157:H7 and bovine viral diarrhea virus (BVDV) cells. The biosensor was assembled by attaching the three membrane pads (application pad, capture pad and absorption pad) onto the polyvinylidene chloride substrate as shown in Fig. 11 .11 a. Figure 11 .11 b shows the pathogen detection principle of the biosensor. The electrospun biosensor exhibits a linear response to both microbial samples, the E. coli O157:H7 and BVDV. The detection time of the biosensor is 8 min, and the detection limit is 61 CFU/mL and 103 CCID/mL for bacterial and viral samples, respectively (see Fig. 11 .11 c, d). With the advantage of effi cient antibody functionalization, excellent capillary capability and relatively low cost, the electrospinning process and surface functionalization method can be implemented to produce nanofi brous capture membranes for different immunodetection applications.

Electrospinning is a versatile method of creating high-functional and highperformance nanofi bers that can revolutionize the world of structural materials. It is obvious that the current research has focused on seeking the possible applications of these resultant nanofi bers with broad functionalities. The utilization of electrospun nanofi bers with a large specifi c surface as sensing materials has received great attention since 2004. The fi ne structure of electrospun fi bers makes them excellent candidates to replace the current widely used solid fl at membranes, promising to further increase sensor sensitivity; this opens a new way to fabricate ultrasensitive sensors . It is important to note that the application of electrospun nanomaterials for sensors still faces many challenges. Preparation of sensing materials in the form of nanostructures may signifi cantly improve their performances in the existing devices or open doors to new types of applications. However, integration of nanomaterials into sensors requires materials of well-controlled orientation, size and other target characteristics, as well as reproducibility locating them in specifi c positions and orientations. The ability to achieve this, however, remains a major challenge in the fi eld (Frenot and Chronakis, 2003) . Fortunately, several approaches have emerged that promise to bridge these gaps and realize the vision of upgraded nanosensors. Given the versatility of electrospinning for generating fi brous membranes with various nanostructures, it is likely that electrospinning will be one of the most signifi cant nanotechnologies in the fabrication of more and more ultrasensitive and practical sensors. 

Electrospinning of manmade and biopolymer nanofi bers-progress in techniques, materials, and applications

Electrospinning has nanofi bers in alignment

A single ZnO nanofi ber-based highly sensitive amperometric glucose biosensor

Fluorine tin oxide as an alternative to indium tin oxide in polymer LEDs

Quartz crystal microbalance sensor based on nano structured IrO 2

Nanowire-inmicrotube structured core/shell fi bers via multifl uidic coaxial electrospinning

Hollow ZnO nanofi bers fabricated using electrospun polymer templates and their electronic transport properties

New organic materials suitable for use in chemical sensor arrays

Buildup of ultrathin multilayer fi lms by a self-assembly process: III. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces

Polyoxometalate nanotubes from layer-by-layer coating and thermal removal of electrospun nanofi bres

Electrospun nanofi brous polyelectrolytes membranes as high sensitive coatings for QCM-based gas sensors

Electrospun nanofibrous membranes coated quartz crystal microbalance as gas sensor for NH 3 detection

Layer-by-layer structured fi lms of TiO 2 nanoparticles and poly(acrylic acid) on electrospun nanofi bres

Formation of novel 2D polymer nanowebs via electrospinning

Biomimetic super-hydrophobic micro/nanoporous fi brous mat surfaces via electrospinning

Fabrication of a super-hydrophobic nanofi brous zinc oxide fi lm surface by electrospinning

Electrospun nanomaterials for ultrasensitive sensors

Gas sensors based on electrospun nanofi bers

Electrospun fi brous polyacrylic acid membrane-based gas sensors

Electrospinning process and application of electrospun fi bers

One-step production of polymeric microtubes by co-electrospinning

Beaded nanofi bers formed during electrospinning

Process and apparatus for preparing artifi cial threads

Functionalization of electrospun TiO 2 nanofi bers with Pt nanoparticles and nanowires for catalytic applications

Polymer nanofi bers assembled by electrospinning

Acoustic wave microsensor arrays for vapor sensing

Electrospinning: A fascinating method for the preparation of ultrathin fi bres

Picosecond nonlinear optical response of a Cu:silica nanocluster composite

Form and function in multilayer assembly: New applications at the nanoscale

Electrospun PVP fi bers and gas sensing properties of PVP/36 YX LiTaO 3 SAW device

Photosensitive gold-nanoparticle-embedded dielectric nanowires

Electrospun polymer nanofi bres with small diameters

Electrospun palladium nanoparticle-loaded carbon nanofi bers and their electrocatalytic activities towards hydrogen peroxide and NADH

Directed assembly of one-dimensional nanostructures into functional networks

A review on polymer nanofi bers by electrospinning and their applications in nanocomposites

Chemical sensors

Modifi cation of ZnO thin fi lms by Ni, Cu, and Cd doping

Fabrication of necklace-like structures via electrospinning

Nanoporous ultra-high specifi c surface inorganic fi bres

Thin multilayer fi lms of weak polyelectrolytes on colloid particles

Microscale polymeric helical structures produced by electrospinning

Multicore cable-like TiO 2 nanofi brous membranes for dye-sensitized solar cells

Chemical sensing and catalysis by one-dimensional metal-oxide nanostructures

Flat polymer ribbons and other shapes by electrospinning

Electrospinning of nanofi bers: Reinventing the wheel?

Collecting electrospun nanofibers with patterned electrodes

Electrospinning of polymeric and ceramic nanofi bers as uniaxially aligned arrays

A surface acoustic wave humidity sensor based on electrosprayed silicon-containing polyelectrolyte

Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties

Facile synthesis of single-crystal and controllable sized silver nanoparticles on the surfaces of polyacrylonitrile nanofi bres

Direct fabrication of highly nanoporous polystyrene fi bers via electrospinning

Magnetic-fi eld-assisted electrospinning of aligned straight and wavy polymeric nanofi bers

Electrospun nanofi brous fi lm doped with a conjugated polymer for DNT fl uorescence sensor

Effect of aluminum oxide doping on the structural, electrical, and optical properties of zinc oxide (AOZO) nanofi bers synthesized by electrospinning

One-dimensional composite nanomaterials: Synthesis by electrospinning and their applications

Surface functionalization of electrospun nanofi bers for detecting E. coli O157:H7 and BVDV cells in a direct-charge transfer biosensor

Electrospun nanocomposite fi ber mats as gas sensors

Colorimetric and fl uorescent sensor constructing from the nanofi brous membrane of porphyrinated polyimide for the detection of hydrogen chloride gas

Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites

Quartz crystal microbalance: A useful tool for studying thin polymer fi lms and complex biomolecular systems at the solution-surface interface

Electrospinning of collagen nanofi bers

Oriented contraction: A facile nonequilibrium heat-treatment approach for fabrication of maghemite fi ber-in-tube and tube-in-tube nanostructures

Humidity sensor based on ultrathin polyaniline fi lm deposited using layer-bylayer nano-assembly

Super-hydrophobic surfaces of layer-by-layer structured fi lm-coated electrospun nanofi brous membranes

Synthesis and toluene sensing properties of SnO 2 nanofi bers

Electrospun nanofi bers: Solving global issues, Materials Today

Electrospun poly(vinyl alcohol)/glucose oxidase biocomposite membranes for biosensor applications

Nanometre diameter fi bres of polymer, produced by electrospinning

Surface acoustic wave chemical sensor arrays: New chemically sensitive interfaces combined with novel cluster analysis to detect volatile organic compounds and mixtures

Deposition of polymer coatings onto SAW resonators using AC electrospray

The use of quartz oscillators for weighing thin layers and for microweighing

ZnO-nanostructures, defects, and devices, Materials Today

The fabrication of photosensitive self-assembly Au nanoparticles embedded in silica nanofi bers by electrospinning

Compound core-shell polymer nanofi bers by co-electrospinning

A review on electrospinning design and nanofi bre assemblies

Self-assembled multilayer fi lms from dendrimers

Fabrication and characterization of polycrystalline WO 3 nanofi bers and their application for ammonia sensing

Predictions of effective physical properties of complex multiphase materials

Nanofi brous polyethyleneimine membranes as sensitive coatings for quartz crystal microbalancebased formaldehyde sensors

Electro-netting: Fabrication of two-dimensional nano-nets for highly sensitive trimethylamine sensing

A highly sensitive humidity sensor based on a nanofi brous membrane coated quartz crystal microbalance

Electrospinning technology: A novel approach to sensor application

Electrospun nanofi brous membranes for highly sensitive optical sensors

Electrostatic assembly of conjugated polymer thin layers on electrospun nanofi brous membranes for biosensors

Highly sensitive optical sensors using electrospun polymeric nanofibrous membranes

Preparation, characterization and sensitive gas sensing of conductive core-sheath TiO 2 -PEDOT nanocables

heterojunction composite nanofi bers

Room temperature gas sensing properties of SnO 2 /multiwall-carbon-nanotube composite nanofibers

Electrospinning of three-dimensional nanofi brous tubes with controllable architectures

ZnO hollow nanofi bers: Fabrication from facile single capillary electrospinning and applications in gas sensors

Bio-mimic multichannel microtubes by a facile method

Formation of nanofi ber crossbars in electrospinning