Electrospinning: a facile technique for fabricating polymeric nano ﬁ bers doped with carbon nanotubes and metallic nanoparticles for sensor applications

The increased interest in electrospinning (ES) and its recent applications for fabrication of sensors and biosensors is driven by the development of materials science and nanotechnology. Compared with other fabrication processes, ES is versatile and superior for producing and constructing ordered and complex nano ﬁ brous materials. The introduction of carbon nanotubes (CNTs) and metallic nanoparticles (MNPs) into the electrospun polymeric nano ﬁ bers (NFs) extends their potential applications as electrical and electrochemical sensors and biosensors. In this review, we summarize the recent progress using the ES technique to fabricate di ﬀ erent polymeric NFs doped with CNTs and various MNPs, as well as their applications for detecting alcohols, H 2 S, H 2 , glucose, H 2 O 2 , and urea. The fabrication, intrinsic fundamentals, and optimization design of the sensors were introduced and discussed in detail. In addition, the improvements and challenges of ES techniques were mentioned. It is expected that this review will promote development in the ES ﬁ eld and guide studies to create nano ﬁ brous hybrid materials as novel sensors and biosensors.


Introduction
Electrospinning (ES) is a technique to spin polymeric bers and nanobers (NFs) by using electrostatic forces, which was rstly introduced by Anton Formhals in 1934. 1 However, it did not attract much attention until the 1990s. The neglect of this technique by researchers is ascribed to two possible reasons. One reason is that the high-voltage electrostatic generation technique was not very mature in the earlier years, and the other is that the small-size effect of the electrospun NFs had not been understood before the development of nanotechnology. With the help of nanotechnology, ES technique was recovered by Darrell et al. in 1996, who found that it is a very simple and powerful technique to generate ultrane polymeric bers with diameters ranging from micrometer to nanometer. 2 Since then, ES technique has been widely utilized for academic research and industry applications.
The standard setup for ES consists of a spinneret with a metallic needle, a syringe pump, a high-voltage power supply, and a grounded collector, as shown in Fig. 1a. A solution of polymer, sol-gel, or composite is loaded into the syringe and this viscous liquid is driven to the needle tip by a syringe pump, forming a droplet at the tip. [3][4][5] When a high voltage is applied to the metallic needle, the droplet is rst stretched into a structure called Taylor cone and nally into an electried jet. The jet is then elongated and whipped continuously by electrostatic repulsion until it is deposited onto the collector. The elongation with instable bending results in the formation of uniform bers and NFs.
Previously we made a new promotion of ES technique by adding a pair of parallel auxiliary electrodes and a mobile platform to get not only well-oriented NF alignment but also controllable NF placement in the large-scale NF mats for extensive applications in electrochemical sensing. [6][7][8][9] Fig. 1b and c show the optical image and schematic presentation of our home-made ES apparatus. The rotating drum collector, the positive high-voltage supply, and the liquid propulsion system are components of the standard ES congurations. During a conventional ES process, the polymer jet exhibits bending instabilities due to the repulsive forces between the charges carried within the jet, so that the electrospun bers always result in a random orientation, which restricted the applications of ES technique for preparing new functional materials. To solve this problem, we added a pair of parallel auxiliary electrodes between the spinneret and drum collector, and replaced the common rotating drum collector with a high speed one. 6 The two symmetric, rectangular, and auxiliary aluminum electrodes were operated at a high voltage of the same polarity as the needle but with an adjustable potential controlled by another independent high-voltage power supply. The alignment of polymeric NFs can be improved by constraining the bending instability, which is thought to be the main reason to cause random orientation. Besides NF mesh without orientation, other NF structures (e.g., parallel and crossed ber arrays, helical or wavy bers, twisted ber yarns, patterned ber web, and 3D brous stacks) can also be fabricated with the modied ES technique. 6,[10][11][12][13][14] In the past years, a variety of microbers and NFs from a remarkable range of organic and inorganic materials have been prepared with ES technique successfully. The specic physical and chemical properties of the created bers make them very versatile for different applications such as optoelectronics, sensors, catalysis, textiles, lters, ber reinforcement, tissue engineering, drug delivery, and wound healing. 8,9,[15][16][17][18][19][20][21][22] There are a variety of parameters that inuence the morphology and diameter of the electrospun bers, such as the intrinsic properties of solution, operational conditions, humidity, and temperature.
Carbon nanotubes (CNTs) have attracted great attentions due to their excellent mechanical, electrical, chemical, and thermal properties. 23,24 By introducing a small amount of CNTs, either in the form of small bundles or individuals, into the electrospun polymeric NFs, the mechanical strength, conductivity, and thermal resistance of the nal NF-CNT composites can be obviously improved compared to those of the pristine polymeric NFs. 25,26 Therefore, the idea of dispersing and aligning CNTs in a NF matrix to form composite materials seems to be very promising. CNTs offer unique advantages including enhanced electronic properties, a large edge plane/basal plane ratio, and rapid electrode kinetics, making them very useful for fabricating functional devices and sensors. 27,28 It is well known that the CNT-based sensors and biosensors generally have higher sensitivities, lower detection limit, and faster electron transfer kinetics than the traditional carbon electrodes. 29 Nanoparticles (NPs), especially metallic nanoparticles (MNPs) with nanoscale size, have unique electrical, optical, magnetic, and chemical properties, and their potential applications in the elds of nanotechnology, materials science, biomedical engineering, and tissue engineering have been widely explored. [30][31][32][33][34] It is well known that the size and shape of NPs, as well as the hybridization of NPs with other components are the key factors for determining, revealing and enhancing the functions of NPbased materials. 33,[35][36][37] In principle, one can accurately tune the physical and chemical properties of NPs by controlling any one Gang Wei is currently a postdoctoral researcher and group leader in the Hybrid Materials Interfaces Group at the University of Bremen, Germany. His research interests include carbon-based nanomaterials, nanobrous biomaterials, biosensors, and single-molecule force spectroscopy. He has published 55 papers in international peer reviewed journals and the published papers have been cited nearly 800 times with an h-index of 18.
of these parameters, but the exibility and scope of change are highly sensitive to some specic parameters. For instance, gold (Au) NPs have size-dependent surface plasmon resonance (SPR) property and generally exhibit visible SPR absorption, whereas Au nanorods, nanocage and hollow nanospheres own strong near-infrared (NIR) absorption. [38][39][40] These novel Au nanostructures with NIR absorption are very important for photothermal therapy and bio-imaging in the NIR region because blood and so tissue in the NIR region are relatively transparent, therefore the collateral damage to surrounding healthy tissue is minimized. Platinum (Pt) NPs with high-index facets or multi-compositions have been proven to exhibit higher electrocatalytic activities toward the oxidation and oxygen reduction reactions of small molecules than some commercial catalysts. [41][42][43] Silver (Ag) NPs with proper size, complex sharp, more edges and corners have higher surface-enhanced Raman scattering activity than the spherical Ag NPs. 44 Therefore, the control of these pivotal parameters of NPs makes it possible to enhance their potential applications in catalysis, electronics, photography, photonics, sensing, imaging, medicine and information storage. [30][31][32][33]35,37,39,43 Sensing with nano-system is a rising interdisciplinary topic, which combines the advantages of traditional analytical techniques (e.g., high sensitivity, rapid detection, and low cost, etc.) with the unique electronic, optical, magnetic, mechanical, and catalytic properties of NPs. 45,46 NP-based electro-analytical techniques show great potentials to construct different platforms for chemical sensing and biosensing. NPs can effectively catalyze the redox processes of some molecules due to their high conductivity, large surface area, and unique surface property, permitting an obvious improvement of analytical performances with lower detection limit and shorter deposition time. 47,48 The interest in ES technique to fabricate functional nanomaterials for sensing is growing fast. A survey of open publications related with "electrospinning" and "sensor" in the past 10 years with SciFinder Scholar was performed and the result is shown in Fig. 2. Meanwhile, the applications of the electrospun nanomaterials for fabricating electrochemical sensors and biosensors are still at an early but promising stage. In this review, we outline the representative progress in fabricating polymeric NFs doped with CNTs and NPs by ES technique. The sensor applications of the electrospun NF-CNT, NF-MNP, and NF-CNT-MNP composites were introduced in detail. Finally, some personal perspectives in this area were given.

Electrospun polymer NFs
Electrospun NFs are featured with very small diameters, extremely long length, large surface area per unit mass, and small pore size. Fibers with different morphologies, like uniaxially and biaxially aligned NF meshes, 6 ribbon, 49 porous bers, 50 Necklace-like, 51 nanowebs, 52 hollow, 53 nanowire-inmicrotube, 54 and multichannel tubular, 55 have been created by controlling the ES parameters (Fig. 3). For example, recently we reported the orientation of poly(ethylene oxide) (PEO) NFs by an advanced ES technique. 6 Fig. 3a and b show the uniaxially and biaxially oriented NF assemblies created by our home-made apparatus, respectively. Furthermore, we created both uniaxially oriented NF mats with different NF density and biaxially oriented NF mats with different layers by adjusting the spinning time, movement speed, and auxiliary voltage.
Meanwhile, ultrathin polystyrene (PS) bers with porous structures on their surfaces have been successfully fabricated by Zhao's group. 50 The porous structure distributed on the brous surfaces was thought to be caused by the volatile solvent. Also, they obtained knotted bers by coaxial ES. 56 In their study, PS  acts as supporting ber on which balsam-pear-like poly(methyl methacrylate) knots are distributed for Rayleigh instability effect. Such spider-silk-like structures exhibit fog harvesting capacity. During the process of solvent evaporation, phase separation and selective removal of a certain phase from the mixed polymers, the bimodal and spinodal structures have been acquired. 57 Thanks to the two great advances in ES, namely the coaxial 58,59 and side-by-side ES techniques, 58,60 not only the multi-level structures on the NF surface but also NFs with controllable inner multi-level structures have been fabricated successfully. Compared to other methods, it is attractive for the fabrication of a number of core-shell or anisotropic materials, since ES is a simple and efficient method. Notable examples include coaxial, microuidic manifold, and triple-layer coaxial structures. 61,62 For instance, Lahann and Bhaskar prepared multi-compartmental bers via co-jetting ES using two or more poly(lactide-co-glycolide) polymers, which yielded biodegradable micro-bers with multiple chemically distinct compartments. 63 The individual compartments can be identied from their chemical compositions, which are controlled by the chemical composition of the initial jetting solutions. Confocal laser scanning microscopy study revealed the extremely wellshaped multi-bers and the clear boundaries between those compartments in the bers. 63 Recently, two-dimensional (2D) nanowebs ( Fig. 3f) have been generated by optimizing the processing parameters in ES technique. 64 The electrospun micro-bers act as a support for the network nanowebs with interlinked 1D NFs. The mean diameter of the NFs contained in the typical nanowebs is about one order of magnitude less than that of conventional electrospun bers. Until now, various nanowebs have been successfully prepared using different polymer systems such as nylon-6, 64 polyacrylic acid (PAA), 64,65 and PAA/nylon-6. 52 The formation, morphology, and density of the nanowebs in the electrospun brous membranes are strongly affected by applied voltage, relative humidity, used solvents, solution concentration, and distance between the capillary tip and the collector.
Three-dimensional (3D) brous macro-structures can be effectively fabricated by ES technique. The most direct way is to create the multilayered NF structures by the sequential ES technique. By this way, brous membrane with a certain thickness up to hundreds of microns can be obtained, although this strategy may take a long time (usually from 20 min to 20 h) to achieve a sufficient 3D structure. Another way is postprocessing of the electrospun bers, such as peeling off the electrospun thin lm from the collector, and then bending or stacking the ber layers into a 3D brous structure like pipe or thick mat. 66,67 3D NF structures can also be successfully obtained by replacing the conventional 2D at collector by a 3D one. 68,69

Electrospun polymer NFs doped with CNTs
It is well known that the lling of CNTs into the electrospun polymeric NFs can greatly inuence the mechanical, electrical, and thermal properties of NFs, but it is not so easy to align CNTs along the axis of the created NFs. To maximize the reinforcement of CNTs in polymer composites, the alignment and dispersion of CNTs in NFs are two important factors should be considered. Recently, CNT-lled NF membranes have been produced using ES technique to improve electrical characteristics. [70][71][72][73][74][75][76][77] In 2004, Reneker's group for the rst time found that the orientation of MWCNTs within the electrospun NFs was much higher than that within the polyacrylonitrile (PAN) polymer crystal matrix (Fig. 4a). 78 This nding suggests that not only the surface tension and jet elongation but also the slow relaxation of MWCNTs in NFs are the key factors for the orientation of MWCNTs. As a result of the highly anisotropic orientation of MWCNTS in the polymer NFs, the fabricated PAN-MWCNT nanobrous mats possessed enhanced properties like electrical conductivity, mechanical strength, thermal stability, and dimensional stability. With natural polymer, Jin's group prepared silk broin bers, 79 where MWCNTs were embedded and well aligned (Fig. 4b). Meanwhile, Haddon's group studied SWCNT reinforced polymer composite membranes using ES technique. 80 NFs with diameters in the range of 50-100 nm were obtained by spinning SWCNTs lled PS composites. TEM observations revealed that the incorporation of small bundles of SWCNTs orient parallel to the NF axis (Fig. 4c). By using a posttreatment process, Lee's group prepared the mesoporous CNTembedded carbon NFs (Fig. 4d). 81 ES can also be used to fabricate NFs with multi-component polymer matrix. For instance, Mallon's group synthesized gra copolymers of polyacrylonitrile-gra-poly(dimethyl siloxane) (PAN-g-PDMS) by ES. 82 The amphiphilic property of the gra copolymers induced phase segregation and selfassembly of copolymers in solution to form a network-like structure. The self-assembly has a direct effect on the nal formation and morphologies of NFs. MWCNTs were also introduced into the amphiphilic copolymer NFs by ES. The addition of MWCNTs produced bers with much smaller diameters, but the porous structure on bers was maintained (Fig. 4e). In the same year, Shim's group fabricated copolymer NFs doped with MWCNTS by ES and utilized the fabricated NFs as a chemiresistor to detect the aromatic volatile organic compounds (Fig. 4f). 83 Previously we reported that both the alignment of MWCNTs in the electrospun PEO NFs and the orientation of electrospun PEO-MWCNT hybrid NFs can be controlled by an advanced ES technique. 6 The embedding and alignment of MWCNTs in PEO NFs were conrmed by TEM, as shown in Fig. 4g and h. With the increasing of content of MWCNTs in the electrospun NFs, MWCNTs were tended to align closely in a line, forming an ideal structure that can best express the unique anisotropic properties of CNTs. In a next study, we further investigated the preparation of polyurethane (PU) NFs lled with MWCNTs and created a novel PU-MWCNT conductive brous membrane with ES technique (Fig. 4i). 8 The use of CNTs to reinforce and enhance the performance of polymer-CNT hybrid NFs can produce a new generation of composite materials. The above introduction highlights the fact that the intrinsic crystalline quality and the straightness of the embedded CNTs are signicant factors inuencing the reinforcement capability. Meanwhile, in the electrospun polymer-CNT composites, polymers not only act as the matrix for the uniform distributions of CNTs and the formation of a thin lm, but also play a signicant role in enhancing the electrochemical characteristics by means of their interactions with CNTs.

Applications as sensors and biosensors
Electrospun polymer-CNT NFs have wide applications for the fabrication of electrochemical sensors and biosensors.
2.3.1 Sensing of alcohols. Shim and co-workers rstly doped MWCNT-OH into nylon-6,6 and created the nylon-MWCNT hybrid NFs by ES technique. 84 They further investigated the sensing properties of the created nylon-MWCNT NFs by measuring their responses upon exposure to low molecularweight alcohol vapours, such as methanol, ethanol, 1-propanol, and 1-butanol (Fig. 5a). The changes of the electrical resistance of nylon-MWCNT hybrid NFs were demonstrated on the basis of hydrogen bonds among the alcohol vapours and hydroxyl groups (-OH) on MWCNT-OH, and amide groups (-NHCO-) in nylon-6,6. CNT is p-type semiconductor at room temperature. Therefore, electron donating groups (alcohols) will donate electrons into the valence band of CNT, resulting in chargecarrier (h+) recombination when exposed to alcohol vapours. The adsorbed alcohol vapours can also reduce the charge mobility of CNTs via providing scattering sites. As a result, the resistance of nylon-MWCNT hybrid NFs is increased during sensing of alcohol vapours. This kind of sensors fabricated by nylon-MWCNT hybrid NFs showed reversible and reproducible responses to ethanol vapours within ve cyclic tests, as shown in Fig. 5b.
Recently, Han et al. developed an integrated sensor system by using electrospun poly(methyl methacrylate) (PMMA)-SWCNT hybrid NFs combined with the inter-digitated electrodes directly printed on a surface to detect volatile organic compounds. 85 When the polymer in the NFs swells due to the vapor adsorption, the CNTs separate from each other and increase the electrical resistance of this kind of material. The conductivity change of the hybrid NF material was recorded when the material was exposed to volatile organic compounds. The response to different vapors showed a linear relationship between resistance change and vapor concentration. In their study, polymer-CNT NFs were electrospun onto a porous gaspermeable membrane. The design of this integrated sensor system (porous membrane backing, NF mat with printed electrodes) enables the ow-through design, as shown in Fig. 5c. The high permittivity of this integrated NF sensor system allows gas to interact with the sensing material in 3D, thus resulting in enhanced sensitivity for sensing. They have obtained a oneorder-of-magnitude sensitivity enhancement with the permeable sensor when compared to the electrodes modied by directly depositing NFs (Fig. 5d)  high-surface area, optimum porosity, high thermal stability, chemical inertness, and negligible swelling in aqueous and non-aqueous solutions. 86 Electrospun brous membrane meets many of the requirements to achieve improved performances as a sensor material. The main benets of electrospun NFs to the fabrication of sensors include design exibility, dimensional stability for gas and liquids, high surface area, safe operations, easy scaling, and high reusability.
Lee et al. constructed CNT-doped PAN porous NFs by ES technique and further modied the NFs with glucose oxidase (GOD) for electrochemical sensing of glucose. 27 The specic surface area and pore volume of the thermal-treated electrospun carbon bers (CNFs) can be increased by immobilizing GOD. CNTs were embedded as an electrically conductive additive to improve the electrical property of the porous CNFs. The effects of CNT addition and oxyuorination on the performance of the fabricated glucose sensor were investigated with cyclic voltammogram (CV). The current peak intensity in the CVs increased due to the effects of CNT additive and oxyuorination treatment. This result is attributed to efficient GOD immobilization and the improved affinity between GOD and the surface resulting from the introduced hydrophilic functional groups on carbon surface and the efficient electron transfer. When glucose was freshly injected to achieve higher glucose concentrations, the stabilization time of the current was approximately 7 s at every step. This rapid stabilization of current is attributed to the enhanced effect of CNTs on the electrical properties of electrode.
In the next study, Lee and co-workers fabricated another glucose sensors by immobilizing GOD onto electrospun polymer (poly(diallyldimethylammonium chloride) (PDDA) and PMMA)-MWCNT nanobrous membranes. 28 The fabricated PMMA(PDDA)-MWCNT/GOD exhibits excellent electrocatalytic activity towards hydrogen peroxide (H 2 O 2 ) with a pronounced oxidation current at +100 mV. Glucose was amperometrically detected at +100 mV in 0.1 M phosphate buffer solution (PBS, pH 7). The linear response for glucose detection is in the range of 20 mM to 15 mM with a detection limit of 1 mM and a response time of $4 s. The superior performance of this kind of sensors is due to the wrapping of PDDA over MWCNTs that bound with GOD.

Electrospun polymer NFs doped with MNPs
It is well-known that the electrochemical activity of MNPs is extremely sensitive to their sizes, sharp, and dispersion. A high dispersion of MNPs in functional materials is important to present high electrochemical activity, while the associated tendency of MNPs to aggregate would lower their catalytic activity and reuse life-time. Therefore, how to design and prepare MNP-based materials with long-term dispersion stability and high catalytic efficiency is a primary challenge for their widely applications. Several strategies have been utilized to synthesize and immobilize MNPs into electrospun polymer NFs. [87][88][89][90] For instance, polymer-MNP hybrid NFs can be prepared by mixing MNPs into a polymer solution and spinning subsequently. 87 MNPs can also be directly deposited onto electrospun polymer NFs via surface modication. 88,[91][92][93] Alternatively, MNPs can be synthesized on the surface of electrospun polymer NFs through thermal and chemical reduction of metallic precursor ions. 89,90 Du et al. prepared the highly uniform and monodisperse noble MNPs (Ag, Au, and Pt) in polyvinyl alcohol (PVA) NFs by combining ES technique and an in situ reduction. 94 The small and stable MNPs can be easily synthesized in aqueous solution using epigallocatechin gallate as both reductant and stabilizer. Through ES technique, uniform and smooth PVA NFs decorated with uniform and highly dispersed MNPs can be obtained (Fig. 6a) Fig. 6b, in which numerous AgNPs were attached onto an PAN NFs. The AgNPs were roughly spherical in shape and were randomly distributed on the surface of the NFs with a moderate density. The size distribution of AgNPs is 23.3 AE 5.3 nm. Lei and co-workers reported the successful preparation of polyaniline (PANi) NFs with integrated Pt nanoowers. 96 PANi was prepared by in situ polymerization of aniline on an electrospun NF template in an acidic solution with ammonium persulfate as the oxidant. Pt nanoowers were further electrodeposited onto the PANi NF backbone by CV, resulting in novel functionalized hybrid NFs. The coverage of Pt nanoowers on PANi NFs can be facilely controlled by adjusting the electrodeposition parameters (Fig. 6c).
You's group prepared PdNP-modied CNFs by ES and subsequent thermal treatments. 97 Fig. 6d shows the SEM image of the electrospun CNF-PdNP hybrid. The surface of CNFs is coarse and the diameter is in the range of 200-500 nm. The spherical PdNPs with a mean diameter of about 75 nm can be observed on the surface of CNFs. Meanwhile, they also prepared NiNP-loaded CNFs (CNF-NiNP) by combination of ES technique and thermal treatment. 98 The TEM image illustrates that the NiNPs with a diameter of about 50 nm are embedded in the CNF matrix (Fig. 6e). Shanmugam et al. fabricated CoNP-doped porous carbon nanorods (CNR-CoNP) by an easy and versatile ES technique and followed by one-step carbonization at 900 C in Ar. 99 FE-TEM analysis clearly shows that the CoNPs are actually embedded in the CNRs (Fig. 6f). The size of CoNPs ranges from 20 nm to 100 nm. Wang's group synthesized Cu-doped ZnO NFs via ES technique. 100 The SEM image of the assynthesized Cu-doped ZnO NFs is shown in Fig. 6g.
Xia's group reported a simple method for functionalizing the surface of ZrO 2 nanobrous membranes with RhNPs. 101 The ZrO 2 membranes were fabricated in the form of nonwoven mats by ES with a solution of polyvinylpyrrolidone (PVP) and zirconium acetylacetonate, followed by calcination in air at 550 C to yield the tetragonal phase. The brous mats were then immersed in a polyol reduction bath to coat the surface of the NFs with RhNPs of 2-5 nm in size. Guo et al. successfully prepared Pd-Ni alloy NPs on CNFs (CNF-PdNi) by a simple method involving ES of precursor PAN/Pd(acac) 2 /Ni(acac) 2 NFs, followed by a thermal process to reduce metals and carbonize PAN (Fig. 6i). 102 The size, composition, and alloy homogeneity of the Pd-Ni alloy NPs could be readily tailored by controlling the feed ratio of metal precursors and the thermal treatment process.

Applications as sensors and biosensors
ES technique can be used to prepare MNP-CNF composites for electrochemical sensing applications by spinning metal precursor-containing polymer NFs, followed by a thermal treatment. 103,104 The advantages of this method include the uniform dispersion of MNPs within the framework of CNF, the high electrical conductivity of CNF, and the highly porous and mechanically strong network structure of the resulting composite, which contribute to a high electrochemical activity by providing a larger active surface area, preventing MNPs from detachment and agglomeration, and facilitating electron and mass transfer within the system.
3.2.1 Sensing of H 2 S and H 2 gas. Wang and co-workers synthesized Cu-doped ZnO NFs via ES technique. 100 The effects of Cu doping on H 2 S sensing properties at low concentration (1-10 ppm) were investigated at 230 C. The results show that the H 2 S sensing properties of ZnO NFs are effectively improved by Cu doping: 6 wt% Cu-doped ZnO NFs show a maximum sensitivity to H 2 S gas, and the response to 10 ppm H 2 S is one order of magnitude higher than that of pure ZnO NFs. Fig. 7a shows the response of Cu-doped ZnO gas sensors exposed to different concentrations of H 2 S. It is observed that the response of the sample increases with the increasing of H 2 S gas concentration. The response time and the recovery time are 18 and 20 s, respectively.
Fong et al. prepared carbon nanobrous mat (nano-felt) surface-attached with PdNPs by ES, and explored its application for hydrogen (H 2 ) sensing. 105 Fig. 7b shows the variations of sensor responses when the device was exposed to H 2 and vacuum alternatively. The results indicated that the resistance of carbon nano-felts decreased quickly upon exposure to H 2 in the testing chamber, but the rate of resistance change was reduced upon prolonged exposure time; the resistance essentially returned to the original value aer H 2 ow was turned off, indicating that the sensor response was reversible. It was evident that the carbon nano-felt with PdNPs had substantially higher sensitivity on H 2 due to the electronic interactions among H 2 molecules, PdNPs, and carbon nano-felt, and the adsorbed H 2 molecules would react with PdNPs to form palladium hydride when the nano-felt was exposed to H 2 .
3.2.2 Sensing of ethanol. You's group developed a novel NiNP-loaded carbon ber paste (CFP-NiNP) electrode for enzyme-free determination of ethanol. 104 An ES technique was used to prepare the CFP-NiNP composite with large amounts of spherical nanoparticles rmly embedded in CFP. In application to electroanalysis of ethanol, the CFP-NiNP-modied electrode exhibited high amperometric response and good operational stability. The calibration curve was linear up to 87.5 mM with a detection limit of 0.25 mM, which is superior to that obtained with other transition metal based electrodes (Fig. 7c).

Sensing of H 2 O 2 and sugars.
Hou and co-workers synthesized PdNP-loaded CNFs (CNF-PdNP) by the combination of ES and thermal treatment processes. 106 The CNF-PdNP-modied carbon paste electrode (CNF-PdNP/CPE) demonstrated direct and mediatorless responses to H 2 O 2 at low potentials. Fig. 7d shows the CVs of CNF-PdNP/CPE in 0.1 M PBS (pH 7.0) (curve a) and PBS containing 5 mM H 2 O 2 (curve b) recorded at 50 mV s À1 . When no H 2 O 2 was added, the peak appeared at À7 mV was the reduction peak of the oxide of Pd. The H 2 O 2 reduction peak potential at the CNF-PdNP/CPE is at À48 mV, which is much lower than that at AgNP-modied glassy carbon electrode (GCE) (À0.68 V vs. SCE), This result suggested a faster electron transfer rate and higher electrocatalytic activity toward the reduction of H 2 O 2 at the CNF-PdNP/ CPE. Fig.7e shows the typical current-time plot of the CNF-PdNP/CPE at À0.2 V with the successive addition of H 2 O 2 . Aer H 2 O 2 was added into the stirring buffer solution, the sensor responded rapidly to reach a steady state. The sensor could achieve 95% of the steady-state current within 5 s. The CNF-PdNP/CPE displayed a wider linear range from 0.2 mM to 20 mM with a correlation coefficient of 0.9991 and a slope of 4.15 mA mM À1 (inset, Fig. 7e).
Liu et al. produced a novel PtNP-loaded carbon NF (CNF-PtNP) electrode with ES technique, and further applied the electrode for sensing of H 2 O 2 . 107 When applied to the electrochemical detection of H 2 O 2 , the CNF-PtNP electrode exhibited low over-potential, fast response, and high sensitivity. In addition, the CNF-PtNP electrode showed good selectivity for H 2 O 2 detection in the presence of ascorbic acid, acetaminophenol and uric acid under physiological pH condition. Fig. 7f shows the amperometric responses of the CNF-PtNP electrode upon successive addition of H 2 O 2 at different concentrations. The electrode exhibited strong and fast response to each injection of analyte, attributing to the high electrocatalytic efficiency of the well-dispersed PtNPs. Fig. 7g indicates the amperometric responses of the CNF-PtNP electrode when injecting 0.1 mM H 2 O 2 for eight times. With this electrospun electrode, a detection limit of 0.6 mM with a linear range of 1-800 mM (R ¼ 0.9991) was obtained.
Guo et al. successfully prepared Pd-Ni alloy NP-doped carbon NFs (CNF-PdNi) composites by a simple method involving ES of precursor PAN/Pd(acac) 2 /Ni(acac) 2 NFs, followed by a thermal process to reduce metals and carbonize PAN. 102 CV studies showed that the CNF-PdNi-based electrodes reveal enhanced redox properties compared to the Ni-metal electrode and show signicantly improved electrocatalytic activity to sugars (e.g., glucose, fructose, sucrose, and maltose) oxidation ( Fig. 7h and i). The application potential of CNF-PdNi-based electrodes in ow systems for sugar detection was explored. A very low limit of detection for sugar (e.g., 7-20 nM), high resistance to surface fouling, excellent signal stability and reproducibility, and a very wide detection linear range (e.g., 0.03-800 mM) were revealed for this new type of CNF-PdNi composite as the detecting electrode.
Devadoss's group reported the electrochemical application of AuNP-decorated Naon NFs (NNF-AuNP) using a facile ES technique. 108 Owing to the uniform distribution and large surface area of the AuNPs in the NNFs, the NNF-AuNP-modied electrodes gave rise to greatly improved electrochemical properties compared to AuNP-free electrodes. When they were employed as reservoirs for immobilizing horseradish peroxidase (HRP), reliable and sensitive electrochemical detection by the enzyme reaction was achieved. The detection sensitivity for H 2 O 2 was determined to be as low as 38 nM. In addition, there was no change in the enzyme stability over three weeks. To determine the electrochemical response to H 2 O 2 , CV was performed in the absence and presence of 0.01 mM H 2 O 2 (Fig. 8a), where HRP-immobilized NNFs and NNF-AuNP-modied electrodes were compared. Upon the addition of 0.01 mM H 2 O 2 , the most noticeable change in the current was observed for the NNF-AuNP electrode. The current density of NNF-AuNP/HRP-modied electrodes was almost 2.4-fold greater than that of NNF/HRP-modied electrodes. This result clearly indicates that the AuNPs play a critical role in H 2 O 2 reduction through catalyzing HRP enzyme on the composite electrode. Furthermore, the NNF-AuNP-modied electrode in the absence of HRP did not show the distinct redox peak even at high concentrations of H 2 O 2 indicating that the reduction of H 2 O 2 at the electrode surface is predominantly catalyzed by the immobilized HRP. This kind of H 2 O 2 sensor has a good linear range from as 6 Â 10 À8 to 8 Â 10 À7 M (inset in Fig. 8b) with a current sensitivity of 0.3242 mA mM À1 .
Yu's group reported a facile route to fabricate water stable AuNP-poly(vinyl alcohol) (PVA-AuNP) hybrid nanobrous mats with tunable densities of AuNPs and further demonstrated the potential application of as-prepared PVA-AuNP nanobrous mats as efficient H 2 O 2 biosensor substrate materials. 109 In the electrospinning process, the uniformity of electrospun PVA nanobers is greatly inuenced by the properties of polymer solution and the operating parameters. In their study, PVA solution (8 wt%) was prepared by dissolving PVA powder into deionized water at 95 C overnight under magnetic stirring, and then the solution was cooled down to room temperature naturally. Subsequently, the glutaraldehyde solution was added to PVA aqueous solution under vigorous stirring to achieve a homogeneous viscoelastic spinnable solution, in which the mass ratio of PVA and GA was 4 : 1. The optimized experimental parameters including ow rate of 0.3 mL h À1 , voltage of 9 kV, and collection distance of 17 cm. Fig.8c shows the amperometric response and calibration curve of steady state current vs. concentration of H 2 O 2 . Stepped increases of the amperometric reduction currents were observed with the addition of H 2 O 2 at a constant potential of À0.2 V. The current response of the sensors was rapidly enhanced and approached about 98% of its steady state current within 1 s. The obvious increase of the reduction current can be observed when the concentration of H 2 O 2 was as low as 1 mM (inset picture in Fig. 8c) for PVA-AuNP/ HRP-modied GCE. The amperometry result indicates that the PVA-AuNP/HRP-modied GCE exhibits very high sensitivity.
Du et al. prepared highly uniform and mono-disperse noble MNPs (Ag, Au, Pt) in PVA NFs by combining the in situ reduction and ES technique, which can be used as efficient biosensor for the detection of H 2 O 2 . 94 In their research, 5 g PVA powder was dissolved in 45 mL deionized water to get a concentration of 10 wt% solution and was stirred at 80 C for 5 h to obtain a transparent homogeneous solution. The solution was then cooled to room temperature and 10 mL 10 wt% PVA solution were injected into three 3-neck ask, respectively. The calculated amounts of AgNO 3 (0.0375 g), HAuCl 4 $3H 2 O (0.0462 g) and H 2 PtCl 6 $6H 2 O (0.0642 g) were added into the asks, respectively. 0.025 g epigallocatechin gallate dissolved in 2 mL deionized water was injected into the above solutions, respectively. The solutions were kept at 65 C under vigorously stirring for 3 h to ensure the complete reduction. Aer that, the precursor solutions for electrospinning were refrigerated at 4 C. The optimized experimental parameters including ow rate of 0.01 mL min À1 , voltage of 12 kV, and collection distance of 12 cm. The fabricated PVA-AgNP functionalized electrodes exhibit remarkably increased electrochemical catalysis toward H 2 O 2 and excellent stability and reusability. The biosensor allows the highly sensitive detection of H 2 O 2 with a broad linear range span of the concentration of H 2 O 2 from 10 mM to 560 mM (Fig. 8d). The PVA-AgNP-based biosensor responded rapidly and approached about 98% of its steady state current less than 2 s. The rapid electrode response to the change of the H 2 O 2 concentration is attributed to the fast diffusion of the H 2 O 2 onto the surface of small AgNPs through the porous NF structures.
3.2.4 Sensing of urea. Lei's group prepared PANi NFs with integrated Pt nanoowers (PANi-Pt). 96 As a demonstration, urease is immobilized onto the PANi-Pt hybrid NFs and the composite was employed as the sensing platform for urea detection in a ow-injection-analysis (FIA) system. The detection of urea shows a wide linear range and an excellent anti-interference property against chloride ion. In addition, it was found that the response to urea was attributed not only to the conductivity change of PANi due to the interaction between PANi and ammonia (liberated from the enzymatic reaction), but also to the interaction between Pt nanoowers and amine groups in urea. The inset of Fig. 8e exhibits the typical FIA amperometric response of the PANi-Pt/GCE to successive injection of urea operated at the optimal applied potential of À0.1 V. The developed urea biosensor displays a linear range up to 20 mM with a correlation coefficient of 0.9968, a sensitivity of 115.6 nA mM À1 cm À2 , and a detection limit of 10 mM (S/N ¼ 3).

Electrospun polymer NFs doped with CNTs and MNPs
The research of electrospun polymer NFs doped with CNTs and MNPs has been reported recently. 110,111 For example, Shi's group reported a new approach to immobilizing zero-valent iron nanoparticles (ZVI NPs) into electrospun polymer NFs (Fig.9a). 110 In their study, MWCNT incorporated PAA/PVA composite NFs were produced by spinning the PAA and PVA mixed solution containing well dispersed MWCNTs, and the thermal treatment was introduced to crosslink the NFs in order to obtain water-stable composite nanobrous mats. Then, the water-stable composite nanobrous mats were immersed into FeCl 3 solution to complex Fe 3+ for subsequent reductive formation and immobilization of ZVI NPs into the composite NF mats. A typical SEM micrograph of the ZVI NP-immobilized nanobrous mats with MWCNTs (Fig. 9c) shows that the hybrid nanobrous mat attains a porous structure with a smooth surface. The diameter of the ZVINP-immobilized NFs is narrowly distributed with a mean diameter of 283 AE 44 nm (Fig. 9b).
Lee's group fabricated a SWNT/silver ber-based transparent conductive lm using silver bers produced by the ES method and post-processing (Fig.9d). 111 In their research, electrospun silver bers provided a segregated structure with AgNPs within the brous microstructures. The individual AgNPs within the electrospun bers on the substrate were interconnected with SWNTs, which resulted in the efficient activation of a conductive network by bridging the gaps among the separated AgNPs ( Fig.9e and f).
Our group also made contributions to the fabrication of polymer NFs doped with CNTs and MNPs. 8,9 For example, we reported the electrospun preparation of PU NFs lled with MWCNTs and AgNPs (PU-MWCNT-AgNP). 8 By simply blending the suspension of modied MWCNTs and commercialized AgNPs with PU solution and then performing ES, PU-MWCNT-AgNP hybrid NFs were successfully prepared (Fig. 10a). It can be clearly seen that Ag clusters spread throughout the PU matrix discretely, without being connected to each other. With this ES technique, both PU-MWCNT and PU-MWCNT-AgNP NFs were fabricated successfully, and MWCNTs were successfully aligned and oriented in the PU matrix ( Fig. 10b and c). It should be noted that in the PU-MWCNT-AgNP NFs, the Ag clusters tend to adhere to the surface of MWCNTs (Fig. 10c), and MWCNTs always act as bridges to combine the discrete Ag clusters together (Fig.10d). The assembly of MWCNTs and AgNPs can effectively facilitate the electron transfer, thus boosting the conductivity and improving the electrocatalytic performance of the materials. Based on the experimental data, we further proposed a potential model to display the distribution of MWCNTs and AgNPs inside the fabricated PU-MWCNT-AgNP NFs (Fig. 10e). The aligned MWCNTs act as bridges and get the discrete Ag clusters in series, which results in a notable synergistic effect and endows the PU-MWCNT-AgNP NFs with good conductivity and great potential in the application of electrochemical sensors.
Recently, we reported a novel b-phase polyvinylidene diuoride (PVDF) nanobrous membrane decorated with MWCNTs and PtNPs. 9 It is interesting to nd that the PtNPs tend to attach onto the side walls of MWCNTs, and at the same time MWCNTs serve as bridges to connect the dispersive PtNPs together in PVDF-MWCNT-PtNP hybrid NFs (Fig.10f and its insets). The assembly of MWCNTs and PtNPs can effectively facilitate the electron transfer, thus boosting the conductivity and improving the electrocatalytic performance of the fabricated materials.

Electrochemical sensing
We are the rst to apply electrospun polymer NFs (PVDF and PU) doped with MNPs and MWCNTs for electrochemical sensors of H 2 O 2 and glucose. 8,9 Considering the good electrocatalytic performance of AgNPs and the excellent conductivity of MWCNTs, the prepared PU-MWCNT-AgNP hybrid NFs are expected to have better performance than either PU-MWCNT or PU-AgNP NFs for the fabrication of electrochemical sensors. In our experiments, PU-MWCNT-AgNP NFs were directly electrospun onto the surface of a polished GCE, and the performance of the NF-modied GCE for H 2 O 2 sensing was investigated. 8 The experimental results show that pure electrospun PU NF-modied GCE shows no apparent redox processes. Aer modi-ed with electrospun PU-MWCNT NFs, the diffused current increased clearly because MWCNTs have a catalytically active surface, and therefore they can increase the active surface area of the modied electrode. Similar to the pure PU NF-modied GCE, no obvious redox current peaks appear in the CV of PU-MWCNT-modied GCE either. For the PU-Ag NPs NF-modied GCE, a large current response and a broad reduction peak from À0.65 to À0.4 V were observed due to the reduction of H 2 O 2 . The presented CV results indicate that the PU-MWCNT-Ag-modied GCE has better electrocatalytic activity than either PU-MWCNT or PU-Ag modied GCE toward the reduction of H 2 O 2 , which conrms the synergistic effect of MWCNTs and AgNPs. PU-MWCNT-AgNP-modied GCE reveals a linear response to H 2 O 2 . The biosensor has a linear detection range from 0.5 to 30 mM(R ¼ 0.9999), and possesses a detection limit of 18.6 mM (S/N ¼ 3) with a sensitivity of 160.6 mA mM À1 cm À2 .
In a further study, we fabricated PVDF-MWCNT-PtNP nanobrous membrane by ES and utilized the membrane for biosensors of H 2 O 2 and glucose. 9 The nonenzymatic amperometric biosensor has highly stable and sensitive, and selective detection of H 2 O 2 and glucose.

Conclusions and future perspectives
The past decade years have witnessed signicant progresses of ES technique to fabricate functional nanomaterials for various applications. There is no doubt that ES has become one of the most powerful techniques for preparing diverse nanostructured materials and highly sensitive sensors in the future. In this review, we present a detailed progress report on how to applying ES technique to create polymer NFs lled with MNPs and CNTs, as well as how to put the fabricated nanobrous materials for electrochemical sensors and biosensors. Essential studies, nevertheless, are still required and many challenges have to be faced. More experimental studies and theoretical modeling are required in order to achieve a better control over the size and morphology of electrospun nanomaterials.
Electrospun nanobrous materials lled with MNPs and CNTs will have other potential applications in solar cells, batteries, hydrogen storage, environmental protection, and tissue engineering.