Introduction to Sensor using Electrospun fibers

SEM images of pure In2O3 nanofibers for formaldehyde sensing. Inset: High magnification TEM image. [Hu et al. Journal of Nanomaterials, vol. 2014, Article ID 431956, 7 pages, 2014. doi: 10.1155/2014/431956. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

High surface area to volume ratio of electrospun nanofibers makes it an attractive material for sensor application. Electrospun fibers have been investigated for use in many sensor applications due to its flexibility in materials selection and ease of incorporating active agents. Sensor techniques that have been used using electrospinning include optical change, electrical resistivity, electrochemical sensing and acoustic waves depending on the materials and its properties. Most sensors using electrospun fibers are in the form of nonwoven mesh which does not require modifications to the standard setup. Performance of sensors made from electrospun fibers are often more sensitive than cast film sensors of the same material [Khoshaman 2011]. Sensors made of both organic and inorganic materials have been electrospun in fibers. While electrospun fibers already have very high surface area, they have been processed into nanotubes to further increase its surface area. Park et al (2022) was able to construct ZnO-ZnFe₂O₄ hollow nanofibers from electrospinning of their precursors followed by a calcination process for the development of a sensor for H₂S. By testing varying ratios of ZnO-ZnFe₂O₄, they found that 80:20 wt% showed better sensing properties than others. The optimized ZnO-ZnFe₂O₄ exhibited a H₂S gas sensing properties of 84.5 (S = R_a/R_g) at 10 ppm at 250 °C and excellent selectivity when tested with a gas mixture containing CH₃COCH₃, NO₂, and C₂H₅OH.

One of the most basic forms of sensors is the lateral-flow analysis (LFA) strips. These strips are usually made of nitrocellulose with the sensing material attached to its surface. However, a limitation is its loading capacity and sensitivity. Wang et al (2021) used electrospinning to coat a nitrocellulose nanofibrous layer on commercial nitrocellulose membrane to increase the surface porosity and surface area which increases protein adsorption. Increasing thickness of the electrospun nitrocellulose nanofibrous layer also increases the hydrophobicity of the membrane and this reduces the flow rate which increases the opportunity of antigen-antibody reaction. Electrospun nitrocellulose fiber coated membrane (ENC) at optimum coating thickness has a protein adsorption rate close to 12% while corresponding commercial nitrocellulose membrane (CNC) has an adsorption rate of only 5.3%. Compared to the lower detection limit of CNC, the sensitivity was increased by 50 times in ENC. Beyond the optimum electrospun fiber thickness, the sensitivity starts to drop. This has been attributed to increasing hydrophobicity as the thickness of electrospun nitrocellulose fiber layer increases which reduces protein adsorption. Targeted application and sensor techniques typically depend on the material selected rather than the form. To construct a sensor, there must be a change in characteristics or properties of the material upon exposure to the target and secondly, there must be a way to measure the change.

There are many ways of constructing a nanofibrous structure based on the sensor material through electrospinning. A sensor material may be electrospinnable on its own. Many functional polymers are electrospinnable given the right molecular weight and solvent combination. Polyacylic acid (PAA) which is reactive to alkaline gas such as ammonia can be electrospun [Khoshaman 2011]. Similarly, polyvinyl alcohol (PVA) which is sensitive to water vapor may be used as a humidity sensor [Khoshaman 2011]. Metal oxides are also often used as sensor material especially in high temperature environment and these too can be electrospun using precursor materials followed by sintering [Ding et al 2010]. Where functional sensor materials are not electrospinnable, they may be incorporated into electrospun fibers using various functionalization methods. The simplest way to introduce functional molecules into electrospun fibers is by blending the material into the polymer solution followed by electrospinning to form nanofibers. Sometimes, these materials are added to amplify the reaction of the base nanofiber material which has responded to the target. Salt ions, nanoparticles and supra-molecules have been incorporated into electrospun nanofibers through blending [Zhang et al 2014, Hendrick et al 2010, Khoshaman 2011]. Khatri et al (2019) blended β-cyclodextrin into polyvinyl alcohol (PVA) solution and electrospun into a fibrous membrane for chiral recognition of histadine and serine amino acids isomers. To maintain stability of the fibrous film in the test medium, cross-linking using gluteraldehyde was carried out. Cyclodextrins are commonly used in chiral separations due to their selectivity and unique ability to form inclusion compounds with smaller hydrophobic molecules. Their tests showed that β-cyclodextrin/PVA electrospun film exhibited excellent chiral recognition and have the potential to for detection of D-isomers as biomarkers in biological fluids. Another way to functionalize electrospun fibers for sensing is by surface adhesion. Jaffal et al (2021) first electrospun nylon-6 into nanofibrous mat. The mat is then dipped into a suspension of multi-walled carbon nanotube (MWCNT) with surfactant. Finally, the MWCNT coated matis dipped in a calixarene solution and dried to form MWCNT/calixarene-functionalized nylon-6 mat for sensing sodium ion concentration. The optimized sensor has a fiber diameter ranging between 250 to 320 nm and covered with MWCNT length less than 200 nm. A maximum sensor sensitivity of about 45 µA/mM to sodium ions was obtained. For integration of the composite sensor to a fabric material, it needs to have sufficient strength and elasticity to withstand stretching during usage while maintaining sensor reading consistency. The nylon-6/MWCNT nanocomposite material has a statistically higher stress at break with no significant difference on elastic modulus compared to neat nylon-6. Interestingly, it was found that the flow rate for electrospinning has a significant influence on the mechanical and sensing properties of the nanocomposite mat. An optimum flow rate of 1.0 ml/h for electrospinning was found with either higher or lower flow rate resulting in stiffer mat and poorer sensing response after multiple stretches. Further, the ultimate strength of the nanocomposite mat was comparable to that of common commercially available fabrics which makes integrated use easier.

Once the material has reacted to the target, there must be a way to detect the change. For changes in the resistivity of the material, the nanofibers may be deposited onto conductive electrodes where the change in resistance can be measured. This is commonly used for electrospun metal-oxide sensors [Leng et al 2011, Santos et al 2014]. Detecting change in resistivity when the material is exposed to the target has also been used for electrospun conductive polymers such as polyaniline and this can be used under room condition [Zhang et al 2014] instead of elevated temperatures as in the case of metal-oxide sensors. Where sufficient target materials are absorbed onto the sensor material, the subtle change in mass may be picked up by changes in its surface acoustic wave. Electrospun fibers have been deposited on quartz crystal microbalances (QCM) where very small mass change in the fibers can be detected by the resonance frequency shifts of QCM [Khoshaman 2011]. Rianjanu et al (2018) used electrospun polyacrylonitrile (PAN) nanofibers on quartz crystal microbalances (QCM) to detect safrole, a main precursor for producing amphetamine-type stimulant (ATS) drug, N-methyl-3,4-methylenedioxyamphetamine (MDMA), also known as ecstasy. Comparing the sensitivity between electrospun PAN nanofiber and spin coated PAN on QCM, the electrospun PAN nanofibers showed a two-fold increase in mass shift compared to spin coated PAN. For each 1 mg/L safrole vapor, a frequency shift of 5 Hz was shown on electrospun PAN coated QCM. The coated PAN was also reported to be selective in its detection towards safrole vapor when compared against various volatile organic compounds (VOCs) of 1 mg/L commonly present in the atmosphere such as ammonia (25% in aqueous solution), formaldehyde (37% in aqueous solution), acetone, water, benzene, toluene, and xylene. For some enzyme-based sensors, reaction with the substrate releases a small current which can be detected. Electrospun fibers incorporated with the enzyme may be connected to a device which shows the current generated. Electrospun glucose sensors are often incorporated with glucose oxidase where detection is by amperometric means [Ding et al 2010]. Another common method of using changes in material resistivity for detection is in electrically conductive polymer composites (ECPC) or inorganic composites. This works by measuring an increment or reduction in the sensor material resistivity due to diffusion of contaminants into its matrix which causes it to swell. As there is a finite amount of conductive fillers in the matrix, the swelling increases the space between the conductive fillers and therefore increases the resistivity of the material. This method has been used in sensing of gas and volatile organic compounds (VOC) where electrospun polymers were loaded with conductive polymers, nanoparticles, graphene [Avossa et al 2019] or carbon nanotubes. Moghaddasi et al (2019) used this method for the detection of vegetable oil in water. In their study, electrically conductive nanocomposites based on styrene-isoprene-styrene copolymer (SIS) and multiwall carbon nanotubes (MWCNT) fabricated by electrospinning were used. Their sensor was able to detect oil contaminants in water from 50 ppm onwards. Avossa et al (2019) constructed a nanofibrous conductive chemical sensor based on combination of two insulating polymers, polystyrene (PS) and polyhydroxybutyrate (PHB), doped with 5,10,15,20-tetraphenylporphyrin (H2TPP) and mesoporous graphene nanopowder (MGC). Electrical conductivity of the composite is given by the presence of MGC in the matrix. However, the electrical conductivity of the constructed chemi-resistor is also influenced by H2TPP. At lower temperature, H2TPP causes the chemi-resistor to be more resistive but at 60 to 70 °C, it causes the chemi-resistor to be more conductive than MGC/PS/PSB fibers. This has been attributed to the rearrangement of MGC in the polymer fiber as the aromatic planes of H2TPP face the graphene surfaces and phenyl groups of PS. The presence of H2TPP also increases the sensitivity of the chemi-resistor compared to MGC/PS/PSB fibers for detection of toluene vapor. It is possible that toluene is able to facilitate electron transfer between H2TPP and MGC. Toluene as an organic compound may also increase conformational changes to both macromolecules and PS chains which favors redistribution of MGC network resulting charge flow.

Electrospun fibers have been used in the development of inorganic solid-state gas sensors. Ghosh et al (2023) showed that in the use of tungsten oxide (WO₃) as acetone sensors for the non-invasive diagnosis of diabetes, electrospun derived WO₃ nanofibers performed better compared to nanoparticles form. WO₃ fiber sensor (WFS) has a higher sensitivity of 90% at a lower operating temperature of 150 °C compared to particle-based sensor (WPS) with 84% sensitivity at 200 °C. The response and recovery time for WFS was faster at 10 s and 40 s respectively compared to 18 s and 90 s for WPS. Better performance of WFS has been attributed to higher porosity, charge confinement and electron transfer of a one-dimensional (1D) structure and exposed highly reactive (002) plane with better crystallinity. In the production of WFS, polyvinylpolypyrrolidone (PVP) was added to ammonium metatungstate hydrate solution for electrospinning into fibers. During the sintering process, PVP interacts with the ammonium tungstate surface causing the stabilization of the (002) facet which leads to better crystallinity and exposure of the (002) plane. This reactive plane facilitates more chemisorbed oxygen molecules at a lower temperature which improves the sensitivity of WFS.

Piezoelectric materials have the unique property of producing charges when it is subjected to mechanical movement. The charges produced may then be picked up by an electrode for signal processing. Wang et al (2020) developed a non-resonating acoustic sensor based on the ability of sparsely distributed freely suspended electrospun piezoelectric fibers to pick up acoustic signals. A nanofiber mesh made of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) with an average fiber diameter of 307 nm was produced by a dynamic near field electrospinning method. This nanofiber mesh was found to be sensitive to acoustic waves from 200 Hz to 500 Hz which covers the most common sound frequencies encountered daily hence the acoustic sensors made from the electrospun (P(VDF-TrFE)) fibers were able to detect changes in the source frequencies. As the sound wave travels perpendicular to the suspended nanofibers, it causes them to vibrate and generate a voltage output. Further tests showed that the nanofiber mats were able to differentiate between different sound frequencies.

An interesting development of electrical sensors is the introduction of self-powered sensors. Peng et al (2020) constructed a self-powered electronic skin using electrospun triboelectric nanogenerators. This triboelectric nanogenerator is made out of electrospun polylactic-co-glycolic acid (PLGA) fibers layer and an electrospun polyvinyl alcohol (PVA) fibers layer with silver nanowires (AgNW) sandwiched between the two layers. To have this AgNW layer, a vacuum filtration method was used to pass a suspension of AgNW solution through the electrospun PVA membrane layer where the nanofibers membrane traps the AgNW. Electrospinning of PLGA was carried out with the electrospun PVA membrane loaded with AgNW as the collector. A copper foil was attached to the surface of the membrane as the lead out. Electricity generation and transmission is realized by the periodic contact and separation between the constructed electronic skin (e-skin) and a contact object. The induced potential difference between the inner AgNW inner electrode and the external ground object will cause a charge transfer between them and generates a current. A maximum areal power density of 130 mW m^-2 at a matched resistance of ~500 megohms was recorded for the PLGA/Ag NWs/PVA e-skin. This e-skin exhibits excellent pressure sensitivity (a voltage response pressure sensitivity of 0.011 kPa) and conformity to surfaces which is vital for wearer comfort. Movement of the wearer's skin such as bending movements of joints can be detected by changes in the output voltage in real time. This has the potential for full body physiological monitoring. Chen et al (2021) constructed an array of electrospinning derived Ag nanofibers electrodes as part of a triboelectric nanogenerator (TENG) technology that is flexible with pressure-mapping sensing capability. The Ag nanofibers were produced by electrospinning polyvinyl alcohol (PVA) followed by sputter coating a layer of Ag on the PVA nanofibers. The PVA nanofiber template was removed by dissolving in water and transferred to a flexible polyethylene terephthalate (PET) layer. The resultant Ag nanofibers layer was modified to form a 4 x 4 electrodes array using lithography and wet etching process. The electrodes array was packaged with polydimethylsiloxane (PDMS) to give a pressure sensor array where each square electrode is linked to a pixel on screen to create a TENG capable of detecting mechanical stimulations. The ability of this pressure sensor array to detect localized mechanical stimulation was demonstrated by pressing a "Z"-shaped stamp on the array. The differential voltage output from each electrode was able to give a visual mapping of the stamp on screen.

Left: output voltage of a single pixel as a function of the applied pressure. The inset shows the experimental setup to apply and measure the pressure sensing. Right: the pressure mapping result of the "Z" shaped stamp with output voltage signals, in which pixels contacted with the stamp ridge area show a higher output [Chen et al 2021].

Li et al (2023) developed a low intensity sensitive heart sound sensor using electrospun PVDF layer as the tribo-negative and electrospun polyurethane (PU) layer as a tribo-positive layer. With this triboelectric pair, the output voltage can be enhanced and the generated charge can be trapped to prevent charge recombination at the electrode. The PU fiber layer was created using near field electrospinning to form ordered grids and the microcavities between the fibers help to reduce air damping and these create resonance and improve sensitivity while the PVDF fibers were randomly oriented from conventional electrospinning setup. The assembled sensor consists of the PVDF tribo-negative layer sandwiched between two PU tribo-positive layers using double sided tapes so that there is an air gap between the layers. These electrospun layers were in turn sandwiched between two electrode layers. When there is an acoustically induced vibration the center tribo-negative layer contacts the upper positive tribo-positive layer thus creating electrification. Separation of the layers result in electrostatic induction and electron flow from the lower electrode to the upper electrode. The bottom tribo-positive layer functions as a barrier to prevent the recombination of the surface charge on the center tribo-negative layer from the induced positive charge on the lower electrode. When the negative tribo-negative layer touches the lower positive tribo-electic material, the upper positive tribo-electric layer is the barrier to prevent recombination at the upper negative tribo-electric layer. The PU fibers grid spacing was also found to affect the sensitivity of the sensor with 0.4 mm grid spacing being the optimum. Lower grid spacing reduces sensitivity due to increased air damping while higher grid spacing reduces the ability of the PU layer to prevent charge recombination.

a) Fabrication of the heart sound sensor. b) Structure of the sensor, including electrodes, PU grid spacer, and a PVDF layer. c) Schematic of near-field/far-field hybrid electrospinning. d) Image of a PU grid spacer. e) Scanning electron microscopy image of a PVDF film [Li et al 2023]. The sensor showed good output at heart sound frequency range of 50-150 Hz with a sensitivity of 7027 mV Pa-1 and a low detection limit of 47 dB. The sensor demonstrated excellent stability with no degradation during 7 h continuous operation [Li et al 2023].

Highly sensitive sensors made from electrospun fibers have given rise to its potential use as wearable sensors. Kim et al (2020) constructed a glucose sensor made of electrospun fibers and glucose oxidase (GOx) which was able to detect glucose at concentration at levels lower than those in the blood. The electrospun fibers were made of poly(vinyl alcohol) (PVA), Au nanoparticles (AuNPs), GOx-β-cyclodextrin (β-CD) complexes and cross linking agent 1,2,3,4-butanetetracarboxylic acid (BTCA). The constructed glucose sensor is thin and flexible, exhibiting high absorbency and a high enzyme activity of 76.3%. The PVA/BTCA/β-CD/GOx hydrogel also showed a linear range for glucose concentration from 0.1 mM to 0.5 mM with a sensitivity of 47.2 µA mM^-1 and a detection limit of 0.01 mM.

Additives may be incorporated into electrospun fibers to amplify the reaction of its sensing material or to give it an alternative detection property. Silver nanoparticles have been added to stabilized polyacrylic acid/β-cyclodextrin nanofibrous membrane for humidity sensing. The presence of silver nanoparticles made it possible to measure optical response of the mat due to changes in humidity [Urrutia et al 2012]. For sensing using amperometric means, conductive nano-materials may be added to the nanofiber matrix to enhance the flow of the electrical current. PVA nanofibers loaded with graphene and glucose oxidases showed increased sensitivity compared to nanofibers without graphene [Wu et al 2014].

When the diameter of electrospun fibers are reduced to the tens of nanometer, it has the potential to be used as optical sensors. In particular, there is an interest in using porous materials for the development of optical sensing structures based on measurement of refractive index changes as light passes through it. Ponce-Alcántara et al (2018) investigated the feasibility of electrospun membrane with its high porosity for this application. By using a thin layer of electrospun polyamide-6 (PA6) nanofibers of diameter smaller than 30 nm, they were able to construct a Fabry-Pérot optical sensing structures, which have shown an experimental sensitivity up to 1060 nm/RIU (refractive index unit). The ability of electrospinning to cover a large area easily makes it a promising candidate for low-cost and high performance optical sensors.

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