Improving Sensor Performance of 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 of electrospun nanofibers makes them a good candidate for improving currently available sensors. Further improvement of electrospun fiber sensors can be realized by optimizing its operating environment and modification of its structure or material. These advancements will increase the attractiveness of using electrospun fibers in sensor and commercial utilization.

Increase Surface Area

Sensors made from electrospun nanofibers are able to demonstrate higher sensitivity due to its high surface area. Increasing its surface area will further increase its sensitivity with more exposed sensing and binding sites. At a few hundred nanometers diameter, sensors made from electrospun fibers have already shown significantly better sensitivity compared to conventional sensor. Zhang et al (2018) was able to bring the fiber diameter of electrospun polyvinyl alcohol/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PVA/PEDOT:PSS) to below 100 nm by a combination of setup modification and introducing pressure airflow to the electrospinning process. The resultant fiber has a diameter of 68 mm compared to 263 nm from conventional electrospinning setup. With smaller fiber diameter, the response speed of the ammonia sensor was faster at less than 6s compared to more this 10s for low ammonia concentration of 50 ppm. The sensor was also very sensitive to changes in ammonia concentration at low level although the changes in resistance quickly approaches zero at more than 10 ppm ammonia concentration. Kang et al (2011) improved the sensitivity of NO gas sensor by activating electrospun carbon fibers. The carbon fibers were fabricated by using polyacrylonitrile as the carbon source. The activation of the carbon nanofibers using H₃PO₄ and heat treatment in a controlled environment creates mesopores on the fibers and loss of semiconducting characteristics. The activated carbon fibers show change of electrical resistance that are dependent on the quantity of adsorbed gas in the pores. Introducing surface projections on electrospun fibers is another of increasing surface area. Fan et al (2019) produced electrospun ZnO nanofibers with two different types of surface projections. ZnO nanofibers were first produced by annealing of electrospun Zn(CH₃COO)₂·2H₂O/polyvinylpyrrolidone (PVP) fibers. A reaction solution was prepared by mixing hexamethylenetetramine (HMTA) and zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) followed by the addition of ZnO. Hydrothermal reaction of this mixture produces fire cracker like surface. When the ZnO nanofibers were sonicated prior to hydrothermal treatment, flower like projections were produced. When tested for their ability to sense H₂S gas, the ZnO nanofibers with flower like projections have the greatest sensing performance followed by fire cracker like surface and ZnO nanofibers without any surface projections had the least sensing performance. The nanofiber with flower like projections have larger size nanorods and greater point to point contacts compared to the fire cracker like structure. Together with higher surface area, this gave the flower like projection ZnO nanofibers the greatest sensing performance.

SEM images of as-prepared samples: (a) ZnO with fire cracker like surface in low magnification, (b,c) ZnO with fire cracker like surface at high magnification, (d) ZnO with flower like projections in low magnification and (e,f) ZnO with flower like projections at high magnification [Fan et al 2019].

Doping

To improve the sensitivity of nanofibrous sensor, one concept is to increase the number of charge carriers for impedance-based sensors. Doping inorganic nanofibers with salt will increase the number of ions available when water is absorbed onto the nanofiber surface. Qi et al (2008) showed that KCL-doped TiO₂ nanofibers demonstrated impedance variance of more than four orders of magnitude in the range of 11% to 95% relative humidity over pure TiO₂ nanofibers. Li et al (2008) also reported improved impedance variance when TiO₂ nanofibers were doped with up to 30% LiCl. Su et al (2012) was able to increase the humidity range of ZrO₂ by doping with Mg salt. The resultant Zr_0.9Mg_0.1O_2δ nanofibers showed an increased in the impedance variance of more than four orders of magnitude in the range of 11% to 97% relative humidity. Wang et al (2019) compared the performance of electrospun graphene fibers with graphene oxide (GO)/SnO₂ hollow fibers from sintering of its electrospun precursor for sensing NO₂ gas. In this case, the presence of SnO₂ nanoparticles enhances the performance of the gas sensor. Sensitivity of GO/SnO₂ increases linearly with increasing gas concentration and at a significantly higher rate than graphene alone. At 100 ppm NO₂ gas, the response (Ra/Rg) from GO/SnO₂ was 14.9 while graphene seems to reach saturation at 3.2. The presence of SnO₂ nanoparticles increases the concentration range of the gas sensor probably because it increases its charge carrier density and reduces the width of the surface depletion layer. These help to provide more electrons for the adsorption of gases. GO/SnO₂ was also found to be more sensitive towards NO₂ gas compared to other gases such as SO₂ and Cl₂.

Functionalization

Functionalizing the surface of electrospun fibers will improve the interface between the immobilized enzymes and catalyst and the output response. This in turn will improve the sensitivity and rate of detection. The stability of the sensor is also expected to be better due to if the catalyst or enzyme is covalently bonded to the fiber surface. Mondal et al (2014) introduces functional group such as -COOH and -CHO to the surface of TiO₂ nanofibers from electrospinning via oxygen plasma treatment. Cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) were covalently bonded to the surface of the functionalized nanofibers using cross linking agent. The resulting sensor showed better detection limit down to 0.49 mM, excellent sensitivity (181.6 µA/mg.dL^-1/cm²) and rapid detection (20 s) for esterified cholesterol. Sui et al (2017) showed that using foaming agent, they are able to encourage the migration of TiO₂ nanoparticles to the surface of carbon nanofibers during carbonization of polyacrylonitrile (PAN). With higher carbonization temperature, the amount of TiO₂ nanoparticles on the surface increases. This led to an increase in photocatalytic activity toward the degradation of rhodamine B. However, when the temperature reaches 1000 °C, TiO₂ particles were agglomerated and this caused a reduction in its photocatalytic activity. Hu et al (2019) proposed using electron beam treatment on calcinated electrospun TiO₂ nanofibers to make them more reactive. The electron beam irradiation also form Ti³⁺ on the nanofibers and together with the surface defects, lowers the band gap width of the electron beam modified TiO₂ (mTDNF) to 2.846 eV, a reduction of 0.36 eV from pristine TiO₂ (pTDNF). This reduces the transmission barrier and increases electron diffusion path. Surface defects and Ti³⁺ also helps to inhibit recombination of electron-hole pairs and this enhances chemical activity. When tested for use as glucose biosensor, glucose oxidase (GOD)/m-TDNF/Nafion/ glassy carbon electrode (GCE) assembly showed anode and cathode response currents that were 1.6 times higher than that of GOD/p-TDNF/Nafion/GCE. This has been attributed to the stronger electrostatic interaction between GOD and mTDNF. The (GOD)/m-TDNF/Nafion/ glassy carbon electrode (GCE) biosensor also showed a higher sensitivity of 12.5 µA.mM^-1cm^-2, a low limit of detection of 0.9 µM and fast current response of less than 3s.

In polymeric sensors, the sensitivity of the material may be dependent on its molecular arrangement and structure. Poly(inylidene fluoride) (PVDF) is known for its piezo-electric property and its performance is dependent on its molecular phase. The semi-crystalline polymer exhibits a nonpolar crystalline α phase, a crystalline polar β phase, crystalline γ phase and a crystalline δ phase. For piezoelectric performance, β phase is preferred. Li et al (2014) was able to increase the amount of β phase by adding silver nanowires (AgNWs) to the solution for electrospinning. The resultant electrospun membrane was able to show pressure sensitivity of 29.8 pC/N with optimum amount of 1.5% AgNW added.

Contact points for microelectronics

Smooth nanofibers used in microelectronics and sensors may face a practical difficulty in establishing proper contact between the fiber and the probe. Where there are beads on the fiber, the beads offer a much bigger target for contact with the probe. Good contact between the probe and the beads on the fiber will facilitate transfer of signal from the fiber to the signal receiver. Xue et al (2013) deliberately fabricated beads on fibers by mixing polystyrene (PS) particles into poly(vinylpyrrolidone) (PVP) solution and electrospinning. The PS particles form aggregates which are distributed periodically along the fiber. The size of the aggregates and the distance between them gets smaller and longer respectively as the volume ratio of PS particles against PVP gets lower. Sensor made from these beads on PVP fibers loaded with FeCl₃ and coated with polypyrrole was shown to be highly sensitive with NH₃ detection limit down to 1 ppb level.

Electrode Material

For sensor depending on changes in impedance for detection, the electrode material plays an important role in its sensitivity. Batool et al (2013) examined the effect of Ti, Ni and Au electrodes on the characteristics of TiO₂ nanofibers for use as humidity sensor. Their setup comprises of a base silicon substrate where the electrospun fibers were deposited. The 80 nm thick metal electrodes (Ti, Ni and Au) were deposited on top of the fibers. In their study, Ti was found to be the best electrode compared to the electrode made from Ni and Au. They attribute this to the more porous structure of Ti electrode which allows more water vapour to diffuse through the pores to reach the sensing material layer.

Intact fibers

Conventional method of coating sensor substrate with inorganic fibers involved using short strand fibers. However, there are several advantages if intact fibers are used instead. In gas sensor application, an intact fiber network allows better gas diffusion through the pores between the fibers. With an intact fiber network, the connections to the two electrodes are better as the long fibers are better able to bridge the gap. In contrast, electron transport may be inefficient in a fractured network as there are numerous joints, points of poor contact and dead end paths. Ning et al (2021) constructed a H₂S sensor by directly coating a ceramic tube with CuO-doped SnO₂ precursors using electrospinning and sintering to form CuO-doped SnO₂ fibers. By direct coating of the sensor part using electrospinning, it avoided the need for pulverizing the fibers for coating. Comparing the sensors constructed by in situ electrospun fibers on the ceramic tube carrier and coated with pulverized electrospun fibers, the former showed significantly better performance with faster response and recovery.

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