A Review on Electrospinning as Versatile Supports for Diverse Nanofibers and Their Applications in Environmental Sensing

Characteristics of the Electrospinning Solution

Polymer: Electrospinning has been recognized as a low-cost, convenient, and environmentally friendly method that is widely adopted to produce nanofibers with various morphologies and functions [12]. Water-soluble polymers, such as poly (vinylpyrrolidone), polyethylene oxide, poly (vinyl acetate), and polyvinyl alcohol (PVA), and solvent-soluble polymers, such as polyimide, PAN, polyvinylidene fluoride (PVDF), poly (methyl methacrylate), and polystyrene (PS) are the most common polymers used in electrospun nanofibers. Notably, the addition of multifunctional materials, such as metal oxides, inorganic non-metals, metal–organic frameworks, and covalent organic frameworks, to polymers allows the preparation of composite nanofibers with different morphologies, structures, and functionalities under controlled conditions [37–39].

Conductivity: The conductivity of the electrospinning solution is another key parameter that affects the morphology of electrospun fibers and contributes to jet formation. The addition of salts, ionic polymers, or conductive polymers to the electrospinning solution increases the solution conductivity and supports the formation of high-quality fibers with fewer defects and smaller diameters [40].

Surface Tension: In electrospinning, a trickle is ejected when the charged solution overcomes its surface tension. Generally, high surface tension in a liquid reduces its surface area, maintains its spherical shape, and is the predominant contributing factor towards the formation of liquid beads [40]. Therefore, the surface tension of the electrospinning solution is minimized to enlarge its surface area as much as possible to prevent the formation of liquid beads and reduce the fiber diameter. Routine approaches to reduce the surface tension of the solution include (1) the use of a solvent with low surface tension, such as ethanol or acetone, and (2) the addition of surfactants into the electrospinning solution.

Concentration and Viscosity of the Electrospinning Solution: Low polymer concentration typically results in low viscosity that causes the jet to be unstable and discontinuous. Such a low concentration also results in the formation of beaded nanofibers in which this phenomenon is known as the "flying filaments". Viscosity increases with increasing polymer concentration [41]. However, the flow of the solution is hindered when the polymer concentration exceeds a certain limit, causing nozzle blockage or what some authors refer to as "non-spinning" [13].

Equipment Parameters

Electric Field Intensity: Low voltage applied cannot overcome the surface tension of the electrospinning solution due to a low electric field, which hinders the stretching and splitting of the solution, resulting in larger nanofiber diameters [35]. As the electric field strength increases, higher surface charge density and electrostatic repulsion accumulate on the jet of the polymeric electrospinning solution. Simultaneously, a higher electric field strength accelerates the jet to a higher velocity. These factors contribute to the formation of fibers with greater tensile stress and finer textures. However, a high electric field strength over a certain limit induces a very fast jet flow and hampers its stretching and splitting, which consequently results in larger fiber diameters, worse uniformity, and beaded fibers [42–44].

Needle Pitch: The distance between the electrospinning needle and collector affects the electric field strength and volatilization of the electrospinning solution, thereby altering the diameter of the nanofibers. When the voltage is kept constant, the electric field strength is inversely proportional to the distance. In a typical electrospinning setup, the distance ranges from 10 to 15 cm, which usually allows sufficient evaporation of the solvent from the jet before being deposited as a dried fiber bundle. If the distance is too short to allow sufficient evaporation of the solvent of the jet, unfavorable fused fibers may be formed [44, 45].

Flow Rate: The electrospinning rate is controlled by the flow rate of the syringe. The electrospinning rate not only governs the production of high-quality nanofibers, but more importantly influences the jet stability and fiber diameter, which increases with decreasing electrospinning speed [32, 46].

Environmental Factors

Environmental temperature and humidity in the electrospinning process have a significant impact on the evaporation rate of jet solvents and the morphology and diameter of the produced fibers. Temperature is known to affect solvent evaporation, Brownian motion, and the diffusion of ions, atoms, molecules, and particles. The increased temperature of the electrospinning solution reduces its viscosity and elevates its solvent evaporation and Brownian motion, ultimately altering the fiber diameter [47]. Humidity also changes the volatilization rate of solvents in the spinning solution, affecting the void distribution on the surface of the electrospun nanofibers. Several studies have revealed a relatively smoother surface of electrospun fibers produced with less than 25% humidity and a rougher surface with more pores on the fiber surface as humidity exceeds 30%. Notably, very low humidity speeds up the solvent evaporation of a jet and increases its viscosity, both of which may cause nozzle clogging at the capillary tip. In contrast, high humidity aggravates the formation of dangerous conducting air and slows solvent volatilization [38, 48].

Individual Single Types of Electrospinning

Beaded Fiber: Beaded fibers are formed by unstable fluid jets under an external electric field. Owing to their unique structure, they can be applied in different applications. Moreover, it has become possible to carry out the corresponding theoretical modelling on the formation of beaded fibers with the rapid advancement in electrospinning technology in the recent years [30, 49, 50].

Core-Shell/Hollow Nanofiber: Electrospun core-shell/hollow nanofibers have been widely assessed and applied by many researchers in sensing systems. These nanofibers can be produced by coaxial or emulsion electrospinning. In the former process, the outer and inner electrospinning solutions are delivered to two capillaries with different inner diameters. Thereafter, they combine to form a coaxial Taylor cone followed by coaxial fluid jets in a high-voltage electrostatic field, of which the jets solidify into core-shell nanofibers with different interior and exterior contents. If the core layer is removed via heating or dissolution, hollow nanofibers are formed with a hollow inner lumen surrounded by an intact outer layer.

Prior studies revealed that polymeric electrolytes based on ionic liquids do not only have high conductivity, but also excellent mechanical properties [51–53]. Ionic gels with high capacitance can be used as thin films to prepare capacitive sensors [54]. However, the surface of a hydrogel (ionic gel) is prone to dehydration. Therefore, a protective mechanism must be developed to prevent water from evaporating from its surface [55]. In addition, surface area affects the capacitance of an ionic hydrogel matrix film and limits the sensitivity and stability of its capacitive sensor device. Lin et al. [29] overcame the above-mentioned problems using the coaxial electrospinning method. An ionic gel was employed as the core with the copolymer PVDF as the outer shell to produce a core-shell nanofiber mat. The advantages of a co-polymer shell include its larger surface area and sealed protective layer that effectively prevents dehydration of the core-shell ionic gel.

Liu et al. [56] developed a chemical sensor based on the core-shell structured semiconductor nanofibers through rapid single-nozzle electrospinning process and spontaneous phase separation. The rich active sites of the core-shell structured nanofibers effectively promote interactions between charge carriers in their conductive channels and chemical analytes, and change the output current and charge carrier mobility that lead to a high sensitivity in the resulting sensor. The shell thickness in a core-shell structure can be adjusted by changing the ratio of the outer and inner electrospinning solutions in the mixed solution. A nanofiber-based sensor with an appropriate core-shell ratio exhibits high sensitivity toward low concentrations of ammonia (the limit of detection was as low as 50 ppb).

Hollow nanofibers with porous surfaces and tubular structures also exhibit outstanding properties. Electric current can easily flow around their disturbance zone and target contaminants can freely diffuse in and out of them, thereby shortening the response and recovery time [57]. Park et al. [28] took advantage of the Kirkendall effect to fabricate porous hollow nanofibers of SnO₂-CuO nanocomposite by single-needle electrospinning. The Kirkendall effect arises from the supersaturation of lattice vacancies caused by the different diffusivities of distinct atoms. A nanotube structure is formed after calcination, which is attributed to the Kirkendall effect [58]. A fast response was achieved with the prepared SnO₂-CuO hollow nanofiber-felt for the detection of H₂S gas with a rapid detection time of 5.27 s and a low concentration range of 1–40 ppm.

Porous Fiber: Emerging porous materials generally have channels, pores, and gaps on their surfaces or interiors. These materials also usually have the characteristics of low density, high specific surface area, high porosity, and high adsorption capacity and thus widely adopted in sensors, filtration, catalysis, and other applications [59–61] reported that the calcination of electrospun PVA composite nanofibers containing silica sol nanoparticles at 450 °C produced porous inorganic nanofibers with high specific surface areas. Liu et al. [62] prepared porous alumina nanofibers with hollow structures by sintering aluminum-nitrate/PAN nanofibers through high-temperature calcination. Ma et al. [63] prepared the porous PAN nanofibers by washing electrospun PAN/NaHCO₃ composite nanofibers with a 10% hydrochloric acid solution to dissolve and remove NaHCO₃, and the escaping CO₂ gas "blown" nanopores in the PAN nanofibers. The electrospun porous nanofibers also had discontinuous pore structures in the interior or open pore structures on the surface. The porous structure of nanofiber materials provides superior properties, including large specific surface areas and reduced thermal conductivity, which further broadens their application range.

Others: Electrospinning can be employed to generate fibers of various shapes, such as beaded, core-shell, ribbon, porous, spiral, square, and honeycomb (Fig. 2). Zhang et al. [22] prepared membranes with electrospun ribbon nanofibers and suggested that the formation of a fiber-ribbon shape could be attributed to the positive charge of the polymer precursor. According to the study, a jet carrying negative charge on its surface attracts the positively charged polymer precursor to its surface during the electrospinning process, leaving its center solvent-rich and forming a tube-shaped structure with a thin outer layer. When the residual solvent in the tube center evaporated, a partially collapsed hollow tube was formed with wrinkles on the surface. When the hollow tube collapsed completely, a ribbon-like structure was formed. Shin et al. [26] reported a facile process for producing electrospun spiral nanofibers with a single polymer and demonstrated the transformation of nanofibers from spiral to linearly oriented with an adjustment in electric field strength.

Aggregate Electrospinning Structure

Figure 2g–j shows the great diversity in the structures and morphologies of electrospun fiber membranes. The structural diversity mainly depends on the collector type, which is one of the most important parts of the electrospinning equipment and determines the distribution and macrostructure of the electrospun fibers. Other aspects of the processing equipment, such as the electric field strength, presence of a magnetic field, and composition of the electrospinning solution, can be optimized to obtain the desired fiber mat structures. The collected electrospun fibers or fiber mats are generally divided into three groups: random fibers, arranged fibers, and pattern fibers.

Random Electrospinning: Random fibers are the most common form of electrospun fiber collection. Randomly oriented fibers can be obtained simply using a traditional electrospinning collector. Charged polymer jets naturally travel along spiral trajectories during the electrospinning process and finally solidify into randomly oriented continuous fine fibers on a ground plate substrate.

Oriented Electrospinning: Regularly oriented electrospun nanofibers can be prepared using a special collector and electrodes. Park et al. [25] prepared a hybrid scaffold with a dual configuration of aligned and random electrospun fibers. Wang et al. [62] prepared ordered parallel arrays of PVA nanofibers using similar hybrid scaffolds and successfully applied them to the detection of low concentrations of NO₂.

Patterned Electrospinning: Patterned electrospun nanofibers can be produced using appropriate solutions and a special collecting device. Liang et al. [24] prepared an oriented square microfiber mat using an electrostatic template for nanofibers deposition on a pattern collector made of regularly distributed protrusions. Gangolphe et al. [23] developed honeycomb electrospun scaffolds and successfully applied them to prepare honeycomb fibrous membranes with appropriate macropore size and fiber arrangement. These researchers also assessed the anisotropic and mechanical properties, and the degradation mechanism of honeycomb membranes.

Resistive Gas Sensor

Compared with traditional gas detection equipment, portable and inexpensive gas sensors have outstanding advantages in terms of shorter detection times, without sacrificing detection accuracy. Resistive gas sensors are one of the most common types of gas detection sensors [76–79]. A resistive sensor is based on changes in the electrical resistance or conductivity caused by the adsorption of gas molecules. Therefore, the response rate depends on the interaction speed and strength between the target gas molecules and the sensor material surface, and the specific surface area available for adsorption and desorption [80]. In an oxygen-rich environment, oxygen molecules adsorbed onto sensor surfaces extract electrons from the conduction band and trap them as negatively charged ions on the surface, forming different surface energy levels. Accordingly, an electron depletion layer and a hole accumulation layer are formed on the sensor surface for the n- and p-type materials, respectively. When switched to a target gas-rich environment, the target gas molecules are adsorbed onto the sensor surface and exchange electrons with it, causing a conductivity change in the sensing material. The resistance/conductivity of gas sensors depends on the charge transfer mechanism between the adsorbed gas molecules and gas-sensitive adsorbents, as well as their surface reactions. When an n-type sensing material is exposed to oxidizing gas, its resistance increases, whereas exposure to reducing gas decreases its resistance [81, 82]. Figure 3 shows the principle of the gas-sensing mechanism of a resistive gas sensor based on the n-type and p-type sensing materials [83].

According to the configuration, resistance gas sensors can be further divided into ceramic tubular, flat, and flexible sensors. The structure and physical diagrams of various resistance gas sensors are shown in Fig. 4. For a long time, the ceramic tubular sensor was the main type owing to its simple preparation process and low cost [85, 86]. As shown in Fig. 4a, the ceramic tube is composed of four parts: ceramic tube, Ni-Cr heater, Au electrode, and Pt wire. The heater is placed in a ceramic tube to maintain the working temperature. Two rings of gold electrodes are placed on the outer wall of the ceramic tube, on top of which is the deposited sensing material for electrical signal production.

When the sensing interface of the gas sensor is porous, the target gas molecules can penetrate the entire sensing layer, thereby improving sensitivity. In addition, a large specific surface area facilitates the interaction between the gas molecules and the sensing material. Commonly used chemical solution deposition technology and ceramic slurry sintering technology for preparing gas-sensitive materials can hardly meet the requirements of porosity and large specific surface area at the same time. However, it is difficult to uniformly coat gas-sensitive materials onto the ceramic tube surface using traditional techniques. Hence, they easily aggregate, their pore sizes are not uniform, and gas molecules at their sensing interface cannot be well diffused. In contrast, sensing layers prepared via electrospinning can fulfill the requirements of both high porosity and large specific surface area, and nanofibers have controllable fiber and pore sizes. For different gases, sensor layers with suitable pore sizes can be developed to effectively overcome the gas diffusion limitation, enabling unimpeded penetration of the sensor layer by gas molecules. Han et al. [90] used atomic layer deposition to deposit n-type ZnO on the surface of CuO-polymer hybrid nanofibers. Subsequently, the polymer molecules were removed via calcination to form hollow p-CuO/n-ZnO nanofibers (Fig. 5a, b). The optimal responses of the gas sensor based on the p-CuO/n-ZnO heterostructure were approximately 6- and 45-fold higher than those of pure ZnO and CuO, respectively. Similarly, Zhang et al. [91] reported the use of electrospinning to prepare nanofibers with hierarchical porous heterostructures (p-type Co₃O₄ and n-type In₂O₃). Based on their findings, Co²⁺/Co³⁺ with high catalytic activity diffused in In₂O₃ with uniform distribution (Fig. 5c). Their results revealed that the gas sensor had a remarkably high response to VOCs, including acetone and formaldehyde. In addition to the heterogeneous structure, the size of the sensing material has a significant impact on its sensitivity. Li et al. [92] successfully prepared electrospun ZnO/ZnFe₂O₄/Au nano-mixtures with controllable particle sizes using a combination of electrospinning and atomic layer deposition techniques (Fig. 5d). Their results showed that the response of the prepared ZnO/ZnFe2O4/Au nanocomposite to acetone was 3- and 5.5-fold higher than that of the ZnO/ZnFe₂O₄ composites and ZnO, respectively. More interestingly, Zhang et al. [93] synthesized Cr₂O₃-TiO₂ core-shell fibers with different shell thicknesses through a simple coaxial electrospinning technology and discovered that the shell thickness affected the gas sensor performance. As the TiO₂ shell thickness increased, the response characteristics of the prepared core-shell fiber gas sensor to acetone showed a transition from the p-type to n-type. In addition, the formation of a Cr₂O₃-TiO₂ heterojunction in the gas sensor enabled a quicker response to the target gas molecules and a shorter recovery time.

In flat-type sensors, sensing materials are often deposited on the insulating substrate surface of an interdigital electrode (IDE) as shown in Fig. 4b. IDE sensors are favored in the fields of biomedicine and the environment owing to advantages such as miniaturization, low-cost, and large-scale production [94, 95]. Bulemo et al. [96] suggested that gas analytes diffusing into semiconductor metal oxide-based sensing layers require gas-sensing materials to be thin and porous, and exhibit high performance. In their study, hollow SiO₂-SnO₂ core-shell microstrips were prepared by etching away the SiO₂ core with NaOH solution (pH 12) from electrospun calcined highly porous SiO₂-SnO₂ core-shell microstrips (Fig. 6a, b). After sensitizing with platinum nanoparticles (Pt NPs), the prepared hollow nanofibers were used to detect acetone, exhibiting a highly stable response (R_a/R_g = 93.70 ± 0.89) and significant selectivity to as low as 2 ppm acetone in air. Electrospinning has also been employed as an innovative functionalization method for manufacturing polymer nanofibers with a diameter range of 50–500 nm and a nano-network on the movable plate of a capacitive micromachined ultrasonic transducer (CMUT). Zhao et al. [97] reported a CMUT-based resonant biochemical sensor that was prepared to detect SO₂. The resonance frequency shift of this sensor rapidly changed as the SO₂ concentration changed (Fig. 6c, d). Bai et al. [98] synthesized a layered heterostructure composed of thin MoS₂ nanosheets vertically grown on SnO₂ nanotubes through simple electrospinning and hydrothermal strategies (Fig. 6e). This heterojunction expands the surface area of MoS₂@SnO₂, thereby providing more adsorption sites for gas adsorption. The heterostructure formed by the nanofibers facilitated electron communication between MoS₂ and SnO₂, which promoted charge generation and enhanced the charge separation efficiency (Fig. 6f). The prepared sensing interface MoS₂@SnO₂ responded to 34.67–100 ppm NO₂ at room temperature in a time-period as less as 2.2 s.

In recent years, flexible sensors have attracted considerable attention. Flexible gas sensors are key components of portable electronic devices, which have been widely used for on-site environmental and health monitoring owing to their small size, light weight, and strong flexibility [99–101]. Nair et al. [102] fabricated a CNFs@Ni-Pt nanohybrid on a flexible polyester substrate by electrospinning and chemical reduction and tested it with an H₂ sensor (Fig. 7a, b). The integration of bimetallic Ni and Pt nanocatalysts on CNF provides more active sites for hydrogen sorption and improves the hydrogen sensing performance of the sensor. Based on this sensor, the research team achieved a wide range of concentration (0.01–4%) of hydrogen detection. Furthermore, after several bending cycles, a good response to H₂ (50% strength compared with that before bending) was still exhibited owing to the high aspect ratio of the carbon nanofibers. Khalifa and Anandhan [103] developed a gas sensor based on electrospun blend nanocomposite film (EBNC), which was shown in Fig. 7c, d. The EBNC sensor showed good response (approximately 92% saturation with 108 ppm analyte) and high selectivity for NO₂ gas. Furthermore, this sensor was stable for more than 30 days and 100 operation cycles, demonstrating excellent durability. Notably, even after repeated bending cycles, the performance of the sensor did not decrease. This result demonstrates that the electrospun nanofibers were flexible, and their sensing performance was not affected by the bending stress.

Optical Gas Sensor

Although optical sensing technology appeared relatively late, its development is the fastest in gas-sensing technology. Commonly used types in the industry include ultraviolet analyzers, infrared gas analyzers, light scattering analyzers, photoelectric colorimetric analyzers, chemiluminescence analyzers, etc.

Fluorescent probes have been widely explored because of their convenient non-invasive operations [104]. The fluorescence of traditional organic fluorophores is often affected by aggregation-caused quenching (ACQ), hence, the stability of sensors made of them is also seriously affected. Fluorescent agents that exhibit aggregation-induced emission (AIE) characteristics have attracted much attention because they can overcome the ACQ problem [105–107]. In addition, volatile acidic and alkaline gases such as hydrochloric acid and ammonia are common hazardous components in flue gases emitted from industrial production processes. They are usually toxic, harmful, and corrosive, and are extremely detrimental to human health. It was demonstrated that organosilicon precursors with AIE characteristics covalently attached to periodic mesoporous organosilicon (PMO) frameworks had high fluorescence efficiency [108, 109]. Electrospun three-dimensional ordered porous materials provide abundant active sites for gas adsorption/desorption, allowing them to be used for the quick and sensitive monitoring of pollutant gases. Gao et al. [110] prepared a flexible film of dispersed periodic mesoporous organosilicas (PMOs) nanospheres in a mixed fiber matrix (PMO-CFs) by electrospinning and assembly technologies and successfully applied it as an efficient fluorescent probe for the detection of ammonia and hydrochloric acid vapor by naked eyes, which was shown in Fig. 8a–c. In addition, the PMO-CF sensor was easy to be regenerated and very stable to light over a long period of time (fluorescence still above 94% after 10 cycles of regeneration).

Polymers that have been utilized in optical oxygen sensors thus far include polystyrene, hydrogel, silicone rubber, various copolymers, and fluorinated polymers [112–114]. The inclusion of fluorine can increase the rate of diffusion of oxygen molecules in fluorinated copolymers [115], which promotes the sensor to have higher sensitivity owing to the high quenching efficiency [114]. Polymer microfibers, notably fluorocopolymer fibers, are promising functional materials for optoelectronic devices, sensing, and energy storage due to their different surface properties [116, 117]. However, the existing fluorine-containing copolymers broadly face notorious drawbacks of low fluorine content, poor fiber morphology and non-uniformity, hampering their application in sensor devices [118, 119]. For this reason, Mao et al. [111] originally proposed a thin porous microfiber membrane of platinum porphyrin grafted poly(isobutyl methacrylate-co-dodecafluoromethacrylate) copolymer with high fluorine content and applied it to an optical oxygen sensor (Fig. 8d, e). Unlike many solid sensor membranes, porous sensor membranes can promote the permeation of oxygen molecules, and the large surface area of fluorine in the fiber can offer more sites for analyte interactions and signal transduction [120, 121]. Therefore, the sensitivity of the fabricated optical gas sensor with a fluorine-containing microfiber membrane was greatly enhanced to be 584% higher than that of the solid sensor membrane.

Electrospun liquid crystal fiber mats have also been established to be a promising medium for gas sensing [122, 123]. Liquid crystals can operate at room temperature and do not require any input of energy as they are powered only by thermal energy and provide a strong optical response. They can conveniently detect gases without intricate spectroscopy equipment. Even low-concentration gases will strongly affect the self-assembly of liquid crystals [124, 125]. Reyes et al. [126] demonstrated the application of a polyvinylpyrrolidone-5CB fiber mat to qualitatively detect the presence of toluene gas by optical measurement. Wang et al. [127] suggested that the electrospun fiber mat made from polylactic acid (PLA) and amyl-cyanobiphenyl (5CB) could quantitatively detect toluene and acetone vapor. Agra-Kooijman et al. [128] reported a resistive liquid crystal core polymer fiber mat (LCC PFM) sensor, in which LC/polymer fibers were obtained after electrospinning on the IDE substrate. The results showed that they responded well to low concentrations of acetone at room temperature. These studies have revealed that chemicals can pass through the polymeric shell of the fiber and be absorbed by the liquid crystal in the core. Upon absorption, the optical properties of the fiber mat were altered by phase transition, which could be adopted for the sensitive and selective detection of volatile organic compounds.

Lead(II) acetate ((Pb(Ac)₂) reacts with hydrogen sulfide to form a brown lead sulfide precipitate. Up to now, Pb(Ac)₂ has been used as an indicator in the form of test papers with a detection limit of 5 ppm to detect H₂S gas leaks. The Cha research group [129] successfully fabricated porous nanofibers with high surface area and thermal stability using Pb(Ac)₂, overcoming the limitations of traditional Pb(Ac)₂-based fibers H₂S sensors (Fig. 9). The nanostructure could not only prevent the aggregation of particles but also provided a variety of reaction sites. This sensor material detected H₂S as low as 400 ppb at 90% relative humidity.

QCM Gas Sensor

Quartz crystal microbalances (QCM) have attracted attention in gas detection owing to their high accuracy, low power consumption, and high stability. The detection mechanism is based on the load mass changes after the sensor layer absorbs gas molecules, which translates to changes in the resonant frequency of the quartz crystal [130–132]. The gas detection capability is highly dependent on the appropriate choice of nanomaterials, including metal oxides, organic polymers, and carbonaceous materials that make up the QCM sensor layer. For instance, a tungsten oxide film can detect NO₂, while graphene oxide can detect formaldehyde (HCHO) [133–135]. Despite the plethora of nanomaterials to select from, they lack porous structure and sufficient adsorption sites to absorb a considerable amount of target gas molecules. To further improve the ability of QCM sensor materials to absorb more gas molecules and their sensitivity, electrospinning technology has been introduced to produce highly porous sensor materials.

Kang et al. [136] used electrospinning and oxidative polymerization techniques to successfully prepare SnO₂ nanofibers (NFs)/PDA composites that were quantitatively deposited onto the QCM electrode surface. The SnO₂ nanofibers prepared in polymeric composites had rough morphology, which promoted the adsorption of gas molecules. The agglomerated PDA microspheres adhered to the surface of SnO₂ NFs and revealed a compact composite structure with a larger specific surface area exposing more active sites. The gas sensor prepared in this way showed a high sensitivity for formaldehyde sensing. Gas adsorption capacity is generally related to the surface functional groups and defect sites of the sensor materials. In addition to the porous nanofibers fabricated by electrospinning technology, the active metal oxides on the fiber surface were also crucial for gas adsorption, wherein the oxygen atom and imine functional group hydrogen-bonded with formaldehyde molecules [137, 138]. Moreover, the amine functional group in PDA nanospheres had undergone condensation with the aldehyde groups in formaldehyde to some extent [139, 140], thereby greatly improving the sensitivity. The fabricated nanofibrous QCM sensor achieved a detection limit of 500 ppb formaldehyde.

Diltemiz’s research team [141] employed similar principles to prepare CuO-ZnO nanofibers for detecting formaldehyde as low as 41 ppb and the optimal detection frequency reached 8 Hz ppm⁻¹. Zhang et al. [142] made use of centrifugal electrospinning equipment to fabricate aligned nanofibers to selectively adsorb CO molecules against control nitrogen gas. Based on the Sauerbrey equation, it was found that an exceptionally high amount of CO (72.36 ng) was adsorbed on the nanofibers with a maximum frequency change of − 54 Hz for 50 ppm of CO feed, which was attributed to the large nanofiber surface area.

Electrochemical Sensors

Electrochemical techniques have been extensively studied for their application in the detection and speciation of heavy metals. These electrochemical analyses normally use cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), chronoamperometry (i–t), linear sweep voltammetry (LSV), and stripping voltammetry (SWSV) for that purpose. The measurements from the electrochemical analysis are typically registered as electrical signals such as resistance and current. Therefore, the electrochemical analysis equipment is relatively simple to analyze, easy to automate, and performs continuous analysis.

Chemically modified electrodes refer to the molecular design and modification of the electrode surface by chemical means to produce the electrode with specific electrochemical properties [159]. By endowing materials with excellent properties (catalytic effect, photoelectric effect, etc.) on the electrode surface, the conductivity of the electrode can be greatly improved, accelerating the electron transfer efficiency, while increasing the electrode response, thereby improving their selectivity and sensitivity as potential sensors [159, 160].

Common materials used in electrospinning are conductive polymers, carbonaceous, and metal nanoparticles [160]. Among the polymeric nanofibers, polyaniline (PANi) turned out to be the most commonly investigated conductive polymer owing to its one-dimensional structure that ensures high conductivity and surface area, which plays an extremely important role in the preparation of the electrode. Promphet et al. [161] uncovered the development of graphene/polyaniline/polystyrene (GO/PANi/PS) nanoporous fiber-modified screen-printed carbon electrode (SPCE), which was successfully solved by square wave anodic stripping voltammetry (SWASV) for the simultaneous determination of lead (Pb²⁺) and cadmium (Cd²⁺) in the range between 10 and 500 μg L⁻¹, with detection limits of 3.30 μg L⁻¹ (Pb²⁺) and 4.43 μg L⁻¹ (Cd²⁺). Similarly, Huang et al. [162] used phytic acid-doped polyaniline nanofiber-based nanocomposites to modify glassy carbon electrodes for Cd²⁺ and Pb²⁺ detection with EIS and differential pulse anodic stripping voltammetry (DPASV) proved that the synergistic effect of polyaniline nanofibers and phytic acid had enhanced the charge transfer efficiency of metal ions. Due to the synergistic effect of PANi, the detection limit of Cd²⁺ in the range of 0.0560 μg L⁻¹ (S/N = 3) was 0.02 μg L⁻¹, and that of Pb²⁺ in the range of 0.160 μg L⁻¹ was 0.05 μg L⁻¹.

Besides PANi, carbon nanofibers (CNFs) have attracted attention because of their high mechanical strength, large specific surface area, excellent electrical conductivity, and strong corrosion resistance. The PAN-based CNFs prepared by Zhao et al. [163] using electrospinning technology was eight times more sensitive to trace amounts of Pb²⁺ with a detection limit of 0.9 nM under anodic dissolution voltammetry analysis. Furthermore, Gao et al. [164] prepared nitrogen- and sulfur-codoped PAN-based CNFs by the pyrolysis of trithiocyanuric acid and silica nanospheres, which showed the advantages of exhibiting large surface area (109 m² g⁻¹), porous structure and high proportion of heteroatoms (19 at.% of N and 0.75 at.% S). The electrochemical sensor fabricated using the high-performance porous CNFs was highly responsive to trace Cd²⁺ in the range of 2.0500 μg L⁻¹ using DPASV. Tang et al. [165] formed PANi nanosheet arrays on electrospun Fe-CNFs substrates and deposited AuNPs uniformly on the nanosheet surfaces with reducing characteristics, which was shown in Fig. 10a, b. The presence of Fe in CNF accelerated the growth of PANi nanosheets and improved their adsorption of As³⁺, responding to a wide linear range (5–400 ppb) and low detection limit of 0.5 ppb (S/N ≥ 3).

Metal oxides and metal nanoparticles are also widely applied in electrochemical sensors due to their significant electron transfer kinetic ability, larger specific surface area, and availability of adsorption sites [166]. ZnO is an excellent sensor material due to its high adsorption capacity, biocompatibility, and high chemical stability. The one-dimensional nanostructure can facilitate the diffusion of ions from the electrolyte to the surface of the sensor material more rapidly. Therefore, the production of nanofibers with highly uniform one-dimensional nanostructures by electrospinning is a feasible approach. Oliveira et al. [167] reported the synthesis of ZnO nanofibers/l-cysteine (ZnO/l-cys) nanocomposite electrode for electrochemical sensing of Pb²⁺. Under the SWASV analysis, highly sensitive quantification of Pb²⁺ (LOD = 0.397 μg L⁻¹) was achieved in the linear range of 10,140 μg L⁻¹. Girija et al. [168] corroborated that cobalt and zinc ions form a stable tetrahedral structure with the zeolite imidazole ester framework under electrostatic action. On that basis, the cobalt–zinc–zeolite imidazole framework (Co/Zn-ZIF) nanofibers were successfully synthesized to detect heavy metal ions using CV analysis, uncovering the catalysis effect of the electrode to facilitate electron transfer between the electrode surface and metal ion. The Co/Zn-ZIF nanofibers effectively detected Cd²⁺ with low interference in the range from 100 nM to 1 mM and a detection limit of 27.27 nM. Teodoro et al. [169] used polyamide (PA6), cellulose nanowhiskers (CNW), and silver nanoparticles (AgNPs) to fabricate electrospun nanofiber electronic tongues for the quality assessment of water samples. It was found that the nanofiber mat was rich with pores and channels, which provided a multidisciplinary interaction between the binding site and analytes. With high aspect ratio, high biodegradability, and low density, CNW was uniformly blended in a co-system with metal nanoparticles for increased conductivity, enabling the prepared electronic tongue sensor to effectively discriminate different heavy metal ions with detection limit for Pb²⁺ in aqueous solution only 10 nM.

Colorimetric and Fluorescence Sensor

The binding constant of the analyte of interest and the nature of sensor molecules are the main factors affecting the sensitivity, specificity, reusability, and stability of the sensor. For the colorimetric sensor interface, the signaling molecule would need to induce responsive color change [173, 174]. Electrospinning has the advantages of fabricating a three-dimensional structure with a large specific surface area and excellent biocompatibility, which can provide more binding sites in the sensor molecules, and effectively improve the sensitivity and response rate of the sensor [174, 175]. Moreover, the low cost, simple operation and facile surface functionalization of electrospinning make it widely employed in many fields [176]. In recent years, colorimetric sensors constructed using electrospinning as the sensor interface have been used to detect metal ions with careful design of organic chemistry. Thence, with the development of nanotechnology, colorimetric methods based on surface plasmon resonance (SPR) of metal nanomaterials have progressively been introduced.

The core of the color sensing system is represented by the specific recognition molecules that can generate and transmit color signals, mainly using organic dyes or transition metal complexes. The recognition molecule and the analyte selectively interact by non-covalent bonding or covalent bonding, which induces the charge transfer or spatial structural change of the recognition molecule leading to the macroscopic manifestation in color change.

Rhodamine B (RhB), widely used for industrial coloring and as a fluorescent probe, is revealed to be a reliable color signal transmitter because of its long absorption and emission wavelengths, high photostability, high absorption coefficient, and quantum efficiency. Parsaee et al. [177] prepared Au NPs using Gracilaria under ultrasonication, and used electrospinning technology to modify silica gel membranes in the presence of RhB. The nanofibers immobilized with gold nanoparticles and RhB were used to detect Hg²⁺ in real water samples using colorimetry and fluorescence at detection limits of 2.21 nM and 1.10 nM, respectively. Moreover, the sensor can be reused multiple times with high integrity upon regeneration under oxidation by air. Similarly, Li et al. [170] used copolymers of rhodamine and quinoline propylene-based monomers to produce electrospun NFMs that acted as solid-state sensors and exhibited a high degree of Fe³⁺ selectivity, which was shown in Fig. 10c. In addition, the NFM realized the obvious color change from colorless to pink within a minute and its LOD was 1.19 μM.

Poly(aspartic acid) (PASP) is a synthetic poly(amino acid) with several unique properties, including strong metal ion binding ability, biocompatibility, biodegradability, water solubility, and low toxicity. Zhang et al. [172] developed a reuseable PASP-based colorimetric sensor with nanofiber structure for Cu²⁺ and Fe³⁺ detection, with a large amount of Cu²⁺ or Fe³⁺ accumulating on the PASP-based NFM after filtration, which was shown in Fig. 10e, f. Upon exposure to an aqueous Cu²⁺ solution, the color of the sensor changed from white to blue and the detection limit was reported to be 0.3 mg L⁻¹. In contrast, the membrane changed color from white to yellow, with a detection limit of 0.1 mg L⁻¹ for Fe³⁺ aqueous solution.

Noble metal nanoparticles have a small size effect, surface effect, and quantum tunneling effect, and exhibit unique physicochemical properties. SPR can be described by the reaction of a large number of freely conducting electrons in precious metals to external incident electromagnetic waves. When the electronic oscillation frequency is equal to the incident light frequency, surface plasmon oscillation occurs, enabling the noble metal nanoparticles to produce strong absorption and scattering spectra in the visible light range. The spectral peak position is highly dependent on the size and distribution of the nanoparticles, and any changes in the external environment. When the radius of the nanoparticles is larger than 3.5 nm, the clustering of nanoparticles causes surface plasmon coupling, and a strong color change is observed in the visible light range.

The Au/Ag NPs were fixed on the aminated PAN nanofiber membrans (NFMs) to obtain a test strip with a porous structure of a large surface area (38.6 m² g⁻¹). The color of the NFM measured at a wavelength of 420 nm, underwent a redshift when exposed to Cu²⁺ and the color changed from yellow to pink to colorless. This effect was due to the leaching of Au/Ag NPs from NFM in the presence of ammonium chloride, thiosulfate, and Cu²⁺ and the formation of soluble thiosulfate complexes of Ag⁺, Au³⁺, and Cu²⁺. On that basis, Abedalwafa et al. [171] achieved colorimetric detection of Cu²⁺ in drinking water samples by developing electrospun nanofiber membranes with Au/Ag core-shell nanoparticles, which was shown in Fig. 10d. Under optimized conditions, this method has the advantages of low detection limit (50 nM at S/N = 3), fast determination time (3 min), good specificity, and excellent reversibility.

Spectrometric Sensor

Surface-enhanced Raman Scattering (SERS) technology overcomes the inherent shortcomings of traditional Raman spectroscopy with weak signals and can increase the Raman intensity by several orders of magnitude. One of the main research directions in the SERS research area is the development of SERS substrates. The fabrication of SERS substrates usually relies on the surface plasmonic properties of the noble metal nanoparticles [178]. The nanoparticles loaded onto the electrospun polymer fiber mat, provides a larger surface area that can effectively enhance the detection signal and is considered to be an effective SERS substrate [179]. Xu et al. [180] fabricated a 3D SERS substrate composed of electrospun polycaprolactone (PCL) fibers and silver-coated gold nanorods (Ag/AuNRs) to detect traces of heavy metals in the environment. The successful fabrication and fixation of Ag/AuNRs on PCL fibers benefited from electrostatic interaction and excellent charge transfer between the gold core and the silver layer in the bimetallic structure. Following that, the detection of trace concentrations of organic arsenic, arsenic acid and roxarsone was achieved. Zhang et al. [181] used electrospinning technology to produce for the first time polyacrylonitrile (PAN)/noble metal/SiO₂ nanofiber mats with plasma-enhanced fluorescence activity. These nanofiber mats selectively increased the fluorescence intensity of conjugated polyelectrolytes (CPE), making the polymer/noble metal nanofibers a promising substrate with improved sensitivity for metal ion detection.