Application Area for the Electrospun-Nanofibers-Based Chemosensors

With the acceleration of the industrialization process, pollution control has become a hot issue worldwide. Electrospinning technology has been widely applied in pollution treatment due to its own properties, including the adsorption of heavy-metal substances in water [171], filtering of the pollutants in the air [57], and catalytic degradation of organic pollutants [172]. At the same time, chemosensors prepared by electrospinning technology play an important role in environmental pollution detection. This paper introduces the application of electrospinning technology in chemosensors and explores the advantages and future prospects of different structural NFs in the development of environmental-monitoring chemosensors. Electrospun NFs often have some remarkable properties, including a special three-dimensional morphology, a large specific surface area, and ease of preparation. Excellent biocompatibility, degradability, and the ability to repeat NFs make environmental monitoring more environmentally friendly and convenient, greatly reducing the pollution in the detection process. The following presents chemosensors of NF platforms for monitoring water, air, and land pollution

1.Water Pollutant

At present, due to the rapid increase of pollution sources caused by excessive industry, a large number of harmful pollutants flow into surface water and groundwater, and the concentration levels of various pollutants are increasing; particularly, dyes, drugs, pesticides, and other industrial products make water pollution a key problem in the world. Water pollutants are harmful and can have detrimental effects on both human health and the environment. They can be absorbed by plants, and through the food chain, they can cause indirect land pollution. Certain pollutants have been linked to cancer and mutagenesis, posing a significant risk to human beings. To address this problem, electrospinning technology has been utilized to prepare NFs with adjustable properties, such as a specific surface area, composition, porosity, thickness, and diameter. These NFs have been used to develop chemosensors with excellent sensing properties, allowing for the detection and analysis of water pollutants. Due to their large specific surface area and large active site, the various functional metal-modified NFs have high chemical and electrochemical stability, which helps to improve the high stability and significant conductivity of the sensor. NF-based sensors have the advantages of specificity, stability, and low cost. NFs with specific structures can be used to directly capture specific targets, bind to nanomaterials, amplify response signals, and also serve as oxygen functional groups at binding and analyte recognition sites to improve detection performance. Hua et al. demonstrated the preparation of a sulfonylcalix[4]arene-functionalized NF membrane (sulfonylcalix–NFM) as a sensor and adsorbent for the detection, identification, and enrichment of Tb3+ ions through electrostatic attraction loading of anion sulfonyl-calix[4]arene onto the surface of cationic NFs [173]. The adsorption

mechanism of Tb3+ ions was attributed to the complex formation between sulfonylcalix[4]arene loaded on NFM and Tb3+ ions. The combination of Tb3+ on sulfonylcalix–NFM and sulfonylcalix[4]arene exhibited a good photoluminescent performance and selective fluorescence recognition, with a low detection limit for Tb3+ ions under ultraviolet radiation. The adsorption of Tb3+ ions onto sulfonylcalix–NFM is believed to occur via electrostatic interactions between SO3 groups and Tb3+ ions, as well as through synergistic coordination of sulfonyl O and two adjacent phenoxy O− (Figure 5). NF membranes were prepared by using electrospinning technology to achieve fluorescence recognition and enrichment adsorption of heavy metal ions, demonstrating a novel method for the development of electrospinning technology in the field of environmental detection. Chen et al. prepared porous NFs by using ultra-efficient liquid chromatography and mass spectrometry to analyze and detect residual sulfonamide in ambient water samples [174]. They found that polystyrene (PS) porous NFs were effective materials for preparing different polar sulfonamides in wastewater due to their high specific surface area, and the interaction of the formation mechanism causes phase separation of solvent and non-solvent evaporation. The surface morphology of the NFs may improve the absorption efficiency of the drugs. These results suggest that porous electrospinning NFs can be a promising adsorbent with great potential in detecting drug residues in a complex wastewater matrix. The porous structure of the NFs facilitates and accelerates the interaction between the analyte and recognition site. Qi et al. prepared core-and-shell-structure polypyrrole (PPy)-functionalized NFs that could detect trace amounts of sulfated azo dye in water in the environment [175]. The PA6/PPy core–shell-structure NFs composed of PPy NFs have good plasticity and mechanical properties, providing perfect ductility and mechanical properties for better extraction of trace sulfated azo dyes from aqueous solution. Core–shell-structured NFs can improve the recovery and repeatability of sulfated azo dyes, and PPy NFs not only extract target compounds but also effectively remove interference. Simple, fast, sensitive, and uninterrupted, this method can provide a good alternative tool for the determination of contaminants in water samples. Abedalwafa et al. made aptamer immobilized electrospun-NF membranes (A-NFMs) with signal probes (DNA-conjugated gold nanoparticles (AuNPs)) combined with a colorimetric measurement of kanamycin (KMC) to prepare a portable biosensor [176]. The A-NFMs were modified with the complementary single-stranded DNA (cDNA) of the KMC aptamer-coupled AuNP (cDNA @ Au) as a colorimetric agent. The prepared colorimetric sensor finally can visually observe the resulting color changes with good selectivity and applicability. Compared to the traditional two-dimensional planar film, the special structure of electrospun NFs, due to their versatility, high porosity, and specific surface area, can improve its sensitivity and responsiveness; they not only have an excellent sensing performance but also a good adsorption performance and can continuously analyze trace pollutants in polluted water, thus making them more suitable for water detection.、

Figure 1. Electrospun-NFs-based chemosensors for water pollutant. (a) Preparation procedure and photoluminescent behavior of sulfonylcalix[4]arene-functionalized APAN nanofibrous for the adsorptive removal Tb3+ ions. (b) Photographs of NFs after immersing into Tb3+ aqueous solutions (0–100 ppm). (c) Fluorescence spectra of NFs (reproduced with permission from [173], copyright © 2018, Royal Society of Chemistry).

1.1. Gas Pollutant

Toxic gases in the environment can affect the human respiratory system and even contribute to global warming, making the monitoring of polluting gases a crucial aspect of air quality management. A gas sensor is a device that provides a signal in response to a specific target gas, and gas-sensing technology has made significant progress in environmental testing, enabling the effective detection of air pollutants. Gas sensors are often required to detect toxic, highly corrosive, and irritating gases, such as inorganic and organic gases, in environmental pollution detection. Electrospinning technology can be used to fabricate NF-based gas chemosensors that enhance sensing performance, improve chemical and physical properties, increase specific surface area and interaction sites, and enhance the diffusion coefficient. Compared to other types of nano-based sensors, this sensing system can improve the sensitivity to gases and can be combined with other nanomaterials to enhance electrospinning-based sensing parameters. NF-based chemosensors can detect a variety of analytes in different states without interference. However, the response time during gas sensing may be limited due to factors such as slow gas diffusion, the adsorption characteristics, and the working environment, which can affect the real-time responsiveness of analytes [177]. Terra et al. used p-electron conjugation in conjugated polymers to design highly fluorescent PMMA/polyfluorene (PFO) electrospinning NFs for optical gas sensing through luminescence quenching [178]. These NFs can detect different volatile organic compounds (VOCs), such as chloroform, toluene, acetone, and ethanol. When the NFs are exposed to certain vapors, they undergo a conformational change from the glass phase to B phase under VOCs, causing fluorescence quenching that can be analyzed. The flexible morphological structures of PMMA/PFO NFs constructed by electrospinning provide large surface-to-volume ratios, greatly enhancing chemical functionalization and interaction with target analytes (VOCs), which can improve the sensor’s performance (Figure 6A). Han et al. prepared a sensor based on In2O3/ZnO yolk–shell NFs [179]. These NFs can improve the gas-sensing performance due to their heterojunction effect, which enhances the efficiency of photogenerated charge separation, increases reaction sites, and enhances gas adsorption. The In2O3/ZnO-sensing material with the YS heterostructure was synthesized using the traditional electrospinning method, resulting in an ordered semiconductor heterostructure with a hollow structure (Figure 6B). The hollow structure and abundant pores significantly increase the inward diffusion rate of the gas, resulting in more active sites, which is conducive to charge separation and gas transfer. The increase of adsorption sites of hollow structures directly enhances the surface reaction involved in the gas to be measured, thereby showing a high sensing performance. The fast detection response and complete recovery characteristics of the sensor greatly improve its sensing performance. Ngoensawat et al. prepared 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT), poly(vinyl alcohol) (PVA), and silver nanoparticles (AgNPs) in PEDOT/PVA/AgNPs composite fibers by emulsion electrospinning [180]. PEDOT/PVA/AgNPs composite fibers detected heavy metal ions (Zn(II), cd(II), and Pb(II)) in analytes by positive wave anode stripping voltammetry (SWASV). It was demonstrated that electrospinning combined with solid-state polymerization (SSP) is a simple, catalyst-free, and complex instrument-free method for the preparation of PEDOT-based electrochemical analysis platforms (Figure 6C). Deng et al. prepared the electrospun core–shell multiwalled carbon nanotubes (MWCNTs)/gelatin–hemoglobin (Hb) nanobelts on an electrode surface [181]. The electrocatalytic activity of the nanobelts toward the reduction of H2O2 is improved by protein adsorption, which enhances electron-transfer kinetics. The core–shell structure of the nanobelts, with MWCNTs providing conductivity and the gelatin– Hb shell conferring biocompatibility and protein adsorption capability, plays a critical role in this process. The preparation method based on electrospinning can be directly deposited on the electrode surface, adjustable-size NFs can be prepared at the specified deposition position, and the fibers can be adapted to the auto-preconcentration of the analyte on the electrode, thereby improving the sensitivity of electrochemical detection of real trace sample analytes.

Figure 2. Electrospun-NFs-based chemosensors for gas pollutant. (A) PL spectra. (a) PMMA_0.25%PFO NF exposition to chloroform vapor; it was excited at 390 nm. (b) The dependence of the emission intensity as function of chloroform concentration for PMMA_0.25%PFO NF (monitored at 420 nm) (reproduced with permission from [178], copyright © 2017, Wiley Periodicals). (B) Schematic diagrams of (a) NO2 detection process, (b) In2O3/ZnO heterojunctions, and corresponding energy band diagram (reproduced with permission from [179], copyright © 2021, Elsevier). (C) The PEDOT/PVA/AgNPs-fibers-modified SPCE preparation. (a) Bi-alloy formation on electrode surface, followed by stripping. (b) Electrochemical detection of heavy metal ions via SWASV (reproduced with permission from [180], copyright © 2022, Elsevier).

1.2. Soil Pollutant

Soil pollution caused by agricultural production and industrial development has become an increasing environmental concern. Due to the pollution caused by pollutants, the self-purification ability of the soil itself will change in space and time. Therefore, a continuous and accurate temporal and spatial monitoring of the physical properties of the soil is required. However, compared with water- and air-pollution monitoring, the development of soil-sensing monitoring is not fast. The challenges of soil-sensing monitoring include the difficulties in sample separation and treatment compared to water- and air-pollution monitoring. Dielectric assays can be useful for characterizing soil pollutants by relying on information about the dielectric properties of contaminated soil. Electromagnetic methods can measure the soil dielectric properties and determine the substances included in the dielectric properties. In addition, biosensors, particularly enzyme biosensors, can be highly specific and sensitive to analytes and can be used to systematically evaluate contaminated soil based on the fluctuation of enzyme activity detected in the presence of contaminants. Furthermore, NFs can act as a catalytic medium due to their high specific surface area and active site characteristics, thus improving the conductivity and catalytic activity and enabling high-precision detection of soil pollutants. Jin et al. prepared a ratiometric fluorescent probe for the detection of copper ions (Cu2+), using a combination of electrospinning, strip testing, and hydrogel-encapsulation techniques [182]. The probe was composed of ATP and a fluorophore, which exhibited a reversible response to Cu2+ ions. The probe was able to detect Cu2+ ions with high sensitivity and selectivity in living cells and in soil samples, making it a useful tool for environmental safety assessment. Electrospinning has the potential to be a powerful tool

for developing novel probes and sensors for various applications, including environmental monitoring and biomedical diagnostics (Figure 7). Srinivasan et al. developed a composite electrode by using copper and reduced graphene oxide (Cu-rGO) NFs for the ultralow-level detection of the pesticide imidacloprid (IMD) in soil, with high catalytic activity provided by the combination of NFs with redox copper and rGO [183]. The use of electrospun composite NFs provides necessary chemical sensing and electrocatalytic activity for the accurate detection of soil pollutants in real time. Therefore, the composite NFs prepared through the electrospinning technique offer both the chemical sensing and electrocatalytic activity necessary for soil pollutant detection.

Figure 3. Schematic illustration of a ratiometric fluorescent sensing platform that uses a probe-doped PMMA material for the specific and visual detection of Cu2+ and ATP sequentially, with photographs of the nanofibers exposed to different metal ions that confirm the selectivity of the platform (reproduced with permission from [182], copyright © 2020, Elsevier).

From the above discussion, it can be seen that the NFs prepared via the electrospinning process can effectively solve the manufacturing problems of such sensors due to their unique surface morphology and a variety of nanostructures, such as the core–shell structure, Janus structure, tertiary core–shell/Janus structure, hollow structure, porous structure, etc. Both the sensing methods used and the multiple structural NFs can play different roles in detecting various analytes in different states. An example includes optical sensors that detect solutions and gases with less interference and that can achieve color change by adding chromogenic agents, MOF, QDs, and other optically active functional groups to NFs. However, in soil detection, the sensing characteristics of NFs, such as the response time, may be limited due to the slow diffusion and adsorption of analytes, which reduces the interaction time between the analyte and reactant and limits the applicability of the analyte. To overcome this limitation, NFs with different structures can be utilized to improve the active adsorption site and shorten the response time, thus enhancing the real-time reliability and stability of detection (Table 1). Currently, electrospun nanofiber-based sensors are mostly prepared by single and coaxial electrospinning to obtain nanofibers with uniaxial, core–shell, porous, and hollow structures. The good electron-transfer characteristics of various nanostructured NFs enable them to have excellent electrocatalytic activity, including sensitivity, selectivity, stability, repeatability, and multiplexing capabilities. Electrospinning sensors are effective at adsorbing and sensing a range of analytes, with low detection limits and fast response times. With the advancement of electrospinning technology, the unique NF structure is improving the sensing performance of chemosensors.

Table 1. Electrospun-NFs-based chemosensors for environmental pollution detections.

Article Source: https://www.mdpi.com/2227-9040/11/4/208