Advances in electrospun nanofibrous membrane sensors for ion detection

Harmful metal ions and toxic anions produced in industrial processes cause serious damage to the environment and human health. Chemical sensors are used as an efficient and convenient detection method for harmful ions. Electrospun fiber membranes are widely used in the field of solid-state chemical sensors due to high specific surface area, high porosity, and strong adsorption. This paper reviews the solid-state chemical sensors based on electrospinning technology for the detection of harmful heavy metal ions and toxic anions in water over the past decade. These electrospun fiber sensors have different preparation methods, sensing mechanisms, and sensing properties. The preparation method can be completed by physical doping, chemical modification, copolymerization, surface adsorption and self-assembly combined with electrospinning, and the material can also be combined with organic fluorescent molecules, biological matrix materials and precious metal materials. Sensing performance aspects can also be manifested as changes in color and fluorescence. By comparing the literature, we summarize the advantages and disadvantages of electrospinning technology in the field of ion sensing, and discuss the opportunities and challenges of electrospun fiber sensor research. We hope that this review can provide inspiration for the development of electrospun fiber sensors.


Introduction
With the development of industry, while bringing progress to human society, it also causes damage to the environment. For example, the harmful metal ions (Cu 2+ , Zn 2+ , Fe 3+ , Al 3+ , Pb 2+ , Hg 2+ ) and anions (CN − , F − , HSO 3 − , ClO − ) in wastewater produced in industrial processes will enter the environment and cause serious health problems to animals and plants in water bodies, including humans. [1][2][3][4][5] Among them, iron, copper and zinc are essential trace elements for human beings and play vital roles in human physiological activities. [6][7][8][9] However, high levels of them can also affect the normal physiological activities of the body, causing a series of diseases. Excessive intake of Cu 2+ may cause Alzheimer's disease, familial amyotrophic lateral sclerosis, and Liangqiang Wu is currently a doctoral student in organic chemistry at Jilin University, under the guidance of Professor Yang Qingbiao. At present, he mainly studies the application and performance of polymer electrospun ber materials in chemical sensors.
Hai Xu is currently an associate professor at Jilin University. The main research direction is molecular simulation and quantitative calculation of organic optoelectronic functional materials.
. 41,42 These harmful ions pose a threat to human health. Therefore, it is an inevitable trend to develop a method that can quickly detect the type and content of harmful anions and metal ions in water. At present, plasma emission spectroscopy, 43 mass spectrometry, 44 chromatography 45 and electrochemical analysis methods 46 have been widely used in the detection of harmful ions in the environment. Although these detection methods can well complete the qualitative and quantitative detection of harmful ions in the environment, they have the disadvantages of relying on large-scale equipment, complex sample pretreatment, low detection efficiency, and high cost. Despite its advantages of simple operation, fast response time, low cost, naked eye identication and colorimetric detection, uorescent probes have the disadvantage that they cannot be recovered and separated from the detection system. 47,48 Solid-phase sensors have the advantages of convenient operation, environmental friendliness, easy separation, and reusability, and are widely used in environmental monitoring. Therefore, the development of solid-phase composite nano-uorescence sensors has become an important direction for uorescence sensing. 49,50 Electrospinning technology is an efficient method for the preparation of nanobers. Compared with traditional nano-bers, electrospun nanobers have the advantages of high porosity, ultra-high specic surface area and uniform ber diameter, which are ideal sensing substrates. In addition, electrospinning technology is simple to operate, low cost, widely applicable (almost all linear polymers and sols) and capable of continuous production in large batches. 51, 52 Reneker once pointed out that electrospinning technology can prepare nanobers up to one kilometer long. 53 Yuris Dzenis dene it as an effective method that can realize the preparation of "Continuous Fibers for Nanotechnology". 54 It has been widely used in the elds of environment, 55,56 energy, 57,58 biomedicine, 59,60 sensing, 61,62 EMI shielding, 63 ber reinforced composites, 64,65 smart textiles, 66 food packaging, 67 electrocatalysts, 68 actuator, 69,70 water treatment. [71][72][73] Usually, some sensing units (uorescent dyes, quantum dots, metal-organic framework luminescent metal nanoclusters, etc.) and nanober materials are combined to form a composite sensing material and prepared into test strips for sensing (Scheme 1). The current electrospun ber test strips for colorimetric or uorescent detection have made great progress in sensing pH, 74 Scheme 1 Preparation and application of electrospun fiber membrane ion sensor.
Qingbiao Yang is currently a professor and doctoral supervisor at Jilin University. The main research direction is [1] organic uorescent probes and biological imaging: synthesis of organic uorescent probes and contrast agents with specic structures based on metal ions, anions, and biomolecular detection, and their applications in cell imaging and biological imaging. [2] Functional nanobers: based on electrospinning technology, study the application of nanobers in the elds of environment and sensing. temperature, 75 harmful ions, 76 toxic gases 77 and biomolecules. 78 Aer the test, the test strip can be directly removed from the water, and can even be prepared as a reusable test strip. There are many ways to combine the materials of electrospun nano-bers with chemical sensor, such as physical doping, chemical modication, copolymerization, surface adsorption and selfassembly.
Physical doping method is through the sensing unit (uorescent dye, quantum dots, metal-organic framework, etc.) directly dispersed in a certain concentration of polymer solution, and then using electrospinning technology to prepare nanober membrane. This method has the advantages of simple preparation and wide application. However, the binding mode of sensing units and polymers only relies on weak intermolecular interactions, so some hydrophilic sensing units fall off the nanobers in aqueous environment, which affects the sensing effect. Moreover, many luminescent units will undergo aggregation-induced quenching in the aggregated state, which is not suitable for direct physical doping. The utilization rate of the large number of sensing units wrapped inside the ber is not high.
In the copolymerization method, the sensing unit is prepared into monomer and copolymerized with some high performance polymer monomer to form block polymer, which is then prepared into nanober lm. This method effectively avoids the shortcomings of the sensor unit falling off the ber, but the problems of aggregation-induced quenching and the inability to utilize the sensor unit inside the ber are not solved.
Chemical modication is made by graing sensing units on the polymer and reusing electrospinning technology to prepare nanobers or graing sensing units on the surface of nano-bers. The idea is to form strong chemical bonds between the sensing unit and the polymer chain. Among them, the method of chemical modication of sensing unit on ber surface can reduce the inuence of aggregation induced quenching and make the sensing unit directly contact with the measured object, so that the sensing efficiency is higher.
The method of surface adsorption is similar to surface chemical modication, except that one uses strong chemical bond binding while surface adsorption uses electrostatic interaction binding between ber surface and sensing unit. This method can also effectively reduce the effect of aggregation-induced quenching and enhance the utilization of sensing units.
The host-guest self-assembly method makes use of the iondipole, hydrogen bond, van der Waals force and hydrophobic interactions of the host and guest to form substances with specic structures. In the preparation of electrospun nano-bers, we synthesize polymers and sensing units with subject or guest structures, respectively. The modied polymer is then prepared into nanobers, which are combined by host-guest interaction. This method is similar to surface chemical modi-cation and surface adsorption, except that we can change the structure of host and guest by changing the conditions to regulate the binding and separation between them. This article will review the related nanober sensors in the past ten years (2012-2022) from the aspects of ber material, preparation method, sensing mechanism, and sensor performance based on the different types of ions to be detected.

Electrospun nanofiber fluorescent sensor for detection of harmful metal ions
Generally, transition metal ions have empty orbitals and usually have strong coordination. Based on this characteristic, many compound of ligands with N, S and O atoms can be designed to detect metal ions such as organic molecular probes, surface-modied noble metal clusters and quantum dots. In order to enhance the convenience of use and sensing sensitivity of the sensor and reduce the pollution to the environment, metal-ion composite sensors based on electrospun nanobers have made remarkable progress in the past decade. Some electrospun ber membrane metal ion sensor base on organic molecular probes, metal nanoclusters, nanoparticles and quantum dots have been prepared by physical doping, chemical modication, copolymerization and self-assembly.
Electrospun nanober uorescent sensor for detection of Cu 2+ Physical doping method. Some organic molecular and nanoparticle probes that respond to Cu 2+ can be combined into nanobers by physical doping to prepare nanober membrane sensors for detecting Cu 2+ , due to the strong interaction between sensing molecules and polymers.
Min et al. 79 doped salicylaldehyde-modied rhodamine dye into poly(ether-sulfone) (PES) to prepare a sensor for copper ion response in aqueous medium by electrospinning technology. The nanober membrane sensor can achieve high sensitivity and selective response to copper ions. The detection limit is as low as 1.1 nM and it exhibits remarkable uorescence enhancement and colorimetric effect. The ber membrane shows good reusability aer treatment with EDTA ( Fig. 1(A) and 2(A)). Zhang et al. 80 fabricated an electrospun ber membrane from chitosan/polyacrylonitrile (CS/PAN) doped with rhodamine hydrazide salicylaldehyde Schiff base as a Cu 2+ nanober membrane (NFM) sensor. The response mechanism of the NFM to copper ions is the coordination of copper ions and oxygen leads to the opening of the cyclic lactam, which causes the color change. The naked-eye detection limit of NFM is as low as 10 −8 M, which is about three orders of magnitude lower than that of pure probe solution. The NFM shows good reusability aer treatment with EDTA ( Fig. 1(B) and 2(B)). In 2020, Jin et al. 81 doped rhodamine-phenothiazine-based Schiff base derivative RB into polymethyl methacrylate (PMMA) to prepare a reusable ratio uorescence-colorimetric nanobrous membrane sensor by electrospinning technology. The excitedstate intramolecular proton transfer (ESIPT) process of the probe molecule was inhibited by the coordination of copper ion with the oxygen atom on the carbonyl group on RB and the nitrogen atom on the pyridine, which manifested as a red shi of the emission peak and enhanced UV absorption and the limit of detection (LOD) is 0.11 mM/0.28 mM. Aer adding adenosine triphosphate (ATP), the copper ions will fall off from the RB molecule, and the ESIPT effect of the molecule will restore the blue-shi of the emission peak to the original uorescence and weakened ultraviolet absorption ( Fig. 1(C) and 2(C)). It can be seen that the Schiff base derivative based on rhodamine is a good copper ion recognition unit and the sensing mechanism of copper ion is the same. Moreover, the ber membrane sensor prepared by physical doping method still shows good uorescence and colorimetric detection effects, which indicates that rhodamine derivatives are not easily affected by aggregationinduced quenching. It showed good reusability aer strong ligand treatment. It provides a good model for fabricating ber membrane copper ion sensor easily. Some phenols, pyridines and salicylaldehyde-based Schiff base derivatives can also be used as recognition probes for copper ions. Wang et al. 82 prepared 1,4-dihydroxyanthraquinone (1,4-DHAQ) and cellulose acetate (CA) co-doped nanobrous membranes (1,4-DHAQ@CA) by electrospinning technology. 1,4-DHAQ@CA was then deacetylated to obtain a microporous brous membrane (1,4-DHAQ@CL). Since 1,4-DHAQ can coordinate with Cu 2+ in aqueous solution to form phenolate, resulting in obvious uorescence quenching. The experimental results show that the uorescence intensity of the ber membrane has a good linear relationship in the range of copper ion concentration of 2.5 to 37.5 × 10 −9 M, the LOD is 3 × 10 −9 M. The nanober membrane can complete the specic response to copper ions under a variety of interfering metal ions. Therefore, the 1,4-DHAQ@CL ber membrane prepared by simple doping method and deacetylation can achieve high sensitivity and selectivity for Cu 2+ uorescence detection. Under the treatment of Cr 3+ , the sensor shows good reusability ( Fig. 1(E) and 2(E)). Lin et al. 83 doped 1,10-phenanthroline uorescence sensor (F-phen) into poly (N-isopropylacrylamideco-N-methylol acrylamide) (P(NIPAAm-co-NMA)). A nanober membrane copper ion sensor with a thermally responsive switching mechanism was fabricated by electrospinning technology. The sensor exhibits a dynamic linear relationship in the range of copper ion concentration from 1.0 × 10 −5 to 1.0 × 10 −4 M and exhibits signicant uorescence quenching. When the temperature exceeds lower critical solution temperature (LCST), the conformational transition of poly (N-isopropylacrylamide (PNIPAAm) exhibits shrinkage and hydrophobic state, which leads to the inhibition of uorescence emission by F-phen aggregation and the brous membrane in the hydrophobic state is also unfavorable for binding to copper ions, so the sensor exhibits characteristics of temperaturecontrolled copper ion detection. This approach opens up the possibility for more temperature-controlled sensor designs ( Fig. 2(D)). Jin et al. 84 synthesized nanoparticles (PEI-EAS NPs) using 4(N,N-diethyl) salicylaldehyde and poly(ethylene imine) (PEI). The PEI-EAS NPs were doped into PMMA and the nano-brous membrane sensor PEI-EAS@NF was fabricated by electrospinning. PEI-EAS@NF can respond specically to copper ions, showing the phenomenon of green uorescence quenching. The uorescence can be recovered by the treatment of sulfur ions, and the repeated detection of copper ions can be realized ( Fig. 1(D) and 2(F)). Rhodamine derivatives have poor photostability and the spiral rings are easy to be opened under light conditions. Rhodamine-based sensors require strict light avoidance during synthesis and storage. The Schiff base derivative sensor mentioned above not only has the same good sensing performance as rhodamine derivative, but also has better photostability, which is more suitable for the preparation of solid-phase nanober membrane sensor.
Chemical modication method. Some copper ion recognition molecules can also be graed on the surface of nanobers by chemical modication methods. Due to the high specic surface area and porosity of electrospun nanobers, the surface-graed sensing unit can be well combined with copper ions, and has the advantages of fast response speed and high sensitivity.
In 2012, Wang et al. 85 reported a rhodamine surface-modied poly (methyl methacrylate co 4-aldehyde-3-hydroxy phenyl acrylate)nanober membrane (PMAR), which enables real-time colorimetric-uorescence sensing of copper ions. PMAR can sense copper ions with high efficiency and selectivity, show obvious uorescence enhancement and colorimetric effect. Experiments show that the response time of PMAR to copper ions is very short (<10 s), with a good linear relationship in the copper ion concentration range of 1.0 × 10 −6 -2.0 × 10 −4 M, the detection limit is 1.5 × 10 −6 M. Copper ions can be detected repeatedly by EDTA-treated PMAR (Fig. 3(A) and 4(A)). Cho et al. 86 graed pyrene derivatives (PyDAN2) on the surface of electrospun nanobers using poly (2-hydroxyethylmethacrylic acid-co-N-methacrylic acid acrylamide) (P(HEMA-co-NMA)) as raw material prepared a uorescent ber membrane sensor (Fiber-g-PyDAN2) with high sensitivity to copper ions. Fiber-g-PyDAN2 nanobers can chelate with copper ions in aqueous solution, resulting in a blue uorescence enhancement. The lowest and highest LOD of 10 −7 to 10 −6 M and 10 −2 to 10 −1 M, respectively. Treat it with EDTA and it can be reused at least four times ( Fig. 3(B) and 4(B)). Gao et al. 87 synthesized a tri-ethoxy silane-modied triphenylamine- based symmetric Schiff base derivative (L) using ultrasoundassisted technology, and graed L onto the surface of polyvinyl alcohol (PVA) electrospun nanober membranes. A highly sensitive copper ion colorimetric sensor (PTLNFM) was prepared. PTLNFM can chelate Cu 2+ in aqueous solution, resulting in changes in UV absorption. It exhibits a good linear relationship in the copper ion concentration range of 9.34 × 10 −8 to 1.15 × 10 −5 M and the detection limit is 1.27 × 10 −8 M to achieve an ultrasensitive response to copper ions ( Fig. 3(C) and 4(C)).
PEI is rich in amino groups and can form tetra-amino-Cu 2+ complexes with copper ions, showing a special dark blue color, which can realize copper ion adsorption and colorimetric detection. In 2021, Shao et al. 88 graed branched polyethyleneimine (BPEI) onto the surface of polyacrylonitrile (PAN)-based electrospun nanobrous membranes to prepare copper ion colorimetrically responsive nanobrous membrane sensor (aPAN/BPEI NMs). The aPAN/BPEI NMs were able to adsorb copper ions in water, and the color of the ber membrane changed from yellow to blue. With the addition of copper ions, the UV absorption peaks at 280 nm and 636 nm gradually increased, and showed a good linear relationship in the range of copper ion concentration from 0 to 700 mM. The detection limits of Cu 2+ is l 280 nm = 11.5 mM, l 636 nm = 4.8 mM.
And it also showed a remarkable adsorption effect on copper ions, and the adsorption capacity reached 209.53 mg g −1 (Fig. 3(D) and 4(D)).
Noble metal nanoparticles can be used for sensor design due to their easy surface modication and good stability. Abedalwafa et al. 89 modied Au/Ag NPs with shell-core structure onto porous ammoniated PAN electrospun bers, and successfully prepared a ber membrane sensor Au/Ag NPs@a-PAN for colorimetric detection of copper ions. With the increase of copper ions, the yellow color of the ber membrane gradually faded. The mechanism of discoloration is the leaching of Au/Ag NPs from NFM in the presence of ammonium chloride, thiosulfate and Cu 2+ , forming a soluble complex of Ag + /Au 3+ / Cu 2+ -thiosulfate on their surface. The discolored ber membrane can be restored to its original yellow color by placing it in the Au/Ag NP solution, that is, it can be regenerated. The detection limit of Au/Ag NPs@aPAN for copper ions is 50 nM (Fig. 3

(E) and 4(E)).
Graing on the ber surface with rhodamine derivatives, Schiff base compounds, gold nanoparticles is a good way to prepare nanober membrane sensors that respond to copper ions. This method allows the sensing unit to be distributed on the surface of the ber, and the detected object can fully contact the identication site. In addition, for some uorescent dyes that are induced by aggregation, a small amount of graing on the surface can also slow down the quenching phenomenon.
Copolymerization method. Wu et al. 90 rstly copolymerized thermally responsive N-isopropylacrylamide (NIPAAm), chemically cross-linked N-methylol acrylamide (NMA) and copper ion-responsive rhodamine derivative (RHPMA) to obtain polymer (PNNR). Then, a nanober membrane sensor (PNNR2@NFs) with temperature-controlled on and off and copper ion response was fabricated by electrospinning technology. The uorescence intensity of PNNR2@NFs gradually weakened with the addition of copper ions, showing a good linear relationship in the concentration range of copper ions from 1 to 10 mM. The change of temperature leads to the hydrophilic-hydrophobic transition of the nanowire bers, which shows the characteristics of temperature on and off. The sensor can be reused under the action of EDTA (Fig. 5(A) and 6(A)).
It is also an effective method to prepare the electrospun nanober membrane sensor. Moreover, this method makes the copper ion probe less susceptible to detachment from the ber due to solubility.
Surface adsorption. Quantum dots (QDs) can be used to design chemical sensors due to their high uorescence  quantum yield and photostability. Li et al. 91 used electrostatic interaction to assemble negatively charged mercaptopropionic acid (MPA)-coated CdTe QDs with surface positively charged polyethyleneimine (PEI)/polyvinyl alcohol (PVA) electrospun nanobers to prepare a solid-state uorescent sensor that can respond to copper ions. The uorescence quenching degree of CdTe QDs-electrospun nanober assemblies increased, with the increase of Cu 2+ concentration, and the uorescence intensity at 652 nm showed a good linear relationship with the Cu 2+ concentration in the range of 0.08-800 mmol L −1 . The detection limit was 11.1 nM (S/N = 3) (Fig. 5(B)). Senthamizhan et al. 92 prepared a copper ion sensor by adsorbing dithiothreitol (DTT)coated gold nanoclusters (DTT.AuNC) onto the surface of porous cellulose acetate bers (pCAF). With the increase of copper ions, the color of the ber membrane gradually changed from red to blue under ultraviolet light, showing the quenching of red uorescence, the visual detectable limit of 1 ppm (16 mM). This is due to the coordination of sulydryl groups on DTT surface-modied with gold nanoclusters to copper ions ( Fig. 6(B)).
Using electrostatic interactions, van der Waals forces, and coordination to adsorb sensors onto nanober membrane surfaces is also a method to fabricate probe/nanober membrane composite sensors. This method is easier to operate, and the surface-adsorbed probe can quickly contact with copper ions in water, enhancing the sensing efficiency.
Electrospun nanober uorescent sensor for detection of Fe 3+ As a transition metal, iron has abundant empty orbitals and is easy to form complexes with many ligands. Therefore, the related iron ion sensor is prepared according to this characteristic.
Physical doping method. Some organic small molecules have more lone electron pairs, which can form chelates with paramagnetic iron ions, resulting in changes in uorescence or color. The selective detection of iron ions can be achieved. Kacmaz et al. 93 synthesized an iron-responsive uorescent dye N-(4-cyanobenzylidene) isonicotinohydrazide (CBINH). By codoping CBINH with modied ethylcellulose (EC), a nano-brous membrane sensor CBINH@EC NFs with ultrasensitive response to iron ions was fabricated by electrospinning. CBIN-H@EC NFs exhibit an ultrasensitive response to Fe 3+ , in the iron ion concentration range of 10 −12 to 10 −6 M exhibited good linearity with detection limits as low as 0.07 fM (7 × 10 −14 M). Under the treatment of acetate buffer, CBINH@EC NFs can achieve reversible detection of iron ions (Fig. 8(A) and (B)). Mun et al. 94 synthesized naphthalene-based probe molecules 1 and 1A, and doped 1 and 1A into PMMA in DMF solution, respectively, followed by electrospinning to prepare two electrospun ber membrane sensors NF-1 and NF-1A that could respond to iron ions. The LOD was found at 174 ppb for NF-1 and 59 ppb for NF-1A. The iron ion caused uorescence quenching through 1 : 1 coordination with the N atom of the naphthalene ring and the carbonyl oxygen of the amide moiety ( The nitrogen atoms on the four cyano groups in the FPN molecule can chelate with two iron ions through weak van der Waals forces, resulting in uorescence quenching. The response of PCL/FPN to Fe ions was tested in the concentration range of Fe ions from 10 to 70 nM, and the results showed that the detection limit of Fe ions was 2.9413 nM (Fig. 7(B)).
The complexes of rare earth elements usually have good luminescent properties. Lanthanide rare earth MOF materials have the characteristics of large Stokes shi and narrow emission wavelength, which can be used for ion detection. 96 Zhou et al. 97 prepared an Eu-MOF material using Eu 3+ of the lanthanide series. The Eu-MOF@PAN nanober membrane sensor responsive to iron ions was prepared by electrospinning Eu-MOF-doped PAN solution. Aer adsorption of iron ions in Eu-MOF@PAN, effective collisions between Fe 3+ and Eu 3+ occur. This process will cause the energy of the excited state to be damaged and lead to the quenching of the uorescence of Eu 3+ -MOF. The sensor exhibits excellent selectivity and sensitivity for iron ions with a detection limit of 63 nM. Bai et al. 98 doped a chain complex (1-Eu 3+ ) formed by Eu 3+ and organic molecular ligands into PAN solution, and fabricated an iron ionresponsive nanobrous membrane sensor (1-Eu 3+ @PAN). The sensor showed obvious red uorescence quenching under iron ion treatment, and the detection limit was 6.685 × 10 −4 M.
As an environmentally friendly material, biomaterials have good application prospects and can be used in the elds of environment, energy and sensing. 99,100 Saithongdee et al. 101 prepared a nanobrous membrane sensor from a curcumindoped zein solution by electrospinning and amidation crosslinking under the conditions of citric acid and heating. The sensor is able to achieve a colorimetric response to iron ions with a color change from yellow to brown with an optical detection limit of 0.4 mg L −1 (Fig. 8(D)).
Many probe molecules, MOFs, and biological matrices etc. Which can selectively detect iron ions can be mixed with some polymers by means of physical doping to prepare nanober membrane sensors that respond to iron ions by electrospinning. This simple mixing method is suitable for a wide range of linear polymers with a wide range of applications and is easy to operate. However, due to simple doping, there is no strong connection between the sensing unit and the polymer, and the polymer and the sensing unit are easily separated by solvents. For some sensing units that are easily soluble in water, they are easy to dissolve in water under the water system, which affects the sensing effect.
Chemical modication method. Direct chemical modication of sensing molecules onto polymers and subsequent fabrication into nanobrous membranes can also be used for the detection of iron ions.
Wang et al. 102 used chemical modication to gra pyrene derivatives to triblock copolymers to form a pyrene-containing copolymer (PPy-b-PNIPAAm-b-PNMA). The pyrene-containing copolymers were then prepared into nanobrous membranes by electrospinning. The bers are aggregated from nanospheres formed by self-assembly of PPy-b-PNIPAAm-b-PNMA copolymer, and the pyrene-containing molecules are on the outermost side of the nanospheres. The ber membrane can quench the uorescence in response to iron ions and temperature. Even at the concentration of 10 −5 M iron ion, the uorescence of the ber membrane can be quenched (Fig. 9(A) and 10(A)). Zhou et al. 103 modied coumarin derivatives on copolymers of acrylic acid and acrylonitrile (PANA) to prepare iron ion-responsive nano-brous membranes (PANADC) by electrospinning. The ber membrane can be used for the adsorption and detection of iron ions, and the uorescence is quenched with the addition of iron ions to the ber membrane, and the detection limit and adsorption amount are 10.63 mM and 13.93 mg g −1 , respectively ( Fig. 9(B) and 10(B)).
Utilizing the adsorption of some amino acids to iron ions can also be used to detect iron ions by chemical cross-linking aer some amino acids are prepared into polymers. Zhang et al. 104 prepared poly(aspartic acid) (PASP) into nanobrous membranes by electrospinning and then cross-linked with ethylenediamine to obtain an electrospun brous hydrogel lm (PASP-ENHM). PASP-ENHM showed a colorimetric change from white to yellow under iron treatment with a detection limit of 0.1 mg L −1 . And the ber membrane can continue to be used for the detection of iron ions aer being treated with EDTA ( Fig. 9(C) and 10(C)).

Copolymerization method
Directly polymerizing molecular probes with other monomers is also a method for preparing sensing materials. Li et al. 105 synthesized a rhodamine Schiff base derivative monomer RQ, RQ and methyl acrylate were prepared by emulsion polymerization to obtain a block copolymer poly(MMA-co-RQ). Poly(-MMA-co-RQ) was obtained by electrospinning prepared nano-lms stacked with nano-microspheres. The lm can be used for rapid colorimetric detection of iron ions from white to pink with a detection limit of 1.19 mM, and can be reused aer treatment with ppi solution (Fig. 9(D) and 10(D)).
The iron ion sensor prepared by the above chemical modi-cation method has good sensing performance. And it solves the problem that the sensing unit is easy to fall off in the physical co-doping method.
Electrospun nanober uorescent sensor for detection of Hg 2+ Gold nanoparticles, rhodamine derivatives and quantum dots can form composite ber membrane sensors that respond to mercury ions through physical doping, chemical modication, surface adsorption, self-assembly, and copolymerization.
Physical doping method. Senthamizhan et al. 106 prepared a nanobrous membrane responsive to mercury ions by doping gold nanoclusters (AuCN) into polyvinyl alcohol solution. The ber membrane showed signicant uorescence quenching effect on mercury ions with a detection limit of 1 ppb (Fig. 12(A)).
Rao et al. 107 synthesized a mercury ion chemical sensor RIM using rhodamine 6G as a raw material. The RIM and polyurethane were mixed uniformly, and a nanober membrane was prepared by electrospinning. The brous membrane is able to respond to mercury ion, the mechanism of sensing of mercury ion by RIM is the ring opening of spirolactum resulting in enhanced uorescence (Fig. 11(A) and 12(B)). Girdthep et al. 108 doped rhodamine B hydrazide (RBH) and rhodamine 6G hydrazide (R6GH) into PMMA, respectively. And prepared PMMA/RBH and PMMA/R6G mercury ion colorimetric and uorescent nanober membrane sensor by electrospinning. Both sensors can chelate with mercury ions in a 2 : 1 ratio, resulting in a white-to-pink color change and uorescence Liang et al. 109 fabricated a temperature-on-off magnetic nanober membrane sensor (P2-5%ESNFs) for mercury ion response. The sensor was prepared by electrospinning by mixing poly (NIPAAm-co-NMA-co-AA) (P2), naphthalimide derivative (BNPTU) and Fe 3 O 4 nanoparticles. P2-5% ESNFs showed the performance of temperature control in response to mercury ions. The temperature change changed the hydrophilic-hydrophobic state of the ber membrane, and the chelation of mercury ions and BNPTU was more likely to occur in the hydrophilic state of the ber membrane. Therefore, the on and off effects of temperature are exhibited. The response of P2-5% ESNFs to mercury ions changed from green to blue. The addition of Fe 3 O 4 makes the ber membrane magnetic for easy removal from water ( Fig. 11(C) and 12(D)). Li et al. 110 prepared a one-dimensional electrospun ber membrane material using a mixed solution of polyethylene oxide (PEO), chitosan (CS), Fe 3 O 4 and CQDs. The ber membrane material can monitor the adsorption of mercury ions by real-time uorescence monitoring. The adsorption of mercury ions reaches equilibrium within 100 min, and the maximum single-layer adsorption capacity is 148.148 mg g −1 . The uorescence of the ber membrane was gradually quenched with the adsorption of mercury ions. The ber membrane is magnetic and can be easily removed from the aqueous solution, due to the addition of Fe 3 O 4 ( Fig. 11(D)). Mercury ion probes based on rhodamine derivatives, naphthalimide derivatives, quantum dots and gold nanoparticles can be simply doped into polymers to quickly prepare electrospun ber membrane sensors with good sensing effects. However, the sensing unit inside the ber in this preparation method is not used, and the sensing unit on the surface of the ber is easy to fall off due to the combination of the polymer only by weak molecular force, which affects the sensing effect.
Chemical modication method. Cho et al. 86 fabricated a mercury ion sensor (Fiber-g-RhBN2) by modifying a rhodamine B derivative (RhBN2) on the surface of poly (HEMA-co-NMA) electrospun nanobers. With the addition of mercury ions, the uorescence intensity of Fiber-g-RhBN2 at 580 nm gradually increased. The lowest and highest detection limits were 10 −5 to 10 −6 M and 10 −2 to 10 −1 M, respectively. Furthermore, treatment of Fiber-g-RhBN2 with EDTA enables the reuse of the sensor. Cai et al. 111 reduced HAuCl 4 to Au-NCs with BSA by in situ preparation on bovine serum albumin/ polyethylene oxide (BSA/PEO) electrospun brous membrane and immobilized on the surface and inside of the brous membrane. The modied ber membranes (BSA/PEO-Au-NCs) can exhibit uorescence quenching in the presence of mercury ions with a detection limit of 57 pM (Fig. 13(A) and 14(A)). Deng et al. 112 graed 4-(2-pyridylazo)-resorcinol (PAR) onto PAN electrospun bers to prepare a sensor (PANMW-PAR) for colorimetric detection of mercury ions and adsorption of mercury ions. PANMW-PAR bers have selective detection ability for Hg 2+ with a detection limit of 35 mg L −1 (Fig. 13(B) and 14(B)).
Copolymerization method. Chen et al. 113 synthesized a rhodamine derivative (RhBN2AM). Poly(N-isopropylacrylamide, poly(N-methylolacrylamide) and RhBN2AM were synthesized by free radical solution polymerization to obtain a copolymer (poly(NIPAAm-co-NMA-co-RhBN2AM)), and the copolymer was dot-spun to prepare a nanober membrane P3. The brous membrane can turn into orange uorescence in response to mercury ions. In addition, the NIPAAm in P3 also has the function of temperature-controlling the response of mercury ions on and off. When the temperature exceeds the LCST (45°C), the uorescence of the ber membrane increases instantaneously (Fig. 13(C)).
Surface adsorption. Ma et al. 114 synthesized a dithioacetal-modied pyreneimide uorescent sensor DTPDI. Subsequently, DTPDI was adsorbed on the surface of PAN electrospun ber membrane by electrostatic interaction to form a ber membrane sensor FNFM that could respond to mercury ions. FNFM produced a lipid-soluble dye AL in the presence of mercury ions, which was exfoliated from the surface of the brous membrane upon treatment with dichloromethane. The mercury ion content can be determined by measuring the UV absorption intensity of AL exfoliated in the solution, and the detection limit is 1 ppb (Fig. 13(D) and 14(C)). Ghosh et al. 115 prepared an electrospun nanobrous membrane by blending nylon 6 (N6) and uorescein isothiocyanate (FITC). Then, Au@BSA nanoclusters were adsorbed by nanober membrane to prepare a sensor Au@BSA/FITC/N6 for the selective detection of mercury ions. Under the treatment of mercury ions, the red uorescence was gradually quenched and returned to green uorescence, which could also respond at a concentration of 1 ppt mercury ions (Fig. 14(D)). Senthamizhan et al. 116 fabricated a Hg 2+ sensor (AuNC*PCL-NF) by adsorbing a layer of gold nanoclusters on the surface of polycaprolactone electrospun bers (PCL-NF). The amalgam caused the red uorescence of the gold nanoclusters to be quenched in a very short time, and the detection limit was at the ppt level ( Fig. 14(E)).
Self-assembly. Wang et al. 117 used b-cyclodextrin-modied rhodamine derivatives (thiocarbamido-SRhB-b-CD) as the host and poly (MMA-co-ADMA) electrospun nanobers as the guest. A sensor for colorimetric and uorescence detection of mercury ions was prepared by host-guest self-assembly. With the addition of mercury ions, the sensor exhibited an orange uorescence enhancement, and the color changed from white to pink under visible light. The detection limit was 6.0 × 10 −5 mol L −1 (Fig. 13(E) and 14(F)).
Nanober membrane sensors were prepared by copolymerization to form block polymers between mercury ion probes and polymers. It can effectively prevent the probe from wandering into the detection environment, but the probe utilization rate and physical doping method are not much different. Chemical modication, surface adsorption and self-assembly methods can modify the mercury ion sensing unit on the surface of the ber membrane to increase the utilization rate of the mercury ion sensing unit and reduce the aggregation and quenching phenomenon of uorescent molecules.

Electrospun nanober uorescent sensor for detection of Pb 2+
Some biological matrix materials and noble metal nanoclusters are very useful for preparing lead ion nanober membrane sensors by co-doping, chemical modication and other methods.
Physical doping method. Raj et al. 118 prepared a ber membrane sensor CC-CA for colorimetric detection of lead ions by electrospinning a mixed solution of curcumin and cellulose acetate (CA). CC-CA changed from yellow to orange under the treatment of lead ions with a detection limit of 20 mM (Fig. 15(A)  and 16(A)).
Chemical modication method. Li et al. 119 prepared cellulose acetate (CA) into nanobrous membranes by electrospinning technology, and then deacetylated the CA nanobrous membranes to obtain DCA brous membranes. A sensor DCA-PMAD for colorimetric detection of lead ions can be obtained by graing pyromellitic anhydride (PMAD) onto the surface of DCA ber membrane. With the addition of lead ions, DCA-PMAD changed from white to dark yellow-brown, and the detection limit was 0.048 mM. And DCA-PMAD also had a strong adsorption capacity for lead ions, and the adsorption capacity was 326.80 mg g −1 . Under the treatment of EDTA, the sensor is reusable (Fig. 15(B) and 16(B)).
Surface adsorption method. Sheikhzadeh et al. 120 prepared electrospun nanobrous membranes (BCNFs) from bacterial cellulose. The lead ion colorimetric sensor (BCNF-CU) was then prepared by adsorbing curcumin with BCNF. The naked eye detection limit and the calculated detection limit of BCNF-CU were 9 mM and 0.9 mM, respectively. And applied to the detection of lead ions in rice samples (Fig. 15(C) and 16(D)). Li et al. 121 prepared polyamide-6/nitrocellulose (PA-6/NC) electrospun ber membranes by bicomponent alternating two-ow electrospinning technique. The bovine serum albumin-modied gold nanoparticles (BAu probe) were adsorbed onto the surface of the ber membrane by assembly method to obtain a sensor BAu probe@PA-6/NC that can be used for the colorimetric detection of lead ions. The color of the ber membrane changed from pink to white with increasing lead ion concentration, and the detection limit was 0.2 mM. And it can be reused aer soaking with BAu probe solution (Fig. 15 (D) and 16 (C)).

Electrospun nanober membranes for Al 3+ detection
Kim et al. 122 blended a rhodamine-based colorimetric-uorescent chemical sensor (R2PP) with polyurethane to prepare an aluminum ion-responsive ber membrane sensor by electrospinning. Under the treatment of aluminum ions, the brous membrane changed from white to pink and showed a signicant enhancement of yellow uorescence, and the detection limit was 8.5 × 10 −9 M. The sensing mechanism is due to the formation of a 2 : 1 complex between R2PP and aluminum ions, which results in the opening of the spiro ring structure of R2PP, which changes the conjugation structure and changes the UV and uorescence. In addition, it can be reused under the treatment of EDTA (Fig. 17(A), (B) and 18(A), (B)).

Electrospun nanober membranes for Zn 2+ detection
Syu et al. 123 doped the zinc ion probe meso-2,6dichlorophenyltripyrrinone (TPN-Cl 2 ) into poly(2-hydroxyethyl methacrylate) (PHEM) solution and fabricated a ber membrane sensor by electrospinning. The sensor can respond to zinc ions in the normal physiological range, and the time resolution can reach 5 min at the zinc ion concentration of 10 −6 M (Fig. 18(C)). Zhou et al. 124 obtained poly (MMA-co-Sal) by polymerizing and modifying 2-hydroxy-4-acryloyloxybenzaldehyde (HAB), methyl methacrylate (MMA) and salicylaldehyde-hydrazine as raw materials. The polymer poly (MMA-co-Sal) can be prepared into nanobrous membranes by electrospinning technology. The ber membrane can be used for uorescence detection and adsorption of zinc ions. The uorescence at 504 nm gradually increased with the addition of zinc ions, the detection limit was 1.95 × 10 −5 mol L −1 , and the adsorption capacity was 11.45 mg g −1 (Fig. 17(C), (D) and 18(D)).
Regardless of the form of binding, the sensing of metal ions is always inseparable from the coordination and oxidation of metal ions. Therefore, the design of metal ion sensing mainly considers the inuence of the way the sensing unit is introduced on the sensor performance.

Electrospun nanober uorescent sensor for detection of harmful anions
The preparation of electrospun ber membrane sensor for harmful anion detection can utilize physical doping, chemical graing, and host-guest self-assembly to combine CDs, organic molecular probes, and noble metal nanoparticles with polymers.   minutes, then used the PVA aqueous solution in which CDs and AuNC were uniformly dispersed for 30 minutes, and nally used the organic cellulose acetate solution. Solution electrospinning for 30 min. A uorescent nanober membrane sensor (CDs/ AuNCs-PVA@CA NFM) for ratiometric detection of cyanide was successfully fabricated. The CDs/AuNCs-PVA@CA NFM uorescence intensity of 445 nm/650 nm gradually increased with the addition of cyanide, showing a ratiometric detection of cyanide. Since the relative content of Au (0) decreased due to the etching effect of cyanide on AuNCs, the uorescence intensity at 650 nm of AuCNs decreased, while the uorescence intensity at 445 nm of CDs was not affected, so NFM showed the effect of ratiometric detection. Experiments show that the detection limit of NFM is 0.15 mM, which is much lower than the WHO standard for cyanide content in drinking water (Fig. 19(A) and 20(A)).
Kim et al. 126 synthesized an imidazole derivative (M) based on naphthalimide and anthraquinone for cyanide detection. A cyanide detection test strip was prepared by blending M and polyurethane in dimethylacetamide and electrospinning. The test strip showed the effect of colorimetric and uorescence detection under the treatment of cyanide. It changes from green to orange under visible light, and exhibits an orange-red uorescence enhancement under UV light irradiation. The reason for this change is that the cyanide can strongly interact with the H on the N-H unit of imidazole and dissociate the proton, resulting in an enhanced ICT effect but a sharp change in UV and uorescence (Fig. 19(B) and 20(B)).
Chemical modication method. Dreyer et al. 127 graed 4-(2-(pyridin-4-yl)vinyl) phenol (PBM) onto ethyl(hydroxyethyl)cellulose (EHCE). The modied EHCE and PMMA were blended at a mass ratio of 5 : 2 (wt/wt) to obtain a cyanide ber membrane sensor PMMA/EHEC-PBM by electrospinning. PMMA/EHEC-PBM has good hydrophilicity and can be used for the detection of enhancers in total aqueous solution. With the addition of cyanide, the uorescence at 486 nm is gradually quenched, and the uorescence at 588 nm is gradually enhanced. The sensing mechanism is due to the change of uorescence caused by the proton detachment of the phenolic hydroxyl group in the presence of cyanide. 31 The detection limit and quantication limit of this sensor were 2.15 × 10 −5 and 7.17 × 10 −5 mol L −1 , respectively ( Fig. 19(C) and 20(C)).
Electrospun nanober uorescent sensor for detection of F − As a strong electronegative element, F − easily forms a hydrogen bond with N-H or O-H to deprotonate the molecule, thereby causing the change of the conjugated structure and the change of uorescence and color. In addition, uoride, as a lewis acid, can react with silicon-containing compounds, and uoride sensors can be designed according to this characteristic.
Physical doping method. The (M) synthesized by Kim et al. 126 can also be used for the detection of uoride compounds, and the test strip prepared by electrospinning technology can also be used for the detection of uoride compounds from the perspective of colorimetry and uorescence. The detection mechanism is due to the strong interaction of the uoride ion with the H atom on the N-H leading to the dissociation of the proton. This change can enhance the ICT effect of the molecule and achieve a signicant change in UV uorescence (Fig. 19(B) and 20(B)). Qiu et al. 128 fabricated AgPt-Fe 3 O 4 nanoparticles by modifying iron tetroxide nanoparticles with Ag and Pt. Then, AgPt-Fe 3 O 4 was wrapped with SiO 2 to prepare AgPt-Fe 3 O 4 @SiO 2 NPs. They were doped into the DMF solution of PAN, and then the nanober membrane F − sensor was fabricated by electrospinning technology. In the presence of F − , the sensor realizes a good colorimetric effect from light pink to blue, and the limit of detection is 13.73 mM. The mechanism of this color development is due to the etching effect of F − on SiO 2 , which makes the outer shell of SiO 2 fall off and exposes the active part of AgPt-Fe 3 O 4 inside, which oxidized and changes color. Can be used for colorimetric detection of F − in total aqueous solution ( Fig. 19(D) and 20(D)).
Self-assembly. Zhang et al. 129 used a b-cyclodextrin (b-CD)-modied copolymer of acrylonitrile and acrylic acid (PAN-co-AA) electrospun ber membrane (PAN-co-AA/b-CD) as the host, and the conjugated A nitrobenzene-modied naphthalimide derivative (intermediate 3) served as a guest. The host-guest self-assembly method was used to combine them to form a ber membrane sensor for F-high selective detection. Due to the strong electronegativity of F − , it will form a hydrogen bond interaction with the N-H unit in the intermediate 3 on the surface of the ber membrane, and make the proton on the N-H dissociate, which makes the electron transfer to the homoorbital of the excited state conjugated structure through the PET effect, and hindering the movement of electrons reduces the uorescence. The ber membrane senses for F − and also has good adsorption capacity for F − , with an adsorption capacity of 16.67 mg g −1 (Fig. 19(E) and 20(E)). Virginia et al. 130 used electrospinning technology to encapsulate active chlorine-responsive graphene quantum dots (GQDs) in PAN to prepare a nanobrous membrane chemical sensor with superior stability. The GQDs-PAN nanobrous membrane exhibited irreversible uorescence quenching effect and excellent selectivity under free active chlorine treatment. The GQDs-PAN nanobrous membrane exhibited better photostability and lower detection limit than the single GQDs solution, also had good luminescence performance under light irradiation for 2 months and the detection limit for hypochlorite was as low as 2 mM. Based on its excellent stability and low detection limit, it has a good application prospect in practical environmental monitoring (Fig. 21(A)).
Electrospun nanober sensor for detection of SO 3

2−
Gao et al. 131 prepared a CNFs/MnCo 2 O 4.5 composite ber membrane material by electrospinning technology and electrochemical deposition technology. The ber membrane can be used for the colorimetric detection of sulte, showing a good linear relationship in the sulte concentration range of 0-1 mM, and the detection limit was calculated to be 15.9 nM. Its excellent colorimetric effect and high sensitivity enable the application of manganese cobalt oxide/carbon nanober composites in chemical sensing (Fig. 21(B), (C) and (D)).
Cyanide, as a strong electron-withdrawing ion, can achieve sensing by forming strong interactions with active N-H and phenolic hydroxyl groups to deprotonate them, resulting in changes in uorescence and color. It is also possible to use the etching effect of cyanide on the metal to cause the change of uorescence to realize the sensing of the reinforcement. Hypochlorite and sulte except according to the inorganic sensing unit described above. It is also possible to use the oxidation of hypochlorite to design some organic dyes containing sulfur and selenium as sensing units, or to use the nucleophilicity of sultes to design organic dyes containing carbon positive structure as sensing units.

Conclusion and future prospects
In this paper, we summarize and compare the recent advances in electrospun ber membrane chemical sensors in the eld of ion detection in the past decade ( Table 1). The preparation of electrospun ber membrane sensors is a unit that will be able to act as sensing through physical doping, chemical modication, copolymerization, surface adsorption and self-assembly, such as organic molecular probes, MOFs, carbon quantum dots, noble metal nanoparticles and natural biomaterials etc. are immobilized on the electrospun bers.
In conclusion, electrospun nanober membranes for ion detection can be prepared using physical doping, copolymerization, chemical modication, self-assembly and surface adsorption. Among them, the physical doping method is the simplest and the widest range of use, but there are also shortcomings such as low utilization rate of the sensing unit and easy separation of the sensing unit from the polymer. The copolymerization method enables the sensing unit and the polymer to be tightly bonded, but the low utilization of the sensing unit is not improved. Chemical modication, surface adsorption and self-assembly can solve the above problems and improve the utilization rate of the sensing unit while making the combination more stable. This provides some ideas for the development of ion electrospun ber sensors. The electrospun ber membrane sensor can rapidly and sensitively detect metal ions and anions in water by colorimetric-uorescence, due to the large specic surface area and high permeability of the ber membrane and most of the ber membrane sensors also have good regeneration ability. It is not easy to cause pollution in the process of detecting harmful substances and it has the characteristics of environmental friendliness, because solid-state sensors can be easily separated from water.
However, there are some challenges in the development of electrospun membrane sensors. At present, most of the electrospun bers use polymers that are not easy to degrade, which are easy to form microplastics in the environment. The pollution of microplastics has become a major environmental problem in today's world. Although electrospun membrane sensors have a very high specic surface area and porosity, the heterogeneous system formed with water limits the contact between the detected ions and the sensor itself. Therefore, the detection limit of the sensor is still relatively high, and the detection of trace ions cannot be completed. In addition, the current electrospun ber membranes for ion detection mainly focus on the detection of metal ions, but for the case of water pollution, some electrospun ber membranes for common highly toxic anion detection have not been reported. The reusability of the sensor is an important indicator. However, only a very small number of electrospun ber membrane sensors have been reported to study their reusability (Scheme 2).
At the same time as the challenges, there are many opportunities for electrospun membrane sensors. There are many polymeric materials with good degradability. Examples include chitosan, cellulose, protein and amino acid polymers. 132 These polymeric materials can be decomposed by microorganisms in the environment into environmentally friendly inorganic substances such as carbon dioxide, nitrogen and water. Moreover, in order to improve the detection sensitivity of nanober membrane sensors to various ions, electrochemical methods can be introduced to improve the sensing sensitivity. Alternatively, a ber membrane material that can enrich ions in water can be designed based on the electrostatic interaction of anions and cations so that low concentrations of ions can also respond to the ber membrane sensor. [133][134][135] Expanding the detection of ion types of sensors is also the future development direction of electrospun ber membrane sensors. Some anions containing arsenic and chromium in the environment pose a serious threat to human health and the environment. It is necessary to design ber membranes that can efficiently detect and remove these ions. Finally, for the sustainable development of electrospun membrane sensors, exploring their reusability is also a highlight.