Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions

Novel multifunctional switchable chemosensors based on fluorescent electrospun (ES) nanofibers with sensitivity toward magnetism, temperature, and mercury ions (Hg2+) were prepared using blends of poly(N-isopropylacrylamide)-co-(N-methylolacrylamide)-co-(Acrylic acid), the fluorescent probe 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]-thiourea (BNPTU), and magnetite nanoparticles (NPs), and a single-capillary spinneret. The moieties of N-isopropylacrylamide, N-methylolacrylamide, acrylic acid, BNPTU, and Iron oxide (Fe3O4) NPs were designed to provide thermoresponsiveness, chemical cross-linking, Fe3O4 NPs dispersion, Hg2+ sensing, and magnetism, respectively. The prepared nanofibers exhibited ultrasensitivity to Hg2+ (as low as 10−3 M) because of an 80-nm blueshift of the emission maximum (from green to blue) and 1.6-fold enhancement of the emission intensity, as well as substantial volume (or hydrophilic to hydrophobic) changes between 30 and 60 °C, attributed to the low critical solution temperature of the thermoresponsive N-isopropylacrylamide moiety. Such temperature-dependent variations in the presence of Hg2+ engendered distinct on–off switching of photoluminescence. The magnetic ES nanofibers can be collected using a magnet rather than being extracted through alternative methods. The results indicate that the prepared multifunctional fluorescent ES nanofibrous membranes can be used as naked eye sensors and have the potential for application in multifunctional environmental sensing devices for detecting metal ions, temperature, and magnetism as well as for water purification sensing filters.


Introduction
In recent years, identifying the presence and concentration of heavy metal ions has received gradually increasing levels of attention because of emerging environmental and human health issues [1,2]. Heavy transition metal (HTM) cations and their derivatives are used extensively in the industrial sector, and can cause adverse environment and health problems [3,4]; thus, developing sensitive and selective fluorescent chromogenic probes composed of chelating ligands that are used to detect HTM cations in biological and environmental sensory devices is crucial. Colorimetric and Fe 3 O 4 NPs were designed to achieve thermoresponsiveness, chemical cross-linking, hydrophilicity, Hg 2+ sensing, and magnetism, respectively. A single-capillary spinneret was used to fabricate ES nanofibers from poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe 3 O 4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe 3 O 4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices.
Polymers 2017, 9,136 3 of 21 poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe3O4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe3O4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices.   poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe3O4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe3O4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices.

Synthesis of Magnetic Iron Oxide (Fe 3 O 4 ) Nanoparticles (NPs)
Magnetic nanoparticles, Fe 3 O 4 , were prepared by a coprecipitation method as reported [27]. A 100-mL round-bottomed flask was charged with FeCl 3 ·6H 2 O (380 mg, 1.4 mmol), FeCl 2 ·4H 2 O (140 mg, 0.7 mmol) and 80 mL of water. The reaction mixture was stirred at reflux under N 2 until the mixture had completely dissolved. The ammonium hydroxide (8 mL) was added dropwise under mechanical stirring at 60 • C for 30 min under N 2 . Then, the magnetic iron oxides were isolated from the solution by a magnet bar and dried in a vacuum oven at 40 • C for 24 h.

Synthesis of Poly(NIPAAm-co-NMA-co-AA)
Poly(NIPAAm-co-NMA-co-AA) was synthesized through free-radical copolymerization of the following three monomers: NIPAAm, NMA, and AA ( Figure 2) Poly(NIPAAm-co-NMA-co-AA) with different monomer ratios were denoted P1-P2, as listed in Table 1. The concentration of AIBN used as the initiator was 0.004 M. The reaction mixture, containing dimethylformamide (DMF) and monomers, was degassed by first bubbling nitrogen through for 30 min and then left to react at 70 • C for 24 h. The reaction mixture was quenched by exposure to air. The mixture was diluted with methanol to remove the unreacted monomers. The light white filtrate was concentrated, re-precipitated from diethyl ether, collected by filtration, and dried under vacuum to obtain the polymer product. The synthesis and characterization of P1-P2 are described in the following sections. The number-averaged molecular weight (M n ) and polydispersity index (PDI) estimated from gel permeation chromatography (GPC) (THF eluent) are listed in Table 1 and Figure S1.

Synthesis of Poly(NIPAAm-co-NMA) Random Copolymers (P1)
A reaction mixture of 1.131 g (9.99 mmol) of NIPAAm, 505 mg (4.99 mmol) of NMA, 5 mg (0.03 mmol) of AIBN, and 7.5 mL of EtOH was used to produce an orange solid (yield: 86%). Figure 4 and Table 1 present the molecular weight and chemical structure characterization of poly(NIPAAm-co-NMA) obtained using GPC with THF as the eluent and 1 H-NMR, respectively. The copolymer composition, estimated by performing peak integration, was consistent with the proposed structure. The estimated copolymer ratio of poly(NIPAAm-co-NMA) based on the NMR spectrum was 81:19, and the number-averaged molecular weight M n and PDI estimated using GPC were 26,856 and 2.01, respectively.

Synthesis of Poly(NIPAAm-co-NMA-co-AA) Random Copolymers (P2)
A reaction mixture of 1.131 g (9.99 mmol) of NIPAAm, 505 mg (4.99 mmol) of NMA, 0.204 mL (0.0029 mmol) of AA, 5 mg (0.03 mmol) of AIBN, and 9 mL of EtOH was used to produce an orange solid (yield: 82%). Figure 4 and Table 1 present the molecular weight and chemical structure characterization of poly(NIPAAm-co-NMA-co-AA) obtained using GPC with THF as the eluent and 1 H NMR, respectively. The copolymer composition, estimated by performing peak integration, was consistent with the proposed structure. The copolymer ratio of poly(NIPAAm-co-NMA-co-AA) based on the NMR spectrum was 72:15:13, and the number-averaged molecular weight Mn and PDI estimated using GPC were 18041 and 1.76, respectively.

Preparation of Electrospun (ES) Nanofibers
As shown in Figure 2b, the ES nanofibers were prepared using a single-capillary spinneret in a procedure similar to that described in our previous papers [19][20][21][22][23][24][25][26]. The poly(NIPAAm-co-NMA) (P1), poly(NIPAAm-co-NMA-co-AA) (P2), BNPTU, and Fe3O4 NPs blend was dissolved in methanol (MeOH) as solvent at 250 mL h −1 and stirred overnight. The blend composition (wt %) of (P1 or P2), BNPTU, and Fe3O4 NPs for preparing the ES nanofibers was 85/10/5. The polymer solution was fed into a metallic needle using syringe pumps (Model 100, KD Scientific, Holliston, MA, USA) at a feed rate of 0.8-1.0 mL h −1 . The tip of the needle was connected to a Chargemaster CH30P high voltage power supply (Simco, Hatfield, PA, USA) set at 13.4 kV during electrospinning. A piece of aluminum foil or quartz was placed 15 cm below the tip of the needle for 30 min to collect the ES nanofibers. All experiments were performed at room temperature and a relative humidity of approximately 30%. P1 and P2 blended with the Fe3O4 NPs at a 5 wt % ratio are denoted by P1-5% and P2-5%. The ES nanofibers were annealed at 100 °C for 24 h in an oven for chemical cross-linking.

Characterization
1 H-NMR data were recorded at room temperature using an AM 300 (300 MHz) spectrometer (Bruker, Billerica, MA, USA) and the residual proton resonance of deuterated chloroform and deuterated dimethyl sulfoxide. High resolution electrospray ionization mass spectrometry spectra were recorded using an ion-trap time-of-flight liquid chromatograph mass spectrometer (Shimadzu, Kyoto, Japan). Gel permeation chromatography (GPC) analysis was performed using a Lab Alliance RI2000 instrument (two-column, MIXED-C and MIXED-D from Polymer Laboratories, Theale, UK)

Preparation of Electrospun (ES) Nanofibers
As shown in Figure 2b, the ES nanofibers were prepared using a single-capillary spinneret in a procedure similar to that described in our previous papers [19][20][21][22][23][24][25][26]. The poly(NIPAAm-co-NMA) (P1), poly(NIPAAm-co-NMA-co-AA) (P2), BNPTU, and Fe 3 O 4 NPs blend was dissolved in methanol (MeOH) as solvent at 250 mL h −1 and stirred overnight. The blend composition (wt %) of (P1 or P2), BNPTU, and Fe 3 O 4 NPs for preparing the ES nanofibers was 85/10/5. The polymer solution was fed into a metallic needle using syringe pumps (Model 100, KD Scientific, Holliston, MA, USA) at a feed rate of 0.8-1.0 mL h −1 . The tip of the needle was connected to a Chargemaster CH30P high voltage power supply (Simco, Hatfield, PA, USA) set at 13.4 kV during electrospinning. A piece of aluminum foil or quartz was placed 15 cm below the tip of the needle for 30 min to collect the ES nanofibers. All experiments were performed at room temperature and a relative humidity of approximately 30%. P1 and P2 blended with the Fe 3 O 4 NPs at a 5 wt % ratio are denoted by P1-5% and P2-5%. The ES nanofibers were annealed at 100 • C for 24 h in an oven for chemical cross-linking.

Characterization
1 H-NMR data were recorded at room temperature using an AM 300 (300 MHz) spectrometer (Bruker, Billerica, MA, USA) and the residual proton resonance of deuterated chloroform and deuterated dimethyl sulfoxide. High resolution electrospray ionization mass spectrometry spectra were recorded using an ion-trap time-of-flight liquid chromatograph mass spectrometer (Shimadzu, Kyoto, Japan). Gel permeation chromatography (GPC) analysis was performed using a Lab Alliance RI2000 instrument (two-column, MIXED-C and MIXED-D from Polymer Laboratories, Theale, UK) connected to a refractive index detector (Schambeck SFD GmbH, Bad Honnef, Germany). All GPC analyses were performed using a polymer and THF solution at a flow rate of 1 mL min −1 at 40 • C and calibrated with methyl methacrylate. The thermal decomposition temperature was determined using a Q50 thermal gravimetric analyzer (TGA) (TA Instruments, Lukens Drive, New Castle, DE, USA) over a heating range of 100-800 • C at a heating rate of 10 • C min −1 in a nitrogen atmosphere. The LCST of the prepared copolymer solution was recorded by monitoring the transmittance of a 520 nm light beam on Shimadzu UV-Vis spectrophotometer. The copolymer concentration in water was 1 wt %, and the temperature was raised from 10 to 70 • C in 2.5 • C increments every 10 min. A plot of the changes in optical transmittance as a function of temperature for polymers in water was made, the LCST corresponds to the first turning point of the transition curve [21]. Magnetic properties of Fe 3 O 4 nanoparticles or ES nanofibers were measured on a Lake Shore VSM-7407 instrument (MPMS (SQUID) VSM, Quantum Design, San Diego, CA, USA). The magnetization hysteresis loops were measured at 300 K [30].
The morphologies of ES nanofibers were characterized using an S-520 scanning electron microscope (Hitachi, Tokyo, Japan) equipped with X-ray microanalysis capability. Samples were coated with platinum prior to scanning electron microscopy (SEM) characterization, and analysis was conducted at an increased voltage of 10 kV. Fluorescence optical microscopy images were captured using an LCS SP5 two-photon laser confocal microscope (Leica, Mannheim, Germany). The morphologies of the ES nanofibers were similar to those reported in our previous studies [15,20,24].
Ultraviolet-visible (UV-Vis) absorption and PL spectra were measured to examine photophysical properties and recorded using a UV-Vis spectrophotometer (Shimadzu) and a Fluorolog-3 spectrofluorometer (Horiba Jobin Yvon, Edison, NJ, USA), respectively. Variations in the optical absorption and PL of the prepared ES nanofibers with different metal ion concentrations are described as follows. To ensure that the beam excited the same point on the prepared samples during each measurement, the ES nanofibers were fixed in cuvettes with adhesive tape, and the cuvette was filled with a basic aqueous metal ion solution at 10 −4 -10 −2 M. Each measurement was maintained for 15 min to ensure that the chelating reaction reached equilibrium. All PL spectra of the ES nanofibers were recorded using the spectrofluorometer, and the samples were excited at a suitable wavelength, as described in our previous studies [19][20][21][22][23][24][25][26].

Characterization of BNPTU and Poly(NIPAAm-co-NMA-co-AA)
The chemical structure of BNPTU was characterized using 1 H-NMR ( Figure 3). The synthesis routes of BNPTU ( Figure 1) are similar to those reported in a previous study [8]. Figure 2a shows the route by which poly(NIPAAm-co-NMA-co-AA) copolymers were synthesized through free radical polymerization. The two types of copolymers (P1 and P2) were synthesized and their composites are listed in Table 1. Figure 4 shows the 1 H-NMR spectrum of poly(NIPAAm-co-NMA) (P1) and poly(NIPAAm-co-NMA-co-AA) (P2). The proton peaks at 7.23-7.59 ppm (Peak e) and 3.71-3.82 ppm (Peak c) correspond to the methylene neighbors of the secondary amine moiety and the alkyl chains on NIPAAm, respectively. The proton peaks at 5.34-5.53 ppm (Peak b), 7.92-8.14 ppm (Peak f), and 4.36-4.65 ppm (Peak g) correspond to the methylene neighbors of the hydroxyl, secondary amine moiety, and alkyl chains on NMA, respectively. The proton peak at 11.92-12.05 ppm (Peak d) corresponds to the methylene neighbor of hydroxyl on the AA in P2. The peak at 0.82-1.12 ppm (Peak a) and those at 1.23-2.1 ppm (Peaks h, i, and j) correspond to the alkyl chains on the copolymers. The copolymer ratios of P1 and P2 were estimated from NMR spectra, and were 81:19 and 72:15:13, respectively. The favorable agreement between the feeding ratio and experimental composition suggests the successful preparation of the target copolymers. The molecular weights, thermal properties, and LCSTs of P1 and P2 are listed in Table 1. The number-averaged molecular weights and polydispersity indices (PDIs) of P1 and P2 were 26,856 and 2.01, and 18,041 and 1.76, respectively. Notably, Figure S1 shows that the accurate PDI (M n /M w ) data of P1 were 26,856/53,980 equal to 2.01 and that of P2 was 18,041/31,752 equal to 1.76. The thermal decompositon curves of the prepared polymers are presented in Figure S2 (Supplementary Materials). The identical thermal decomposition temperature of 210 • C for P1 and P2 is attributable to their similar NIPAAm compositions. All decompostion temperatures for P1 and P2 were higher than 270 • C, and thus, they exhibited favorable and stable thermal properties. Figure S3 shows the typical optical transmittance (520 nm) versus temperature curves of P1 and P2, which exhibit thermoresponsive soluble-to-insoluble phase transitions in an aqueous medium. The copolymers are soluble in water below their LCSTs. The LCSTs of P1 and P2 are both approximately 55 • C, slightly higher than that of PNIPAM (32 • C) because of the hydrophilic characteristics of the NMA and AA compounds, as stated in our previous study [23] and one other study [34]. Figure 5a illustrates that BNPTU in CH 3 CN had a maximum in the UV-Vis absorption peak (λ abs max ) of 430 nm at pH 7 and emitted green fluorescence under UV light (inset) as it is a fluorescent dye. Figure 5b shows variations in the UV-Vis spectra of BNPTU in a CH 3 CN solution containing various metal ions at a concentration of 10 −5 M (pH 7). The λ abs max of BNPTU was blueshifted from 430 to 350 nm when the Hg 2+ ion was added. The Hg 2+ ion transformed the thiourea unit of BNPTU under aqueous conditions into an imidazoline moiety with considerably weakened electron-donating ability ( Figure 2c) [5,[35][36][37]. However, no change in the absorption peak was observed when other metal ions such as Co 2+ , Ni 2+ , Pb 2+ , Zn 2+ , Mg 2+ , Cu 2+ , Fe 2+ , and Cd 2+ were added, suggesting that BNPTU exhibited high selectivity and sensitivity toward Hg 2+ . polymers are presented in Figure S2 (Supplementary Materials). The identical thermal decomposition temperature of 210 °C for P1 and P2 is attributable to their similar NIPAAm compositions. All decompostion temperatures for P1 and P2 were higher than 270 °C, and thus, they exhibited favorable and stable thermal properties. Figure S3 shows the typical optical transmittance (520 nm) versus temperature curves of P1 and P2, which exhibit thermoresponsive soluble-to-insoluble phase transitions in an aqueous medium. The copolymers are soluble in water below their LCSTs. The LCSTs of P1 and P2 are both approximately 55 °C, slightly higher than that of PNIPAM (32 °C) because of the hydrophilic characteristics of the NMA and AA compounds, as stated in our previous study [23] and one other study [34]. Figure 5a illustrates that BNPTU in CH3CN had a maximum in the UV-Vis absorption peak (λ abs max) of 430 nm at pH 7 and emitted green fluorescence under UV light (inset) as it is a fluorescent dye. Figure 5b shows variations in the UV-Vis spectra of BNPTU in a CH3CN solution containing various metal ions at a concentration of 10 −5 M (pH 7). The λ abs max of BNPTU was blueshifted from 430 to 350 nm when the Hg 2+ ion was added. The Hg 2+ ion transformed the thiourea unit of BNPTU under aqueous conditions into an imidazoline moiety with considerably weakened electron-donating ability (Figure 2c) [5,[35][36][37]. However, no change in the absorption peak was observed when other metal ions such as Co 2+ , Ni 2+ , Pb 2+ , Zn 2+ , Mg 2+ , Cu 2+ , Fe 2+ , and Cd 2+ were added, suggesting that BNPTU exhibited high selectivity and sensitivity toward Hg 2+ .    Figure 6 presents field-emission SEM (FE-SEM) images of the ES nanofibers prepared using P1 and P2 at a solution concentration of 250 mg mL −1 with MeOH. When dry, the P1 and P2 ES nanofibers had average diameters of 416 ± 31 and 437 ± 42 nm, respectively. The recorded average diameter was a statistical average of 50 fibers from each sample. Moreover, all of the ES nanofibers from pure MeOH solvent were smooth and nonporous. The diameters of the P1 and P2 ES nanofibers were similar when they were dry because the concentration of the ES solution was fixed and the molar ratios of the NMA and AA moieties in the copolymers were lower than that of NIPAAm. To observe the morphologies of the cross-linked P1 and P2 ES nanofibers in pure water at various temperatures, the fibers were collected on a small piece of aluminum foil and immersed in water at 30 • C or 60 • C. After 10 min, the samples were solidified by placing in a flask containing liquid nitrogen, and the residual water was removed using a vacuum for 30 min to retain the original morphology. Figure 6 shows the FE-SEM images of the P1 and P2 ES nanofibers in a wet state after they were immersed in water at 30 or 60 • C. As shown in Figure 6 (wet state), the P1 and P2 fibers exhibited diameters of 1250 ± 205 and 1420 ± 282 nm at 25 • C and 830 ± 124 and 920 ± 138 nm at 60 • C, respectively. The diameters were substantially enlarged after the nanofibers were immersed in water (416 ± 31, 437 ± 42 nm, respectively; Figure 6) because the hydrophilic NIPAAm chain swelled in the water. However, these swollen fibers maintained their cylindrical shape and did not dissolve in water, a phenomenon attributed to the efficient chemical cross-linking of the NMA moiety [23]. Furthermore, the fiber diameters at 30 • C were greater than those at 60 • C because of the hydrophilic NIPAAm chain, the temperature of which was below the LCST. By contrast, the nanofiber diameters decreased from approximately 1.5 µm to 1.0 µm as the temperature increased from 30 to 60 • C, due to the temperature of which was above the LCST resulting in the hydrophobic NIPAAm chain.

Morphology and Characterization of ES Nanofibers
Polymers 2017, 9,136 10 of 21 a statistical average of 50 fibers from each sample. Moreover, all of the ES nanofibers from pure MeOH solvent were smooth and nonporous. The diameters of the P1 and P2 ES nanofibers were similar when they were dry because the concentration of the ES solution was fixed and the molar ratios of the NMA and AA moieties in the copolymers were lower than that of NIPAAm. To observe the morphologies of the cross-linked P1 and P2 ES nanofibers in pure water at various temperatures, the fibers were collected on a small piece of aluminum foil and immersed in water at 30 °C or 60 °C. After 10 min, the samples were solidified by placing in a flask containing liquid nitrogen, and the residual water was removed using a vacuum for 30 min to retain the original morphology. Figure 6 shows the FE-SEM images of the P1 and P2 ES nanofibers in a wet state after they were immersed in water at 30 or 60 °C. As shown in Figure 6 (wet state), the P1 and P2 fibers exhibited diameters of 1250 ± 205 and 1420 ± 282 nm at 25 °C and 830 ± 124 and 920 ± 138 nm at 60 °C, respectively. The diameters were substantially enlarged after the nanofibers were immersed in water (416 ± 31, 437 ± 42 nm, respectively; Figure 6) because the hydrophilic NIPAAm chain swelled in the water. However, these swollen fibers maintained their cylindrical shape and did not dissolve in water, a phenomenon attributed to the efficient chemical cross-linking of the NMA moiety [23]. Furthermore, the fiber diameters at 30 °C were greater than those at 60 °C because of the hydrophilic NIPAAm chain, the temperature of which was below the LCST. By contrast, the nanofiber diameters decreased from approximately 1.5 μm to 1.0 μm as the temperature increased from 30 to 60 °C, due to the temperature of which was above the LCST resulting in the hydrophobic NIPAAm chain.   Figure 7c shows that the Fe3O4 NPs precipitated out of the solution and dropped to the bottom because of higher density (left image) and then accumulated on the side of the bottle because of a magnetic bar placed next to the bottle (right image), indicating that magnetic Fe3O4 NPs can be absorbed using a magnet [38].     Figure 8a presents an SEM image of P2-5% ES nanofibers prepared from P2 copolymers blended with Fe3O4 NPs at a 5% weight ratio. The strong stretching force associated with electrospinning induces orientation of these Fe3O4 NPs along the axis of the fiber. The numerous carboxyl groups of PAA in P2 inhibited the aggregation of Fe3O4 NPs, a result similar to those of several previous studies [27][28][29][30][31]. The result can be attributed to P2-5% containing carboxyl groups of PAA that interact with the Fe3O4 NPs, thereby enabling the PAA to be easily adsorbed by Fe3O4 NPs. Figure 8b shows an SEM image of P2-5% ES nanofibers before elemental mapping, and Figure 8c,d shows the corresponding SEM images of P2-5% ES nanofibers after carbon (C) and iron (Fe) elemental mapping, respectively, by using energy dispersive X-ray spectroscopy (EDS). Notably, the C and Fe are contributed by the P2 and Fe3O4 NPs, respectively. The C and Fe mapping images in Figure 8b-d indicate that Fe3O4 NPs were present within the P2-5% ES nanofibers because no Fe species remained on the substrate, as shown by the dark region. The Fe content within the P2-5% ES nanofibers was 14.54 wt % as estimated according to the EDS spectrum ( Figure 8e). Furthermore, the amounts of Fe3O4 NPs within the P2-5% ES nanofibers were confirmed through TGA in an oxygen atmosphere ( Figure S4). Figure S4 shows the stages within the temperature range 100-750 °C, including the decomposition of poly(N-methylolacrylamide)(PNMA) and PAA, and the oxdiation of the Fe3O4 NPs. Weight loss initially occurred below 300 °C, and originated from either the decomposition of noncoordinated carboxyls or the evaporation of trace water. Weight loss that occurred between 300 and 450 °C was caused by the cleaving of PNIPAM chains [31]. Finally, the copolymers were degraded and Fe3O4 was completely transformed into Fe2O3 at 700 °C [30]. The calculated weight percentage of the remaining content, consisting of carbon residue and Fe2O3, was approximately 14 wt %.   [27][28][29][30][31]. The result can be attributed to P2-5% containing carboxyl groups of PAA that interact with the Fe 3 O 4 NPs, thereby enabling the PAA to be easily adsorbed by Fe 3 O 4 NPs. Figure 8b shows an SEM image of P2-5% ES nanofibers before elemental mapping, and Figure 8c,d shows the corresponding SEM images of P2-5% ES nanofibers after carbon (C) and iron (Fe) elemental mapping, respectively, by using energy dispersive X-ray spectroscopy (EDS). Notably, the C and Fe are contributed by the P2 and Fe 3 O 4 NPs, respectively. The C and Fe mapping images in Figure 8b-d indicate that Fe 3 O 4 NPs were present within the P2-5% ES nanofibers because no Fe species remained on the substrate, as shown by the dark region. The Fe content within the P2-5% ES nanofibers was 14.54 wt % as estimated according to the EDS spectrum ( Figure 8e). Furthermore, the amounts of Fe 3 O 4 NPs within the P2-5% ES nanofibers were confirmed through TGA in an oxygen atmosphere ( Figure S4). Figure S4 shows the stages within the temperature range 100-750 • C, including the decomposition of poly(N-methylolacrylamide)(PNMA) and PAA, and the oxdiation of the Fe 3 O 4 NPs. Weight loss initially occurred below 300 • C, and originated from either the decomposition of noncoordinated carboxyls or the evaporation of trace water. Weight loss that occurred between 300 and 450 • C was caused by the cleaving of PNIPAM chains [31]. Finally, the copolymers were degraded and Fe 3 O 4 was completely transformed into Fe 2 O 3 at 700 • C [30]. The calculated weight percentage of the remaining content, consisting of carbon residue and Fe 2 O 3 , was approximately 14 wt %.

Hg 2+ Sensing, Thermoresponsiveness, and Magnetic Properties of ES Nanofibers
P1 or P2 blended with 10% BNPTU and 5% Fe 3 O 4 NP ES nanofibers, which can sense metal ions because of the ability of PAA and BNPTU to adsorb Hg 2+ , was systematically explored [8,33]. Figure 9a shows the PL spectra of ES nanofibers prepared from P1 and P2 blended with 10% BNPTU in an aqueous solution without Hg 2+ (blank) or with Hg 2+ at 10 −2 M and subjected to 430-nm excitation. The maximum emission peak (λ PL max ) of the P1 and P2 ES nanofibers blueshifted substantially from 530 nm in a non-Hg 2+ aqueous solution to 450 nm in an Hg 2+ aqueous solution. This change corresponded to the thiourea unit of BNPTU, which transformed the imidazoline moiety (with the Hg 2+ ion), thereby causing a significant reduction in electron delocalization within the fluorophore (Figure 2c) [5,[35][36][37]. Moreover, the PL intensity of the P2 ES nanofibers was 1.3 times that of P1 ES nanofibers, indicating that the AA moiety of P2 enhances the adsorption of mercury ions, similar to one report [33].

Hg 2+ Sensing, Thermoresponsiveness, and Magnetic Properties of ES Nanofibers
P1 or P2 blended with 10% BNPTU and 5% Fe3O4 NP ES nanofibers, which can sense metal ions because of the ability of PAA and BNPTU to adsorb Hg 2+ , was systematically explored [8,33]. Figure  9a shows the PL spectra of ES nanofibers prepared from P1 and P2 blended with 10% BNPTU in an aqueous solution without Hg 2+ (blank) or with Hg 2+ at 10 −2 M and subjected to 430-nm excitation. The maximum emission peak (λ PL max) of the P1 and P2 ES nanofibers blueshifted substantially from 530 nm in a non-Hg 2+ aqueous solution to 450 nm in an Hg 2+ aqueous solution. This change corresponded to the thiourea unit of BNPTU, which transformed the imidazoline moiety (with the Hg 2+ ion), thereby causing a significant reduction in electron delocalization within the fluorophore (Figure 2c) [5,[35][36][37]. Moreover, the PL intensity of the P2 ES nanofibers was 1.3 times that of P1 ES nanofibers, indicating that the AA moiety of P2 enhances the adsorption of mercury ions, similar to one report [33].   Figure 9b shows the PL spectra of ES nanofibers prepared from P2 blended with 10% BNPTU and 5% Fe3O4 NPs. P2-5% was placed in aqueous solutions without Hg 2+ (blank), with Hg 2+ at 10 −4 , 10 −3 , and 10 −2 M, and under neutral conditions at 430-nm excitation. The presence of the highest Hg 2+ concentration (10 −2 M) led to a λ PL max blueshift from 530 nm to 450 nm in the emission spectra, similar to that of the P2 ES nanofibers without Fe3O4 NPs, indicating that the metal ion sensing ability was unaffected by Fe3O4 NPs. As shown in the figure, the concentration of Hg 2+ increasing from 0 (blank) to 10 −4 , 10 −3 , and 10 −2 M resulted in clear blueshifts in the λ PL max from 530 to 500, 450, and 450 nm, respectively. Furthermore, when the mercury ion concentration was at the extremely dilute level of 10 −3 M, the emission maximum shift Δλmax was observed to be as high as 80 nm, resulting in a color change easily observable by the naked eye. Moreover, the lowest detectable Hg 2+ concentration for the ES nanofibers was 10 −4 M, suggesting that the P2-5% ES nanofibers were highly sensitive to Hg 2+ but not to general ions, even when the system contained Fe3O4 NPs. Figure 9c shows the changes of the fluorescence intensity ratio, I450/I530 (I450 is the fluorescence intensity of BNTPU after the detection of Hg 2+ emission at 537 nm, whereas I530 is the fluorescence intensity of BNTPU before the detection of Hg 2+ emission at 530 nm) of the P2-5% ES nanofibers when subjected to Hg 2+ . As the concentration of Hg 2+ was increased, BNTPU emission at 530 nm gradually decreased, and BNTPU-Hg 2+ emission at 450 nm gradually increased. These changes corresponded to the fluorescence of BNTPU chelated with Hg 2+ ions, causing the I450/I530 to increase from approximately 0.1 to 1.2 as the Hg 2+ ion concentration increased from 0 to 10 −2 M. The P2-5% ES nanofibers exhibited high sensitivity to Hg 2+ ions between 10 −4 and 10 −2 M. Moreover, the titration data of Figure 9c show that the Kd was calculated as 1.58 mM [24]. Figure 9d shows fluorescence variations in the P2-5% ES nanofibers in a 10 −2 M Hg 2+ environment as the temperature was varied from 30 to 60 °C. At 30 °C, the temperature at which the ES nanofibers chelate Hg 2+ , the λ PL max was blueshifted substantially from 530 nm (blank, green emission) to 450 nm (blue emission) in the emission spectra. When the temperature was increased from 30 to 45 °C, only a mild decrease in PL intensity at 450 nm (λ PL max) was observed, exhibiting almost no change, as shown in the enlarged inset image. However, when the temperature was increased from 45 to 50 °C, a dramatic quenching of the PL intensity of the ES nanofibers was observed. The PL intensity decreased as the temperature increased from 50 to 60 °C, reaching its lowest point at 60 °C. Notably, Figure S5 shows no changes in the PL intensity of the pristine BNPTU compound as the temperature was increased from 30 to 60 °C because the chemical stability of BNTPU is favorable at room temperature or higher temperatures (e.g., 60 °C), indicating that the quenching of the PL intensity of the ES nanofibers (Figure 9d) was not caused by the pristine BNPTU compound.  Figure 9b shows the PL spectra of ES nanofibers prepared from P2 blended with 10% BNPTU and 5% Fe 3 O 4 NPs. P2-5% was placed in aqueous solutions without Hg 2+ (blank), with Hg 2+ at 10 −4 , 10 −3 , and 10 −2 M, and under neutral conditions at 430-nm excitation. The presence of the highest Hg 2+ concentration (10 −2 M) led to a λ PL max blueshift from 530 nm to 450 nm in the emission spectra, similar to that of the P2 ES nanofibers without Fe 3 O 4 NPs, indicating that the metal ion sensing ability was unaffected by Fe 3 O 4 NPs. As shown in the figure, the concentration of Hg 2+ increasing from 0 (blank) to 10 −4 , 10 −3 , and 10 −2 M resulted in clear blueshifts in the λ PL max from 530 to 500, 450, and 450 nm, respectively. Furthermore, when the mercury ion concentration was at the extremely dilute level of 10 −3 M, the emission maximum shift ∆λ max was observed to be as high as 80 nm, resulting in a color change easily observable by the naked eye. Moreover, the lowest detectable Hg 2+ concentration for the ES nanofibers was 10 −4 M, suggesting that the P2-5% ES nanofibers were highly sensitive to Hg 2+ but not to general ions, even when the system contained Fe 3 O 4 NPs. Figure 9c shows the changes of the fluorescence intensity ratio, I 450 /I 530 (I 450 is the fluorescence intensity of BNTPU after the detection of Hg 2+ emission at 537 nm, whereas I 530 is the fluorescence intensity of BNTPU before the detection of Hg 2+ emission at 530 nm) of the P2-5% ES nanofibers when subjected to Hg 2+ . As the concentration of Hg 2+ was increased, BNTPU emission at 530 nm gradually decreased, and BNTPU-Hg 2+ emission at 450 nm gradually increased. These changes corresponded to the fluorescence of BNTPU chelated with Hg 2+ ions, causing the I 450 / I 530 to increase from approximately 0.1 to 1.2 as the Hg 2+ ion concentration increased from 0 to 10 −2 M. The P2-5% ES nanofibers exhibited high sensitivity to Hg 2+ ions between 10 −4 and 10 −2 M. Moreover, the titration data of Figure 9c show that the Kd was calculated as 1.58 mM [24]. Figure 9d shows fluorescence variations in the P2-5% ES nanofibers in a 10 −2 M Hg 2+ environment as the temperature was varied from 30 to 60 • C. At 30 • C, the temperature at which the ES nanofibers chelate Hg 2+ , the λ PL max was blueshifted substantially from 530 nm (blank, green emission) to 450 nm (blue emission) in the emission spectra. When the temperature was increased from 30 to 45 • C, only a mild decrease in PL intensity at 450 nm (λ PL max ) was observed, exhibiting almost no change, as shown in the enlarged inset image. However, when the temperature was increased from 45 to 50 • C, a dramatic quenching of the PL intensity of the ES nanofibers was observed. The PL intensity decreased as the temperature increased from 50 to 60 • C, reaching its lowest point at 60 • C. Notably, Figure S5 shows no changes in the PL intensity of the pristine BNPTU compound as the temperature was increased from 30 to 60 • C because the chemical stability of BNTPU is favorable at room temperature or higher temperatures (e.g., 60 • C), indicating that the quenching of the PL intensity of the ES nanofibers (Figure 9d) was not caused by the pristine BNPTU compound.
The aforementioned thermoreversible luminescence characteristics are explained as follows. As shown in Figure 6, compared with those in the dry state, the ES fibers soaked in water (wet state) underwent morphological change at 30 • C (below the LCST). At 30 • C, the hydrophilic groups in the PNIPAM interacted easily with water molecules to form intermolecular hydrogen bonds. However, the swollen ES fibers were insoluble in water because of the chemically cross-linked NMA segment. When the temperature was raised to 60 • C (above the LCST), the intermolecular hydrogen bonds between the PNIPAM and water were broken, leading to the release of water molecules from the fibers. However, Hg 2+ was still present within the fibers. Furthermore, as the temperature was 60 • C (above the LCST), the PNIPAM was collapsed and densely packed, potentially suppressing absorption of incident light by the BNPTU moiety with Hg 2+ , resulting in a reduction of the PL intensity. Thus, the prepared ES fibers exhibited an on-off PL intensity profile with temperature variation (Figure 9d), indicating that the ES nanofibers have thermoresponsive properties.
The selectivity of P2-5% ES nanofibers toward Hg 2+ over other common metal ions was also studied ( Figure 10). Figure 10a shows that among all tested metal ions, namely Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ (10 −2 M; all metal ion test solutions were controlled at pH 4), a substantial blueshift in PL was observed only in the presence of Hg 2+ . In Figure 8b, I 450 /I 530 indicates the ratio of a PL intensity of 530 nm (λ PL max ), corresponding to other metal ions (Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ ), to a PL intensity of 450 nm (λ PL max ), corresponding to Hg 2+ . The presence of Hg 2+ induced the most prominent I 450 /I 530 enhancement (approximately 2.5-fold), resulting in blue emission. For all of the other metal ions, the nanofibers exhibited reduced I 460 /I 510 values (approximately 0.1-fold), resulting in green emission (Figure 10b). In addition, the fluorescence spectra recorded in the presence of Hg 2+ ions and the other metal ions revealed that none of the other metal ions interfered with the Hg 2+ ion-induced fluorescence enhancement (inset Figure 10b), indicating that the sensing of Hg 2+ by the P2-5% ES nanofibers was virtually unaffected by commonly coexisting ions. The aforementioned thermoreversible luminescence characteristics are explained as follows. As shown in Figure 6, compared with those in the dry state, the ES fibers soaked in water (wet state) underwent morphological change at 30 °C (below the LCST). At 30 °C, the hydrophilic groups in the PNIPAM interacted easily with water molecules to form intermolecular hydrogen bonds. However, the swollen ES fibers were insoluble in water because of the chemically cross-linked NMA segment. When the temperature was raised to 60 °C (above the LCST), the intermolecular hydrogen bonds between the PNIPAM and water were broken, leading to the release of water molecules from the fibers. However, Hg 2+ was still present within the fibers. Furthermore, as the temperature was 60 °C (above the LCST), the PNIPAM was collapsed and densely packed, potentially suppressing absorption of incident light by the BNPTU moiety with Hg 2+ , resulting in a reduction of the PL intensity. Thus, the prepared ES fibers exhibited an on-off PL intensity profile with temperature variation (Figure 9d), indicating that the ES nanofibers have thermoresponsive properties.
The selectivity of P2-5% ES nanofibers toward Hg 2+ over other common metal ions was also studied ( Figure 10). Figure 10a shows that among all tested metal ions, namely Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ (10 −2 M; all metal ion test solutions were controlled at pH 4), a substantial blueshift in PL was observed only in the presence of Hg 2+ . In Figure 8b, I450/I530 indicates the ratio of a PL intensity of 530 nm (λ PL max), corresponding to other metal ions (Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ ), to a PL intensity of 450 nm (λ PL max), corresponding to Hg 2+ . The presence of Hg 2+ induced the most prominent I450/I530 enhancement (approximately 2.5-fold), resulting in blue emission. For all of the other metal ions, the nanofibers exhibited reduced I460/I510 values (approximately 0.1-fold), resulting in green emission (Figure 10b). In addition, the fluorescence spectra recorded in the presence of Hg 2+ ions and the other metal ions revealed that none of the other metal ions interfered with the Hg 2+ ion-induced fluorescence enhancement (inset Figure 10b), indicating that the sensing of Hg 2+ by the P2-5% ES nanofibers was virtually unaffected by commonly coexisting ions.   Figure 11a shows the CIE coordinates of the P2-5% ES nanofibers in 0-10 −2 M Hg 2+ aqueous solutions. All of the inset figures show corresponding photographs captured under UV light. The CIE coordinates show a strong blueshift from (0.24, 0.61) (blank) to (0.16, 0.08) (pH 4, Hg 2+ 10 −2 M) as the concentration of Hg 2+ was varied from 0 to 10 −2 M because of the detection of BNPTU by Hg 2+ , resulting in a color change from green to blue in the P2-5% ES nanofibers. Furthermore, as shown in the confocal microscopy images in Figure 11b, the color emission of the P2-5% ES nanofibers varied from green to blue as the concentration of Hg 2+ was increased. A microfluidics system was constructed, in which a P2-5% ES nanofiber filter membrane with an area of 3 cm 2 placed in the middle of a tube was used to rapidly absorb and sense Hg 2+ in a solution flowing through the tube (Figure 11c). Figure 11d depicts the measured time-dependent variation in the solution conductivity. The prepared Hg 2+ solution contained 1 ppm Hg 2+ (6.3 × 10 −3 M Hg 2+ in 0.5 L of water) and the conductivity of the Hg 2+ solution in its initial state (0 min) was 131.2 µS cm −1 . The solution conductivity gradually decreased to 101.4 uS cm −1 . Table S1 contains raw data on the conductivity changes over 20 min, and Figure 11d depicts C t /C 0 (%) versus time (C 0 : original conductivity at 0 min; C t : conductivity at time t). The solution conductivity decreased as the flow time increased, indicating that an increasing number of Hg 2+ ions was absorbed by the P2-5% ES nanofiber membrane, yielding less Hg 2+ in the solution. Thus, the 100% conductivity of the solution in its initial state (0 min) had decreased to 77.2% after 20 min. This rapid change in conductivity was caused by the high surface-to-volume ratio of the P2-5% ES nanofibers. The sensory filter membrane based on the P2-5% ES nanofibers specifically absorbed Hg 2+ in an aqueous solution that contained a variety of metal ions and had a dual fluorescent chemosensory function for Hg 2+ . Figure 11a shows the CIE coordinates of the P2-5% ES nanofibers in 0-10 −2 M Hg 2+ aqueous solutions. All of the inset figures show corresponding photographs captured under UV light. The CIE coordinates show a strong blueshift from (0.24, 0.61) (blank) to (0.16, 0.08) (pH 4, Hg 2+ 10 −2 M) as the concentration of Hg 2+ was varied from 0 to 10 −2 M because of the detection of BNPTU by Hg 2+ , resulting in a color change from green to blue in the P2-5% ES nanofibers. Furthermore, as shown in the confocal microscopy images in Figure 11b, the color emission of the P2-5% ES nanofibers varied from green to blue as the concentration of Hg 2+ was increased. A microfluidics system was constructed, in which a P2-5% ES nanofiber filter membrane with an area of 3 cm 2 placed in the middle of a tube was used to rapidly absorb and sense Hg 2+ in a solution flowing through the tube (Figure 11c). Figure 11d depicts the measured time-dependent variation in the solution conductivity. The prepared Hg 2+ solution contained 1 ppm Hg 2+ (6.3 × 10 −3 M Hg 2+ in 0.5 L of water) and the conductivity of the Hg 2+ solution in its initial state (0 min) was 131.2 μS cm −1 . The solution conductivity gradually decreased to 101.4 uS cm −1 . Table S1 contains raw data on the conductivity changes over 20 min, and Figure 11d depicts Ct/C0 (%) versus time (C0: original conductivity at 0 min; Ct: conductivity at time t). The solution conductivity decreased as the flow time increased, indicating that an increasing number of Hg 2+ ions was absorbed by the P2-5% ES nanofiber membrane, yielding less Hg 2+ in the solution. Thus, the 100% conductivity of the solution in its initial state (0 min) had decreased to 77.2% after 20 min. This rapid change in conductivity was caused by the high surfaceto-volume ratio of the P2-5% ES nanofibers. The sensory filter membrane based on the P2-5% ES nanofibers specifically absorbed Hg 2+ in an aqueous solution that contained a variety of metal ions and had a dual fluorescent chemosensory function for Hg 2+ .   Figure 12a shows the saturation magnetization and coercivity of the P2-5% ES nanofibersnanocomposite, which were determined using a vibrating sample magnetometer in the field range of ±20,000 Oe at 300 K. The results indicate that the P2-5% ES nanofibers exhibited magnetic properties, and the saturation magnetization was 4.8 emu g −1 . Moreover, the magnetic P2-5% ES nanofibers specifically adsorbed Hg 2+ in an aqueous solution containing a variety of metal ions and had a fluorescent chemosensory response to Hg 2+ ( Figure 11). Thus, filter membranes based on the P2-5% ES nanofibers with porous architectures can specifically chelate with Hg 2+ in an aqueous solution containing many types of metal ion and serve as fluorescent chemosensors for Hg 2+ . These results could assist researchers in cleaning water while simultaneously chelating and sensing Hg 2+ . In addition, rather than removal through alternative methods, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe3O4 NPs. The results in Figure 12b and Figure 2c indicate that P2-5% ES nanofibers have the potential for application in multifunctional sensory filter membrane devices for HTM ion chelation, temperature sensing, and magnetism. In conclusion, these copolymer inorganic NP-based sensory fibers have considerable potential for application in water purification sensing filters for the filtration of industrial wastewater with HTM, and may assist researchers in purifying water while simultaneously chelating and sensing Hg 2+ ; moreover, used magnetic fluorescent switchable chemosensors can be collected using magnets (noncontact force).  Figure 12a shows the saturation magnetization and coercivity of the P2-5% ES nanofibers-nanocomposite, which were determined using a vibrating sample magnetometer in the field range of ±20,000 Oe at 300 K. The results indicate that the P2-5% ES nanofibers exhibited magnetic properties, and the saturation magnetization was 4.8 emu g −1 . Moreover, the magnetic P2-5% ES nanofibers specifically adsorbed Hg 2+ in an aqueous solution containing a variety of metal ions and had a fluorescent chemosensory response to Hg 2+ ( Figure 11). Thus, filter membranes based on the P2-5% ES nanofibers with porous architectures can specifically chelate with Hg 2+ in an aqueous solution containing many types of metal ion and serve as fluorescent chemosensors for Hg 2+ . These results could assist researchers in cleaning water while simultaneously chelating and sensing Hg 2+ . In addition, rather than removal through alternative methods, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe 3 O 4 NPs. The results in Figures 12b and 2c indicate that P2-5% ES nanofibers have the potential for application in multifunctional sensory filter membrane devices for HTM ion chelation, temperature sensing, and magnetism. In conclusion, these copolymer inorganic NP-based sensory fibers have considerable potential for application in water purification sensing filters for the filtration of industrial wastewater with HTM, and may assist researchers in purifying water while simultaneously chelating and sensing Hg 2+ ; moreover, used magnetic fluorescent switchable chemosensors can be collected using magnets (noncontact force). (b) Schematic of a filter sensory membrane prepared from P2-5% ES nanofibers composed of poly(NIPAAm-co-NMA-co-AA)), BNPTU, and Fe3O4 NPs blends to simultaneously chelate and sense Hg 2+ . A magnet can directly attract the P2-5% ES nanofibers because of the magnetism of Fe3O4 NPs.

Conclusions
Novel magnetic fluorescent switchable chemosensors for the simultaneous detection of temperature, magnetism, and Hg 2+ based on fluorescent ES nanofibers were prepared using blends of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 NPs by employing a single-capillary spinneret. The NIPAAm, NMA, AA, BNPTU, Fe3O4 NPs moieties were designed to provide thermoresponsiveness, chemical cross-linking, dispersion of Fe3O4 NPs, sensing of Hg 2+ , and magnetism, respectively. Crosslinked ES nanofibers maintained their structure in water and exhibited sensitivity toward temperature variations and Hg 2+ because of the sufficient NMA composition. The fluorescence emission of BNPTU within the ES nanofibers exhibited strong selectivity toward Hg 2+ with green emission in aqueous solutions without Hg 2+ (thiourea-derived), shifting to blue emission in aqueous solutions with Hg 2+ (imidazoline-derived)). The P2-5% ES nanofibers exhibited considerable blueshifts in photoluminescence spectra and enhanced emission intensity for detecting an extremely dilute concentration of Hg 2+ (10 −3 M). Furthermore, the LCST of the NIPAAm moiety in the P2-5% nanofibers showed a significant temperature-dependent variation in PL intensity due to fiber volume change (or hydrophilic to hydrophobic change), engendering distinct on-off switching of photoluminescence when the nanofibers were exposed to Hg 2+ . Furthermore, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe3O4 NPs, which serves as a substitute for removal through other methods. The present study demonstrated that the prepared

Conclusions
Novel magnetic fluorescent switchable chemosensors for the simultaneous detection of temperature, magnetism, and Hg 2+ based on fluorescent ES nanofibers were prepared using blends of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe 3 O 4 NPs by employing a single-capillary spinneret. The NIPAAm, NMA, AA, BNPTU, Fe 3 O 4 NPs moieties were designed to provide thermoresponsiveness, chemical cross-linking, dispersion of Fe 3 O 4 NPs, sensing of Hg 2+ , and magnetism, respectively. Cross-linked ES nanofibers maintained their structure in water and exhibited sensitivity toward temperature variations and Hg 2+ because of the sufficient NMA composition. The fluorescence emission of BNPTU within the ES nanofibers exhibited strong selectivity toward Hg 2+ with green emission in aqueous solutions without Hg 2+ (thiourea-derived), shifting to blue emission in aqueous solutions with Hg 2+ (imidazoline-derived). The P2-5% ES nanofibers exhibited considerable blueshifts in photoluminescence spectra and enhanced emission intensity for detecting an extremely dilute concentration of Hg 2+ (10 −3 M). Furthermore, the LCST of the NIPAAm moiety in the P2-5% nanofibers showed a significant temperature-dependent variation in PL intensity due to fiber volume change (or hydrophilic to hydrophobic change), engendering distinct on-off switching of photoluminescence when the nanofibers were exposed to Hg 2+ . Furthermore, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe 3 O 4 NPs, which serves as a substitute for removal through other methods. The present study demonstrated that the prepared magnetic fluorescent ES nanofibers can be used as naked eye sensors and have potential for application in multifunctional environmental sensing devices.
Supplementary Materials: The following are available online at www.mdpi.com/2073-4360/9/4/136/s1. Figure S1: GPC profiles of P1 and P2 copolymers. Figure S2: TGA curves of P1 and P2 copolymers with a heating rate of 10 • C min −1 in a nitrogen atmosphere. Figure S3: Variations in optical transmittance of P1 and P2 in pH 7 water solutions with temperatures between 30 and 60 • C. Figure S4: TGA curves of P2-5% blended with 5 wt % Fe 3 O 4 NPs nanofibers with a heating rate of 10 • C min −1 in a nitrogen atmosphere. Figure S5: PL intensity of pristine BNPTU compound as the temperature is increased from 30 to 60 • C. Table S1: Time-dependent solution conductivity of the prepared Hg 2+ solution.