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

Abstract

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.

1. 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 derivative probes based on 1,8-naphthalimide have been used extensively in fluorescence detection for HTMs such as Hg²⁺, Cd²⁺, and Cu²⁺ because of their many excellent optical properties such as high quantum yields and high photo stability [5,6,7,8,9,10,11]. A new fluorescent chemodosimeter, namely a 1,8-naphthalimide-based colorimetric derivative, 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]-thiourea (BNPTU), with high selectivity for Hg²⁺ in aqueous solutions and based on the reactivity of thiourea derivatives toward Hg²⁺ was reported by Liu et al. [5]. Mercury-triggered intramolecular cyclization of thiourea results in the formation of a highly blue fluorescent naphthalimide derivative, whereas the dosimeter itself fluoresces yellowish green.

Poly(N-isopropyl acrylamide) (PNIPAM) is a well-known thermosensitive polymer that exhibits a low critical solution temperature (LCST) of 32 °C, close to the healthy human body temperature. PNIPAM exhibits a hydrophilic extended structure below its LCST, whereas above its LCST, it dehydrates and exhibits a compact structure [12]. Chen reported that introducing hydrophilic acrylic acid (AA) monomers with different compositions can change the thermal responses of PNIPAM microgels. Furthermore, the AA content of poly(NIPAM-co-AA) can be adjusted to change the LCST, with the hydrogen bonding interaction being the fundamental cause of the change [13]. However, all of the aforereferenced studies adopted solutions and polymer composites rather than nanofibers in sensory applications. The high surface-to-volume ratio of nanofibers could induce responses in metal ions-sensitive, temperature-sensitive, and multifunctional sensory materials.

Electrospinning is an easy, versatile, and inexpensive technique that enables the assembly of various functional nanofibers [14,15,16,17]. The high surface-to-volume ratio of electrospun (ES) nanofibers has encouraged extensive research on sensory applications including sensors for pH levels [18], temperatures [19,20], nitric oxide [21], and metal ions [22,23,24,25,26]. Recently, various fluorescent sensor-based ES polymer nanofibers for sensing pH or different metal ions such as Hg²⁺, Fe³⁺, Zn²⁺, and Cu²⁺ were successfully prepared by our research group [24,25,26]. These multifunctional fluorescent ES nanofibers exhibited distinct on–off switching capabilities such as quenching–enhancing photoluminescence (PL) intensity and changed–recovered PL colors for sensing various pH levels or metal ions.

Poly(acrylic acid) (PAA) is a weak polyelectrolyte often used to change the surface properties of inorganic particles, because of the strong chelation of its numerous carboxyl groups with metal ions. Iron oxide (Fe₃O₄) nanoparticles (NPs) obtained through a one-pot method with PAA can solve the agglomeration problem [27,28,29,30,31]. Recently, Liu reported the fabrication of Fe₃O₄ NP, PAA, and polyvinyl alcohol composite ES nanofibers with a homogeneous dispersion of Fe₃O₄ NPs and high water resistance [32]. In addition, Xu et al. developed a material for industrial wastewater treatment combining polydopamine-coated mesh with PAA. This material not only selectively detects Hg²⁺ but also effectively separates oil and water, a process attributed to the hydrophilicity of PAA and the change in mesh wettability based on the chelation between Hg²⁺ and PAA [33]. However, most studies on polymer-inorganic composites have focused on exploring their physical properties, which have seldom been studied in terms of their combination with metal ions, temperature, and magnetic mulfunctional materials in sensory applications. This paper reports the production of novel multifunctional switchable ES nanofiber sensors from blends of multifunctional copolymers and Fe₃O₄ NPs. Comprising traditional polymer-inorganic composite ES nanofibers, our chemosensors can simultaneously detect metal ions, temperature, and magnetism.

In this study, we developed magnetic fluorescent sensory ES nanofibers by using blends of poly(N-isopropylacrylamide)-co-(N-methylolacrylamide)-co-(acrylic acid) (poly(NIPAAm-co-NMA-co-AA)), fluorescent BNPTU, and Fe₃O₄ NPs and a single-capillary spinneret. The nanofibers, which can sense Hg²⁺, temperature, and magnetism simultaneously, were prepared by combining synthesis and electrospinning, and their optical applications and morphological characteristics were analyzed. The process for synthesizing fluorescent BNPTU probes is shown in Figure 1. The poly(NIPAAm-co-NMA-co-AA) was synthesized through free radical polymerization (Figure 2a). The moieties of N-isopropylacrylamide (NIPAAm), N-methylolacrylamide (NMA), AA, BNPTU, and Fe₃O₄ NPs were designed to achieve thermoresponsiveness, chemical cross-linking, hydrophilicity, Hg²⁺ 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 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 Hg²⁺ dependent. When BNPTU is used to detect Hg²⁺, 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 Hg²⁺, 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.

2. Experimental Section

2.1. Materials

N-Isopropylacrylamide (NIPAAm) and N-methylolacrylamide (NMA) were provided by Tokyo Chemical Industry Co. Japan. NIPAAm was re-crystallized three times from hexane–toluene (10/1, v/v) prior to use. Acrylic acid (AA, Aldrich, Saint Louis, MO, USA, 99%) was dried over CaH₂ to distill in a round-bottomed flask overnight, then purified by passing it through a short aluminum-oxide column (50–200 μm), and stored at 4 °C prior to use. The radical initiator 2,2′-azobis(2-methylpropionitrile) (AIBN) was purchased from UniRegion Bio-Tech and was recrystallized twice in ethanol prior to use. Iron (II) (FeCl₂∙4H₂O) and Iron (III) (FeCl₃∙6H₂O) chloride hexahydrate (Aldrich, 99%), dichloromethane (Tedia, Fairfield, OH, USA, 99.9%); methanol (Tedia, HPLC/SPECTRO); NH₄OH (Alfa, 28%); anhydrous tetrahydrofuran (99.9%); acetone (99.8%); calcium hydride (reagent grade 95%); diethyl ether (99%); 4-bromo-1,8-naphthalic anhydride (95%); benzoyl isothiocyanate (98%); allylamine (98%); 1,2-ethylenediamine (99%); chloroform (anhydrous, 99%); N,N-dimethylformamide (99%); 1,4-dioxane (99.5%); acetonitrile (99%); ethanol (99.8%); and hydrazine hydrate (reagent grade, N₂H₄ 50–60%) were used as received. Perchlorate salts of metal ions (Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Hg²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Ni²⁺) were purchased from Sigma-Aldrich (Saint Louis, MO, USA).

2.2. Synthesis of 4-Bromo-N-allyl-1,8-naphthalimide

Fluorescent BNPTU was prepared according to a previously reported method [8]. The process of its synthesis is illustrated in Figure 1. A 50-mL round-bottomed flask was charged with 4-bromo-1,8-naphthalic anhydride (1.385 g, 5 mmol), allylamine (0.317 g, 5.18 mmol), and 20 mL of 1,4-dioxane. The reaction mixture was stirred at reflux for 8 h. After cooling to room temperature, the suspension was poured into 600 mL of ice water and then filtrated. After drying in a vacuum oven overnight at room temperature, 4-bromo-N-allyl-1,8-naphthalimide (BN-Br) was obtained as a slightly gray solid (1.12 g, yield: 85%). ¹H-NMR (300 MHz, CDCl₃, TMS, Figure 3a): δ = 5.33 (a, 2H, –CH₂CHCH₂–); δ = 5.91 (b, 1H, –CH₂CHCH₂–); δ = 4.58 (c, 2H, –CH₂CHCH₂–); δ = 8.56 (d, 1H, 7–CH); δ = 8.03 (e, 1H, 3–CH); δ = 8.64 (f, 1H, 5–CH); δ = 7.94 (g, 1H, 6–CH ); δ = 8.45 (h, 1H, 2–CH).

2.3. Synthesis of 4-(Aminoethylene)amino-N-allyl-1,8-naphthalimide

A 25-mL round-bottomed flask was charged with NP-Br (500 mg, 1.57 mmol) and an excess of 1,2-ethylenediamine (8 mL). The reaction mixture was stirred at reflux for 6 h. The residues were dissolved in CH₂Cl₂ (200 mL) and extracted with water. Finally, 4-(aminoethylene) amino-N-allyl-1,8-naphthalimide (NP-NH₂) was obtained as an orange crystal (400 mg, yield: 80%). ¹H-NMR (300 MHz, DMSO-d₆, TMS, Figure 3b): δ = 5.33 (a, 2H, –CH₂CHCH₂–); δ = 5.91 (b, 1H, –CH₂CHCH₂–); δ = 4.58 (c, 2H, –CH₂CHCH₂–); δ = 8.56 (d, 1H, 7–CH); δ = 8.03 (e, 1H, 3–CH); δ = 8.64 (f, 1H, 5–CH); δ = 7.94 (g, 1H, 6–CH); δ = 8.45 (h, 1H, 2–CH); δ = 2.85 (i, 2H, –CH₂NH₂).

2.4. Synthesis of 1-Benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]-thiourea (BNPTU)

A 25-mL round-bottomed flask was charged with NP-NH₂ (400 mg, 1.36 mmol), benzoyl isothiocyanate (0.22 g, 1.36 mmol), and 12 mL of acetone. The reaction mixture was stirred at reflux for 1 h. After cooling to room temperature, the solution was filtrated and washed with ethanol. The product was purified using chromatography with CH₂Cl₂ (320 mg, yield: 80%). ¹H-NMR (300 MHz, DMSO-d₆, TMS, Figure 3c): δ = 5.33 (a, 2H, –CH₂CHCH₂–); δ = 5.91 (b, 1H, –CH₂CHCH₂–); δ = 4.58 (c, 2H, –CH₂CHCH₂–); δ = 8.41 (d, 1H, 7–CH); δ = 7.62–7.88 (e + n, 3H, 6–CH; –COCCHCHCHCH–); δ = 8.72 (f, 1H, 5–CH); δ = 7.03 (g, 1H, 3–CH); δ = 8.26 (h, 1H, 2–CH); δ = 4.02 (i, 2H, –NCH₂CH₂N–); δ = 3.95 (j, 2H, –NCH₂CH₂N–); δ = 11.08 (k, 1H, –CSNH–); δ = 11.38 (l, 1H, –CONH–); δ = 7.48 (m, 1H, –COCCHCH–); δ = 7.93 (o, 2H, –COCCH–; –COC(CH)₄CH–).

2.5. Synthesis of Magnetic Iron Oxide (Fe₃O₄) Nanoparticles (NPs)

Magnetic nanoparticles, Fe₃O₄, were prepared by a coprecipitation method as reported [27]. A 100-mL round-bottomed flask was charged with FeCl₃∙6H₂O (380 mg, 1.4 mmol), FeCl₂∙4H₂O (140 mg, 0.7 mmol) and 80 mL of water. The reaction mixture was stirred at reflux under N₂ until the mixture had completely dissolved. The ammonium hydroxide (8 mL) was added dropwise under mechanical stirring at 60 °C for 30 min under N₂. 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.

2.6. 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. Polymerization conditions and molecular weights of poly(NIPAAm-co-NMA-co-AA) random copolymers.

Polymer No.	Composition a NIPAAm:NMA:AA	Mn b	PDI	Td (°C)	LCST (°C) (in pH = 7)
P1	10:5:0	26,856	2.01	375	54.5
P2	10:5:3	18,041	1.76	374	58.1

^a Molar ratio (%) estimated from ¹H-NMR spectra. ^b Determined using THF eluent.

. 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.

2.7. 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 ¹H-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. ¹H-NMR (300 MHz, DMSO-d₆, TMS, Figure 4): δ = 0.82–1.12 (a, 6H, –CH(CH₃)₂); 1.23–1.46 (h + i, 4H, –CH₂CH–, –CH₂CH–); 1.84–2.10 (h + i, 2H, –CH₂CH–, –CH₂CH–,); 5.34–5.53 (b, 1H, –NHCH₂OH); 3.71–3.82 (c, 1H,–CH(CH₃)₂); 7.23–7.59 (e, 1H, –CONHCH–); 7.92–8.14 (f, 1H, –NHCH₂OH); 4.36–4.65 (g, 2H, –NHCH₂OH–).

2.8. 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 ¹H 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 M_n and PDI estimated using GPC were 18041 and 1.76, respectively. ¹H-NMR (300 MHz, DMSO-d₆, TMS, Figure 4): δ = 0.82–1.12 (a, 6H, –CH(CH₃)₂); 1.23–1.46 (h + i + j, 6H, –CH₂CH–, –CH₂CH–, –CH₂CH–); 1.84–2.10 (h + i + j, 3H, –CH₂CH–,–CH₂CH–, –CH₂CH–); 5.34–5.53 (b, 1H, –NHCH₂OH); 3.71–3.82 (c, 1H, –CH(CH₃)₂); 11.92–12.05 (d, 1H, –COCH₂OH); 7.23–7.59 (e, 1H, –CONHCH–); 7.92–8.14 (f, 1H, –NHCH₂OH); 4.36–4.65 (g, 2H, –NHCH₂OH–).

2.9. 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.

2.10. Characterization

¹H-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 Fe₃O₄ 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].

3. Results and Discussion

3.1. Characterization of BNPTU and Poly(NIPAAm-co-NMA-co-AA)

The chemical structure of BNPTU was characterized using ¹H-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 ¹H-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 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 Hg²⁺ ion was added. The Hg²⁺ 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 Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Cu²⁺, Fe²⁺, and Cd²⁺ were added, suggesting that BNPTU exhibited high selectivity and sensitivity toward Hg²⁺.

3.2. Morphology and Characterization of ES Nanofibers

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.

Figure 7a,b shows FE-SEM images and transmission electron microscopy images of the Fe₃O₄ NPs, respectively. The diameters of the nanoparticles were monodispersed within the range 15–25 nm. 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 %.

3.3. Hg²⁺ 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 Hg²⁺, 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 Hg²⁺ (blank) or with Hg²⁺ 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-Hg²⁺ aqueous solution to 450 nm in an Hg²⁺ aqueous solution. This change corresponded to the thiourea unit of BNPTU, which transformed the imidazoline moiety (with the Hg²⁺ 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 Hg²⁺ (blank), with Hg²⁺ at 10⁻⁴, 10⁻³, and 10⁻² M, and under neutral conditions at 430-nm excitation. The presence of the highest Hg²⁺ 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 Hg²⁺ 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 Δλ_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 Hg²⁺ concentration for the ES nanofibers was 10⁻⁴ M, suggesting that the P2-5% ES nanofibers were highly sensitive to Hg²⁺ 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 Hg²⁺ emission at 537 nm, whereas I₅₃₀ is the fluorescence intensity of BNTPU before the detection of Hg²⁺ emission at 530 nm) of the P2-5% ES nanofibers when subjected to Hg²⁺. As the concentration of Hg²⁺ was increased, BNTPU emission at 530 nm gradually decreased, and BNTPU-Hg²⁺ emission at 450 nm gradually increased. These changes corresponded to the fluorescence of BNTPU chelated with Hg²⁺ ions, causing the I₄₅₀/I₅₃₀ to increase from approximately 0.1 to 1.2 as the Hg²⁺ ion concentration increased from 0 to 10⁻² M. The P2-5% ES nanofibers exhibited high sensitivity to Hg²⁺ 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 Hg²⁺ environment as the temperature was varied from 30 to 60 °C. At 30 °C, the temperature at which the ES nanofibers chelate Hg²⁺, 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, Hg²⁺ 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 Hg²⁺, 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 Hg²⁺ over other common metal ions was also studied (Figure 10). Figure 10a shows that among all tested metal ions, namely Hg²⁺, Pb²⁺, Co²⁺, Cd²⁺, Mg²⁺, Ni²⁺, Zn²⁺, Fe²⁺, and Cu²⁺ (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 Hg²⁺. In Figure 8b, I₄₅₀/I₅₃₀ indicates the ratio of a PL intensity of 530 nm (λ^PL_max), corresponding to other metal ions (Hg²⁺, Pb²⁺, Co²⁺, Cd²⁺, Mg²⁺, Ni²⁺, Zn²⁺, Fe²⁺, and Cu²⁺), to a PL intensity of 450 nm (λ^PL_max), corresponding to Hg²⁺. The presence of Hg²⁺ 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 Hg²⁺ ions and the other metal ions revealed that none of the other metal ions interfered with the Hg²⁺ ion-induced fluorescence enhancement (inset Figure 10b), indicating that the sensing of Hg²⁺ 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 Hg²⁺ 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, Hg²⁺ 10⁻² M) as the concentration of Hg²⁺ was varied from 0 to 10⁻² M because of the detection of BNPTU by Hg²⁺, 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 Hg²⁺ was increased. A microfluidics system was constructed, in which a P2-5% ES nanofiber filter membrane with an area of 3 cm² placed in the middle of a tube was used to rapidly absorb and sense Hg²⁺ in a solution flowing through the tube (Figure 11c). Figure 11d depicts the measured time-dependent variation in the solution conductivity. The prepared Hg²⁺ solution contained 1 ppm Hg²⁺ (6.3 × 10⁻³ M Hg²⁺ in 0.5 L of water) and the conductivity of the Hg²⁺ 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/C₀ (%) versus time (C₀: 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 Hg²⁺ ions was absorbed by the P2-5% ES nanofiber membrane, yielding less Hg²⁺ 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 Hg²⁺ in an aqueous solution that contained a variety of metal ions and had a dual fluorescent chemosensory function for Hg²⁺.

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 Hg²⁺ in an aqueous solution containing a variety of metal ions and had a fluorescent chemosensory response to Hg²⁺ (Figure 11). Thus, filter membranes based on the P2-5% ES nanofibers with porous architectures can specifically chelate with Hg²⁺ in an aqueous solution containing many types of metal ion and serve as fluorescent chemosensors for Hg²⁺. These results could assist researchers in cleaning water while simultaneously chelating and sensing Hg²⁺. 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 Hg²⁺; moreover, used magnetic fluorescent switchable chemosensors can be collected using magnets (noncontact force).

4. Conclusions

Novel magnetic fluorescent switchable chemosensors for the simultaneous detection of temperature, magnetism, and Hg²⁺ 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 Hg²⁺, and magnetism, respectively. Cross-linked ES nanofibers maintained their structure in water and exhibited sensitivity toward temperature variations and Hg²⁺ because of the sufficient NMA composition. The fluorescence emission of BNPTU within the ES nanofibers exhibited strong selectivity toward Hg²⁺ with green emission in aqueous solutions without Hg²⁺ (thiourea-derived), shifting to blue emission in aqueous solutions with Hg²⁺ (imidazoline-derived). The P2-5% ES nanofibers exhibited considerable blueshifts in photoluminescence spectra and enhanced emission intensity for detecting an extremely dilute concentration of Hg²⁺ (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 Hg²⁺. 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 magnetic fluorescent ES nanofibers can be used as naked eye sensors and have potential for application in multifunctional environmental sensing devices.