Introduction

Chromogenic and fluorogenic signaling probes co.mposed of chelating ligands for detecting metal ions have attracted extensive scientific interest in biological and environmental sensory device applications because their photophysical properties can be tuned markedly by detecting various metal ions such as Zn2+, Hg2+, Cu2+ and Fe2+. Various chemosensors based on small molecules,1, 2, 3, 4, 5, 6, 7, 8, 9 nanomaterials,10, 11, 12, 13, 14, 15 and polymers or conjugated copolymers16, 17, 18, 19, 20, 21 have been reported. Conjugated copolymers, namely polyfluorene- and poly(phenylenevinylene)-type copolymers containing metal ion binding sites, such as 1,10-phenanthroline and 2,2′-bipyridine (bpy), exhibited a strong ionochromic effect. These copolymers can be applied in metal ion sensors because they exhibit sensitive, selective and highly reversible ion-responsive performance.22, 23, 24, 25 Wang and Wasielewski22 reported that the fluorescence emission band of bpy-poly(phenylenevinylene)-based polymers upon exposure to pollutant metal ions includes Zn2+, Cd2+ and Hg2+ and could be red-shifted by up to 120 nm because of ion-recognition-induced conjugation enhancement and electronic density changes on the main chains of the polymers. Ma and colleagues reported that, by adjusting the bpy content in the backbone of bpy-polyfluorene-based copolymers, the emission color of a polymer solution could be tuned from blue to green when it interacted with Zn2+, suggesting that the resultant emission colors depend on the number of chelating units in the copolymer backbone.25 However, most of the mentioned studies focused on the solution, thin film and bulk solid states.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 The morphology and photophysical properties for sensing the metal ions of polyfluorene-type conjugated copolymers containing bpy-based electrospun (ES) nanofibers have yet to be explored.

Electrospinning is a simple, versatile, low-cost technique for assembling various functional nanofibers that enables flexible and nanoscaled morphology tuning and high-throughput continuous production.26, 27, 28, 29, 30, 31, 32, 33 The high surface-to-volume ratio of ES nanofibers has motivated extensive studies on sensory applications such as pH, temperature and volatile gas sensing.34, 35, 36, 37, 38, 39, 40 However, few studies have reported fluorescent ES nanofibrous membranes for use in metal-ion-sensitive optical sensors and water purification filters.41, 42, 43, 44 Samuelson and colleagues reported highly responsive fluorescence-quenching-based optical sensors for Fe3+ and Hg2+ that were prepared from poly(acrylic acid)-poly(pyrene menthanol) ES nanofibrous membranes.41 Wang and colleagues demonstrated that a novel napthalimide-functionalized nanofibrous film prepared through copolymerization and electrospinning exhibited high sensitivity on 48 nm blue-shifted emission in fluorescence spectra when detecting Cu2+ in an aqueous solution.42 Recently, Meng and our research group developed a real-time Zn2+ sensor with high sensitivity and cell medium compatibility by using ES nanofibers, which were prepared from blends of a fluorescent probe molecule for sensing Zn2+ and a hydrogel polymer, poly(2-hydroxyethyl methacrylate)(poly HEMA).45 Metal-ion-sensitive probes composed of bipyridine-containing fluorene (bpy-F) exhibited various fluorescence emission characteristics including color changing and quenching when they interacted with different metal ions because of the high sensitivity of fluorene to external structural perturbations and electron density changes within the fluorene backbone. Therefore, combining the superhydrophilic property of poly HEMA, the bpy-F metal-ion-tunable luminescence of sensing and chelating pollutant metal ions, and the high surface-to-volume ratio and nonwoven structure of ES nanofibers would create a new type of material for application in simultaneous metal ion sensing and water purification. In addition, it is of value to explore the effect of various metal ions on the emission of bpy-F-based ES nanofiber membranes as well as their sensing efficiency in a real-time detecting system.

In this study, we developed ES nanofibrous membranes consisting of copolymers containing a hydrophilic hydrogel material (HEMA) and chemical cross-linkable segment, N-methylolacrylamide (NMA), blended with a metal-ion-chelating probe, 9,9-dihexylfluorene-2,7-bipyridine (bpy-F-bpy). These membranes can be used as both sensors and filters. Figure 1 illustrates the fabrication process. We combined synthesis, electrospinning, optical sensor application and morphological characteristics to realize these membranes. As shown in Scheme 1, random copolymers of poly(HEMA-co-NMA) were synthesized through free-radical copolymerization, and bpy-F-bpy was synthesized. Photoluminescence (PL) and UV–visible (UV–vis) spectra obtained through experiments and simulations of a bpy-polyfluorene-bpy solution in which the membranes were used in detecting various metal ions were compared. ES nanofibers composed of poly(HEMA-co-NMA) blended with 1, 3 or 5% bpy-F-bpy were fabricated using a single-capillary spinneret and then posttreated to enhance their stability in water through chemical cross-linking. The nanofiber morphologies were studied using a field-emission scanning electron microscope (FE-SEM) and laser confocal microscope. The fluorescence emission transitions of the response of various metal ions in the PL and UV–vis spectra were studied. The experimental results suggest that the ES nanofibrous membranes have high reversibility and real-time metal ion detection ability; thus, they exhibit potential for application in simultaneous environmental sensing and water purification.

Figure 1
figure 1

Design of multifunctional sensory ES nanofibers from poly(HEMA-co-NMA)/bpy-F-bpy blends with fluorescence emission in various colors for sensing different metal ions. (a) Variation of fluorescence emission colors of the bpy-F-bpy probe during a metal ion and EDTA cycle. (b) Polymerization and chemical structure of poly(HEMA-co-NMA). (c) Fabrication of ES nanofibers from the blends. (d) Microfluidics system for real-time metal ion sensing by the ES nanofiber membrane. bpy-F-bpy, 9,9-dihexylfluorene-2,7-bipyridine; EDTA, ethylenediaminetetraacetic acid; ES, electrospun; HEMA, 2-hydroxyethyl methacrylate; NMA, N-methylolacrylamide. A full color version of this figure is available at Polymer Journal online.

Full size image

Experimental Procedure

Materials

2-Hydroxyethyl methacrylate, HEMA (97%, Aldrich Chemical, St Louis, MO, USA) was passed through an Al2O3 column before polymerization. N-methylolacrylamide (NMA) was provided by Tokyo Chemical Industry, Tokyo, Japan. 2,2′-Azobis(2-methylpropionitrile) (AIBN, Acros, 99%) was recrystallized from an ethanol solution. 5-Bromo-2,2′′-bipyridine was prepared from 2-bromopyridine and 2,5-dibromopyridine through the Stille coupling reaction, according to the literature.46 (9,9-Dihexyl-9H-fluorene- 2,7-diyl)diboronic acid, sodium carbonate and tetrakis(triphenylphosphine) palladium(0) were purchased from Aldrich Chemical and used without purification. Methanol (MeOH) and tetrahydrofuran (THF) were dried and distilled under N2. The perchlorate salts of all metal ions were purchased from Aldrich Chemical. Ethylenediaminetetraacetic acid tetrasodium salt hydrate (EDTA, 98%) was purchased from Alfa Aesar (Ward Hill, MA, USA).

Synthesis of poly(HEMA-co-NMA) copolymer

Poly(HEMA-co-NMA) was synthesized through free-radical copolymerization of the monomers HEMA and NMA, as illustrated in Scheme 1. Poly(HEMA-co-NMA) copolymers with different monomer ratios were denoted P1 and P2, as listed in Table 1. The concentration of AIBN used as the initiator was 0.007 m. A reaction mixture containing MeOH and three monomers was degassed by bubbling nitrogen for 30 min and then reacted at 70 °C for 24 h. The reaction mixture was poured into ether, precipitated, filtered and finally dried at 30−35 °C under vacuum to obtain the two polymers. Table 1 lists the number-averaged molecular weight and polydispersity index estimated through gel permeation chromatography (GPC) of both P1 and P2.

Table 1 Results of poly(HEMA-co-PNMA) synthesis under various conditions
Full size table

Synthesis of P1

A reaction mixture of 5000 mg (38.42 mmol) of HEMA, 388 mg (3.84 mmol) of NMA, 8.7 mg (0.053 mmol) of AIBN and 22 ml of methanol was used to afford a white solid (3.5 g, yield: 64.9%). Number-averaged molecular weight and polydispersity index estimated from GPC are 52 830 and 2.66, respectively. 1H NMR (d-dimethyl sulfoxide, 300 MHz): δ (p.p.m.)=0.78–1.92 (peak a, b, c), 3.50–3.62 (peak e), 3.76–3.90 (peak d), 4.50–4.62 (peak g), 4.84–5.16 (peak f), 5.36–5.52 (peak h).

Synthesis of P2

A reaction mixture of 4000 mg (30.74 mmol) of HEMA, 932 mg (9.22 mmol) of NMA, 8.7 mg (0.053 mmol) of AIBN and 20 ml of methanol was used to afford a white solid. (2.9 g, yield: 58.7%) Number-averaged molecular weight and polydispersity index estimated from GPC are 60290 and 2.82, respectively. 1H NMR (d-dimethyl sulfoxide, 300 MHz): δ (p.p.m.)=0.78–1.92 (peak a, b, c), 3.50–3.62 (peak e), 3.76–3.90 (peak d), 4.50–4.62 (peak g), 4.84–5.16 (peak f), 5.36–5.52 (peak h).

Synthesis of bpy-F-bpy

Bpy-F-Bpy probe was synthesized by the Suzuki coupling reaction between 9,9-dihexylfluorene-2,7-diboronic acid (4) and 5-bromo-2,2-bipyridine (5). Typically, 4 (0.45 g, 1.07 mmol), 5 (0.5 g, 2.13 mmol) and Pd(PPh3)4 (0.036 g, 0.03 mmol) were dissolved in THF (10 ml) under nitrogen atmosphere. Then, Na2CO3(aq) (0.44 g, 2 M in H2O) was added, and the flask was immersed in a preheated oil bath at 70 °C overnight. The reaction was quenched by exposure to air, extracted with EA/H2O (100 ml/50 ml, v/v) and the Pd complex was removed through a Kieselguhr column. The crude product was separated from the crude reaction mixture by chromatography and further purified by precipitation from hexane. The slightly yellow solids of 9,9-dihexylfluorene-2,7-bipyridine (6) was obtained (0.264 g, 38%). 1H NMR (CDCl3, 300 MHz): δ (p.p.m.)=0.78–1.20 p.p.m. (11H, Ar-(CH2(CH2)4CH3)2), 2.08–2.1 (2H, Ar-(CH2(CH2)4CH3)2), 7.26–9.0 (10H, pyridine and fluorine aromatic protons).

Fabrication of cross-linked ES nanofibers

Poly(HEMA-co-NMA) (200 mg ml−1; P1 and P2) and P1 and P2 blended with 1, 3 or 5 wt% (with respect to the polymer) of the bpy-F-bpy probe were dissolved in MeOH to prepare the ES nanofibers. A single-capillary spinneret electrospinning device was used to produce the ES nanofibers, as described in our previous reports.28, 30, 37, 39 The polymer solution was fed into a metallic needle by using syringe pumps (KD Scientific Model 100, Holliston, MA, USA) at a feed rate of 1 ml h−1. The tip of the metallic needle was connected to a high-voltage power supply (Chargemaster CH30P, SIMCO, Santa Clara, CA, USA) and set at 7.7 kV. A flat aluminum foil and quartz was placed 15 cm below the tip of the needle (working distance) to collect the nanofibers for 30 min. All the experiments were conducted at room temperature and a relative humidity of approximately 30%. The ES nanofibers were annealed at 100 °C for 24 h in an oven for chemical cross-linking, as described by Chiu’s group47 and our previous report.43, 44 ES nanofibers from P2 blended with 1, 3 and 5% bpy-F-bpy were denoted P2–1, P2–3 and P2–5, respectively.

Fluorescence and conductivity measurements for sensing metal ion solutions

Each metal ion titration experiment was initiated using a 3.0 ml bpy-F-bpy probe solution (in THF) with a concentration of approximately 10−5 m. All metal ion solutions were diluted from standard solutions with a concentration of approximately 10−2 m, and the titration method was similar to that reported in a previous paper.6 The changes in all UV–vis absorption and PL spectra of the bpy-F-bpy probe solution resulting from the addition of the aforementioned metal ion solutions at room temperature were measured using a UV–vis spectrophotometer (Hitachi U-4100, Tokyo, Japan) and Fluorolog-3 spectrofluorometer (Horiba Jobin Yvon, Edison, NJ, USA), respectively. The fabricated ES nanofiber membrane was inserted into a quartz cell with its surface facing the excitation light source. The position of the nanofiber membrane was fixed in cuvettes by using adhesive tape, and various metal ion solutions with concentrations of 10−7 to 10−3 m were then added to measure PL change, as described in our previous papers.37, 39, 40 To measure the filter efficiency of the nanofiber membrane, a cycling microfluidics system was used, and the time-dependent change in the conductivity of metal ions in the solution was measured using the Suntex Conductivity/Resistivity Meter SC-2300 Taipei, Taiwan (R.O.C).

Characterization

1H NMR spectra were obtained at room temperature by using a Bruker AM 300 (300 MHz) spectrometer and d-dimethyl sulfoxide as a deuterated solvent. The molecular weight and molecular weight distribution of poly(HEMA-co-NMA) were determined by performing GPC by using a Lab Alliance RI2000 instrument (two column, MIXED-C and -D from Polymer Laboratories, Milford, MA, USA) connected to a Schambeck SFD Gmbh refractive index detector. All GPC analyses were performed using a polymer/N,N-dimethylformamide solution at a flow rate of 1 ml min−1 and 70 °C, and calibrated using polystyrene. A Perkin-Elmer 7 thermogravimetric analyzer (TA Instruments, Inc., New Castle, DE, USA) was used in a heating range of 100 to 800 °C and at a heating rate of 10 °C min−1. The viscosities of the polymer solutions used for electrospinning were measured using the TA-AR2000ex. The shear viscosities were recorded in the frequency sweep mode at room temperature (25 °C) by using a parallel plate (diameter: 40 mm; gap: 0.075 mm). A solvent trap was used around the sample during measurements.

The morphologies of ES nanofibers were characterized using an FE-SEM. Images were captured at an accelerating voltage of 15 kV. Before imaging, the samples were sputtered with Au/Pt. Fluorescence optical microscope images were captured using a two-photon laser confocal microscope (Leica LCS SP5, Wetzlar, Germany).

Theoretical calculation

All molecular orbital (MO) and time-dependent density functional theory calculations for UV–vis regions were performed using the ORCA package.48 The relativistic effect with zeroth-order regular approximation was included in all the calculations. The coordinates used for optimizing the geometry of complexes were initially set in the Avogadro program49 and optimized using PM6 parameters in the G09 program.50

The geometry was further optimized using the BP86 exchange function in the ORCA program. The def2-TZVP(-f) basis set was used for Cu, Zn, N, O, C and H, but the def2-aug-TZVPP basis set was used for Hg. The electronic structure and time-dependent density functional theory calculations were performed using the B3LYP exchange function with the basis set used for geometry optimization. Isosurface plots of the MOs were generated using Molekel Version 4.3 with an isovalue surface at 0.04 a.u.

Results and Discussion

Characterization of poly(HEMA-co-NMA) and bpy-F-bpy

The random poly(HEMA-co-NMA) copolymers with different monomer ratios were denoted P1 and P2, as listed in Table 1. P1 and P2 were obtained through simultaneous free-radical polymerization of HEMA and NMA by using AIBN as an initiator (Scheme 1a). The feeding molar ratios of the two monomers (HEMA: NMA) were 10:1 and 10:3 for P1 and P2, respectively; thus, random copolymers with different NMA compositions were prepared. The effect of the NMA molar fraction on the thermal cross-linking behavior of ES nanofibers was investigated. Table 1 lists the molecular characteristics of poly(HEMA-co-NMA). The chemical structure and composition were characterized using 1H NMR spectra (Figure 2), with the signals being attributed to PHEMA (δ=4.8 p.p.m., peak f) and PNMA (δ=5.4 p.p.m., peak h). The molar ratios of HEMA to NMA were 90:10 and 77:23 for P1 and P2, respectively. The results reveal that the compositions of copolymers could be manipulated by adjusting the feeding molar ratio of monomers in free-radical polymerization. The molecular weight and polydispersity index were studied using GPC (Table 1). The metal-ion-sensitive fluorescent probe, bpy-F-bpy (6), was synthesized through the Suzuki coupling reaction between 9,9-dihexylfluorene-2,7-diboronic acid (4) and 5-bromo-2,2-bipyridine (5) (Scheme 1b). The 1H NMR spectra (Figure 3) illustrate the successful synthesis of bpy-F-bpy. This fluorescent probe is composed of two 2,2′-bipyridine units that serve as metal-recognizing sites and a fluorene moiety that serves as a fluorogen.

Figure 2
figure 2

1H NMR spectra of poly(HEMA-co-NMA) random copolymers in DMSO. DMSO, dimethyl sulfoxide; HEMA, 2-hydroxyethyl methacrylate; NMA, N-methylolacrylamide. A full color version of this figure is available at Polymer Journal online.

Full size image
Figure 3
figure 3

1H NMR spectra of bpy-F-bpy in CDCl3. bpy-F-bpy, 9,9-dihexylfluorene-2,7-bipyridine. A full color version of this figure is available at Polymer Journal online.

Full size image

Figure 4 shows the PL spectra of bpy-F-bpy in a THF solution containing different types of metal ions at a concentration of 10−4 m and excited at 340 nm. The probe exhibited bright and sky-blue fluorescence in the THF solution (blank) with emission peaks at 394 and 414 nm. The fluorescence intensity was slightly quenched by adding K+, Mn2+, Mg2+ or Pb2+ ions. Severe quenching of the intensity and red shifts of the emission peak to around 480 nm were observed when the probe was mixed with Hg2+ and Zn2+ ions. Complete quenching of fluorescence was observed when the probe was chelated by Ni2+, Co2+ or Cu2+ ions.

Figure 4
figure 4

Variation of PL spectra of bpy-F-bpy in a THF solution (10−5 m, pH 7) with different metal ions at 10−4 m. The corresponding inset figures show color changes under 254 nm UV light (Sample 1 is a blank and Samples 2–10 are Zn2+, Hg2+, Ni2+, Co2+, Cu2+, Pb2+, Mg2+, Mn2+ and K+, respectively.). bpy-F-bpy, 9,9-dihexylfluorene-2,7-bipyridine; PL, photoluminescence; THF, tetrahydrofuran. A full color version of this figure is available at Polymer Journal online.

Full size image

Figure 5 depicts PL spectra of the titration of bpy-F-bpy in a THF solution by three metal ions at concentrations varying from 5 × 10−7 to 5 × 10−4 m, and the corresponding UV–vis spectra are shown in the inset figure. For the titration of the probe with Zn2+ ions (Figure 5a), the fluorescence intensity decreased as the ion concentration increased initially, and an additional fluorescence band was observed when the ion concentration reached 10−5 m. The fluorescence of the probe was completely shifted to the additional band when the ion concentration was higher than 5 × 10−5 m. For the titration of the probe with Hg2+ ions (Figure 5b), the fluorescence spectra showed quenching and red-shift behavior similar to those of the titration with Zn2+ ions; however, the red shift of the fluorescence peak was observed at a higher ion concentration of 10−4 m. For the titration of the probe with Cu2+ ions (Figure 5c), the fluorescence was quenched as the ion concentration increased, and the fluorescence peak did not shift. The UV–vis spectra of all titrations exhibited a red shift in the absorption peak maximum as the concentration of metal ions was increased, as illustrated in their respective inset figures.

Figure 5
figure 5

Variation in the PL spectra of the titration of bpy-F-bpy in a THF solution containing (a) Zn2+, (b) Hg2+ and (c) Cu2+ at concentrations varying from 5 × 10−7 to 5 × 10−4 m. All of the inset figures show the corresponding UV–vis spectra of bpy-F-bpy in the THF solution under the aforementioned conditions. bpy-F-bpy, 9,9-dihexylfluorene-2,7-bipyridine; PL, photoluminescence; THF, tetrahydrofuran. A full color version of this figure is available at Polymer Journal online.

Full size image

Simulated UV–vis spectra of bpy-F-bpy and its binding with Zn2+, Hg2+ and Cu2+ are depicted in Figures 6a–d, respectively. Experimental UV–vis spectra with saturated concentrations of metal ions were used for comparison with the time-dependent density functional theory calculations. The results indicate that the absorption bands at approximately 350–380 nm are mainly related to the transition from the highest occupied MO (HOMO) to the lowest unoccupied MO (LUMO). Overall, the major contributions to the HOMO and LUMO are provided by the fluorene and bpy units, respectively. Close examination of the HOMO and LUMO with and without chelating metal ions revealed that the electrons in the HOMO and LUMO are distributed across the entire metal-free bpy-F-bpy ligand, but become localized on fluorene and bpy units when the ligand binds with metal ions. This effect is particularly obvious in the Zn2+ chelating compound, indicating that this compound has a charge-transfer excited-state characteristic. In Figure 6d, which shows the spectra of the Cu2+ complex, the minor contributions of 3d orbitals in MO263α indicate that the transition involves partial metal-to-ligand charge transfer. Table 2 lists the energy gaps ΔE (ELUMO−EHOMO) of all compounds, and the calculation results indicate that the energy levels of MOs decrease substantially as bpy-F-bpy binds to metal ions and the energy gap ΔE decreases; consequently, red shift effects are observed in absorption spectra. This explains the ionochromic and red-shift effects in the UV–vis titration curves depicted in the inset of Figure 5. Moreover, the smaller energy gap in Hg2+ compared with that in Zn2+ is consistent with the relatively larger red-shift effects observed in absorption spectra. This is owing to the average Mulliken charge of Hg2+ (~0.83) being greater than that of Zn2+ (~0.50), reducing the HOMO–LUMO gaps. These results are consistent with those obtained by Tian et al.25 in a simulation by increasing the charge at metal sites.

Figure 6
figure 6

Simulated UV–vis spectra of (a) bpy-F-bpy in a THF solution and its binding with (b) Zn2+, (c) Hg2+ (d) and Cu2+ at a concentration of 5 × 10−4 m. bpy-F-bpy, 9,9-dihexylfluorene-2,7-bipyridine; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; TDDFT, time-dependent density functional theory; THF, tetrahydrofuran. A full color version of this figure is available at Polymer Journal online.

Full size image
Table 2 Energy gap between the HOMO and LUMO in all calculated compounds
Full size table

In the PL spectra depicted in Figure 5, the strong blue emission can be attributed to π→π* transitions in the bpy-F-bpy ligand.25 Chelation with metal ions, except Cu2+, induces a substantial decrease in the blue emission bands and an increase in a new emission band at approximately 480 nm. This new emission band could be owing to the energy levels newly induced by Zn2+ and Hg2+ chelation. The complete intensity quenching in the Cu2+ case could be attributed to the existence of unpaired electrons in the open shell system of Cu2+ complexes. Wang and Wasielewski22 reported similar phenomena.

Morphology of electrospun nanofibers

Figure 7 shows SEM images of the ES nanofibers prepared from P1, P2 and P2–5. Note that ES nanofibers from P2 blended with 1, 3 and 5% bpy-F-bpy were denoted P2–1, P2–3 and P2–5, respectively. The inset figure shows an enlarged FE-SEM image of the nanofibers. In the initial state (no cross-linking or un-immersion in water), the diameter ranges of the as-prepared ES nanofibers from P1, P2 and P2–5 were estimated to be 250±62, 500±111 and 502±98 nm, respectively. The average diameter value was calculated using a statistical average of 50 fibers from each sample; this calculation method is similar to that described in our previous report.6 The diameter of the P1 nanofibers was markedly smaller than that of the P2 nanofibers, possibly because the molecular weight and, thus, viscosity of P1 (1.4 × 10−3 kg (m·s)−1) are lower than those of P2 (6.6 × 10−3 kg (m·s)−1), as illustrated in Table 1, even though they were both dissolved at the same polymer concentration in the same solvent. However, the diameters of the P2 and P2–5 nanofibers were similar because blending 5% bpy-F-bpy into the P2 polymer does not affect the fiber diameter. Although rapid evaporation of the low-boiling-point solvent (MeOH) and subsequent rapid solidification during electrospinning yields a porous structure, all of the ES nanofibers from the MeOH solvent were smooth and nonporous because the relative humidity was maintained at approximately 30% throughout electrospinning, as described in the literature.48 As-prepared ES nanofibers that were not subjected to cross-linking but immersed in water were dissolved by the water, and the aluminum substrate exhibited a smooth surface in the wet state (not cross-linked), as shown in the SEM images. The effects of different NMA molecular ratios on the ES fiber morphology after cross-linking and the formation of hydrogel-like nanofibers in water were investigated using an NMA moiety and a heating process. After the as-prepared ES fibers were cross-linked when heated at 100 °C for 24 h, the diameter and morphology of the fibers did not change, as illustrated in Supplementary Figure S1. The cross-linked P1, P2 and P2–5 ES fibers were estimated to be 257±51, 511±93 and 522±88 nm, respectively. The mechanism of the intermolecular cross-linking reaction of NMA is shown in Supplementary Scheme S1 of the Supplementary Information. Furthermore, after a fixed cross-linking duration, fiber mats were flushed with water approximately 10 times at 30 °C and then dried at the same temperature by using a vacuum pump. Figure 7 depicts their morphologies in the wet state (cross-linked). The P1 ES nanofibers developed an uneven and porous film-like structure after being extracted using water, suggesting that the cross-linkable PNMA moiety of P1 can resist the solubility of water after cross-linking, but the amount of NMA incorporated is too low to maintain a stable fiber shape. However, the P2 and P2–5 ES nanofibers exhibited a good fiber morphology with cylinder confinement, enabling them to maintain a high surface-to-volume ratio for response in water environments. This observation indicates that the stability of the nanofibers (or degree of cross-linking) was enhanced as the amount of the NMA moiety was increased and when the cross-linking duration was suitable.49 Conversely, the diameters of the P2 and P2–5 ES nanofibers increased to approximately 1000 nm, which is two times greater than their diameter in the initial state. This observation suggests that the cross-linked P2 and P2–5 ES nanofibers exhibited a swelling fiber morphology because their HEMA has a superhydrophilic property and, therefore, absorbs water. This beneficial hydrophilic property enhances the efficiency of the P2–5 ES nanofibers in absorbing metal ions in water.

Figure 7
figure 7

FE-SEM images of (a) P1, (b) P2 and (c) P2–5 ES nanofibers in the initial state (dry state without cross-linking), wet state with no cross-linking and wet state after cross-linking. ES, electrospun; FE-SEM, field-emission scanning electron microscope. A full color version of this figure is available at Polymer Journal online.

Full size image

Sensing property of electrospun nanofibers

Figure 8a depicts the variation in PL spectra of the P2–5 ES nanofibers in the presence of cations in water at a concentration of 10−3 m and excited at 340 nm. The measurement results shown in Figure 8a and summarized in Figure 8b illustrate that the x axis and y axis correspond to a shift in the emission maximum (ΔλPLmax) and the PL intensity decreases with respect to that in the PL spectra of nanofibers that are not in the presence of metal ions. As shown in Figure 8, according to the spectroscopic effects of metal ions on bpy-F-bpy, the metal ions can be categorized into three groups. The first group of metal ions (Zn2+ and Hg2+) yields substantial red-shift performance but does not markedly quench the fluorescence of the P2–5 ES nanofibers. The range of the red shift in the emission spectra (ΔλPLmax) depends on the metal ion. The ΔλPLmax of Zn2+ is 70 nm, which is higher than that of Hg2+, 40 nm. The second group of metal ions, consisting of Ni2+, Co2+ and Cu2+, quenches the fluorescence of the P2–5 ES nanofibers completely or nearly completely. The final group of metal ions, comprising K+, Mn2+, Mg2+ and Pb2+ ions, only slightly quenches the fluorescence intensity. These results demonstrate that the P2–5 ES nanofibers exhibit different photophysical properties when they detect different metal ions and, therefore, can be used as metal ion sensors.

Figure 8
figure 8

(a) Variation in the PL spectra of P2–5 ES nanofibers used for sensing various metal ions at a concentration of 10−3 m. (b) The x axis and y axis correspond to the shift in the emission maximum (λmax) and the enhancement of PL intensity under the aforementioned condition with respect to the PL spectra of nanofibers not exposed to metal ions. ES, electrospun; PL, photoluminescence. A full color version of this figure is available at Polymer Journal online.

Full size image

Figures 9a–c shows the change in the fluorescence spectra of the P2–5 ES nanofibers as a function of different concentrations of Zn2+, Hg2+ and Cu2+, respectively. Images of the corresponding P2–5 ES nanofibers vary with the concentration value under UV irradiation (365 nm), as illustrated in the inset figure. Under the initial condition with no metal ions, the PL spectra of the P2–5 ES nanofibers exhibited a main emission peak corresponding to an emission maximum (λPLmax) of approximately 450 nm, which is attributed to the bpy-F-bpy. Figure 9a shows that the P2–5 ES nanofibers exhibited a high sensitivity for Zn in the 10−7–10−3 m response range. In contrast to the colors in the original state (without Zn2+), the emission colors markedly changed as the concentration of Zn2+ was increased. As Figure 9a illustrates, increasing the Zn2+ concentration reduced the PL intensity and caused a red shift in the emission of the ES nanofibers. A red shift in the emission maximum (λPLmax) from 450 to 520 nm indicated sensitivity for Zn2+. The Zn2+ concentration at which the PL intensity began to decrease was 10−7 m, suggesting that the ES nanofibers exhibited a high sensitivity for Zn2+. The emission maximum of the ES nanofibers substantially shifted from 450 nm in 0 m Zn2+ to 520 nm in 10−3 m Zn2+. The considerable red shift in the emission peaks, high Δλmax (70 nm), and change in the luminescence color from blue (450 nm) to green (520 nm) indicate the high sensitivity for Zn2+. Similar Δλmax red-shifting by 34 nm was observed for the ES nanofibers chelated with Hg2+, as Figure 9b illustrates. However, compared with Zn2+ sensing, the shift of Δλmax decreased from 70 to 34 nm in Hg2+ sensing. The greater shift of Δλmax in Zn2+ sensing than in Hg2+ sensing may be partially attributed to the smaller element size of Zn2+, which leads to more efficient chelation by bpy-F-bpy and promotes metal ion sensing characteristics. Conversely, Figure 9c shows that the fluorescence intensity decreased as the Cu2+ concentration was increased. This fluorescence intensity decrease was expected and believed to be owing to the quenching of the bpy-F-bpy indicator by Cu2+. The fluorescence quenching is owing to the interactions between the electron-rich bpy-F-bpy indicator(−) and electron-deficient quencher(+), and the degree of quenching depends on the amount of Cu2+.10

Figure 9
figure 9

Variation in the PL spectra of P-2 ES nanofibers in aqueous solutions containing (a) Zn2+, (b) Hg2+ and (c) Cu2+ with concentrations varying from 10−7 to 10−3 m. All of the inset figures show corresponding photographs recorded under UV light. ES, electrospun; PL, photoluminescence. A full color version of this figure is available at Polymer Journal online.

Full size image

Figure 10 shows a laser confocal image of the P2–5 ES nanofibers in the presence of no metal ions, 10−3 m Hg2+ and 10−3 m Zn2+, respectively. These nanofibers had visually distinguishable luminescence colors of blue, blue-green and green, suggesting that they can become efficient sensory materials for detecting various metal ions. The P2–5 ES nanofibers exposed to 10−3 m Zn2+ exhibited apparent color change from blue to green. In addition, the porous structure of ES nonwoven fibers can be applied in filters for purifying water containing heavy metal ions. Therefore, the P2–5 ES nanofibers can serve as both smart sensors and efficient filters. The experimental results pertaining to these nanofibers are discussed in the following paragraphs.

Figure 10
figure 10

Confocal microscopy images of (a) blank P-2 ES nanofibers and (b) P-2 ES nanofibers used for sensing Hg2+ and (c) Zn2+. ES, electrospun.

Full size image

To explore the effect of different blending ratios of bpy-F-bpy on the sensing characteristics of the P2 ES fibers, we summarized the PL spectra of the P2–1, P2–3 and P2–5 ES nanofibers in the presence of 10−7–10−3 m Zn2+ excited at 340 nm. The fiber morphology of P2–1 and P2–3 is similar to that of P2–5, as the SEM images in Supplementary Figures S2 and S3 depict, and raw data on the measured PL spectra of the P2–1, P2–3 and P2–5 ES nanofibers are provided in Supplementary Figures S4a and b and Figure 9a, respectively. Figure 11a summarizes the PL results of the three nanofibers, showing that increasing the Zn2+ concentration reduced the PL intensity and resulted in a red shift in the emission of the ES nanofibers. A red shift in the emission maximum (λPLmax) from 450 to 520 nm indicated sensitivity for Zn2+. For the P2–5 ES nanofibers, the PL intensity If/I0 (I0 denotes the original PL intensity at 450 nm of λPLmax and If denotes the PL intensity at 520 nm of λPLmax at 10−3 m Zn2+) was 1.60, whereas the If/I0 values of the P2–3 and P2–1 ES nanofibers were 1.32 and 1.21, respectively, as Figure 11a shows. This observation indicates that the P2–5 ES nanofibers contain a higher amount of bpy-F-bpy than the other nanofibers do, thus yielding higher performance. The results indicate that the bpy-F-bpy composite of the P2–5 ES nanofibers absorbed more Zn2+ than those of the P2–3 and P2–1 nanofibers did, considerably enhancing the Zn2+-sensing efficiency. The 5% blending ratio of bpy-F-bpy is low, but yielded high sensing performance. Therefore, increasing the blending ratio of bpy-F-bpy can enhance the efficiency of Zn2+ sensing.

Figure 11
figure 11

(a) Curves of PL intensity versus wavelength when the Zn2+ ion concentration was increased from 10−7 to 10−3 m. (b) Reversible sensing of Zn2+ ions. (b) Reversibility of the Zn2+-dependent ‘on-off-on’ fluorescence intensity profile of the P-2 ES nanofibers; the inset figures show corresponding photographs recorded under UV light. EDTA, ethylenediaminetetraacetic acid; ES, electrospun; PL, photoluminescence. A full color version of this figure is available at Polymer Journal online.

Full size image

Reversibility is another crucial factor determining the usability of sensing nanofibers. Therefore, the variation in the emission maximum (λPLmax) of the P2–5 ES nanofibers during a Zn2+-EDTA cycle was tested and is shown in Figure 11b. The original P2–5 ES nanofibers exhibited λPLmax at 450 nm, emitting blue fluorescence. Chelating Zn2+ caused the emission peaks of the P2–5 ES nanofibers to red-shift substantially, causing Δλmax to be high (70 nm) and the luminescence color to change from blue (450 nm) to green (520 nm). Furthermore, adding EDTA resulted in the fluorescence emission of the P2–5 ES nanofibers considerably switching from green to blue because of the Zn2+ chelation of EDTA from the bpy-F-bpy of the nanofibers. This observation indicates that, on the addition of EDTA, emission is almost completely restored to the original value (blue, 450 nm) obtained in the absence of Zn2+, which is denoted by Cycle 1. Such on–off switchable fluorescence emission of the P2–5 ES nanofibers in aqueous media on the sequential addition of Zn2+ and EDTA can be cycled at least five times.

Figure 12a depicts a microfluidics system in which a P2–5 ES nanofiber membrane with an area of 16 cm2 set in the middle of a tube is used to rapidly chelate and sense Zn2+ in a solution flowing through the tube. The time-dependent variation of the solution conductivity was tested. The prepared Zn2+ solution contained 1 p.p.m. Zn2+ (7.5 × 10−3 mole Zn2+ in 0.5 liter of water). The conductivity of the Zn2+ solution in the initial state (0 min) was 163.2 μS cm−1. As the flow time increased, the solution conductivity decreased to 94.0 μS cm−1. Supplementary Table S1 in the Supplementary Information contains raw data on the entire change in conductivity over 50 min, and Figure 12b depicts Ct/C0 (%) versus time (C0 denotes the original conductivity at 0 min, and Ct denotes the conductivity at time t). As Figure 12b shows, the solution conductivity decreased as the flow time increased, indicating that an increasing number of Zn2+ ions were chelated with the P2–5 ES nanofiber membrane, causing less Zn2+ to be present in the solution. Thus, the 100% conductivity of the solution in the initial state (0 min) decreased to 57.6% after 50 min. The rapid conductivity variation is owing to the high surface-to-volume ratio of the ES nanofibers. These results suggest that the studied ES fibers consisting of blends of PHEMA-co-PNMA and bpy-F-bpy have potential for application as efficient and multifunctional sensor filters.

Figure 12
figure 12

(a) Microfluidics system for examining real-time metal ion sensing by the ES nanofiber membrane. (b) Relative conductivity versus time of P-2 ES nanofibers in the microfluidics system. ES, electrospun. A full color version of this figure is available at Polymer Journal online.

Full size image

Conclusions

New multifunctional ES nanofibers with high sensitivity in detecting various metal ions were prepared by blending poly(NIPAAm-co-NMA) copolymers with various amounts of bpy-F-bpy by using a single-capillary spinneret. The HEMA, NMA and bpy-F-bpy moieties were designed to exhibit hydrophilic properties, chemical cross-linking and metal ion sensing ability, respectively. Three types of optical change, namely slightly quenched, red-shifted and completely quenched emission, in bpy-F-bpy used for detecting metal ions in a THF solution were determined according to experimental optical spectra and simulation results. Cross-linked ES nanofibers maintained their fiber structure in water and sensitivity for metal ions because of the sufficient NMA composition. In an aqueous solution containing metal ions, the emission of the ES nanofibers varied from blue (without metal ions) to green (Zn2+) to blue-green (Hg2+) and was quenched (Cu2+). The P2–5 ES nanofibers exhibited the highest Δλmax of 70 nm, and the luminescence color changed from blue to green when they were used for detecting Zn2+. Moreover, these nanofibers exhibited substantial reversible dual PL between blue and green in aqueous Zn2+ and EDTA solutions. According to the experimental results obtained using a microfluidics system, the solution conductivity decreased as the flow time increased, indicating that an increasing number of Zn2+ ions in the solution was chelated with the P2–5 ES nanofiber membrane. The high surface-to-volume ratio of the P2–5 ES nanofibrous membranes, which can be used as ‘naked eye’ sensors, indicates that these membranes have potential for application in multifunctional environment chemosensor-filtering devices.

scheme 1

Synthesis of (a) poly(HEMA-co-NMA) random copolymers and (b) bpy-F-bpy.