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Aug 18, 2016 - ABSTRACT: Combined with the features of electrospun nanofibers and the nature of hydrogel, a novel choreographed poly(acrylic acid)−s...
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Electrospun Poly(acrylic acid)/Silica Hydrogel Nanofibers Scaffold for Highly Efficient Adsorption of Lanthanide Ions and Its Photoluminescence Performance Min Wang, Xiong Li, Weikang Hua, Lingdi Shen, Xufeng Yu, and Xuefen Wang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, PR China S Supporting Information *

ABSTRACT: Combined with the features of electrospun nanofibers and the nature of hydrogel, a novel choreographed poly(acrylic acid)−silica hydrogel nanofibers (PAA-S HNFs) scaffold with excellent rare earth elements (REEs) recovery performance was fabricated by a facile route consisting of colloid-electrospinning of PAA/SiO2 precursor solution, moderate thermal cross-linking of PAA-S nanofiber matrix, and full swelling in water. The resultant PAA-S HNFs with a loose and spongy porous network structure exhibited a remarkable adsorption capacity of lanthanide ions (Ln3+) triggered by the penetration of Ln3+ from the nanofiber surface to interior through the abundant water channels, which took full advantage of the internal adsorption sites of nanofibers. The effects of initial solution pH, concentration, and contact time on adsorption of Ln3+ have been investigated comprehensively. The maximum equilibrium adsorption capacities for La3+, Eu3+, and Tb3+ were 232.6, 268.8, and 250.0 mg/g, respectively, at pH 6, and the adsorption data were well-fitted to the Langmuir isotherm and pseudo-second-order models. The resultant PAA-S HNFs scaffolds could be regenerated successfully. Furthermore, the proposed adsorption mechanism of Ln3+ on PAA-S HNFs scaffolds was the formation of bidentate carboxylates between carboxyl groups and Ln3+ confirmed by FT-IR and XPS analysis. The welldesigned PAA-S HNFs scaffold can be used as a promising alternative for effective REEs recovery. Moreover, benefiting from the unique features of Ln3+, the Ln-PAA-S HNFs simultaneously exhibited versatile advantages including good photoluminescent performance, tunable emission color, and excellent flexibility and processability, which also hold great potential for applications in luminescent patterning, underwater fluorescent devices, sensors, and biomaterials, among others. KEYWORDS: colloid-electrospinning, hydrogel nanofibers, adsorption, lanthanide ions, photoluminescent

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INTRODUCTION Currently, rare earth elements (REEs) have been widely applied in mundane (glassmaking, lighting, ceramics, and agriculture) and high-tech (lasers, nuclear industry, energy conservation, and permanent magnets) fields relying on their specific chemical, optical, electrical, magnetic, and catalytic properties.1−5 Particularly, outstanding REEs luminescent materials have received startling interest due to their fantastic photochemical stability, high luminescence quantum yield, narrow emission bandwidth, and low toxicity.6,7 However, with the widespread utilization of REEs and the consequent rapid depletion of global reserves in the past decades, REEs recovery has become an urgent task and gained considerable attention in current research.8,9 Meanwhile, the severe environment pollution and the terrible health threats of REEs effluent will further aggravate the challenge of developing a highly efficient, environmentally friendly, and cost-effective novel functional material to separate REEs.2,10 Accordingly, numerous techniques have been performed to recover REEs, such as adsorption,9,11−13 ion exchange,14 solvent extraction,2 coprecipitation,15 and hydrometallurgy,10 © 2016 American Chemical Society

among others. It is noteworthy that adsorption has been proven to be a well-established and economical approach for the wastewater treatment due to its simple operation, high efficiency, and low cost.4,16,17 Up to now, various adsorbents including inorganic nanoparticles, resin, biomaterials, and functional polymer materials,3−5,9,13,15,18,19 have been reported for REEs and heavy metals recovery. These materials are often limited by the low adsorption capacity and/or second pollution.17 Significantly, electrospun nanofibers, possessing remarkable structural features such as high porosity, large surface-tovolume ratio, facile functionalization, convenient recyclability, and especially safety of use, exhibited great potential in the field of REEs recovery.20−24 Recently, Wang et al. have successfully constructed a polydopamine (PDA)-wrapped PAN/PSU composite nanofiber mat with heterogeneous structural protuberances, and a micro-/nanostructured p-sulfonatocalix[8]arene Received: July 7, 2016 Accepted: August 18, 2016 Published: August 18, 2016 23995

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Illustration for the Fabrication Procedure of PAA-S Hydrogel Nanofibers (PAA-S HNFs) for the Adsorption of Ln3+ and Its Photoluminescent Performance

groups,39,40 plenty of PAA-based composite materials have been investigated for wastewater treatment due to the carboxylic acid that can interact with various metal ions and dyes.16,19 Silica nanoparticles (SiO2 NPs) will play crucial roles in both cross-linking the PAA component through esterification to form a water-insoluble system and restricting the mobility of PAA chains13,16 (i.e., acting as a rigid support to prevent the fibrous networks from collapsing).41 The resultant PAA-S HNFs with loose and spongy porous network structure exhibited a remarkable adsorption capacity of Ln3+ triggered by the penetration of Ln3+ from the nanofiber surface to interior through the abundant water channels. Luminescent materials generally including organic42,43 (e.g., polyfluoranthene, polypyrene, polynaphthalene, oligofluoranthene, and arylaminiothiazole derivatives) and inorganic44,45 (e.g., lanthanide ions, transition metal ions, and actinide ions) materials have aroused tremendous attention for decades owing to their broad applicability in numerous fields such as photoluminescent devices, flat displays, medical diagnostics, and chemical sensors.7,30,46,47 In particular, due to the specific photoluminescent properties of Ln ions and rapid advances in nanotechnology, there has been growing interest in the controlled synthesis of Ln3+-doped materials on the nanoscale.1,6 In this study, benefiting from the unique features of Ln3+, the Ln3+-loaded PAA-S HNFs simultaneously possess characteristics of good photoluminescent performance and tunable emission color. Furthermore, the free-standing Ln-PAA-S HNF scaffold can be folded and bent optionally without damage owing to its excellent flexibility and processability, which also hold great potential for applications in luminescent patterning, underwater fluorescent devices, sensors, and biomaterials, among others.

(calix8) complex nanofiber scaffold through electrostatic selfassembly of calix8 and aminated polyacrylonitrile (APAN) nanofibers, which all showed relatively higher adsorption capacity for La3+ and excellent regeneration capability.25,26 Nevertheless, previous efforts focused excessively on the increase of specific surface area and adsorption sites on the nanofiber surface to promote the recover efficiency,21,25−27 ignoring the potentially abundant available adsorption sites in the interior of nanofibers (i.e., the adsorption activity between host and guest only implemented on the normal nanofiber surface due to the dense skin layer). Thus, taking full advantage of the internal adsorption sites of the nanofibers might be proposed as an innovative strategy for further enhancing the adsorption capacity, and this challenge could be theoretically overcome by constructing a 3D fiber configuration with loose and spongy porous network structure and abundant water channels to induce the penetration of metal ions from the nanofiber surface to interior. Notably, hydrogels, well-known “soft and wet” materials, are constituted by chemically or physically cross-linked water-swollen 3D macromolecular networks, which could allow the free molecules or ions to diffuse throughout the whole network system, resulting in their widespread applications in biomedicine and engineering.28−30 For instance, a poly(vinyl alcohol) (PVA) network trapped cyanobacterial polysaccharide sacran heterogel could permit the metal ions permeate into the aggregates smoothly and tended to adsorb a larger amount of Nd ions than the stoichiometric amount.31 Moreover, hyaluronic acid32,33 and poly(oligoethylene glycol methacrylate) fibrous hydrogels,34 Na-alginate,35 and polyacrylamide hydrogel fibers36 have been successfully fabricated and demonstrated their promising biomedical applications. Therefore, a choreographed hydrogel fiber combining with the features of electrospun nanofibers37,38 and the nature of hydrogel28,30 could be expected to make the best use of the internal adsorption sites of fibers for further enhancing the adsorption capacity. Herein, we describe a novel PAA−SiO2 hydrogel nanofibers (PAA-S HNFs) scaffold with excellent REEs recovery performance fabricated by a facile approach that was composed of colloid-electrospinning of PAA/SiO2 precursor solution, moderate thermal cross-linking of PAA-S nanofiber matrix, and full swelling in water. Upon exposure to water, PAA-S electrospun matrix quickly hydrated and swelled but still maintained the integrated 3D fibrous morphology (as shown in Scheme 1). Wherein PAA is a highly water-absorbing and electrospinnable hydrogel material with abundant carboxyl

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EXPERIMENTAL SECTION

Materials. Poly(acrylic acid) (PAA, Mw = 250 000 g/mol) was purchased from Wako; silica nanoparticles (SiO2 NPs, size of particle 10−20 nm, 99.5% trace metal basis), LaCl3•6H2O (99.9%), EuCl3•6H2O (99.9%), and TbCl3•6H2O (99.9%) were supplied by Sigma-Aldrich (China). Absolute ethanol and HCl (aqueous solution, 36.5%) were received from China State Medicinal Group Chemical Reagent Co., Ltd. Ultrapure water with a resistance of 18.2 MΩ was prepared by Easy pure II, Thermo. All chemicals were of analytical grade and were used as received without further purification. Fabrication of PAA-SiO2 (PAA-S) Hydrogel Nanofibers. Typically, PAA was dissolved in absolute ethanol by mild stirring for 6 h to obtain 5, 8, 10, and 12 wt % homogeneous transparent solutions, and silica suspensions with concentrations of 5, 10, 15, and 23996

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

Research Article

ACS Applied Materials & Interfaces 20 wt % were obtained by dispersing SiO2 NPs in absolute ethanol and ultrasonic treatment for 3 h. In this study, all the PAA/SiO2 precursor solutions for electrospinning were prepared by mixing PAA solutions and silica suspensions with a certain weight ratio of 2:1. First, the effect of PAA concentration on the PAA−SiO2 electrospun morphology was investigated, wherein the content of SiO2 NPs in the precursor solutions was fixed at ∼3.3 wt % (10 wt % silica suspension was applied). Then, the obtained optimal PAA concentration was utilized to study the effect of silica content on the cross-linking degree and swelling property of the PAA-S nanofibers, and the content of SiO2 NPs in the precursor solutions was regulated to ∼1.7, 3.3, 5.0, and 6.7 wt %. Finally, all the mixtures of colloid solutions were homogenized via vigorous stirring in a water bath at 45 °C for 12 h and ultrasonic treatment for 2 h prior to the colloid-electrospinning process. About 5 mL of PAA/SiO2 colloid solution was loaded into a 5 mL syringe equipped with a blunt metal needle of 0.37 mm inner diameter. The syringe was placed in a syringe pump that maintained a solution feeding rate of 20 μL/min. The anode of a high-voltage power supply was clamped to the syringe needle, the cathode was connected to a metallic rotating roller collector, and the distance from the tip to the collector was 15 cm. During the electrospinning process, the applied voltage was set at 24 kV, and the relevant temperature and humidity were kept at 25 ± 2 °C and 42 ± 5%, respectively. The obtained electrospun nanofibrous mats were denoted as non-cross-linked PAA-S nanofibers. The as-prepared PAA-S nanofibrous mats were cross-linked via thermal treatment at 150 °C for 1−6 h in an oven and denoted as cross-linked PAA-S nanofibers, and then fully swelled in ultrapure water for 3 h to form the resultant PAA-S hydrogel nanofibers (PAA-S HNFs), which were used for the following adsorption experiments. The swelling ratio and gel fraction were calculated by eqs 1 and 2:48

Swelling ratio = Ws/Wd

(1)

Gel fraction = Wd /W0

(2)

(W0) were immersed in ultrapure water with different pH’s for 12 h, and then the mats were dried by lyophilization until constant weight (Wc) was achieved. The pH was adjusted with HCl (1 M) and NaOH (1 M), and the measurement of the membrane was carried out three times for each pH value. The loss of nanofibrous mats was calculated by eq 3:

loss of nanofibrous mats = (W0 − Wc)/W0

(3)

Adsorption and Regeneration Experiments. The adsorption of Ln3+ on PAA-S HNFs was carried out in a series of 250 mL flasks containing 100 mL of Ln3+ solutions, and the flasks were equilibrated in a thermostatic water-bath shaker operated at 25 °C and 85 rpm. The effects of different pH’s (1−7), initial concentrations (25−400 mg/L), and contact times (0.25−8 h) on the adsorption performances of cross-linked PAA-S nanofibers with certain dosages of 0.1 g/L were investigated in detail. After adsorption, the hydrogel nanofibers were taken out and rinsed with ultrapure water and then dried in vacuum oven at 40 °C for further analyses. The amount of Ln3+ uptake was calculated according to

qe = (C0 − Ce) × V /m

(4)

where C0 and Ce are the initial and equilibrium concentration of Ln3+ ions (mg/L), respectively. V is the volume of the solution (L), and m is the weight of the cross-linked PAA-S nanofibrous mats (g). The pH of Ln3+ solutions were adjusted by 0.1 mol/L HCl and NaOH solutions, and the different concentrations of Ln3+ solutions were diluted by the stock solution of 1000 mg/L. Zhu et al.19 reported that Ln3+ could be efficiently dissociated from PAA complex adsorbents in 0.5 mol/L HCl solution. Thus, 0.5 mol/L HCl solution was chosen as eluant in desorption and regeneration experiments. The PAA-S HNFs were equilibrated with Ln3+ in aqueous solution and then put into 100 mL of desorption solutions for 3 h to release the bonded Ln3+. Thereafter, the PAA-S HNFs were washed with ultrapure water three times and reused in the next cycle of adsorption experiment. The consecutive adsorption−desorption experiments were performed for 5 cycles. The regeneration rate (Rr) was calculated from

where W0 is the original weight of cross-linked PAA-S nanofibers, Ws is the weight of the sample swelling in ultrapure water for 3 h, and Wd is the weight of the swollen sample dried by lyophilization. The stability of the PAA-S nanofibrous mats in acidic and alkaline solutions was tested. PAA-S nanofibrous mats with a certain weight

R r = (qr /qe) × 100%

(5)

Figure 1. FE-SEM images of PAA-SiO2 nanofibers electrospun from PAA/SiO2 colloid solutions by mixing different PAA solutions with concentration of (A) 5, (B) 8, (C) 10, and (D) 12 wt % with a fixed 10 wt % silica suspension at a certain weight ratio of 2:1, respectively (the contents of SiO2 NPs in all precursor solutions for electrospinning were all fixed at ∼3.3 wt %). 23997

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

Research Article

ACS Applied Materials & Interfaces where qe is the maximum uptake capacity of Ln3+ onto fresh PAA-S HNFs before desorption (mg/g) and qr is the uptake capacity of Ln3+ onto regenerated PAA-S HNFs (mg/g). Characterization and Measurements. The surface morphologies of PAA-S nanofibers and PAA-S HNFs before and after adsorption Ln3+ were examined by field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Ltd., Japan). The surface chemical compositions of all samples were measured by a Nicolet 8700 FT-IR spectrometer in the range of 650−4400 cm−1 (attenuated total reflectance mode, Thermometer, USA). The X-ray photoelectron spectroscopy (XPS) measurements were executed to analyze the surface element compositions and chemical state of the samples by using a Kratos Axis UltraDLD spectrometer (Kratos Analytical-A Shimadzu Group Company, Japan) with monochromatic Al Ka radiation as the excitation and an X-ray power of 75 W. Elemental peaks were fitted using Casa XPS software (Casa Software Ltd., Teignmouth, Devon, UK). The concentrations of Ln3+ in aqueous solution were investigated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Prodigy, USA). Photoluminescent (PL) Performance. The photoluminescence measurements of Ln-PAA-S HNFs and LnCl3 aqueous solution were recorded by a fluorescence spectrometer (QM/TM, PTI, USA) equipped with a xenon lamp as the excitation source. Both excitation and emission slit widths for all samples were set at 1.5 nm. For the measurements by confocal laser scanning microscopy (CLSM), PAA-S nanofibers were directly electrospun on separate glass slides followed by cross-linking, and then adsorption of Ln3+ for 20 min. After drying, images were captured by a Zeiss LSM 700 at ×40 magnification, wherein excitation sources were set at 405 nm.

elliptical-bead-on-string structure. A randomly oriented 3D nonwoven scaffold with uniform nanofibers (with average fiber diameter of 325 nm) was obtained when the PAA concentration was increased to 10 wt % as shown in Figure 1C. Nevertheless, with regard to the situation from higher concentration of PAA solution (12 wt %), irregular electrospun morphology with microfibers/ribbons, adhesions, and ultrafine nanofibers were generated due to the gel-like and highly viscous property of the precursor solution (Figure 1D). Therefore, the optimized PAA solution with concentration of 10 wt % will be chosen to prepare PAA/SiO2 precursor solution and to investigate the effect of silica content for the resultant PAA-S nanofiber fabrication. The effect of cross-linking time on the cross-linking degree and swelling property of the PAA-S nanofibers will also be discussed subsequently. PAA-S hydrogel nanofibers (PAA-S HNFs) were achieved through the thermal treatment of PAA-S nanofibers followed by fully swelling the PAA component under water, wherein SiO2 NPs played a crucial role as cross-linking agent due to its abundant hydroxyl groups. Significantly, the content of SiO2 NPs in the PAA-S nanofiber matrix and thermal cross-linking time are commonly recognized as the key factors to achieve an excellent swelling property while maintaining the intrinsic fibrous morphology. The swelling ratio and gel fraction of PAA-S nanofibers can be regulated by the SiO2 NPs content and thermal cross-linking time.50 Figure 2A presented the typical comparison of the gel fraction and swelling ratio for the cross-linked PAA-S nanofibers (electrospun from PAA/SiO2 colloid solutions which was prepared by mixing 2:1 weight ratio of 10 wt % PAA solution and the SiO2 suspension with various concentrations, followed by thermal treatment at 150 °C for 5 h) as a function of silica content in the precursor solutions. It is clearly shown that the swelling ratio and gel fraction exhibited an opposite variation tendency with the increase of SiO2 content in the precursor solutions. In the case of SiO2 content of 1.7 wt % (from 5.0 wt % SiO2 suspension), the insufficient cross-linking degree of PAA-S nanofibers would result in the dissolution of the mat in water and further loss of its inherent fibrous morphology (see Supporting Information, section S1, Figure S1A,E). With the increase of the SiO2 content to 3.3 wt % (from 10 wt % SiO2 suspension), the adjacent nanofibers intertwined with each other and exhibited adhesion morphology on the PAA-S mat surface (Figure S1B,F). With further augmentation of SiO2 content to 5.0 wt % (from 15 wt % SiO2 suspension), PAA-S HNFs exhibited an appropriate swelling ratio and gel fraction, which were 4.02 and 95.1%, respectively, and this result could be confirmed from its good resistance to water and the textured

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RESULTS AND DISCUSSION PAA-S Hydrogel Nanofibers Fabrication and Characterization. The surface morphology of electrospun product is particularly influenced by the solution concentration due to the variation of the viscosity and surface tension under a given electrospinning circumstance. For the dilute solution, the low viscosity will result in electrosprayed microparticle topography or bead-on-string structure; once the solution concentration increases to a certain degree, stable and uniform fiber morphology forms.49 Here, the effect of PAA concentration on the PAA-SiO2 electrospun morphology at fixed silica content (∼3.3 wt %) in colloid precursor solutions was first investigated to obtain the uniform nanofibers. Figure 1 showed the representative FE-SEM images of PAA-SiO2 electrospun morphologies fabricated from various PAA/SiO2 colloid solutions prepared by mixing 2:1 weight ratio of PAA solution with different concentrations and the SiO2 suspension with fixed 10 wt % concentration. As can be seen from Figure 1A,B, the electrospun surface morphology from the dilute PAA solutions with concentrations of 5 and 8 wt % exhibited an

Figure 2. Effect of (A) SiO2 content in the precursor solution (thermal treatment at 150 °C for 5 h) and (B) cross-linking time at 150 °C (5.0 wt % SiO2 content) on the gel fraction and swelling ratio of the cross-linked PAA-S nanofibers. Stability of the well cross-linked (C) and poorly crosslinked (D) PAA-S nanofiber mats when immersed into water. 23998

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

Research Article

ACS Applied Materials & Interfaces fibrous morphology in the swollen state (Figure S1C,G). When 6.7 wt % SiO2 was reached (from 20 wt % SiO2 suspension), the swelling ratio of PAA-S HNFs decreased to 2.24 (i.e., the nanofibers were slightly swollen in water), which could be ascribed to the higher cross-linking degree hindering the relaxation of the PAA-S network chains and thus impeding the mobility of water molecules (Figure S1D,H).51,52 The sacrifice of abundant carboxyl groups that reacted with the cross-linking agent would result in the loss of free carboxyl group as adsorption site.36,48 Thus, the optimal content of SiO2 in the precursor solution (5 wt %) for the fabrication of PAA-S HNFs was determined and utilized for the following discussion. Here, it should be emphasized that the following involved PAA-S nanofibers that were all electrospun from the optimized precursor solution prepared by mixing 2:1 weight ratio of 10 wt % PAA solution and 15 wt % SiO2 suspension. Additionally, thermal cross-linking time is another important factor to guarantee the excellent swelling property and integrated fibrous morphology of PAA-S HNFs. Figure 2B illustrated the relationship of the swelling ratio and gel fraction of the cross-linked PAA-S nanofibers with different crosslinking times at 150 °C, which presented a similar variation tendency as the above discussion of the silica content in the precursor solution. As can be seen from Figure S2A,B, the PAA-S nanofibrous mat was completely swollen into a film during the early cross-linking stage (0).13,62 After four cycles of adsorption−desorption process, the regeneration rate of PAA-S HNFs was still kept at above 90% without significant loss of initial active adsorption sites (Figure 6D), suggesting that PAA-S HNFs can be used as a promising adsorbent for the recovery of REEs. Adsorption Mechanism. To identify the adsorption mechanism of PAA-S HNFs, FT-IR spectral analysis was employed to characterize the changes of surface chemical compositions of PAA-S HNFs before and after Ln3+ adsorption. As can be seen from Figure 7, the CO of carboxyl at 1710 cm−1 (shown in curve a)53,63 shifted to a lower wavenumber of 1657 cm−1,55 and the vibration intensity of the CO decreased significantly after Ln3+ adsorption, indicating that the carboxyl groups were involved to form complexes with Ln3+. Furthermore, with respect to Ln3+-loaded PAA-S HNFs (shown in curve b-d), the appearance of additional absorption peaks at 1544 and 1422 cm−1 were assigned to the asymmetric stretching vibration (νasym) and symmetric stretching vibration (νsym) of carboxylate, respectively, and the separation (Δ) between νasym and νsym was 122 cm−1.59,63 Xu et al.63 have reported that the Δ(νasym − νsym) value of bidentate carboxylate was less than sodium carboxylate (164 cm−1), while the Δ value of unidentate carboxylate was more than 164 cm−1. Thus, it is clearly demonstrated that the interaction between COO− and Ln3+ was bidentate coordination including chelating and bridging, as displayed in Figure 8I. XPS analyses were also performed to further verify the adsorption mechanism of Ln3+ on PAA-S NHFs. The typical wide-scan XPS spectra of the virgin and Ln3+-loaded PAA-S HNFs were shown in Figure 8A−D. It could be clearly observed that La 3d,26 Eu 4d,64 and Tb 4d65 orbital were detected at the characteristic binding energy of 838.4, 137.4, and 154.1 eV, respectively, which confirmed the adsorption of Ln3+ on PAA-S HNFs (see details in the Supporting Information, section S6). Figure 8E−H depict the high-resolution scan of the O 1s core-level spectra of PAA-S HNFs before and after Ln3+ adsorption. As shown in Figure 8E, the deconvoluted O 1s spectra of virgin PAA-S HNFs was composed of three curve-fitted peaks at the binding energies of 531.17, 532.28, and 533.53 eV, which were assigned to the CO, C−O (ether and/or alcohol hydroxyl), and Si−O, respectively,53,54 whereas

Table 2. Comparison of the Adsorption Capacity of Different Adsorbents for the Removal of Ln3+ adsorbents calix8-APAN complex nanofibers PDA-wrapped PAN/PSU composite nanofiber Fe3O4@ TMS-EDTA magnetic nanoparticles Fe3O4@humic acid magnetic nanoparticles Fe3O4@AMPSNa magnetic nanocomposite hydrogel P(VP-AMPS-SiO2) composite hydrogel hollow core/shell hematite DTPA-functionalized chitosan biopolymer EDTA-β-cyclodextrin biopolymer CA/PEG membranes HPC-g-PAA this study

Ln3+ qm (mg/g) 3+

155.1 (La ) 59.5 (La3+) 113 (Gd3+) 10.6 (Eu3+) 58.8 (La3+) 116 (La3+) 14.48 (Sm3+) 77 (Nd3+) 55.48 (Eu3+) 27.4 (Eu3+) 195.8 (Ce3+) 232.6 (La3+) 268.8 (Eu3+) 250.0 (Tb3+)

refs 21 22 13 11 5 3 12 9 15 4 19

indicating that the adsorption process supported the chemisorption as the controlling mechanism.51,59 Adsorption Kinetics. To further investigate the adsorption performance of Ln3+ on PAA-S HNFs, the effect of contact time on the adsorption rate was conducted with initial Ln3+ concentration of 250 mg/L at pH 6. As shown in Figure 6C, the plot clearly indicated that a high adsorption rate was observed in the initial 1 h, and about 90% of the adsorption capacity was achieved due to the abundant adsorption sites.20,60 With the extension of contact time, the adsorption rate dropped off due to the decrease of available sorption sites and the enhanced resistance for Ln3+ traversing the surface into the interior of hydrogel fibers. Then, the adsorption capacity gradually reached adsorption equilibrium after 3 h, indicating that PAA-S HNFs can be used as efficient adsorbents for the fast recovery of REEs. The adsorption kinetics was analyzed to elucidate the possible rate-controlling step of the adsorption process,25,61 and three typical kinetic models, pseudo-first-order (pfo),16 pseudosecond-order (pso),58 and intraparticle diffusion models,51 have been employed to fit the experimental adsorption data as shown in Figure S5 (see details in the Supporting Information, section S5). The corresponding kinetic parameters were summarized in Table 3. As compared to the correlation coefficient R2 of pfo, the pso model (R2 > 0.9997) was more suitable to explain the adsorption kinetics of Ln3+ on PAA-S HNFs, suggesting that intraparticle diffusion process was the ratecontrolling step of the adsorption process.27,51 Thus, it is of vital importance to analyze the intraparticle diffusion model, which can describe the adsorption process more clearly. As shown in Figure S5C, the plots exhibited two linear portions without pass through the origin, which revealed that the adsorption process was mainly controlled by two steps involving film diffusion and intraparticle diffusion.27,51 In detail, Phase I depicted the instantaneous adsorption of Ln3+ on the fiber external surface, and the diffusion of Ln3+ through the loose and spongy fibrous hydrogel network, which was in 24002

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

Research Article

ACS Applied Materials & Interfaces

Table 3. Adsorption Kinetic Parameters for the Adsorption of Ln3+ onto PAA-S HNFs by Pseudo-First-Order, Pseudo-SecondOrder, and Intra-Particle Diffusion Models pseudo-first-order model adsorbates 3+

La Eu3+ Tb3+

intraparticle diffusion model

pseudo-second-order model

k1 (h−1)

qe (mg/g)

R2

k2 (g/mg h)

qe (mg/g)

R2

kp1 (mg/g h)

R2p1

kp1 (mg/g h)

R2p2

0.9710 0.8856 1.038

45.84 36.51 50.55

0.8979 0.8866 0.9297

0.0397 0.0530 0.0408

229.36 267.38 250

0.9997 0.9998 0.9998

78.40 40.59 74.22

0.9948 0.9951 0.9916

3.26 2.86 4.77

0.9978 0.9729 0.9725

Figure 7. FT-IR spectra of PAA-S HNFs before (a) and after (b) La3+, (c) Eu3+, and (d) Tb3+ adsorption.

the O 1s core-level spectra of Ln3+-loaded PAA-S HNFs could be split into four individual peaks, as shown in Figure 8F−H, where the appearance of new curve-fitted peaks around at 532.14, 532.23, and 532.25 eV were assigned to La−O, Eu−O, and Tb−O, respectively,59 indicating the chemical bond between the oxygen and lanthanide atoms. Meanwhile, the relative content of oxygen atom from C−O and CO decreased due to a certain sacrifice of C−OH and CO for the formation of coordination bond with Ln3+. Thus, it could be deduced that the carboxyl groups formed bidentate carboxylates with Ln3+, and the two oxygen atoms on the carboxyl group turned equivalent, as presented in Figure 8I. Furthermore, the binding energy of both C−O and CO exhibited a certain degree of shift to the higher values, which was presumably ascribed to the decreased electron density of the oxygen atoms bonded with Ln3+.55,66 Consequently, these obtained results further confirmed the formation of the strong bidentate coordination bonds between carboxyl and Ln3+, which were fully in accordance with the FT-IR analysis. Photoluminescent Properties of Ln3+-Loaded PAA-S HNFs. It is well-known that the unique electronic structure of Ln ions enables it to emit photons efficiently in the spectral region from ultraviolet to visible or infrared.1 Herein, the resultant Ln-PAA-S HNFs (Ln = Eu, Tb, and Eu/Tb) could exhibit excellent photoluminescence (PL) due to the typical 4f transition luminescence features of Ln.44,47 Figure 9A described the PL emission spectra of EuCl3 solution and Eu-PAA-S HNFs under excitation at 390 nm. The emission spectrum of Eu-PAA-S HNFs showed several 5D0/7FJ (J = 0, 1, 2, 3, and 4) characteristic emission lines of Eu3+ at about 578, 592, 616, 656, and 695 nm,7,45 which were similar to that of EuCl3 solution, demonstrating that Eu-PAA-S HNFs could retain the PL property of Eu3+ ions. The relative PL intensity ratios of 616 nm (5D0−7F2) and 592 nm (5D0−7F1) (I616/I592) was about 0.4 (less than 1) for EuCl3 solution, whereas a reverse I616/I592 of about 2.32 (greater than 1) was observed for Eu-PAA-S

Figure 8. XPS wide-scan spectra of PAA-S HNFs before (A) and after adsorption of (B) La3+, (C) Eu3+, and (D) Tb3+. (E−H) The corresponding O 1s core-level spectrum. (I) Possible adsorption mechanism of the PAA-S HNFs and Ln3+.

HNFs that resulted from the strong complexation of COO− and Eu3+ in PAA-S HNFs, which primarily enhanced the PL intensity at 616 nm (5D0−7F2). Meanwhile, the greater I616/I592 was beneficial to the better red color purity.30 The photoluminescent performances of TbCl3 solution and Tb-PAA-S HNFs were also investigated (Figure 9B). Upon excitation at 305 nm, the emission spectrum of TbCl3 solution 24003

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

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Figure 9. Emission spectra of (A) EuCl3 solution and Eu-PAA-S HNFs (λex = 390 nm) and (B) TbCl3 solution and Tb-PAA-S HNFs (λex = 305 nm); the concentration of EuCl3 and TbCl3 aqueous solutions are 250 mg/L. Confocal laser scanning microscopy images of (C) Eu-PAA-S HNFs and (D) Tb-PAA-S HNFs.

was composed of a series of typical emission peaks at 489, 544, 585, 620, and 649 nm, which originated from the 5D4−7F6, 5 D4−7F5, 5D4−7F4, and 5D4−7F2−0 transitions of Tb3+, respectively, and the dominant peaks were all the green emissions of 544 nm (5D4−7F5).30 The emission spectrum of Tb-PAA-S HNFs was similar to that of TbCl3 solution, demonstrating that Tb3+ ions could maintain the photoluminescent performance in the Tb-PAA-S HNFs. In addition, the PL intensities of Ln-PAA-S HNFs were much stronger than that of LnCl3 aqueous solution, revealing that the PL intensity could be dramatically enhanced by Ln3+ coordination with COO− groups.30 As can be seen from the confocal microscopy images of Eu-PAA-S HNFs and Tb-PAA-S HNFs (Figure 9C−D), the Ln-PAA-S HNFs exhibited favorable fibrous morphology and uniform bright photoluminescence due to the appropriate cross-linking and the homogeneous adsorption of photoluminescent Ln3+ emitters, which was consistent with the above discussions. To determine the tunable photoluminescent properties of Ln-PAA-S HNFs, the emission spectra of Ln-PAA-S HNFs excited at 277 nm were investigated. As shown in Figure 10A, it could be clearly observed that Eu-PAA-S HNFs and Tb-PAA-S HNFs exhibited red and green emissions, respectively, while Eu/Tb-PAA-S HNFs showed a series of characteristic peaks of not only the red emission at 616 nm (5D0−7F2) from Eu3+ but also the green emission at 544 nm (5D4−7F5) from Tb3+. Furthermore, the CIE chromaticity diagram of the Ln-PAA-S HNFs was shown in Figure 10B; the CIE chromaticity coordinates shifted from orange-red (x = 0.657, y = 0.3427) to orange (x = 0.4955, y = 0.4552) and green (x = 0.3459, y = 0.5695) with adsorptions of Eu3+, Eu3+/Tb3+, and Tb3+, respectively. Meanwhile, the corresponding luminescence colors changed from bright red to orange and green, which could be clearly identified from the photographs presented in Figure 10C. Thus, it could be further anticipated that the emission colors can be tuned by reasonably adjusting the adsorption concentration ratio of Tb3+/Eu3+ on the basis of using a single wavelength as pumping source. Additionally, the Ln-PAA-S

Figure 10. (A) Emission spectra of Eu-PAA-S, Tb-PAA-S, and Eu/Tb-PAA-S HNFs excited at 277 nm. (B) CIE chromaticity diagram of Eu-PAA-S, Tb-PAA-S, and Eu/Tb-PAA-S HNFs. (C) Photographs of Eu-PAA-S, Tb-PAA-S, and Eu/Tb-PAA-S HNFs with various shapes before and after 254 nm UV radiation.

HNFs could be readily knotted (folded and bent) and optionally cut to a desired shape without damage due to its excellent flexibility and processability.6 Consequently, the Ln-PAA-S HNFs simultaneously possessed the characteristics of good photoluminescent performance, tunable emission color, and excellent flexibility and processability, indicating its great potential for applications in luminescent patterning, underwater fluorescent devices, sensors, and biomaterials, among others. 24004

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

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CONCLUSIONS We have described the facile fabrication of a novel PAA-S hydrogel nanofibers (PAA-S HNFs) with excellent REEs adsorption performance through moderate thermal cross-linking of PAA-S electrospun nanofibrous matrix followed by fully swelling under the water circumstance. The swelling ratio and gel fraction analyses concluded that the SiO2 content in the precursor solutions (5 wt %) and thermal cross-linking time (5 h) at 150 °C contributed significantly to the excellent swelling property and the intrinsic fibrous morphology of the resultant PAA-S HNFs. Due to the penetration of Ln3+ from the hydrogel nanofiber surface to interior through the loose and spongy porous network, the resultant PAA-S HNFs scaffold exhibited high adsorption capacity for Ln3+. The Ln3+ adsorption isotherms on PAA-S HNFs were well-fitted to the Langmuir model with the maximum adsorption capacity of 232.6, 268.8, and 250.0 mg/g for the La3+, Eu3+, and Tb3+ solutions, respectively, and the adsorption kinetics followed the pseudosecond-order model and the intraparticle diffusion model. The FT-IR and XPS results have revealed that the complex of Ln3+ and carboxyl groups was via bidentate coordination. Finally, PAA-S HNFs adsorbed Ln3+ that could be desorbed in 0.5 mol/L HCl and regenerated successfully. Consequently, the proposed PAA-S HNFs provided a new prospect to construct electrospun-nanofibers-based adsorption materials for highly efficient recovery of Ln3+. Furthermore, the Ln-PAA-S HNFs simultaneously possessed good photoluminescent performance, tunable emission color, and excellent flexibility and processability, which demonstrated the feasibility of design and manufacture of novel hydrogel nanofibers for luminescent applications.

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Education Commission, and Program of Changjiang Scholars and Innovative Research Team in University (IRT1221).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08294. FE-SEM images of cross-linked PAA-S nanofibers with different silica contents in the precursor solutions before and after immersing into water, FE-SEM images of crosslinked PAA-S nanofibers with different cross-linking times after immersing into water, FT-IR spectra of PAA-S nanofibers before and after cross-linking and specific analysis of the FT-IR spectra, detailed description of adsorption isotherms, adsorption isotherms fitted curve for adsorption of Ln3+ onto PAA-S HNFs, detailed description of the adsorption kinetics model, adsorption kinetics fitted curve for adsorption of Ln3+ onto PAA-S HNFs, core-level spectra for La 3d, Eu 4d, and Tb 4d of Ln3+-PAA-S HNFs and specific analysis of the core-level spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-67792860. Fax: +86-21-67792855. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (51273042), Program for New Century Excellent Talents in University, Innovation Program of Shanghai Municipal 24005

DOI: 10.1021/acsami.6b08294 ACS Appl. Mater. Interfaces 2016, 8, 23995−24007

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