Silica Hydrogel Nanofibers Scaffold for

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08294 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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ACS Applied Materials & Interfaces

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, P.R. China

ACS Paragon Plus Environment

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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 crosslinking of PAA-S nanofiber matrix and fully swelling under the water circumstance. The resultant PAA-S HNFs with 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 Langmuir isotherm model and pseudo-second-order model. 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 well designed 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, etc. 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, the 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.89

Meanwhile, the severe environment pollution and the terrible health threats of the 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 co-precipitation,15 hydrometallurgy,10 and etc. 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 like high porosity, large surface-to-volume ratio, facile functionalization, convenient recyclability, and especially safety to be used, exhibited great potential in the field of REEs recovery.20-24 Recently, Wang et al. have successfully constructed a polydopamine (PDA) wrapped PAN/PSU


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composite nanofiber mat with heterogeneous structural protuberances, and a micro/nanostructured p-sulfonatocalix[8]arene (calix8) complex nanofiber scaffold through electrostatic self-assembly 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 three-dimensional (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, the 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 diffusion throughout the whole network system, resulting in their widespread applications in biomedicine and engineering.28-30 For instance, a polyvinyl 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-alginate35 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


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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 nanofiber (PAA-S HNF) 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 PAAS nanofiber matrix and fully swelling under the water circumstance. Upon exposure to water, PAA-S electrospun matrix quickly hydrated and swelled but still remained the integrated 3D fibrous morphology (as shown in Scheme 1). Wherein, PAA is a highly water-absorbing and electrospinnable hydrogel material with abundant carboxyl 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 On the other hand, silica nanoparticles (SiO2 NPs) will play crucial roles in both cross-linking PAA component through esterification to form water-insoluble system, and restricting the mobility of PAA chains,13, 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, arylaminiothiazole derivatives) and inorganic44-45 (e.g. lanthanide ions, transition metal ions, 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 sensors7, 30, 46-47. In particular, due to the specific photoluminescent properties of Ln ions and rapid advances in nanotechnology, there has


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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 bended 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, etc. 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 %), TbCl3•6H2O (99.9 %) were supplied by Sigma-Aldrich (China). Absolute ethanol, HCl (aqueous solution, 36.5 %) were received from China State Medicinal Group Chemical Reagent Co., Ltd. Ultra-pure water with a resistance of 18.2 MΩ was prepared by Easy pureⅡ, 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 concentration of 5, 10, 15 and 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. Firstly, the effect of PAA concentration on the PAA-SiO2 electrospun morphology was investigated, wherein, the content of SiO2 NPs in the


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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 uncross-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 ultra-pure 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 equations (1) and (2):48 Swelling ratio = Ws Wd

(1)

Gel fraction = Wd W 0

(2)


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where Wo is the original weight of cross-linked PAA-S nanofibers, Ws is the weight of the sample swelling in ultra-pure 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. PAAS nanofibrous mats with a certain weight (W0) were immersed in ultra-pure water with different pH for 12 h, and then the mats were dried by lyophilization till constant weight (Wc). 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 equation (3): The loss of nanofibrous mats= (W 0 − Wc) W 0

(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 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 (1-7), initial concentrations (25 ~ 400 mg/L), contact time (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 ultra-pure water, and then dried in vacuum oven at 40 °C for further analyses. The amount of Ln3+ uptake was calculated according to the following equation (4):

qe = (C 0 − C e ) × 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


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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 desorption solutions for 3 h to release the bonded Ln3+. Thereafter, the PAA-S HNFs were washed with ultra-pure 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 was calculated from the following equations (5):

Rr = ( qr qe ) × 100%

(5)

where qe is the maximum uptake capacity of Ln3+ onto fresh PAA-S HNFs before desorption (mg/g), 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


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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 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. 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 the electrospayed microparticle topography or the beadon-string structure, once the solution concentration increases to a certain degree, stable and uniform fiber morphology will be formed.49 Here, the effect of PAA concentration on the PAASiO2 electrospun morphology at fixed silica content (~ 3.3 wt %) in colloid precursor solutions was firstly investigated to obtain the uniform nanofibers. Figure 1 showed the representative FESEM 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 concentration and the SiO2 suspension with fixed 10 wt % concentration. As can be seen from Figure 1A and 1B,


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the electrospun surface morphology from the dilute PAA solutions with concentrations of 5 and 8 wt % exhibited an 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 the fully swelling of PAA component under the water circumstance, 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 crosslinking time.50 Figure 2A presented the typical comparison of the gel fraction and swelling ratio for the crosslinked 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 concentration, and followed by thermal treatment at 150 °C for 5 h) as a function of silica


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content in the precursor solutions. It is clearly showed 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 losing its inherent fibrous morphology (see Supporting Information S1, Figure S1 A and 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 S1 B and F). With further augment of SiO2 content to 5.0 wt % (from 15 wt % SiO2 suspension), PAA-S HNFs exhibited an appropriate swelling ratio and gel fraction, which was 4.02 and 95.1%, respectively, and this result could be confirmed from its good resistance to water and the textured fibrous morphology in the swollen state (Figure S1 C and G). When the 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 hindered the relaxation of the PAA-S network chains and thus impeded the mobility of water molecules (Figure S1 D and H).51-52 On the other hand, 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 were all electrospun from the optimized precursor solution which was prepared by mixing 2:1 weight ratio of 10 wt % PAA solution and 15 wt % SiO2 suspension.


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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 cross-linking time 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 S2 A and B, PAA-S nanofibrous mat was completely swollen into a film during the early cross-linking stage (< 2 h). With the extension of the cross-linking time from 3 to 6 h, the gel fraction was gradually increased to 96%, while the swelling ratio was remarkably decreased to 2.45 due to the increment of the cross-linking degree. Meanwhile, the PAA-S surface topography exhibited an adhesion morphology among the adjacent fibers to individual fibrous structure (Figure S2 C-F). Wherein, the excessive cross-linking of the PAA-S nanofiber will limit its swollen to a relative larger size in diameter in water after 6 h thermal treatment. Consequently, the optimized crosslinking time for the construction of PAA-S HNFs was set at 5 h for thermal treatment. Figure 2C-D clearly showed the dissolution state of the well cross-linked and poorly cross-linked PAAS nanofiber mats in water, which further confirmed the integrity of hydrogel nanofiber for the practical operation. FT-IR spectral analysis was employed to confirm the surface chemical compositions of the uncross-linked and cross-linked PAA-S nanofibers. Both similar characteristic spectra revealed that the chemical compositions of PAA-S nanofibers were essentially maintained after thermal treatment (see supplementary discussion S3). For further investigation of the thermal crosslinking mechanism, the typical X-ray photoelectron spectroscopy (XPS) analysis was performed. As the wide-scan spectra shown in Figure 3A and C, the detected atoms of C and O on the PAAS mats surface demonstrated that the atom molar ratio of C/O increased from 1.561 to 1.882 due


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to the esterification reaction of carboxyl group and hydroxyl group after thermal cross-linking.16 The C 1s core-level XPS spectra of the uncross-linked PAA-S nanofibers could be curve-fitted by three peaks at the binding energy of 284.68, 285.25 and 288.89 eV, corresponding to the carbon from C-C, C-O-H and C=O, respectively,53 whereas that of the cross-linked sample could be curve-fitted by four peaks at the binding energy of 284.68, 285.34, 286.75 and 288.94 eV for the carbon from C-C, C-O-H, C-O-Si and C=O,54-56 respectively (Figure 3B and D), further indicating the esterification reaction and confirming the thermal cross-linking of PAA and SiO2 NPs successfully as proposed (as shown in Figure 3E). The surface morphology of PAA-S and Ln3+-loaded PAA-S HNFs. The representative FESEM images and the corresponding fiber diameter distributions of the as-prepared PAA-S nanofiber matrix, PAA-S HNFs and Ln3+-loaded PAA-S HNFs were shown in Figure 4 to investigate the fiber morphology and swelling behavior. As can be seen from Figure 4A-C, PAAS nanofiber exhibited a rough and uniform nanofiber (~325 nm) morphology with good distribution of SiO2 NPs on the nanofiber surfaces. Upon exposure to water, PAA-S nanofiber matrix quickly hydrated and swelled but still retained the integrated 3D fibrous morphology, and the fiber diameter dramatically increased to 924 nm at equilibrium swelling (Figure 4D-F), indicating more than 2.8-fold enhancement compared to that of the nanofiber before hydration. This could be attributed to the high hydrodynamic free volume of the PAA-S network allowing for the accommodation of adequate water molecules.51-52 Consequently, with the loose and spongy porous network structure and abundant water circumstance of the resultant PAA-S HNFs, lanthanide ions could penetrate into the hydrogel nanofibers by diffusion, and therefore the adsorption process occurred on both the exterior surface and interior of PAA-S HNFs.31, 52 As shown in Figure 4G-O, Ln3+-loaded PAA-S HNFs still maintained their inherent fiber


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morphologies, and the fiber diameters slightly increased to 1.10 µm (La3+), 1.03 µm (Eu3+) and 1.07 µm (Tb3+), respectively. This phenomenon was presumably attributed to the permeation of a large amount of Ln3+ into the PAA-S fibrous hydrogel network (in which the coordination between Ln3+ and the carboxyl group of PAA was achieved) and the consequent penetration of certain amount of water into the fiber interior due to the resultant osmotic pressure between the Ln3+ solution and the hydrogel network,31, 52, 57 finally resulting in the slight expansion of the 3D PAA-S skeleton. The pH stability of PAA-S HNFs. The stability of PAA-S HNFs in acidic and alkaline environments was closely associated with the adsorption and desorption process of Ln3+. Thus, it is of vital importance to investigate the effect of pH value on the loss of hydrogel nanofibers. As can be seen from Figure 5A, it is clearly shown that a negligible loss of PAA-S HNFs was observed at the pH from 0 to 8, indicating a certain stability of the hydrogel nanofibers against the corrosive conditions. And this result could be further confirmed by the mats degradation state in different pH circumstances (as shown in Figure 5B-D). Compared with the two acidic environments (pH of 1 and 6), PAA-S HNFs were almost degraded in water with pH of 10 and could not be removed from the solution completely. Consequently, this wide pH window permitted the adsorption and desorption process of Ln3+ by PAA-S HNFs without damaging the mat integrity, and further guaranteed the regeneration of the PAA-S HNFs. Effect of solution pH on adsorption. Figure 6A demonstrated that the complexation behavior between PAA-S HNFs and Ln3+ exhibited strong dependence on pH values, which would determine the surface charge of PAA-S HNFs and the species of Ln3+ in aqueous solution.13, 58 When the pH value was lower than 2, the amount of Ln3+ adsorbed onto PAA-S HNFs was relatively less, after that, the adsorption capacity increased dramatically with increasing pH from


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2 to 6, whereas a slightly decreased adsorption was observed at pH exceeding 6. The low uptake of Ln3+ on PAA-S HNFs at low pH (6) could be ascribed to the formation of Ln3+ hydroxide adducts (Ln(OH)3), which would diminish the activity of Ln3+.26 Thus, the optimal pH of 6 was selected for subsequent adsorption study. Adsorption isotherm. Initial concentration plays an important role in determining the adsorption capacity of Ln3+ on PAA-S HNFs.8, 20 As shown in Figure 6B, the adsorption capacity of Ln3+ increased remarkably with increasing initial concentration from 25 to 250 mg/L, and then tended to the high-level adsorption with the maximum capacity, which were 221.4, 260.3 and 241.3 mg/g for La3+, Eu3+ and Tb3+, respectively when the initial concentration was 250 mg/L (at pH 6). This variation tendency was attributed to the increase in the driving force of concentration gradient and eventually the saturated adsorption due to the sufficient occupation of chelating sites on the adsorbents.26-27 The interactive behaviors between the PAA-S HNFs and Ln3+ at pH 6 of the adsorption process were predicted by adsorption isotherms.20 Here, three typical models of Langmuir, Freundlich and Dubinin-Kaganer Radushkevich (D-R) isotherms were used to analyze the


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equilibrium data (see details in supplementary discussion S4). As summarized in Table 1, the adsorption behavior of Ln3+ on PAA-S HNFs could be well-fitted by the Langmuir model with a higher correlation coefficient R2 of 0.999 (see the corresponding linear fitting shown in Figure S4 A and B) as compared to the Freundlich model, indicating that the adsorption of Ln3+ was primarily a monolayer adsorption process on the homogeneous surface, i.e., all the adsorption sites exhibited equal adsorbate affinity. The maximum adsorption capacities calculated from the Langmuir model for La3+, Eu3+ and Tb3+ were 232.6, 268.8 and 250.0 mg/g, respectively (Table 1), which showed considerable competitiveness as compared with the adsorbent materials reported so far (Table 2), such as calix8-APAN complex nanofibers,25 Fe3O4@Humic Acid magnetic nanoparticles,11 P(VP-AMPS-SiO2) composite hydrogel,3 DTPA-functionalized chitosan biopolymer9 and CA/PEG membranes.4 This highly efficient adsorption efficiency of the as-prepared PAA-S HNFs could be ascribed to its loose and spongy network structure and abundant water channels permitting the Ln3+ diffusion into the fibers, and thus the adsorption process occurred on both the exterior surface and interior of the hydrogel nanofibers as analyzed previously.52 Additionally, as can be seen from the fitting curve in Figure S4C and the correlation coefficient R2 in Table 1, the adsorption data of Ln3+ on PAA-S HNFs also had a relatively better compliance with D-R model, and the obtained adsorption free energy (E) were all larger than 8.0 kJ/mol, 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


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achieved due to the abundant adsorption sites.20,

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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 of pseudo-first-order (pfo),16 pseudosecond-order (pso)58 and intra-particle diffusion model51 have been employed to fit the experimental adsorption data as shown in Figure S5 (see details in supplementary discussion 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 intra-particle diffusion process was the rate-controlling step of the adsorption process.27, 51 Thus, it is of vital importance to analyze the intra-particle 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 intra-particle diffusion.27, 51 In detail, Phase I depicted the instantaneous adsorption of Ln3+ on the fiber external surface, and the diffusion of Ln 3+ through the loose and spongy fibrous hydrogel network, which was in accordance with the preceding analysis. Phase II corresponded to the equilibrium stage, where the rate of intra-particle diffusion slowed down due to the remarkable decrease of Ln3+ concentration and the active sites available for adsorption. These obtained results further confirmed that the intra-particle diffusion was primarily the ratecontrolling step of the adsorption process.


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Regeneration performance of PAA-S HNFs. Considering the economic plausibility for the potential practical applications, it is necessary to maintain the effective reusability of PAA-S HNFs.20 In this study, the Ln3+ complexed PAA-S HNFs were eluted by 0.5 M HCl for the desorption and regeneration, which was in agreement with the fact that the carboxylic acid groups will be fully protonated below pH 3 (ζ-potential > 0).13, 62 After four cycles of adsorptiondesorption 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 wave number 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 cm-1 and 1422 cm-1 were assigned to the asymmetric stretching vibration (νasym) and symmetric stretching vibration (νsym) of carboxylate, respectively, and the separation (△) between νasym and νsym was 122 cm-1.59, 63 Xu et al.63 have reported that the △ (νasym-νsym) value of bidentate carboxylate was less than sodium carboxylate (164 cm-1), while the

△

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.


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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 La3d26, Eu4d64 and Tb4d65 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 supplementary discussion S6). Figure 8E-H depicted the high-resolution scan of the O1s core-level spectra of PAA-S HNFs before and after Ln3+ adsorption. As shown in Figure 8E, the deconvoluted O1s spectra of virgin PAA-S HNFs was composed of three curve-fitted peaks at the binding energy 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, the O1s core-level spectra of Ln3+-loaded PAA-S HNFs could be split into four individual peaks, as shown in Figure 8F-H, 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 TbO, respectively,59 indicating the chemical bond between oxygen and lanthanide atom. 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.


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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 infrared1. Herein, the resultant Ln-PAA-S HNFs (Ln = Eu, Tb, Eu/Tb) could exhibit excellent photoluminescence (PL) due to the typical 4f transitions luminescence features of Ln.44, 47 Figure 9A described the PL emission spectra of EuCl3 solution and Eu-PAAS HNFs under the excitation at 390 nm. The emission spectrum of Eu-PAA-S HNFs showed several 5D0/7FJ (J=0, 1, 2, 3, 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– 7

F2) 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 HNFs that resulted from the strong complexation of COO− and Eu3+ in PAA-S HNFs, and they 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 was composed of a series of typical emission peaks at 489, 544, 585, 620 and 649 nm, which originated from the

5

D4–7F6,

5

D4–7F5,

5

D4–7F4 and

5

D4–7F2-0 transitions of Tb3+,

respectively, and the dominant peaks were all the green emission 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-


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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 a red and green emission, 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 adsorption 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 HNFs could be readily knotted (folded and bended) and cut optionally 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, etc. CONCLUSIONS


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In summary, we have described the facile fabrication of a novel PAA-S hydrogel nanofibers (PAA-S HNFs) with excellent REEs adsorption performance through moderate thermal crosslinking 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 ℃ 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 pseudo-second order model and the intra-particle diffusion model. The FT-IR and XPS results have revealed that the complex of Ln3+ and carboxyl groups was bidentate coordination. Finally, the PAA-S HNFs adsorbed with Ln3+ 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 nanofiber 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. AUTHOR INFORMATION Corresponding Author


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*Tel.: +86-21-67792860. Fax: +86-21-67792855. E-mail: [email protected]. 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 Education Commission, Program of Changjiang Scholars and Innovative Research Team in University (IRT1221). ASSOCIATED CONTENT Supporting Information FE-SEM images of cross-linked PAA-S nanofibers with different silica content in the precursor solutions before and after immersing into water, FE-SEM images of cross-linked PAA-S nanofibers with different cross-linking time 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. REFERENCES (1) Liu, Y.; Tu, D.; Zhu, H.; Chen, X. Lanthanide-Doped Luminescent Nanoprobes: Controlled Synthesis, Optical Spectroscopy, and Bioapplications. Chem. Soc. Rev. 2013, 42, 6924-6958.


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(34) Xu,

F.;

Sheardown,

H.;

Hoare,

T.

Reactive Electrospinning of Degradable

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Materials

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Ions.

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Graphic for manuscript Scheme 1. Illustration for the fabrication procedure of PAA-S hydrogel nanofibers (PAA-S HNFs) for the adsorption of Ln3+ and its photoluminescent performance. 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 %). Figure 2. The effect of (A) SiO2 content in the precursor solution (thermal treatment at 150 ℃ for 5 h) and (B) cross-linking time at 150 ℃ (5.0 wt % SiO2 content) on the gel fraction and swelling ratio of the cross-linked PAA-S nanofibers. The stability of the well cross-linked (C) and poorly cross-linked (D) PAA-S nanofiber mats when immersed into water. Figure 3. (A, C) XPS wide-scan spectra and (B, D) C1s core-level spectrum of the uncrosslinked and cross-linked PAA-S nanofibers. (E) The proposed thermal cross-linking mechanism for the esterification reaction of PAA and SiO2. Figure 4. FE-SEM images of (A, B) cross-linked PAA-S nanofibers, (D, E) PAA-S HNFs, and (G, H) La3+, (J, K) Eu3+, (M, N) Tb3+ loaded PAA-S HNFs. (C, F, I, L, O) The corresponding fiber diameter distributions. Figure 5. (A) The loss situation of PAA-S HNFs in water with different pH values. The PAA-S HNFs were stable in the acidic environments of pH 1 (B), 6 (C), and were almost disintegrated in basic environment with pH 10 (D). Figure 6. The effect of (A) pH values, (B) initial concentration and (C) contact time on the adsorption capacity of PAA-S HNFs for Ln3+. (D) The regeneration rate of PAA-S HNFs over 5 adsorption/desorption cycles of Ln3+. Figure 7. FT-IR spectra of PAA-S HNFs before (a) and after (b) La3+, (c) Eu3+ and (d) Tb3+ adsorption. Figure 8. XPS wide-scan spectra of PAA-S HNFs before (A) and after adsorption of (B) La3+, (C)


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Eu3+ and (D) Tb3+. (E-H) The corresponding O1s core-level spectrum. (I) The possible adsorption mechanism of the PAA-S HNFs and Ln3+. 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. 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. Table 1. The constant parameters of Langmuir, Freundlich and D-R isotherm model for adsorption of Ln3+ onto PAA-S HNFs. Table 2. Comparison of the adsorption capacity of different adsorbents for the removal of Ln3+. Table 3. Adsorption kinetic parameters for the adsorption of Ln3+ onto PAA-S HNFs by Pseudofirst-order, Pseudo-second-order and Intra-particle diffusion model.


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Scheme 1. Illustration for the fabrication procedure of PAA-S hydrogel nanofibers (PAA-S HNFs) for the adsorption of Ln3+ and its photoluminescent performance.


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A

B

2µm

2µm

20 µm

20 µm

C

D

2µm

20 µm

2µm

20 µm

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 %).


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A

B

Page 38 of 50

C

D

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


38

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A

B

C

D

E

Figure 3. (A, C) XPS wide-scan spectra and (B, D) C1s core-level spectrum of the uncrosslinked and cross-linked PAA-S nanofibers. (E) The proposed thermal cross-linking mechanism for the esterification reaction of PAA and SiO2.


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A

C

B

5 µm

D

5 00nm

E

5 µm

F

5 00nm

I

H

G

5 µm

J

5 00nm

L

K

5 µm

5 00nm

N

M

5 µm

Page 40 of 50

O

5 00nm

Figure 4. FE-SEM images of (A, B) cross-linked PAA-S nanofibers, (D, E) PAA-S HNFs, and (G, H) La3+, (J, K) Eu3+, (M, N) Tb3+ loaded PAA-S HNFs. (C, F, I, L, O) The corresponding fiber diameter distributions.


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B

A

C

D

Figure 5. (A) The loss situation of PAA-S HNFs in water with different pH values. The PAA-S

HNFs were stable in the acidic environments of pH 1 (B), 6 (C), and were almost disintegrated in basic environment with pH 10 (D).


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A

B

C

D

Page 42 of 50

Figure 6. The effect of (A) pH values, (B) initial concentration and (C) contact time on the

adsorption capacity of PAA-S HNFs for Ln3+. (D) The regeneration rate of PAA-S HNFs over 5 adsorption/desorption cycles of Ln3+.


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

adsorption.


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A

E

B

F

C

G

D

H

Page 44 of 50

I

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 O1s core-level spectrum. (I) The possible adsorption mechanism of the PAA-S HNFs and Ln3+.


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A

B

C

D

60µm

60µm

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.


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A

A-Eu B-Eu/Tb C-Tb

Page 46 of 50

B

C

Eu

UV Light 254nm

Eu/Tb

Tb

Eu

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.


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Table 1. The constant parameters of Langmuir, Freundlich and D-R isotherm model for adsorption of Ln3+ onto PAA-S HNFs.

Adsorbates

Langmuir model

D-R isotherm model

Freundlich model

qo (mg/g)

b( L/mg)

R2

KF(mg/g)

qm(mg/g )

β

E(kJ/mol)

R2D-R

La3+

232.6

0.0752

0.9997

83.9

5.6405 0.9359

322.6

0.00154

18.2

0.9831

Eu3+

268.8

0.0773

0.9992

122.8

7.6063 0.9859

357.8

0.00134

19.34

0.9941

Tb3+

250.0

0.0850

0.9994

119.4

8.0302 0.9799

330.6

0.00126

19.92

0.9844

n

R2


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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) 155.1 (La3+) 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


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Table 3. Adsorption kinetic parameters for the adsorption of Ln3+ onto PAA-S HNFs by Pseudo-first-order, Pseudo-second-order and Intra-particle diffusion model.

Adsorbates

Pseudo-first-order model k1(h-1) qe( mg/g )

R2

Pseudo-second-order model k2(g/mg h) qe( mg/g )

Intra-particle diffusion model

R2

kp1(mg/g h)

R2p1

kp1(mg/g h)

R2p2

La3+

0.9710

45.84

0.8979

0.0397

229.36

0.9997

78.40

0.9948

3.26

0.9978

Eu3+

0.8856

36.51

0.8866

0.0530

267.38

0.9998

40.59

0.9951

2.86

0.9729

Tb3+

1.038

50.55

0.9297

0.0408

250

0.9998

74.22

0.9916

4.77

0.9725


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TOC Graphic


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Silica Hydrogel Nanofibers Scaffold for

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