Electrospinning of Calixarene-Functionalized Polyacrylonitrile

Aug 28, 2013 - 2 Experimental Section ..... be important to identify an alternative desorption agent or strategy. .... Cal[8], Ester-Cal[8], and Amide...
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Electrospinning of Calixarene-Functionalized Polyacrylonitrile Nanofiber Membranes and Application as an Adsorbent and Catalyst Support Ming Chen,†,‡ Chengjiao Wang,†,‡ Wei Fang,†,‡ Jing Wang,†,‡ Wang Zhang,†,‡ Gong Jin,†,‡ and Guowang Diao*,†,‡ †

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China Key Laboratory of Environmental Materials & Environmental Engineering of Jiangsu Province, Yangzhou, Jiangsu, 225002, P. R. China

‡

S Supporting Information *

ABSTRACT: Polyacrylonitrile (PAN) nanofiber membranes functionalized with calix[8]arenes (C[8]) were successfully prepared by electrospinning of PAN solutions with addition of various calixarenes. Uniform electrospun C[8]/PAN nanofibers were obtained by incorporating three types of calix[8]arenes into the PAN matrix and characterized by scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared (ATR-FTIR), thermal gravimetric analysis (TGA), and X-ray powder diffraction (XRD). The SEM results showed that the addition of calix[8]arenes resulted in a decrease in the diameter of PAN nanofibers. Static adsorption behavior was studied by using C[8]/PAN nanofibers as an adsorbent and Congo red and Neutral red as model dye molecules. The adsorption of Congo red onto Amide-Cal[8]-15/PAN nanofibers fitted the second-order kinetic model, and the apparent adsorption rate constant was 1.1 × 10−3 g·mg−1·min−1 at 25 °C. Then, by virtue of electrostatic attraction, as-prepared Au nanoparticles were immobilized on Amide-Cal[8]/PAN nanofibers to form Au/Amide-Cal[8]/PAN composite nanofibers. The catalytic activity of the as-prepared Au/Amide-Cal[8]/PAN composite nanofibers was investigated by monitoring the reduction of Congo red in the presence of NaBH4. The reduction kinetics was explained by the assumption of a pseudo-firstorder reaction with regard to Congo red. Au/Amide-Cal[8]/PAN composite nanofibers exhibited high catalytic activity, excellent stability, and convenient recycling.

1. INTRODUCTION The electrospinning technique has become a versatile method for producing multifunctional nanofibers from various polymers, polymer blends, and composites.1−3 Because of many unique nanofibrous properties such as high surface area-tovolume, pore size within a nano range, high porosity, and flexibility for chemical/physical functionalization, nanofibers are used in numerous areas, including catalysis,4,5 biosensors,6 and power sources.7,8 Supramolecular chemistry, defined as “chemistry outside a molecule”, is at the heart of the development of the chemistry of complex systems, molecular devices, ensembles, and nanochemistry.9 Recently, cyclodextrin-functionalized polymeric nanofibers have been studied and widely reported.10−18 Cyclodextrins have been incorporated in electrospun polymeric nanofibers in various forms, including CDs,10−13 CD-inclusion complexes (ICs),14 and CD-pseudopolyrotaxanes.15 The CD and CD-ICs electrospun fibers have been prepared without using a polymeric carrier matrix.16−18 However, as the third generation host compound, calix[n]arene-functionalized poly© 2013 American Chemical Society

mer nanofibers have not been reported until now. Calix[n]arenes are cyclic oligomers synthesized by condensation of a palkylated phenol and formaldehyde. Calixarenes have a central cavity, the dimensions of which can be adjusted to allow formation of inclusion compounds with a great variety of guests, from charged molecules such as anions19,20 and metallic cations to apolar compounds.21−23 This versatility makes the calixarene family the third major class of macrocyclic binding agents after crown-ethers and cyclodextrins. Nowadays many researchers are interested in modifying calixarenes on all kinds of materials, such as nanoparticles,24 resin,25−27 starch,28 and silica gel,29,30 to obtain new composite materials and use as luminescent probes,24 solid phase extractant,25 sorbent,26−30 catalyzer,31 and transducer.32 Therefore, simple nanofibers containing calixarenes should result in composites having a range of useful and interesting properties. Received: October 24, 2012 Revised: August 23, 2013 Published: August 28, 2013 11858

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Figure 1. Chemical structures of (a) p-tert-butylcalix[8]arene (Cal[8], (b) ester-t-butylcalix[8]arene (Ester-Cal[8]), and (c) amide-tbutylcalix[8]arene (Amide-Cal[8]).

Table 1. Properties of Electrospinning Solutions, Fiber Morphology, and Fiber Diameter solutions PAN Cal[8]/PAN

Ester-Cal[8]/PAN

Amide-Cal[8]/PAN

a

weight (wt %) a

8 5b 10 15 20 5 10 15 20 5 10 15 20

fiber morphology

fiber diameter (nm)

zero shear η0(mPa·s)

± ± ± ± ± ± ± ± ± ± ± ± ±

1098 1196 1473 1691 1762 1374 1466 1501 1847 1281 1590 1659 1745

nanofibers nanofibers nanofibers nanofibers beaded nanofibers nanofibers nanofibers nanofibers nanofibers nanofibers nanofibers nanofibers nanofibers

670 421 390 480 520 420 390 390 430 390 370 350 450

163 98 95 104 147 81 83 78 99 82 70 76 82

conductance (μS/cm) 68.9 86.9 88.9 91.7 86.9 78.0 75.9 75.9 74.0 81.9 81.0 81.2 79.9

± ± ± ± ± ± ± ± ± ± ± ± ±

2.0 1.8 2.0 3.2 3.5 1.6 1.2 2.1 2.7 2.3 3.1 2.4 2.7

With respect to solvent (DMF). bWith respect to the polymer (PAN).

The chemical structures of two dyes are shown in Figure S1. Furthermore, Amide-Cal[8]-15/PAN nanofibers were used as catalyst support to immobilize Au nanoparticles by virtue of electrostatic interactions to form Au/Amide-Cal[8]/PAN composite. Finally, the reduction of Congo red with NaBH4 was used to evaluate the catalytic activity of the as-prepared Au/Amide-Cal[8]/PAN composite nanofibers. In this paper, Amide-Cal[8]/PAN nanofibers applied as catalyst support improved the catalytic activity and stability of Au heterogeneous catalysts.

Recently, nanoscale metal particles have become of great interest stemming from their unique catalytic, optical, and electronic properties, which lead to their wide application in various fields, such as catalysis, optoelectronic devices, and sensors. Concerning catalysis applications of metal particles, their high activity comes from the unique crystalline structures, special shapes, and large specific surface areas. However, in most cases, especially in catalytic processes, the aggregation of metal nanoparticles and the separation and recovery of the metal nanoparticles in the reaction mixture solution limit the practical application. To overcome this difficulty, noble metal nanoparticles have been immobilized on a solid support.33−37 In this way, aggregation can be efficiently prevented by dispersing noble nanoparticles finely on supports having a high surface area and good stability, such as polymers,33 silica,34 carbon nanotubes,35 graphene,36 and metal oxides37 and electrosun nanofibers.38−41 In this paper, polyacrylonitrile is chosen as the base material. The nanocomposite fibers based on polyacrylonitrile have been reported widely.42−45 Here we have used the electrospinning technique to develop calixarene-functionalized PAN nanofibers. Three types of calix[8]arenes: Cal[8], Ester-Cal[8], and AmideCal[8] (see Figure 1) were incorporated individually in the PAN nanofibers, and the weight load of calixarenes in the polymer matrix was varied from 5% up to 20% (w/w). We will in the present paper mainly deal with the optimization of electrospinning conditions to produce uniform C[8]/PAN nanofibers with different calixarene content. We find that these nanofibers are used as adsorbent to readily capture organic dye−Congo red (anionic dye) and Neutral red (cationic dye).

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Three types of calix[8]arenes (Cal[8], Ester-Cal[8], Amide-Cal[8]) were synthesized according to the literature.46−48 Polyacrylonitrile (PAN, 99%, MW 150 000) was procured from Aldrich. N,N-Dimethylformamide (DMF) and other reagents were purchased from Shanghai Chemical Reagents Company (China). 2.2. Characterization. The morphologies of the calix[8]arenes functionalized PAN (C[8]/PAN) nanofibers were investigated by a field emission scanning electron microscopy (Hitachi, S-4800). The average fiber diameter (AFD) was determined from the scanning electron microscopy (SEM) images, and around 100 fibers were analyzed. The surface of C[8]/PAN nanofibers (thickness >100 μm) was analyzed by attenuated total reflection Fourier transform infrared (ATR-FTIR). The thermal stability of the nanofibrous membranes was analyzed by thermal gravimetric analysis (TGA) at 30−600 °C with 10 °C/min. The X-ray powder diffraction (XRD) was taken by M03XFH (MAC Science) X-ray diffractometer operated at a voltage of 40 kV and a current of 40 mA with Cu Kα radiation (λ = 0.154056 nm). Steady-state viscosities of electrospinning solutions were measured at 25 °C by a rotational viscometer (Thermo, RS600). The viscosity 11859

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measurements were repeated three times to check the reproducibility and the consistency of the viscosity reading. Conductivities of electrospinning solutions were measured by digital display type electrical conductivity meter (Shanghai Lei-ci, DDS-11A). The UV− vis spectra were recorded on a UV-2550 (Shimazu, Japan) doublebeam spectrophotometer equipped with a stoppered quartz cell with a 1.0 cm optical path length. The transmission electron microscope (TEM) measurements were carried out using a TEM instrument (Philips, Tecnai 12). 2.3. Electrospinning. The homogeneous solutions used for electrospinning were prepared by dissolving calixarenes and PAN in DMF at room temperature under stirring for 2 h. The concentration of PAN was 8 wt %. The amount of calixarenes was 0, 5, 10, 15, or 20 wt % with respect to PAN. The concentrations of PAN and calixarenes are shown in Table 1. The different concentrated homogeneous calixarene solutions were put into a 10 mL plastic syringe fitted with a metallic needle of 0.7 mm inner diameter. The syringe was fixed horizontally with a syringe pump (Baoding longer, LSP01-1A) and the electrode of a high voltage power supply (Tianjin Dongwen, DWLP303-1ACDB) was connected to the metal needle tip. The working distance between the needle tip and the ground electrode was 15 cm. The solution flow rate was 1 mL/h. The electrospinning voltage was 20 kV. The electrospinning temperature and the relative humidity were 25 °C and 50%, respectively. 2.4. Adsorption Experiment. Batch adsorption kinetic experiments of Congo red onto C[8]/PAN nanofibers were performed by agitating 50 mL of dye solution (42 mg·L−1, pH 7.0) with fresh C[8]/ PAN nanofibers (0.02 g) in glass bottles using a laboratory shaker at 160 rpm and room temperature (25 ± 1 °C). The adsorption quantity of Congo red onto C[8]/PAN nanofibers at different adsorption times was estimated by monitoring the residual Congo red in the solution at the maximum absorption wavelength (496 nm) using a UV-2501 spectrophotometer. The equilibrium adsorption quantity of dye, qe (mg·g−1), was calculated using the following relationship:

qe =

(c0 − ce)V W

3. RESULTS AND DISCUSSION 3.1. Characterization of Calix[8]arene-Functionalized PAN Nanofibers (C[8]/PAN). Morphologies of the pure PAN, Cal[8]/PAN, Ester-Cal[8]/PAN, and Amide-Cal[8]/PAN nanofibers were observed by SEM, and the results are shown in Figures 2−4. Figure 2 presents the SEM images of

(1)

where c0 was the initial concentration of dye (mg·L−1), ce was the equilibrium concentration of dye (mg·L−1), V was the volume of the solution (L), and W was the mass of C[8]/PAN nanofibers (g). 2.5. Synthesis of Au/Amide-Cal[8]/PAN Composite Nanofibers. An electrostatic attraction method was employed for the immobilization of Au nanoparticles onto the Amide-Cal[8]-15/PAN nanofibers to form the Au/Amide-Cal[8]/PAN composite nanofibers. First, the Amide-Cal[8]-15/PAN fibers (0.1 g) were immersed in hydrochloric acid (2 M, 100 mL) at room temperature for 24 h. Subsequently, an aqueous solution of Au nanoparticles with an average diameter of 3 nm, and a negative surface charge was prepared according to the literature.49 Finally, the positively charged AmideCal[8]/PAN nanofibers were added to the Au solution (100 mL) under sonication at room temperature for 30 min resulting in electrostatic association of the Au nanoparticles on the surface of the Amide-Cal[8]/PAN nanofibers. The resultant Au/amide-Cal[8]/PAN composite nanofibers were removed from the solution by tweezers, rinsed successively with H2O three times, and dried under vacuum at 60 °C. 2.6. Catalytic Performance Test. In a typical procedure, 20 mg of Au/Amide-Cal[8]/PAN composite nanofibers were added to 20 mL of Congo red aqueous solution (6.0 × 10−5 mol/L). Subsequently, the above solution was mixed with fresh NaBH4 aqueous solution (2 mL, 0.1 mol/L). The reaction was carried out at 25 °C with continuous stirring. The progress of the conversion reaction was then monitored by recording the time-dependent UV−vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was separated from reaction system via a tweezers, washed three times with ethanol, and dried at room temperature for the next cycle.

Figure 2. SEM images of electrospun nanofibers from 8 wt % PAN solutions at different weight % of t-butylcalix[8]arene with respect to PAN: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, and (e) 20 wt %; the insets show higher magnification images. The fiber diameter distributions are shown on the right.

electrospun nanofibers obtained from homogeneous solutions of 8 wt % PAN in DMF with the addition of 0%, 5%, 10%, 15%, and 20% Cal[8] (w/w, with respect to PAN). Regular PANnfm with an average fiber diameter (AFD) of about 670 nm are evident in Figure 2a. Compared with the PAN nanofibers, it was found that the morphological structures of nanofibers were not changed in Cal[8]/PAN system from Figure 2b to e. The average fiber diameters (AFD) of nanofibers are summarized in 11860

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Figure 3. SEM images of electrospun nanofibers from 8 wt % PAN solutions at different weight % of ester-t-butylcalix[8]arene with respect to PAN: (a) 5 wt %, (b) 10 wt %, (c) 15 wt %, and (d) 20 wt %; the insets showed higher magnification images. The fiber diameter distributions are shown on the right side.

Figure 4. SEM images of electrospun nanofibers from 8 wt % PAN solutions at different weight % of amide-t-butylcalix[8]arene with respect to PAN: (a) 5 wt %, (b) 10 wt %, (c) 15 wt %, and (d) 20 wt %; the insets showed higher magnification images. The fiber diameter distributions are shown on the right side.

Table 1. When the amount of Cal[8] was 10% (w/w), the AFD reached the minimum value (390 nm). As the amount of Cal[8] in Cal[8]/PAN system was increased to 20%, the AFD was increased to 520 nm. In Figure 2e, a few small shuttleshaped beads were mingled in with uniform nanofibers, and the surface of the nanofibers appeared rough. The results showed that the addition of Cal[8] did not affect the morphology of the PAN nanofibers, but influenced the AFD of nanofibers. These results contrast with those of β-cyclodextrin systems such as βCD/PEO, β-CD/PMMA, β-CD/Ps, and β-CD/PVA,10−13 where β-CD was found to affect the morphology of electrospun nanofibers through weak intermolecular interactions. Similar findings were observed when Ester-Cal[8] and Amide-Cal[8] were used instead of Cal[8]. From Figure 3a to d, uniform nanofibers with AFD of about 390−430 nm were obtained. In Figure 4, when the amount of Amide-Cal[8] was 15% (w/w), the value of AFD was 350 nm, which was the smallest value in all nanofibers. Compared with the surface of Cal[8]/PAN nanofibers, the surface of Ester-Cal[8]/PAN and Amide-Cal[8]/PAN was much rounder and smoother. To confirm the presence of calix[8]arenes on the fiber surface, the Cal[8]/PAN, Ester-Cal[8]/PAN, and Amide-

Cal[8]/PAN nanofibers were analyzed by a surface sensitive technique, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy.10 The ATR-FTIR spectra of (a) PAN, (b) Cal[8]-15/PAN, (c) Ester-Cal[8]-15/PAN, and (d) Amide-Cal[8]-15/PAN nanofibers are depicted in Figure 5.

Figure 5. ATR-FTIR spectra of (a) PAN, (b) Cal[8]-15/PAN, (c) Ester-Cal[8]-15/PAN, and (d) Amide-Cal[8]-15/PAN nanofibers. 11861

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were present in the fiber matrix.10−13 Figure 7 shows the XRD spectra of PAN, Amide-Cal[8], and Amide-Cal[8]/PAN

Figure 5a exhibits typical bands for PAN. These include stretching vibrations of CH and CH2 groups at 2800−3000 cm−1, intense stretching vibration of CN at 2243 cm−1, and CH/CH2 deformation vibrations at 1250−1500 cm−1. In Figure 5b, the absorption bands observed for Cal[8]-15/PAN nanofibers at around 3462 and 1667 cm−1 (corresponding to stretching vibration of −OH and the phenyl plane bending vibration, respectively) confirmed that Cal[8] molecules were present on the surface of the nanofibers. Similar findings were observed when Ester-Cal[8] and Amide-Cal[8] were used instead of Cal[8]. Figure 5c exhibits typical ATR-FTIR spectrum of Ester-Cal[8]-15/PAN. The absorption bands at around 1755 cm−1 attributed to the −CO indicated that Ester-Cal[8] molecules were present on the surface of the PAN nanofibers. In Figure 5d, the absorption bands observed for Amide-Cal[8]-15/PAN nanofibers at around 3405 and 1755 cm−1 (corresponding to stretching vibration of −NH and −CO, respectively) confirmed that Amide-Cal[8] molecules were present on the surface of the nanofibers. In addition, Amide-Cal[8] displayed the typical stretching vibrations of amide band I at about 1674 cm−1 and amide band II at around 1525 cm−1. The thermal stability of the nanofibers was determined by thermogravimetric analysis in a temperature range of 25−600 °C.50,51 Figure 6 shows the TG curves of (a) PAN, (b) Cal[8]-

Figure 7. XRD spectra of (a) PAN, (b) Amide-Cal[8]-5/PAN, (c) Amide-Cal[8]-10/PAN, (d) Amide-Cal[8]-15/PAN, (e) AmideCal[8]-20/PAN, and (f) Amide-Cal[8] nanofibers.

nanofibers. In Figure 7a, PAN nanofibers show two XRD diffraction patterns with 2θ values of 16.9 and 28.8, respectively. In Figure 7b,c, when a small quantity of AmideCal[8] (5% and 10% with respect to PAN) was mixed into the PAN matrix, the XRD diffraction patterns of the AmideCal[8]/PAN nanofibers were very similar to those for the PAN nanofibers. The lack of any significant XRD diffraction peak for Amide-Cal[8]/PAN nanofibers indicated that the majority of the Amide-Cal[8] molecules were distributed homogeneously in the PAN matrix without forming any crystal aggregates. However, in Figure 7d,e, two weak diffraction peaks at 14.2 and 19.3 are observed and suggest the presence of crystalline Amide-Cal[8] at the highest mixing ratios. When Cal[8] and Ester-Cal[8] were used instead of Amide-Cal[8], similar findings were observed (data not shown). In the electrospinning process, solution viscosity is a very important parameter for the formation of fibers.16−18 To understand the influence of the solution viscosity of electrospinning in the presence of calix[8]arenes, the rheological properties of PAN and C[8]/PAN solutions were studied. Figure S3 shows the shear rate dependence of viscosity for a 8 wt % PAN solution containing different amounts of calix[8]arenes, and Table 1 summarizes the results. With increasing concentration of Cal[8], zero shear (η0) values of Cal[8]/PAN solutions increased. Similar findings were observed when EsterCal[8] and Amide-Cal[8] were used instead of Cal[8]. In addition, Table 1 summarizes the conductivity of the solutions. It was found that the conductivities of C[8]/PAN solutions were higher than pure PAN solution. This is most likely due to the polar functional groups on the calix[8]arenes, specifically −OH, −COOR, and −CO−NH−. Among the four kinds of electrospinning solutions, the order of the conductivity was Cal[8]/PAN > Amide-Cal[8]/PAN > Ester-Cal[8]/PAN > PAN. However, with increasing concentration of C[8], the conductivities of C[8]/PAN solutions were almost unchanged, indicating that the conductivity was not the main reason for the change of nanofiber diameter. 3.2. Dye Adsorption. The adsorption capacity of Congo red onto PAN, Cal[8]-15/PAN, Ester-Cal[8]-15/PAN, and Amide-Cal[8]-15/PAN at different adsorption times are shown in Figure 8A. PAN nanofibers have almost no adsorption capacity for dye, whereas calixarene-modified nanofibers

Figure 6. TGA curves of (a) PAN, (b) Cal[8]-15/PAN, (c) EsterCal[8]-15/PAN, and (d) Amide-Cal[8]-15/PAN nanofibers. Inset: expansion of data from 250 to 350 °C.

15/PAN, (c) Ester-Cal[8]-15/PAN, and (d) Amide-Cal[8]-15/ PAN. In Figure 6a, three main weight loss regions are observed for PAN nanofibers. The weight loss at about 100 °C is mainly due to small molecule (e.g., solvent) evaporation. The rapid weight loss of pure PAN nanofibers continued from 290 to 350 °C, which was due to the side chain degradation of the polymer. The weight loss from 350 to 600 °C was mainly caused by decomposition of the carbon−carbon main chain. Compared with pure PAN nanofibers, it is clearly seen from the inset of Figure 6 that Cal[8]-15/PAN, Ester-Cal[8]-15/PAN, and Amide-Cal[8]-15/PAN nanofibers begin to lose weight at slightly higher temperatures, ca. 310 °C, 306 °C, and 295 °C, respectively, demonstrating a stabilizing effect of calix[8]arenes additives into the polymer. The thermal decomposition behaviors of the three calixarene derivatives in the absence of PAN are higher than PAN alone (Figure S2), consistent with the higher thermal stabilities of the C[8]/PAN nanofiber composites. XRD studies were performed for the C[8]/PAN nanofibers to investigate whether any calix[8]arene crystalline aggregates 11862

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Figure 8. (A) Adsorption kinetics of Congo red on different nanofibers at 25 °C. (B) Adsorption kinetics of Congo red for different compositions of Amide-Cal[8]/PAN nanofibers at 25 °C.

Figure 9. (a) Adsorption kinetics of Neutral red on Amide-Cal[8]-15/PAN nanofibers at 25 °C. (A) The relationship between ln(qe − qt) versus t. (B) The relationship between tqt−1 versus t. (C) The relationship between qt versus t1/2. Data taken from Figure 8.

exhibited increased adsorption with time during the first 80 min after which equilibrium was reached. The equilibrium adsorption capacity of Amide-Cal[8]-15/PAN was approximately 20% greater than for the other calixarene-modified nanofibers and was chosen as the main absorbent in this study. The relationship between adsorption capacity of Congo red on different composition of Amide-Cal[8]/PAN nanofibers is shown in Figure 8B. With increasing amount of Amide-Cal[8], the adsorption capacity increased by approximately 5−10% for each increase in Amide-Cal[8] content. The adsorption kinetic behavior of Neutral red onto C[8]/PAN nanofibers was also studied. As shown in Figure S4 (Supporting Information), Neutral red could be complexed by the calixarene-functionalized nanofibers, although adsorption equilibrium was reached somewhat slower than for Congo red. Many kinetic models have been developed to determine the intrinsic kinetic adsorption constants. Traditionally, the kinetics of metal ion adsorption was described following the expressions originally given by Lagergren which were special cases for the general Lagergren rate equation. A simple kinetic analysis of adsorption is the pseudo-first-order equation in the form52 ln(qe − qt ) = −k1t + ln qe

The possibility of intraparticle diffusion was explored by using the intraparticle diffusion model,54 which is expressed as

qt = k 3t 1/2 + C

where qt (mg·g−1) is the amount of Congo red adsorbed at time t (min), k3 is the initial rate of the intraparticle diffusion (mg· g−1·min1/2), and C is the intercept. According to the adsorption data of Amide-Cal[8]-15/PAN nanofibers, the fitting validity of these models was checked by each linear plot of ln(qe − qt) versus t, (t/qt) versus t, and qt versus t1/2, respectively. Figure 9A, B, and C showed the fits to the three kinetic models: the pseudo-first-order, pseudosecond-order, and intraparticle diffusion adsorption, respectively. From correlation coefficients, the value of R2 (0.996) in the second-order kinetic model was larger than the first-order kinetic model value (0.900) and intraparticle diffusion kinetic model value (0.908), which indicated that the adsorption of Congo red onto Amide-Cal[8]-15/PAN fit a pseudo-secondorder reaction. From the slope and intercept of the straight line (Figure 9B), the pseudo-second-order rate constant k2 was evaluated as 1.1 × 10−3 g·mg−1·min−1. The fitting results of adsorption of Neutral red onto Amide-Cal[8]-15/PAN are shown in Figure S5, which implied that the adsorption of Neutral red onto Amide-Cal[8]-15/PAN also corresponds to a pseudo-second-order reaction. Regeneration is one of the most important characteristics of a sorbent, which may save cost, simplify manipulation of sewage treatment, and enhance disposal efficiency of dye wastewater. The regeneration ability of Amide-Cal[8]-15/PAN was studied, and the details of desorption and regeneration experiment are presented in Supporting Information. In the regeneration experiment, ethanol was chosen as the desorption agent, and the desorption efficiency was close to 80%, which indicated that the adsorption site cannot completely recover due to simple

(2)

−1

where k1 (min ) is the adsorption rate constant and qe and qt (mg·g−1) are the adsorption capacities of dye adsorbed at equilibrium and at any time, respectively. A pseudo-second-order equation based on adsorption equilibrium capacity may be expressed as follows53 t 1 t = + qt qe k 2qe 2

(4)

(3)

where k2 (g·mg−1·min−1) is the adsorption rate constant of a pseudo-second-order equation. 11863

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Figure 10. TEM images of Au/Amide-Cal[8]/PAN with different resolution.

Figure 11. At 25 °C, UV−visible spectra of Congo red catalytically reduced by Au/Amide-Cal[8]/PAN composite nanofibers in the presence of excess NaBH4. Reaction condition: 10 mg Au/Amide-Cal[8]/PAN catalysts; 20 mL, 6.0 × 10−5 mol/L Congo solution. (B) First-order kinetic plot of Congo red reduction catalyzed by Au/Amide-Cal[8]/PAN composite nanofibers.

ethanol treatment. The sorbent was used five times repeatedly, and the adsorption and desorption efficiency are shown in Figure S6. It was clearly found that the adsorption saturation efficiency decreased with each successive trial. After five cycles, the adsorption capacity was reduced more than 50%. To improve the regeneration efficiency, it will be important to identify an alternative desorption agent or strategy. 3.3. Supporting Au Nanoparticles on Amide-Cal[8]/ PAN Nanofibers. The adsorption step described above represents only a transfer of pollutants from the source to the sorbent. Additional steps such as chemical catalytic reduction or oxidation, photocatalysis, electrochemical degradation, or biological methods will be needed to degrade the captured pollutant. To incorporate catalytic functionality to our materials, Amide-Cal[8]/PAN nanofibers were used as a support on which to immobilize Au nanoparticles. Through the process of acidification, the protonation of −CO−NH− of Amide-Cal[8] resulted in Amide-Cal[8]/PAN nanofibers taking a net positive change, which was expected to electrostatically attract Au nanoparticles to the nanofibers. Figure 10 shows TEM images of Au/Amide-Cal[8]/PAN composite nanofibers. It is clear that the Au nanoparticles are dispersed along the surface of the nanofibers (Figure 10A). No individual nanoparticles are found in the solution, indicating that the adsorption of the noble metal nanoparticles took place completely. The Amide-Cal[8]/PAN nanofibers maintained their original fiber shape. The TEM image with higher

resolution (Figure 10B) showed that all of the Au nanoparticles (3 nm) are uniform in size and well-dispersed on the surface with only some aggregation, which was attributed to the nonuniform distribution of Amide-Cal[8] on the surface of nanofibers. 3.4. Catalytic Properties of Amide-Cal[8]/PAN/Au Composite Nanofibers. To evaluate the catalytic performance of the as-prepared Au/Amide-Cal[8]/PAN composite nanofibers, the reduction of Congo red by NaBH4 was chosen as a model reaction, which can be easily monitored by UV−vis absorption spectroscopy because no side reactions exist. If the aqueous solution of Congo red was mixed with a large excess of NaBH4, the color of Congo red remained unaltered for 1 h, which indicated that the reduction of Congo red was insignificant. However, when the solution of Congo red was reduced by NaBH4 in the presence of the Au/Amide-Cal[8]/ PAN composite, the color of Congo red disappeared within 20 min, confirming the catalytic properties of the material (Figure 11). Owing to the presence of excess NaBH4, the reduction rate was assumed to be independent of the NaBH4 concentration, allowing a pseudo-first-order kinetic equation to be applied. The linear relationship between ln(ct/c0) and reaction time (Figure 11B) determined the reaction rate constant to be 0.22 min−1 at 25 °C. The catalytic reduction mechanism of Au/Amide-Cal[8]/ PAN composite nanofibers likely follows several steps. Initially, NaBH4 adsorbs onto the Au nanoparticle surface to donate 11864

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electrons. Simultaneously, Congo red was captured by AmideCal[8] and adsorbed onto the nanofiber surface. Finally, Congo red accepted electrons and was reduced and dissociated from nanofibers to create a free space for the reaction to continue.55 In this case, Amide-Cal[8] might play two roles during the catalytic process. On the one hand, under the acidic conditions, the protonation of −CO−NH− of Amide-Cal[8] resulted in Amide-Cal[8]/PAN nanofibers taking positive change. Then, electrostatic interactions occurred between Amide-Cal[8]/PAN nanofibers and negatively charged Au nanoparticles resulting in Au/Amide-Cal[8]/PAN composite nanofibers. Therefore, Amide-Cal[8] functional PAN nanofibers may be used as a support to immobilize catalytic metal nanoparticles. On the other hand, the molecular recognition properties of the AmideCal[8] hosts could be used to capture dye molecule and shorten the distance between Au nanoparticles (catalyst) and dye molecule (substrate), which should effectively enhance the reaction rate.56−58 The stability and recycling of the catalyst were also investigated. The greatest advantage of the Au/AmideCal[8]/PAN composite catalyst in this work was that the separation could be easily achieved using tweezers. The reactions catalyzed by Au/Amide-Cal[8]/PAN were first monitored by UV−vis spectrometer until their completion. Then, the catalyst was physically separated using tweezers and rinsed with deionized water and ethanol for the next cycle of catalysis. After five cycles of the catalysis reaction, decolorization (%) of dye solution was almost unchanged, which demonstrated that Au/Amide-Cal[8]/PAN composite catalyst exhibited high catalytic activity, excellent stability, and convenient recycling.

Article

ASSOCIATED CONTENT

* Supporting Information S

Desorption and regeneration experiment, structure of Congo red and Neutral red, TGA of the calixarene derivatives, rheological properties of electrospinning solutions, adsorption kinetic of Neutral red on nanofibers, and adsorption and desorption efficiency. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +86 514 87975436. Fax: +86 514 87975244. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledged the financial support from the National Natural Science Foundation of China (Grant No. 20901065, 20973151, and 21273195), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Natural Science Foundation of Education Committee of Jiangsu Province (12KJB150023) and the New Century Talents Project of Yangzhou University. The authors also acknowledge the Testing Center of Yangzhou University for TEM, SEM, and XRD experiments.

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REFERENCES

(1) Guo, Z. H.; Wei, S. Y.; Qu, H. L. Coaxial Electrospun Nanostructures and Their Applications. J. Mater. Chem. A, DOI: 10.1039/C3TA12390A. (2) Zhu, J. H.; Wei, S. Y.; Patil, R.; Rutman, D.; Kucknoor, A. S.; Wang, A.; Guo, Z. H. Ionic Liquid Assisted Electrospinning of Quantum Dots/Elastomer Composite Nanofibers. Polymer 2011, 52, 1954−1962. (3) Chen, M. J.; Qu, H. L.; Zhu, J. H.; Luo, Z. P.; Khasanov, A.; Kucknoor, A. S.; Haldolaarachchige, N.; Young, D. P.; Wei, S. Y.; Guo, Z. H. Magnetic Electrospun Fluorescent Polyvinylpyrrolidone Nanocomposite Fibers. Polymer 2012, 53, 4501−4511. (4) Zhang, Z. Y.; Shao, C. L.; Li, X. H.; Zhang, L.; Xue, H. M.; Wang, C. H.; Liu, Y. C. Electrospun Nanofibers of ZnO-SnO2 Heterojunction with High Photocatalytic Activity. J. Phys. Chem. C 2010, 114, 7920− 7925. (5) Zhu, P. N.; Nair, A. S.; Peng, S. J.; Yang, S. Y.; Ramakrishna, S. Facile Fabrication of TiO2−Graphene Composite with Enhanced Photovoltaic and Photocatalytic Properties by Electrospinning. ACS Appl. Mater. Interfaces 2012, 4, 581−585. (6) Wang, X. Y.; Kim, Y. G.; Drew, C.; Ku, B. C.; Kumar, J.; Samuelson, L. A. Electrostatic Assembly of Conjugated Polymer Thin Layers on Electrospun Nanofibrous Membranes for Biosensors. Nano Lett. 2004, 4, 331−334. (7) Yu, Y.; Gu, L.; Zhu, C. B.; van Aken, P. A.; Maier, J. Tin Nanoparticles Encapsulated in Porous Multichannel Carbon Microtubes: Preparation by Single-Nozzle Electrospinning and Application as Anode Material for High-Performance Li-Based Batteries. J. Am. Chem. Soc. 2009, 131, 15984−15985. (8) Viet, A. L.; Reddy, M. V.; Jose, R.; Chowdari, B. V. R.; Ramakrishna, S. Nanostructured Nb2O5 Polymorphs by Electrospinning for Rechargeable Lithium Batteries. J. Phys. Chem. C 2010, 114, 664−671. (9) Beletskaya, I.; Tyurin, V. S.; Tsivadze, A. Y.; Guilard, R.; Stern, C. Supramolecular Chemistry of Metalloporphyrins. Chem. Rev. 2009, 109, 1659−1713.

4. CONCLUSIONS Electrospinning of calix[8]arene functionalized PAN nanofibers (C[8]/PAN) was carried out with the goal to develop functional nanofibrous membranes. The uniform electrospun C[8]/PAN nanofibers were obtained by incorporating three types of calix[8]arenes: Cal[8], Ester-Cal[8], and Amide-Cal[8] into a PAN matrix. The SEM results showed that the addition of calix[8]arenes resulted in a decrease in the diameter of PAN nanofibers. Furthermore, ATR-FTIR studies revealed that some calixarene molecules were located on the surface of the fiber networks. In this paper, Amide-Cal[8]/PAN nanofibers were applied as adsorbent and catalyst support. The adsorption of Congo red and Neutral red onto Amide-Cal[8]-15/PAN fibers fitted a second-order kinetic model, and the apparent adsorption rate constant for Congo red was 1.1 × 10−3 g· mg −1 ·min −1 at 25 °C. Finally, Au/Amide-Cal[8]/PAN composite nanofibers were prepared by electrostatic attraction and immobilizing Au nanoparticles on Amide-Cal[8]/PAN nanofibers. The catalytic activity of the Au/Amide-Cal[8]/PAN composite nanofibers was investigated by monitoring the reduction of Congo red in present of NaBH4. Significantly, Au/Amide-Cal[8]/PAN composite nanofibers exhibited high catalytic activity, excellent stability, and convenient recycling. These results indicated that Amide-Cal[8] functionalized nanofibers may not only be utilized as molecular filters for the removal of organic wastes but also as catalyst supports to immobilize metal nanoparticles. 11865

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(30) Thompson, A. B.; Cope, S. J.; Swift, D. T.; Notestein, J. M. Adsorption of n-Butanol from Dilute Aqueous Solution with Grafted Calixarenes. Langmuir 2011, 27, 11990−11998. (31) Sahin, O.; Erdemir, S.; Uyanik, A.; Yilmaz, M. Enantioselective Hydrolysis of (R/S)-Naproxen Methyl Ester with Sol−Gel Encapculated Lipase in Presence of Calix[n]arene Derivatives. Appl. Catal. A: Gen. 2009, 369, 36−41. (32) Ohira, S. I.; Wanigasekara, E.; Rudkevich, D. M.; Dasgupta, P. K. Sensing Parts Per Million Levels of Gaseous NO2 by a Optical Fiber Transducer Based on Calix[4]arenes. Talanta 2009, 77, 1814−1820. (33) Miao, S. D.; Zhang, C. L.; Liu, Z. M.; Han, B. X.; Xie, Y.; Ding, S. J.; Yang, Z. Z. Highly Efficient Nanocatalysts Supported on Hollow Polymer Nanospheres: Synthesis, Characterization, and Applications. J. Phys. Chem. C 2008, 112, 774−780. (34) Fazzini, S.; Nanni, D.; Ballarin, B.; Cassani, M. C.; Giorgetti, M.; Maccato, C.; Trapananti, A.; Aquilanti, G.; Ahmed, S. I. Straight forward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads. J. Phys. Chem. C 2012, 116, 25434− 25443. (35) Sanles-Sobrido, M.; Correa-Duarte, M. A.; Carregal-Romero, S.; Rodríguez-González, B.; Á lvarez-Puebla, R. A.; Hervés, P.; Liz-Marzán, L. M. Highly Catalytic Single-Crystal Dendritic Pt Nanostructures Supported on Carbon Nanotubes. Chem. Mater. 2009, 21, 1531−1535. (36) Moussa, S.; Siamaki, A. R.; Gupton, B. F.; El-Shall, M. S. PdPartially Reduced Graphene Oxide Catalysts (Pd/PRGO): Laser Synthesis of Pd Nanoparticles Supported on PRGO Nanosheets for Carbon−Carbon Cross Coupling Reactions. ACS Catal. 2012, 2, 145− 154. (37) Shekhar, M.; Wang, J.; Lee, W. S.; Williams, D.; Kim, S. M.; Stach, E. A.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. Size and Support Effects for the Water −Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al2O3 and TiO2. J. Am. Chem. Soc. 2012, 134, 4700−4708. (38) Demir, M. M.; Gulgun, M. A.; Menceloglu, Y. Z.; Erman, B.; Abramchuk, S. S.; Makhaeva, E. E.; Khokhlov, A. R.; Matveeva, V. G.; Sulman, M. G. Palladium Nanoparticles by Electrospinning from Poly(acrylonitrile-co-acrylic acid)−PdCl2 Solutions. Relations between Preparation Conditions, Particle Size, and Catalytic Activity. Macromolecules 2004, 37, 1787−1792. (39) Horzum, N.; Muñoz-Espí, R.; Glasser, G.; Demir, M. M.; Landfester, K.; Crespy, D. Hierarchically Structured Metal Oxide/ Silica Nanofibers by Colloid Electrospinning. ACS Appl. Mater. Interfaces 2012, 4, 6338−6345. (40) Kim, T. G.; Ragupathy, D.; Gopalan, A. I.; Lee, K. P. Electrospun Carbon Nanotubes−Gold Nanoparticles Embedded Nanowebs: Prosperous Multi-Functional Nanomaterials. Nanotechnology 2010, 21, 134021. (41) Wang, J.; Yao, H. B.; He, D.; Zhang, C. L.; Yu, S. H. Facile Fabrication of Gold Nanoparticles-Poly(vinyl alcohol) Electrospun Water-Stable Nanofibrous Mats: Efficient Substrate Materials for Biosensors. ACS Appl. Mater. Interfaces 2012, 4, 1963−1971. (42) Chen, I.-H.; Wang, C.-C.; Chen, C.-Y. Fabrication and Structural Characterization of Polyacrylonitrile and Carbon Nanofibers Containing Plasma-Modified Carbon Nanotubes by Electrospinning. J. Phys. Chem. C 2010, 114, 13532−13539. (43) Kampalanonwat, P.; Supaphol, P. Preparation and Adsorption Behavior of Aminated Electrospun Polyacrylonitrile Nanofiber Mats for Heavy Metal Ion Removal. ACS Appl. Mater. Interfaces 2010, 2, 3619−3627. (44) Feng, J. Z.; Zhang, C. R.; Feng, J.; Jiang, Y. G.; Zhao, N. Carbon Aerogel Composites Prepared by Ambient Drying and Using Oxidized Polyacrylonitrile Fibers as Reinforcements. ACS Appl. Mater. Interfaces 2011, 3, 4796−4803. (45) Zhu, J. H.; Wei, S. Y.; Rutman, D.; Haldolaarachchige, N.; Young, D. P.; Guo, Z. H. Magnetic Polyacrylonitrile-Fe@FeO Nanocomposite Fibers - Electrospinning, Stabilization and Carbonization. Polymer 2011, 52, 2947−2955. (46) Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishana, R. Calixarenes. 4. The Synthesis, Characterization, And Properties of the

(10) Uyar, T.; Besenbacher, F. Electrospinning of Cyclodextrin Functionalized Polyethylene Oxide (PEO) Nanofibers. Eur. Polym. J. 2009, 45, 1032−1037. (11) Uyar, T.; Balan, A.; Toppare, L.; Besenbacher, F. Electrospinning of Cyclodextrin Functionalized Poly(methyl methacrylate) (PMMA) Nanofibers. Polymer 2009, 50, 475−480. (12) Uyar, T.; Havelund, R.; Hacaloglu, J.; Besenbacher, F.; Kingshott, P. Functional Electrospun Polystyrene Nanofibers Incorporating and β-Cyclodextrins: Comparison of Molecular Filter Performance. ACS Nano 2010, 4, 5121−5130. (13) Zhang, W.; Chen, M.; Diao, G. Electrospinning β-Cyclodextrin/ Poly(vinyl Alcohol) Nanofibrous Membrane for Molecular Capture. Carbohydr. Polym. 2011, 86, 1410−1416. (14) Uyar, T.; Nur, Y.; Hacaloglu, J.; Besenbacher, F. Electrospinning of Functional Poly(Methyl Methacrylate) Nanofibers Containing Cyclodextrin-Menthol Inclusion Complexes. Nanotechnology 2009, 20, 125703. (15) Uyar, T.; Kingshott, P.; Besenbacher, F. Electrospinning of Cyclodextrin-Pseudopolyrotaxane Nanofibers. Angew. Chem., Int. Ed. 2008, 47, 9108−9111. (16) Celebioglu, A.; Uyar, T. Cyclodextrin Nanofibers by Electrospinning. Chem. Commun. 2010, 46, 6903−6905. (17) Celebioglu, A.; Uyar, T. Electrospinning of Polymer-free Nanofibers from Cyclodextrin Inclusion Complexes. Langmuir 2011, 27, 6218−6226. (18) Zhang, W.; Chen, M.; Zha, B. B.; Diao, G. W. Correlation of Polymer-Like Solution Behaviors with Electrospun Fiber Formation of Hydroxypropyl-Β-Cyclodextrin and the Adsorption Study on the Fiber. Phys. Chem. Chem. Phys. 2012, 14, 9729−9737. (19) Staffilani, M.; Hancock, K. S. B.; Steed, J. W.; Holman, K. T.; Atwood, J. L.; Juneja, R. K.; Burkhalter, R. S. Anion Binding within the Cavity of π-Metalated Calixarenes. J. Am. Chem. Soc. 1997, 119, 6324− 6335. (20) Perret, F.; Nishihara, M.; Takeuchi, T.; Futaki, S.; Lazar, A. N.; Coleman, A. W.; Sakai, N.; Matile, S. Anionic Fullerenes, Calixarenes, Coronenes, and Pyrenes as Activators of Oligo/Polyarginines in Model Membranes and Live Cells. J. Am. Chem. Soc. 2005, 127, 1114− 1115. (21) Ling, I.; Alias, Y.; Skelton, B. W.; Raston, C. L. Conformationally Flexible p-Sulfonated Calix[5]arene Binding with Large BisPhosphonium Cations. Cryst. Growth Des. 2012, 12, 1564−1570. (22) Tolpekina, T. V.; den Otter, W. K.; Briels, W. J. Influence of a Captured Solvent Molecule on the Isomerization Rates of Calixarenes. J. Phys. Chem. B 2003, 107, 14476−14485. (23) Bayrakcı, M.; Ertul, S.; Yilmaz, M. Phase Solubility Studies of Poorly Soluble Drug Molecules by Using O-Phosphorylated Calixarenes as Drug-Solubilizing Agents. J. Chem. Eng. Data 2012, 57, 233−239. (24) Li, H. B.; Qu, F. G. Synthesis of CdTe Quantum Dots in Sol− Gel-Derived Composite Silica Spheres Coated with Calix[4]arene as Luminescent Probes for Pesticides. Chem. Mater. 2007, 19, 4148− 4154. (25) Adhikaria, B. B.; Gurunga, M.; Kawakitaa, H.; Jumina; Ohtoa, K. Methylene Crosslinked Calix[6]Arene Hexacaarboxylic Acid Resin: A Highly Efficient Solid Phase Extractant for Decontamination of Lead Bearing Effluents. J. Hazard. Mater. 2011, 193, 200−208. (26) Kamboh, M. A.; Solangi, I. B.; Sherazi, S. T. H.; Memon, S. Synthesis and Application of Calix[4]arene Based Resin for the Removal of Azo Dyes. J. Hazard. Mater. 2009, 172, 234−239. (27) Oshima, T.; Saisho, R.; Ohe, K.; Baba, Y.; Ohto, K. Adsorption of Amino Acid Derivatives on Calixarene Carboxylic Acid Impregnated Resins. React. Funct. Polym. 2009, 69, 105−110. (28) Chen, M.; Shang, T.; Fang, W.; Diao, G. W. Study on Adsorption and Desorption Properties of the Starch Grafted p-tertButyl-Calix[n]arene for Nutyl Rhodamine B Solution. J. Hazard. Mater. 2011, 185, 914−921. (29) Chen, M.; Chen, Y.; Diao, G. W. Adsorption Kinetics and Thermodynamics of Methylene Blue onto p-Tert-butyl-calix[4,6,8]arene-bonded Silica Gel. J. Chem. Eng. Data 2010, 55, 5109−5116. 11866

dx.doi.org/10.1021/la4017799 | Langmuir 2013, 29, 11858−11867

Langmuir

Article

Calixarenes from p-tert-Butylphenol. J. Am. Chem. Soc. 1981, 103, 3782−3792. (47) Arnaud-Neu, F.; Collins, E. M.; Deasy, M.; Ferguson, G.; Harris, S. J.; Kaitner, B.; Lough, A. J.; McKervey, M. A.; Marques, E.; Ruhl, B. L.; Schwing-Weill, M. J.; Seward, E. M. Synthesis, X-ray Crystal Structures, and Cation-Binding Properties of Alkyl Calixaryl Esters and Ketones, A New Family of Macrocyclic Molecular Receptors. J. Am. Chem. Soc. 1989, 111, 8681−8691. (48) Sénèque, O.; Rager, M. N.; Giorgi, M.; Reinaud, O. Calix[6]arenes and Zinc: Biomimetic Receptors for Neutral Molecules. J. Am. Chem. Soc. 2000, 122, 6183−6189. (49) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Two-Dimensional Arrays of Colloidal Gold Particles: A Flexible Approach to Macroscopic Metal Surfaces. Langmuir 1996, 12, 2353−2361. (50) Zhang, X.; Alloul, O.; He, Q. L.; Zhu, J. H.; Verde, M. J.; Li, Y. T.; Wei, S. Y.; Guo, Z. H. Strengthened Magnetic Epoxy Nanocomposites with Protruding Nanoparticles on the Graphene Nanosheets. Polymer 2013, 54, 3594−3604. (51) Zhang, X.; He, Q. L.; Gu, H. B.; Colorado, H. A.; Wei, S. Y.; Guo, Z. H. Flame-Retardant Electrical Conductive Nanopolymers Based on Bisphenol F Epoxy Resin Reinforced with Nano Polyanilines. ACS Appl. Mater. Interfaces 2013, 5, 898−910. (52) Lagergren, S. Zur Theorie der Sogenannten Adsorption Gelöster Stoffe. K. Sven. Vetenskapsakad. Handl. 1898, 24, 1−39. (53) Ho, Y. S.; McKay, G. Pseudo-Second Order Model for Sorption Processes. Process Biochem. 1999, 34, 451−465. (54) Weber, W. J.; Morris, J. C. Kinetics of Adsorption of Carbon from Solution. J. Sanit. Eng. Div. Am. Soc. Civil Eng. 1963, 89, 31−59. (55) Jiang, Z. J.; Liu, C. Y.; Sun, L. W. Catalytic Properties of Silver Nanoparticles Supported on Silica Spheres. J. Phys. Chem. B 2005, 109, 1730−1735. (56) Alvarez, J.; Liu, J.; Román, E.; Kaifer, A. E. Water-Soluble Platinum and Palladium Nanoparticles Modified with Thiolated βCyclodextrin. Chem. Commun. 2000, 1151. (57) Liu, J.; Alvarez, J.; Ong, W.; Román, E.; Kaifer, A. E. Tuning the Catalytic Activity of Cyclodextrin-Modified Palladium Nanoparticles through Host-Guest Binding Interactions. Langmuir 2001, 17, 6762− 6764. (58) Strimbu, L.; Liu, J.; Kaifer, A. E. Cyclodextrin-Capped Palladium Nanoparticles as Catalysts for the Suzuki Reaction. Langmuir 2003, 19, 483−485.

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