inorganic hybrid mesoporous silica membrane

Journal of Membrane Science 457 (2014) 9–18

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Studies on the meso-sized selectivity of a novel organic/inorganic hybrid mesoporous silica membrane Yong Chen n, Shujuan Bian, Kui Gao, Yuanyang Cao, Hongqing Wu, Chuanxiang Liu, Xuheng Jiang, Xiaoling Sun School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 August 2013 Received in revised form 16 January 2014 Accepted 18 January 2014 Available online 26 January 2014

A novel free-standing organic/inorganic hybrid mesoporous silica membrane (HMSM) composed of mesoporous silica rods inside the channels of porous polyethylene terephthalate (PET) membrane was synthesized by employing the aspiration-induced infiltration method combined with solvent extraction. As-synthesized HMSM, in short PET-templated HMSM, was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), N2 adsorption–desorption, thermal gravimetric analysis (TG) and molecule permeation measurements. The results indicate that the PET-templated HMSM presents the structural integrity of membrane material, the mesoporosity of hybrid mesoporous material, and the size-selectivity of nanofilter between small molecule and biomacromolecule. Moreover, the ion transfer behaviors occurring at the liquid/liquid (water/1, 2-dichloroethane) interface supported by PET-templated HMSM were investigated. It is found that such a HMSM-support liquid/liquid interface presents the asymmetric diffusion field with meso-sized selectivity between small ion and biomacromolecule. Both of the molecule permeation and the ion transfer experiment results demonstrate that the PET-templated HMSM can be applied as the nanofiltration membrane not only in aqueous solution but also at the membrane-supported liquid/liquid interface due to the meso-sized selectivity of mesopores formed in the channels of PET. This work firstly extends the applications of HMSMs with unique structure of pores-in-pores to the membrane-supported liquid/liquid interface electrochemistry and the sizeselective ion transfer at the liquid/liquid interface. & 2014 Elsevier B.V. All rights reserved.

Keywords: Hybrid mesoporous membrane Molecule permeation Ion transfer Liquid/liquid interface

1. Introduction During the past decade, lots of free-standing hybrid mesoporous silica membranes composed of mesoporous silica materials inside the confined channels of porous membranes have attracted much attention [1–34] due to their unique structure with pores-in-pores [4,23] and wide applications in the membrane-based syntheses of nanomaterials [6,11,20], adsorption and catalysis [15,18,31], as well as sensors [24,34] and so on. In particular, some hybrid mesoporous silica membranes, in short HMSMs, have presented attractively potential application as the nanofiltration membranes [2,19,22,32, 33]. For example, Yamaguchi and co-workers firstly reported the application of a HMSM as the size-selective nanofiltration membrane [2], which could be further applied in the nanofiltration of nanoparticles and proteins [19,22]. However, such a HMSM seems to be too thick (60 μm) for molecular sieving, which results in much too small molecule transport flux (
2 nmol h  1 cm  2) and limits

n

Corresponding author. Tel./fax: þ 86 21 6087 3565. E-mail address: [email protected] (Y. Chen).

0376-7388/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2014.01.035

its practical application in the nanofiltration as pointed out by Martin [4]. This problem can be solved by employing organic porous membranes as the hard templates to obtain the HMSMs with smaller membrane thickness (9 μm) than that as mentioned above, which can enhance the molecule transport flux [32,33]. Until now, a large number of HMSMs have been synthesized by using two kinds of porous membranes as the hard templates on the basis of various methods including sol–gel method [1,5,7,8,26], aspiration-induced infiltration method [2,14,29,30,32], vapor phase synthesis [12,25], counter diffusion self assembly method [27] and microwave-assisted method [33,34]. According to the employed hard templates, those prepared HMSMs can be classified as two categories. One is the inorganic HMSMs composed of mesoporous materials in the channels of inorganic porous anodic alumina membranes (PAA) employed as the hard templates, that is PAA-templated HMSMs [1–25]. The other is the organic/inorganic HMSMs composed of mesoporous materials in the channels of organic polycarbonate (PC) filtration membranes, namely PCtemplated HMSMs [26–34]. As an organic hard templates used in the syntheses of HMSMs, PC membranes possess some advantages over PAA membranes such as smaller membrane thickness, better

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mechanical flexibility and chemical stability under the acid or basic condition [29–33], but relatively fewer examples of PCtemplated HMSMs [26–34] than that of PAA-templated HMSMs [1–25] have been reported so far, which should be mainly related to some problems or obstacles found in the syntheses of organic/ inorganic HMSMs. Firstly, it often takes a few days to fulfill entire synthesis by some methods such as the sol–gel method [26] and the counter diffusion self assembly method [27], which is not beneficial for the rapid and substantial fabrication of organic/ inorganic HMSMs. Secondly, most of reports on the syntheses of PC-templated HMSMs have adopted the calcination under high temperature to remove surfactants from membranes [26–30], whereas the structural integrity of those as-prepared membranes have been completely destroyed by the calcination due to the low temperature of heat-resistance of PC (o140 1C), which definitely impedes their further applications in the field of membrane science. Therefore, it is necessary to further explore other methods to rapidly synthesize such organic/inorganic HMSMs with structural integrity from the standpoint of their practical applications in the membrane-based filtration and separation. In our recent work, some PC-templated HMSMs have been rapidly synthesized by the aspiration-induced infiltration method [31,32] or the microwave-assisted method [33,34]. Meanwhile, in order to solve the problem related to the calcination as discussed above, solvent extraction instead of calcination has successfully been used to remove surfactants from those PC-templated HMSMs without the destruction of PC and the contraction of silicate framework caused by calcination, which leads to the structural integrity of PC-templated HMSMs [31–34]. Therefore, assynthesized PC-templated HMSMs could be further applied to the membrane-based enzyme adsorption [31], nanofiltration [32,33] and electrochemical catalysis and biosensor [34]. Furthermore, it is found that the transport flux of molecule across PCtemplated HMSMs is larger than that across PAA-templated HMSM mainly owing to much smaller membrane thickness of PC than that of PAA and the removing of surfactants from HMSMs by solvent extraction [32,33]. However, there is an obvious obstacle hindering the wider application of PC-templated HMSMs in the nanofiltration that it is difficult for PC-templated HMSMs to be used in the environment containing organic solvents including the system of liquid/liquid interface because PC is apt to be solved in many organic solvents, such as 1, 2-dichloroethane (DCE). In fact, it has been found that some free-standing porous membranes with organic resistance including track-etched polymer [35], PAA [36,37], zeolite [38] membranes, and silicon micropore array chips [39,40] could be applied to support liquid/liquid interface and study the ion transfer behaviors at the liquid/liquid interface (L/L) also called as oil/water interface (O/W) or the interface between two immiscible electrolyte solutions (ITIES) [41]. Nevertheless, there has been no report on the ion transfer behaviors across the L/L interface supported by such novel hybrid mesoporous silica membranes with unique structure of pores-in-pores until now although they have been widely applied as nanofiltration membranes in aqueous solution [2,19,22,32,33]. Additionally, from the standpoint of detection methods employed for the applications of HMSMs in the nanofiltration, it is difficult to monitor the permeation processes of some substances without characteristic absorption group through such HMSMs only by using UV–vis spectroscopy. Fortunately, the transfer of some ionized substances without characteristic absorption group across the L/L interface can produce the characteristic electrochemical responses, such as multifarious voltammetric curves, as long as the electrochemical responses for the transfer of those targeted ions can be observed within electrochemical window. Therefore, it is expected that the electrochemical methods used in the L/L interface electrochemistry

will play an important role in the studies on the size-selectivity of such HMSMs and help to extend the application range of HMSMs in the membrane-based filtration and separation. Moreover, as far as the present applications of HMSMs as nanofiltration membranes to be concerned, two experimental setups are often used to examine the size-selectivity of HMSMs and the potential applications of HMSMs in the nanofiltration. One is the U-tube without the additional pressure provided by outer vacuum pump [2,33] and the other is the conventional filtration apparatus connected with vacuum pump at moderate pressure [19,22]. If high pressure is applied on those organic/inorganic HMSMs, the possible problems related with the fracture of membrane or the leak of silica rods from the membrane during filtration should be considered because the mechanical strength of organic/inorganic HMSMs is a remarkable problem due to the properties of organic membranes. Thus, the assistance of electric field instead of high pressure employed in the HMSMs-supported liquid/liquid interface will provide a new method to avoid the possible problems related to the application of organic/inorganic HMSMs in the nanofiltration as mentioned above. Herein, a novel organic/inorganic HMSM was synthesized by employing an organic porous membrane with good resistance to organic solvent, polyethylene terephthalate (PET) membrane, as the hard template (abbreviated as PET-templated HMSM) on the basis of the aspiration-induced infiltration method combined with solvent extraction [31,32]. Additionally, the processes of sizeselective molecule permeation through PET-templated HMSM in aqueous solution were investigated according to the method used in our previous work on PC-templated HMSM [33]. Moreover, such a novel organic/inorganic HMSM was further applied to support liquid/liquid interface and investigate the size-selective ion transfer behaviors occurring at the HMSM-supported liquid/liquid interface. To our knowledge, it is the first time to explore the potential application of such free-standing hybrid mesoporous silica membranes in the size-selective ion transfer across the membrane-supported liquid/liquid interface.

2. Experimental 2.1. Synthesis and characterization 2.1.1. Aspiration-induced infiltration method combined with solvent extraction The preparation of precursor solution and the procedure for the synthesis of a PET-templated HMSM by using PET as the hard template are similar with the previous report on the PC-templated HMSM [32]. In brief, the precursor solution containing cetyltrimethylammonium bromide (CTAB), tetraethylorthosilicate (TEOS) was prepared as follows: a mixed solution containing ethanol (7.7 g), TEOS (11.6 g) and 1 ml of HCl (2.8 mM) was refluxed at 60 1C for 90 min. Then, ethanol (15 g), 4 ml of HCl solution (55 mM) and CTAB (1.5 g) were added to the refluxed solution, which was stirred for 30 min to give the precursor solution. Subsequently, the precursor solution was dropped onto a porous polyethylene terephthalate (PET) membrane with pore diameter of 0.5 μm, thickness of 5 μm and porosity of
1% (Nuclepore tracketch membrane, 47 mm, Nuclepore membrane Company of Haoxia, China), which was set in an ordinary membrane filtration apparatus. Under moderate aspiration, two milliliters of the precursor solution filtrated across the membrane, and the membrane was dried for 5 min under vacuum at room temperature. After that, a HMSM containing CTAB was prepared and here called as CTAB-HMSM. Such a CTAB-HMSM was ultrasonically rinsed in doubly distilled water for 15 min to clean the surface of membrane and then extracted by acidic alcohol solution for about 1 h

Y. Chen et al. / Journal of Membrane Science 457 (2014) 9–18

to remove CTAB from HMSM as reported previously [32,33]. After the solvent extraction, the membrane was dried in an oven under 60 1C, a PET-templated HMSM was finally obtained. According to the photos of bare PET membrane and as-prepared PET-templated HMSM with diameter of 47 mm (Fig. S1), it is obvious that PETtemplated HMSM is almost same flat, but not smooth, as bare PET membrane. 2.2. Characterization SEM images were obtained on a HITACHI-3400 equipped with Quantax-400 EDS (Bruker. Ins. Germany). N2 adsorption–desorption measurements were conducted on a micrometrics ASAP2020 instrument (Micromeritics, USA), and the PET-templated HMSMs were cut into small pieces of about 5–6 mm square and placed into the measurement cell. The thermal gravimetric (TG) and differential scanning calorimetric (DSC) analyses were done on a thermogravimetric/differential thermal analyzer (PerkinElmer Pyris Diamond TG/ DTA, USA). The atmosphere used in TG and DSC is air and the heating rate is 10 1C/min. The compactness of PET-templated HMSM was investigated through the molecule permeation experiments with a U-tube molecule transport setup. As shown in Fig. S2, a bare PET membrane or PET-templated HMSM cut with diameter of 18 mm was glued with two glassy tubes (outer diameter¼18 mm and inner diameter¼ 14 mm) by the silicon sealant [33]. One tube containing feed solution is feed half-cell, and the other tube containing permeation solution is permeation half-cell. The feed solutions (15 mL) contains 0.20 mmol L  1 rhodamine B or 8.0  10  3 mmol L  1 horse radish peroxidase (HRP) in a 100 mM phosphate buffer solution prepared by NaH2PO4 and Na2HPO4 with the same concentration (pH 7.0), and the permeation solution (15 mL) only contains the same phosphate buffer solution. The experiments of molecule permeation through membranes were measured by recording UV– vis absorption spectra as a function of time in the permeation halfcell by using a Cary100 UV–vis Spectrophotometer (Varian, USA). 2.3. Ion transfer experiments at the HMSM-supported liquid/liquid interface In order to preliminarily study ion transfer behaviors occurring at the liquid/liquid interface supported by a novel porous membrane, water/1, 2-dichloroethane (W/DCE) interface with extensive studies in the field of liquid/liquid interface electrochemistry [35–39,41–43], and two conventional systems of ion transfer, that is simple ion transfer (IT) of tetraethylammonium (TEA þ ) and facilitated ion transfer (FIT) of K þ by dibenzo-18-crown-6 (DB18C6) [35–39,41], are adopted in this study. As for the sizeselective ion transfer across the HMSM-supported liquid/liquid interface, Cytochrome c (Cyt c), a protein attracting much attention in the studies of adsorption or transfer of proteins at the liquid/liquid interface [43–47], is selected as the targeted biomacromolecule, and double-ion transfer behaviors of two cations with different ion size including small ion TEA þ (diameter
0.62 nm [38]) and Cyt c in aqueous solution with pH
7.0 (charge
þ 9 [43] and hydrodynamic diameter
3.6 nm [47]) are investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The HMSM-supported W/DCE interface is polarized using four-electrode potentiostats (CHI660D, CHI, USA). The homemade four-electrode electrochemical cell is similar to those previous reports [35,36,38] (see Fig. S3), where a pair of Ag/ AgCl electrodes (one in each phase) work as reference electrodes controlling the potential and a pair of Pt-wire electrodes (one in each phase) act as counter electrodes measuring the current. The working electrode is just the membrane-supported liquid/liquid interface.

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All the cell setups employed for voltammetric ion transfer studies can be shown as below. In cell 1, cell 2 and cell 3, the membrane is PET-templated HMSM. In cell 4, the membrane is bare PET or PET-templated HMSM. The organic phase electrolyte salt was bis(triphenylphosphoranylidene)-ammonium tetraphenylborate (BTPPA þ TPB  ) prepared by using bis(triphenylphosphoranylidene)-ammonium chloride (BTPPA þ Cl  , 97%, Aldrich, USA) and sodium tetraphenylborate (Na þ TPB  , 98%, J&KChemica, China) [35,36]. All other chemicals were of analytical grade, which were purchased from Sinopharm, China. High-resistivity distilled water was used to prepare all aqueous solutions.

3. Results and discussion 3.1. Characterization of as-synthesized PET-templated HMSM 3.1.1. SEM and EDS measurements Fig. 1a and b respectively shows the SEM images of the topview on bare PET and PET-templated HMSM. Comparing Fig. 1b with a, it can be observed from the surface of membrane that lots of rods form inside the channels of PET membrane after aspiration-induced infiltration and solvent extraction, which is similar to the phenomenon as observed previously on PCtemplated HMSMs [32]. Notably, it is found that those rods are closely wrapped by PET membrane without obvious gaps between rods and PET channels, which should be owing to that solvent extraction can efficiently prevent the contraction of silica framework as reported previously [31–34]. However, as for the PAAtemplated HMSMs synthesized by using the aspiration-induced infiltration method, the rod-like materials cannot be observed on the surface of membranes and those rods only form in the middle of channels of PAA [2,14]. Moreover, the gaps between silica rods and PAA channels are common for those PAA-templated HMSMs after calcination due to the contraction of silica rods in PAA [8,14,23], which is a key problem for their practical application as nanofilters not only in aqueous solution but also at the liquid/ liquid interface. After completely etching PET to release those rods from membrane, as shown in Fig. 1c and d, a number of onedimensional rods can be collected. According to the insets of Fig. 1b–d, the diameter and the length of those rods formed in the channels of PET can be respectively estimated about 500 nm and 5 μm, which completely accords to the pore structure of PET membrane (Fig. 1a and Fig. S4). Additionally, the elemental composition of PET-templated HMSM and those rods observed above was determined by EDS analyses and mappings. As shown

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Fig. 1. SEM images of the surface of (a) bare PET and (b) PET-templated HMSM; (c, d) the SEM images of released silica rods from PET-templated HMSM. The insets in b, c and d are the SEM images with higher resolution.

Fig. 2. EDS data of the surface of (a) bare PET and (b) PET-templated HMSM.

in Fig. 2b, in addition to the component elements of PET (Fig. 2a), that is carbon and oxygen, silicon can be found on the surface of PET-templated HMSM, which indicates those rods as shown in Fig. 1b contains silicon. On the contrary, silicon cannot be found on the surface of PAA-templated HMSM due to the formation of silica rods in the middle of PAA channels [2]. Different EDS datum of PET-template HMSM from that of PAA-templated HMSM completely agrees with the structural difference between PAAtemplated HMSM and PET-templated HMSM as discussed above. Moreover, EDS mappings further show that those rods formed inside the channels of PET (Fig. S5a–c) or released from membrane (Fig. S5d–f) are composed of silicon and oxygen, which demonstrates that those rods found in as-prepared PET-templated HMSM should be silica rods according to the previous reports [31–34].

3.1.2. N2 adsorption–desorption analysis The mesoporosity of PET-templated HMSM was investigated by N2 adsorption–desorption measurement. As shown as curve A in Fig. 3, the isotherm curve of bare PET membrane (curve A) can be classified as the type II adsorption–desorption behavior for macroporous materials according to the previous reports [33,34,48]. Whereas, the isotherm curve of as-synthesized PET-templated HMSM (curve B) presents the typical type-IV mesopore adsorption–desorption behavior and the inflection position in p/po is close to the previous reports on the HMSMs obtained by using surfactant CTAB as the soft template (CTAB-templated HMSM) [2,32,33]. The adsorption and desorption of PET-templated HMSM is difference from bare PET membrane at the range of pressure area (0–0.7), which should be due to the filling of mesoporous silica rods inside the channels of PET [33,34] and the capillary

Y. Chen et al. / Journal of Membrane Science 457 (2014) 9–18

condensation taking place in mesopores [48]. Therefore, such a PET-templated HMSM can be considered as an organic/inorganic hybrid mesoporous silica membrane due to the formation of inorganic mesoporous silica rods inside the channels of organic PET membrane. The average pore diameter of PET-templated HMSM is estimated as 3.0 nm based on Barret–Joyner–Halenda (BJH) analysis (the inset in Fig. 3), which almost agrees to the results reported previously on other CTAB-templated HMSMs [2,32,33]. The surface area and the pore volume of PETtemplated HMSM are respectively 55 m2 g  1 and 0.049 cm3 g  1, both of which are less than that of PAA-templated HMSMs [2] because of the lower porosity of PET than that of PAA resulting in less mesoporous silicas formed in PET-templated HMSMs and thus lower values of the surface area and the pore volume of PETtemplated HMSMs than PAA-templated HMSMs, which could be also found in the previous reports on PC-templated HMSMs [32,33].

3.1.3. Thermal gravimetric analysis According to the previous reports, thermal gravimetric analysis can be used to examine the effect of solvent extraction on the removing of surfactant from mesoporous silica powders [49–51] and PC-templated HMSM [33,34]. Fig. 4 shows the results of TG and DSC (the inset) analyses of a bare PET, PET-templated HMSM with or without solvent extraction. As shown in Fig. 4, there is obvious difference between the TG curve of PET-templated HMSM with solvent extraction (curve B) and that of PET-templated HMSM

Fig. 3. N2 adsorption/desorption isotherms of bare PET (curve A) and PETtemplated HMSM (curve B) (The inset is the corresponding BJH size distribution of PET-templated HMSM.).

Fig. 4. TG and DSC (the inset) curves of bare PET (curve A, dash line), PETtemplated HMSM with (curve B, dot line) or without (curve C, solid line) solvent extraction.

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without solvent extraction (curve C). The TG curve of the HMSM without solvent extraction presents three main processes of weight loss respectively around 282 1C, 441 1C and 504 1C as indicated by the peaks in the DSC curve (the solid line in the inset of Fig. 4). As reported previously [27,33], those main processes of weight loss should respectively correspond to the decomposition of surfactant at a temperature between 200 1C and 300 1C and the decomposition of polymer between 350 and 550 1C. After 550 1C, the weight of the sample remains constant, which indicates that complete decomposition of polymer and surfactant. However, in the case of the PET-templated HMSM with solvent extraction, there are not obvious weight loss in TG curve and the characteristic peak in DSC curve (the dot line in the inset of Fig. 4) corresponding to the decomposition of surfactant around 282 1C as observed in the TG and DSC curves for the PET-templated HMSM without solvent extraction, and the TG curve of the PETtemplated HMSM with solvent extraction (curve B) almost overlap with that of bare PET (curve A) when the temperature is less than 300 1C, which demonstrates that surfactants can be efficiently removed from such PET-templated HMSMs by solvent extraction as reported previously on mesoporous silica powders [50,51] and PC-templated HMSMs [31–34]. 3.2. Size-selective molecule permeation through and ion transfer across PET-templated HMSM 3.2.1. Size-selective molecule permeation through PET-templated HMSM in aqueous solution Fig. 5a and b respectively shows the UV–vis spectra and the corresponding photos (the insets) of permeation solution for the transport of rhodamine B or HRP through PET-templated HMSM after 4 h. As shown in Fig. 5a, the permeation solution becomes red (the inset) and the absorption peak of rhodamine B (λmax: 552 nm) can be detected in the permeation solution. Additionally, the UV–vis absorbance of rhodamine B in permeation solution increases with time (Fig. S6a). All those results indicate that rhodamine B can transport across PET-templated HMSM. As for biomacromolecule, HRP, as shown in Fig. 5b, the permeation solution keeps colorless (the inset) and the absorption peak of HRP (λmax: 402 nm) is not detected in the permeation solution within 4 h, while it can be detected in the permeation solution when bare PET instead of PET-templated HMSM is used as the membrane filter (Fig. 5c), which illuminates that HRP can transport across bare PET membrane but not across PET-templated HMSM. The different permeation behaviors between HRP and rhodamine B through PET-templated HMSM should be attributed to their different molecule sizes and the existence of mesopores in PET-templated HMSM. The molecule size of rhodamine B (
1.0 nm) is less than the mesopore size (
3.0 nm) of PETtemplated HMSM, but biomacromolecule HRP with asymmetrically three-dimensional structure (
6.0 nm  3.5 nm  3.0 nm) [52,53] is larger than that. As a result, small molecule rhodamine B, not biomacromolecule HRP, can transport through PETtemplated HMSM, which shows that PET-templated HMSM present size-selectivity between small molecule and biomacromolecule. In addition, the double-molecule permeation through PET-templated HMSM shows that only the characteristic absorption peak of rhodamine B can be observed in the permeation solution (Fig. S6b), which further confirms that only small molecule rhodamine B in the mixed solution can transport through PET-templated HMSM. All above results not only demonstrates the permeability and the meso-sized selectivity of PET-templated HMSM owing to the existence of mesopores in HMSM as reported previously on PAA-templated HMSMs [2] and PC-templated HMSMs [32,33], but also further verifies that such a novel organic/inorganic HMSM is gap-free because biomacromolecule

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Fig. 5. The UV–vis spectra and the corresponding photos (the insets) of permeation solution for (a) rhodamine B (0.2 mM in aqueous solution, λmax: 552 nm), (b) HRP (8.0  10  3 mM in aqueous solution, λmax: 402 nm) transport through PET-templated HMSM and (c) HRP transport through bare PET after 4 h. (d) Transport flux plots of HRP (square), rhodamine B (star), as well as rhodamine B (filled circle) in the mixed solution (0.2 mM rhodamine B and 8.0  10  3 mM HRP in aqueous solution) through PETtemplated HMSM (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.).

should be detected in the permeation solution during the process of molecule permeation if there are gaps between silica rods and PET channels as discussed previously on PC-templated HMSMs [32,33], which is significant for the followed studies on the sizeselective ion transfer across the liquid/liquid interface supported by PET-templated HMSM. Moreover, the transport flux of rhodamine B through PETtemplated HMSM was investigated. According to Fig. 5d, the transport flux of rhodamine B in single-molecule permeation through PET-templated HMSM is evaluated to be 7 nmol h  1 cm  2, which is larger than that for the double-molecule permeation (3 nmol h  1 cm  2). The difference of transport flux of rhodamine B between single-molecule and double-molecule permeation across PET-templated HMSM should be due to the transport hindrance caused by HRP as reported previously [32]. Notably, the transport flux of rhodamine B is much larger than the value of rhodamine B through PAA-templated HMSM [2,4]. On the contrary, the lag time of rhodamine B through PET-templated HMSM (30 min) is lower than that reported on PAA-templated HMSM [2]. Both of higher molecule transport flux and lower lag time of rhodamine B through PET-templated HMSM than that through PAA-templated HMSM indicate that it is easier for molecules with suitable size to transport through PET-templated HMSM than PAAtemplated HMSM as previously reported on the PC-template HMSMs [32,33], which should be mainly ascribed to much smaller membrane thickness of PET-templated HMSM (5 μm) than that of PAA-templated HMSM (60 μm) [2] since the transport flux (J) of permeate molecule through a porous membrane is inversely proportional to the membrane thickness, namely J ¼ DKCaq/l where l is the membrane thickness and D, K, Caq are respectively

the diffusion coefficient of the molecule within the membrane, the partition coefficient for the molecule between the aqueous feed solution and the membrane, as well as the concentration of the molecule in the feed solution [54].

3.2.2. Size-selective ion transfer across W/DCE interface supported by PET-templated HMSM Since PET-templated HMSM presents the meso-sized selectivity for the molecule permeation in aqueous solution, it should be possible to observe the meso-sized selective ion transfer behaviors occurring at the HMSM-supported liquid/liquid interface so long as PET-templated HMSM is suitable to construct liquid/liquid interface. Fig. 6a shows the CV response (curve B) for the simple ion transfer of TEA þ at the W/DCE interface supported by PETtemplated HMSM. A typical asymmetric CV curve is observed, namely, the CV response for the ion transfer of TEA þ from W to DCE is peak-shaped, while the CV response for their back transfer from DCE to W is steady-stated, which is similar to the previous reports on the ion transfer across the micro-liquid/liquid interface with asymmetric diffusion field [41]. However, it should be noticed that the CV curves become distorted at the relatively higher scan rate (20, 30 and 50 mV/s) as shown in Fig. 6b, which should be related with the mass transport resistance caused by the silicas formed in HMSM the and the increase of charging current with scan rate. The inset of Fig. 6b shows that the peak current of CV curves at the peak potential around 0.43 V after the subtraction of background [39,42] for the ion transfer of TEA þ from W to DCE increases with the scan rate (v) and displays a linear dependence on v1/2. According to the Randles–Sevcik equation [35], the

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Fig. 6. The CVs of (a) TEA þ (Curve B) across W/DCE interface supported by PET-templated HMSM at 10 mV/s (Curve A is the background and the inset is the simple ion transfer of Cyt c across W/DCE interface supported by PET-templated HMSM); (b) TEA þ transfer across W/DCE interface supported by PET-templated HMSM at different scan rates (5, 10, 20, 30, and 50 mV/s from inner to outside, and the inset is the relationship between Ip and v1/2); (c) FIT of K þ by DB18C6 (Curve B) across W/DCE interface supported by PET-templated HMSM at 10 mV/s (Curve A is the background); and (d) FIT of K þ by DB18C6 at different scan rates (5, 10, 20, 30, and 50 mV/s from inner to outside).

transfer coefficient of TEA þ in aqueous solution is calculated as 1.02  10  5 cm2 s  1, which is almost in agreement with the value reported previously [35]. In the case of FIT, as shown in Fig. 6c, a well-defined steadystate wave is obtained for the FIT of K þ by DB18C6 across the W/DCE interface supported by PET-templated HMSM, which completely accords to the TIC (transfer by interfacial complexation) /TID (transfer by interfacial dissociation) mechanism of FIT occurring at micro-L/L interface with asymmetric diffusion field when the concentration of K þ is much larger than that of DB18C6 in DCE [41]. Fig. 6d shows that the steady-state waves for the FIT of K þ by DB18C6 can be obtained at different scan rates, and the transfer coefficient of DB18C6 in DCE is calculated as 5.17  10  6 cm2 s  1 based on the steady-state current equation [41], which also agrees to the previous report on the FIT of radial diffusion of DB18C6 (in DCE) to micro-W/DCE interface [42]. Asymmetric CV curves for simple IT and well-defined steadystate waves for FIT obtained above can fully illustrate the W/DCE interface supported by PET-templated HMSM should form at the surface of membrane, where the aqueous solution filtrates into the silica mesopores due to their hydrophilicity [32,33] and then forms W/DCE interface with the outside organic solution. If the W/DCE interface forms inside PET-templated HMSM or an aqueous layer forms outside membrane, the CV curves obtained above should be symmetrically peak-shaped as reported previously [35,36]. In addition to the position of interface, the effect of overlap of diffusion field on the voltammetric response often found in the membrane-supported-liquid/liquid interface electrochemistry

[35,36,38] can be ignored here, because the CV curves should change from steady-state to non-steady-state due to the transition from radial diffusion to linear diffusion field caused by the overlap of diffusion layers [35]. Moreover, all above well-defined CV curves obtained from the simple IT of TEA þ and the FIT of K þ by DB18C6 indicate that small ions (TEA þ and K þ ) can transfer across the W/ DCE interface supported by PET-templated HMSM. As for biomacromolecule Cyt c, it has been reported that Cyt c could transfer across the micro-W/DCE interface supported by a micropipette [43], or adsorb at an aqueous-organogel microinterface arrays [46]. However, as shown as the inset in Fig. 6a, there is not any obvious CV response corresponding to the adsorption or the transfer of Cyt c within the similar electrochemical window as that reported previously [43], which infers that it should be impossible for the Cyt c in aqueous solution to transfer across PET-templated HMSM and reach the HMSM-modified liquid/liquid interface, which is due to the size-rejection effect of mesopores formed in such a HMSM to Cyt c because the ion size (
3.6 nm) or molecule weight (12,384) of Cyt c is larger than the mesopore size (
3.0 nm) or the molecule weight cutoff (
11,000) of such a CTAB-templated HMSM [33]. According to all above results of simple IT and FIT, on the one hand, it is found out that such a PET-templated HMSM is a better template than PAAtemplated HMSM to construct liquid/liquid interface because silica rods form through the whole channels of PET without gaps between silica rods and PET channels as discussed above on the structural difference between PET-templated HMSM and PAAtemplated HMSM. On the other hand, PET-templated HMSM also

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Y. Chen et al. / Journal of Membrane Science 457 (2014) 9–18

Fig. 7. The CVs of the double-ion transfer of TEA þ and Cyt c across (a) the W/DCE interface supported by bare PET and (b) the W/DCE interface supported by PET-templated HMSM at 10 mV/s (the insets in Fig. 7a and b are the corresponding DPVs); and (c) is the scheme for the asymmetric diffusion field with meso-sized selectivity of W/DCE interface supported by PET-templated HMSM.

presents meso-sized selectivity for the ion transfer across the HMSM-supported liquid/liquid interface, which can be further confirmed by the followed double-ion transfer experiments. Fig.7a and b shows the CVs and DPVs (the insets) for the double-ion transfer of TEA þ and Cyt c across the W/DCE interface supported by bare PET or PET-templated HMSM. As shown as Fig. 7a, both of peaks corresponding to the transfer of TEA þ and Cyt c across the W/DCE interface supported by bare PET are observed in the CV and DPV curves, which indicates that both of ions, TEA þ and Cyt c, can transfer across such a PET-supported W/DCE interface owing to the large pore size of PET. However, Fig. 7b obviously shows that only one peak corresponding to the transfer of TEA þ across the W/DCE interface supported by PETtemplated HMSM is observed in the CV and more well-defined DPV curves, which indicates that only small ion, TEA þ , not biomacromolecule Cyt c, can transfer across the HMSMsupported W/DCE interface ascribing to the size-rejection effect of mesopores formed in such a HMSM to biomacromolecule. Moreover, in order to further confirm the size-selectivity of liquid/liquid interface supported by PET-templated HMSM, the electrochemically assisted ion extraction of double-ion combined with UV–vis spectroscopy measurement was preliminarily conducted by holding the potential at 0.85 V, which is positive enough for the transfer of Cyt c and TEA þ from W to DCE. As shown in Fig. S7, the absorption peaks of Cyt c (408 nm and 560 nm) can be observed in the DCE solution after electrochemically assisted extraction of double-ion (Cyt c and TEA þ ) from W to DCE by holding the potential at 0.85 V for 30 min, which illuminates that Cyt c can transfer across the liquid/liquid interface supported by

bare PET due to the large pore size of PET as observed above on the transport of HRP through bare PET in aqueous solution (Fig. 5c). Whereas, after PET-templated HMSM instead of bare PET is used to support the W/DCE interface, those absorption peaks for Cyt c cannot be detected in the DCE, which indicates that it is indeed impossible for Cyt c to transfer across the liquid/liquid interface supported by PET-templated HMSM. All above experimental results on the molecule permeation through and the ion transfer across PET-templated HMSM demonstrate that such a HMSM presents meso-sized selectivity between small molecule/ion and biomacromolecule. As far as the liquid/ liquid interface supported by PET-templated HMSM is concerned, as illustrated in Fig. 7c, such a HMSM-supported W/DCE interface is characteristic of the asymmetric diffusion field with meso-sized selective permeability, where the egress transfer of small ion out of HMSM dominated by linear diffusion produces a peak-shaped CV response due to the confinement of mesopores in HMSM, while the ingress transfer of small ion into HMSM controlled by radial diffusion leads to a steady wave. Therefore, the PET-templated HMSM can be used as a meso-sized selective nanofilter not only for the nanofiltration in aqueous solution, but also for the sizeselective ion transfer or extraction at the liquid/liquid interface, which is different from the PAA membrane with macropores [36] and the zeolite membrane with micropores [38]. Moreover, in view of the well-known silane chemistry developed for functionalizing silica surfaces [55], such a PET-templated HMSM is expected to be an attractive membrane material to further enhance the chemical selectivity of membrane-supported L/L interface and even to mimic the ion channels existed in the

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biomembrane after the functionalization of PET-templated HMSM, which can be investigated by using various electrochemical methods and techniques including scanning electrochemical microscopy (SECM) [37,56–58].

4. Conclusions In summary, a novel hybrid mesoporous silica membrane was successfully synthesized by using an organic porous PET membrane as the hard template on the basis of aspiration-induced infiltration method combined with solvent extraction. The mesosized selectivity of such a PET-templated HMSM was investigated by the experiments of the molecule permeation in aqueous solution and the ion transfer across the HMSM-supported liquid/ liquid interface. The results of molecule permeation show that the PET-templated HMSM is permeable for small molecule but impermeable for biomacromolecule due to the structural integrity and the mesoporosity of this hybrid membrane. In addition, the transport flux of small molecule across the PET-templated HMSM is larger than that across the PAA-templated HMSM owing to smaller membrane thickness of PET-templated HMSM than that of PAA-templated HMSM. Moreover, it is found that the liquid/liquid interface supported by PET-templated HMSMs presents the asymmetric diffusion field with meso-sized selectivity between small ion and biomacromolecule, which can offer a new platform to further exploit more potential applications of hybrid mesoporous silica membranes in the membrane-based filtration, separation, and even in the mimic of ion channels of biomembrane after functionalizing PET-templated HMSM in the future.

Acknowledgment This work was supported by the National Science Foundation of China (Grant nos. 21005049 and 21202099), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China (No. ZX2008-04) and the Science Innovation Foundation of Shanghai Educational Committee (No. 09YZ386), China. The authors heartily appreciate all the valuable suggestions from the reviewers.

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