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β-cyclodextrin modified silica nanochannel membrane for chiral separation
Journal of Membrane Science 453 (2014) 12–17
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
β-cyclodextrin modified silica nanochannel membrane for chiral separation Yue Liu, Ping Li, Li Xie, Dingyan Fan, Shasheng Huang n Life and Environmental Science College, Shanghai Normal University, Shanghai 200234, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2013 Received in revised form 16 October 2013 Accepted 28 October 2013 Available online 4 November 2013
The silica nanochannel membrane were prepared by sol–gel template synthesis method. The morphology of the nanochannels, characterized by field emission scanning electron microscope, showed that the SiO2 nanochannel wall was smooth and uniform. The characteristics of the nanochannels were investigated by the Zeta potential measurement, XPS and other methods. The inner wall of the silica nanochannels was modified with β-cyclodextrin (β-CD) and characterized by cyclic voltammetry and AC impedance method. The modification of β-CD provided a chiral environment for the separation of tryptophan enantiomer. The effects of pH and the pore size of the nanotubules on the separation of tryptophan enantiomers were investigated. The β-CD-modified SiO2 nanochannels was verified by the separation of chiral tryptophan. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sol–gel template synthesis method Silica nanochannel Chiral separation
1. Introduction The application of the nanomaterials of one-dimensional structure with unique thermodynamic properties, electronic properties, mechanical properties, and optical properties [1–3] has received intensively attention in many basic disciplines. It has been confirmed that such one-dimensional structure can improve the photoelectric characteristics of nanomaterials [4]. At present, the researchers try to explore the biological characteristics of the one-dimensional structure of nanomaterials. Silica nanochannels [5–8], a kind of one-dimensional structure of inorganic nanomaterials with low cytotoxicity, can be prepared by the template synthesis method [9,10]. These materials show many advantages in the field of biological applications because they are easily dispersed and their surface is easily functionalized. The versatile silica nanochannels has been successfully applied to the separation of biological molecules [10], gene delivery [11], biosensing [12], labeling of magnetic cell [13] and selective identification of cancer cell [14]. Chiral is a universal phenomenon in nature. Amino acids, sugars, proteins and nucleic acids are chiral molecules, they compose the basic substance units of the organism [15–17]. For example, the sugar in nature is the D-configuration, the amino acid is L-configuration, and the protein and the helical conformation of DNA are dextrose. Generally, chiral drugs have the same physical and chemical properties besides different rotary polarization,
n
Corresponding author. Tel./fax: þ 86 021 64321828. E-mail address: [email protected] (S. Huang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.064
physiological activity, metabolism and pharmacokinetics. The separation of chiral drugs has an important significance for the life science. The amino acid enantiomers are important compounds in the fields of medicinal chemistry, agricultural chemistry, food chemistry and biochemistry. It plays an important role in the exploitation of life nature. In the present work, we deposited silica onto the inner wall of the alumina membrane nanopore to prepare silica nanotubes by sol–gel template synthesis method [18]. In our experiment, it was found that the inner wall of silica nanochannels modified with β-cyclodextrin showed a chiral environment. The separation of tryptophan enantiomer based on modified silica nanochannels membrane was discussed. 2. Experimental 2.1. Reagents and apparatus The alumina membranes with pore diameters labeled 100 nm, thickness of 10 μm, were purchased from Whatman company (GE). Tetraethyl orthosilicate (TEOS) was purchased from Sigma-Aldrich (St Louis, MO, USA). Ethanol, acetone, sodium dihydrogen phosphate and disodium hydrogen phosphate were obtained from Shanghai Runjie Chemical Reagent Co. D-tryptophan, L-tryptophan and DLtryptophan was purchased from Shanghai Yuanju biological Technology Co., Ltd. β-cyclodextrin (β-CD) was purchased from Sinopharm. All reagents used in the experiments were of analytical grade. Milli-Q 18.2 MΩ water was used throughout all experiments.
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After the tryptophan racemic mixture in the feed cell was separated, the concentrations of L-tryptophan and D-tryptophan in the permeating cell were detected by a polarimeter (Autopol IV/IV, USA). The electrochemical measurements were carried out at the electrochemical workstation (CHI 760C, Shanghai Chenhua). The field emission scanning electron microscope (FESEM, S4800, HITACHI), transmission electron microscopy (TEM, JEM-2010F, JEOL) and X-ray photoelectron spectroscopy (XPS, 5000C ESCA, PHI) were used to characterize the nanotubules. Zeta potential was measured by a Zeta potential analyzer (ZEN3600, Malvern). 2.2. Procedure 2.2.1. Preparation of silica nanochannels The silica nanotubules were prepared by depositing silica onto the inner wall of nanopore of the alumina membrane using sol–gel template synthesis method [18]. A sol precursor solution was prepared by mixing ethanol, TEOS and 1 M HCl in accordance with the volume ratio at a proportion of 50:5:1. After the mixture solution was set for 30 min, the alumina membrane was immersed in the sol solution and sonicated for 1 min, then sonicated in pure water to remove impurity adsorbed on the surface of nanotubules membrane for 3 min. The alumina film impregnated with the sol solution was set in air for 10 min prior to place alumina membrane in a vacuum oven at 150 1C overnight.
Fig. 1. Relationship between the inner diameter of SiO2 nanochannel and the deposition time.
2.2.2. Functionalization of silica nanochannels The SiO2 nanochannels membrane, after being immersed in acetone for 5 min and then rinsed with water, was immersed in a phosphate buffer saline (PBS, 0.10 M pH 5.90) containing 1.85% β-CD solution for 12 h. In this paper, β-CD-SiO2 was used to denote the SiO2 film modified with β-CD. 2.2.3. Separation and determination of tryptophan enantiomer The apparatus used for the separation of tryptophan enantiomer based on nanochannels membrane was the same as that described before [19]. The membrane was mounted between two halves of a U-tube permeate cell, 10 mM PBS solution (pH 5.9) containing 2.5 10 5 M L-tryptophan, 2.5 10 5 M D-tryptophan or DL-tryptophan to be tested were put into the feed cell while PBS of the same volume as to-be-tested solution was put into permeate cell to keep the same height of liquid surface in both cells. In other words, the equal liquid level between feed cell and permeate cell was maintained during the experiment. The permeate half-cell was sampled, and the UV absorbance was used to determine the flux of the molecules transported periodically at 279 nm when the single component of tryptophan was separated. When a mixture containing 2.5 10 5 M L-tryptophan and 2.5 10 5 M D-tryptophan enantiomer was separated, the tryptophan was detected using a Polarimeter.
3. Results and discussion 3.1. Effect of deposition time to the pore size of SiO2 nanochannel The sol–gel template synthesis method was used to prepare silica nanotubes. Generally, under the same conditions, when a deposition method is used to deposit SiO2 onto the inner walls of nanotubules, the diameter of the nanotubules may be controlled by the deposition time. The longer the time of deposition, the smaller the diameter was (Fig. 1). From Fig. 1, we can see a faster deposition rate in the beginning of deposition. However, the deposition rate of SiO2 begins to slow down when the deposition time was over 40 min. The scanning electron micrographs of the Al2O3 nanoporous membrane with pore diameter of 100 nm after
Fig. 2. FESEM scheme of a SiO2 nanotube array (depositing time, 4 min).
depositing for 0 min (a), 4 min (b), 10 min (c), 20 min (d), 40 min (e) and 60 min (f), respectively showed that the pore size of membrane gradually decreases with increasing deposition time (Fig. S1). From Fig. 1 and Fig. S1, it can be seen that the silica nanochannels membrane has been successfully prepared. 3.2. Characterization of Al2O3 and SiO2 membrane 3.2.1. Morphology of Al2O3 and SiO2 membrane Field emission scanning electron microscope (FESEM) was used to characterize the surface and cross-section of Al2O3 film and SiO2 film. In order to investigate the SiO2 nanotubules, the alumina template membrane was put on the slides surface, and the silica was deposited in the nanopore of alumina membrane. After removing the template, a SiO2 nanotubules array standing on the slide surface was obtained (Fig. 2). The FESEM of a SiO2 nanotubules array after deposition of 40 min showed SiO2 nanotubules were arranged with a smooth wall and a uniform size. The outer diameter of the SiO2 nanotube was consistent with the diameter of the pore in template. In addition, it can be seen that some SiO2 nanochannels are enclosed branch tubes. Perhaps, this phenomenon is caused by the alumina template membrane itself. After plating for 40 min, the silica nanotubules prepared above was dispersed into ethanol. The TEM of ethanol solution shows that the silica nanopipes in good shape can be obtained in this way (Fig. 3).
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3.2.2. X-ray photoelectron spectroscopy analysis of silica nanochannels X-ray photoelectron spectroscopy (XPS) was used to analyze the elements of SiO2 nanochannels (Fig. 4). From Fig. 4, it can be seen that the peaks at 980 eV can be considered to correspond to the Auger peak of oxygen and the peaks at 534 eV, 122 eV and 77 eV belong to O 1s peak, Al 2s peak and Al 2p peak, respectively, for Al2O3 membrane (the curve below). After SiO2 (the curve above) was deposited, the peaks at 978 eV, 532 eV, 285 eV, 155 eV, 120 eV, 104 eV and 75 eV are O Auger peak, O 1s peak, C 1s peak, Si 2s peak, Al 2s peak, Si 2p peak and Al 2p peak, respectively. In Fig. 4, a small peak at 285 eV corresponding to the C 1s peak can be observed, it may be caused by the pollutant of C-containing organics in the sample. Fig. 4 illustrates that the nanochannels prepared here were composed of Si element and O element, i.e. the nanochannels prepared in this way were SiO2 nanochannels. 3.2.3. Cyclic voltammetry (CV) of nanotubules film Two Pt wires, where one was used as the working electrode and another as auxiliary electrode, were inset into the feed cell of the separation apparatus mounted with SiO2 nanochannels film and a saturated calomel electrode used as reference electrode was inset in the permeate cell. The cyclic voltammogram of the SiO2 nanochannels film obtained by different deposition time in 0.1 M K3[Fe(CN)6] solution showed that the peak current decreased gradually when increasing the depositing time (Fig. S2). The reason may be that the pore size of the nanotubules reduced gradually with increasing the coating time, resulting in an increase of the electron transfer resistance through the film. 3.2.4. Zeta potential study of silica nanochannels The zeta-potential is an important measure of particle–particle and particle–surface interactions, and these interactions have been
shown to be of importance to membrane performance [20]. Fig. 5 shows the Zeta potential of SiO2 nanochannels. The Zeta potential of SiO2 nanochannel was 4.01 mV, indicating that the surface of SiO2 nanochannel has a certain amount of negative charge due to a lot of –OH group on the surface of SiO2 nanochannels. 3.3. Modification of nanochannels
β-cyclodextrin in the inner wall of the silica
β-cyclodextrin containing a large amount of hydroxyl group is easy to be modified at the surface of the silica nanochannel with a lot of hydroxyl group (Fig. S3). The cyclic voltammograms (Fig. S4) of SiO2 nanochannel membrane showed that after the SiO2 nanochannel was modified with β-CD, the peak current of the system (The apparatus used for this measurements was the same as described in Section 3.2.3) decreased. The reason may be that upon being modified with β-CD, the inside diameter of the SiO2 nanotubules was decreased to a certain degree, and the modification of β-CD could block the transport of redox through the nanotubules. On the other hand, the electron transfer capacity of β-CD itself is relatively weak, resulting in the decrease of peak current of modified nanochannels. The AC impedance spectra (Fig. S5) of the film showed that the resistance of electron transfer through the nanotubules increased after the SiO2 nanochannel was modified with β-CD. The reason for the impedance changes of the SiO2 nanochannel was the same as the results of peak current of that membrane, indicating that the β-CD was modified onto the inner wall of the silica nanochannels successfully. β-CD modified on the inner wall of the silica nanochannels can be called chiral layer, which can be used to separate chiral enantiomer. Then, the separation of tryptophan enantiomer based on modified silica nanochannels membrane was investigated (Fig. 6). 3.4. Optimization of separation of tryptophan enantiomer 3.4.1. Effect of pH of buffer solution on the tryptophan enantiomers separation The impact of pH value of PBS solution on the separation of tryptophan enantiomers was investigated (Fig. 7). From Fig. 7, it can be seen that at the isoelectric point of tryptophan (pH 5.90), a largest separation efficiency was obtained. The separation efficiency (The definition of the separation efficiency was showed in Section 3.5) decreased whether pH of the buffer solution is less than or greater than the isoelectric point of the tryptophan. 3.4.2. Effect of the pore size on tryptophan enantiomers separation Generally, the size of SiO2 nanotubules can be controlled by the deposition time, and the pore size of the silica nanochannels decreased with increasing the deposition time. The separation
Fig. 3. TEM of a SiO2 nanotube.
Si 2p
O 1s
Intensity/a.u.
Intensity/a.u.
Si 2s
O Auger
Al 2s Al 2p
C 1s
0
200
400
600
Eb/eV
800
1000
1200
0
50
100
150
Eb/eV
Fig. 4. XPS spectra of Al2O3 (the curve below) and after depositing SiO2 (the curve above).
200
Y. Liu et al. / Journal of Membrane Science 453 (2014) 12–17
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Fig. 5. Zeta potential of SiO2 nanochannels.
Fig. 6. Schematic diagram of D-Trp and L-Trp transport through the nanotubules membrane.
2.0
efficiency of tryptophan enantiomers through functionized SiO2 nanochannels obtained at different deposition time was investigated in pH 5.90 PBS solution (Fig. 8). The results showed that the separation efficiency of tryptophan enantiomer increased with the increasing the deposition time. However, when the deposition time of SiO2 was over 60 min (Fig. 1), the decrease of diameter of nanotubules was very slow. In the following experiments, the deposition time was controlled at 60 min.
Separation efficiency
1.8 1.6 1.4 1.2
3.5. A preliminary application of nanotubules membrane, separation of tryptophan enantiomers
1.0 0.8 0.6
5.0
5.5
6.0
6.5
7.0
7.5
pH Fig. 7. Effect of the pH of PBS on the tryptophan enantiomers separation.
According to the procedure described in Section 2.2.3, a ∅ 100 nm Al2O3 film deposited SiO2 for 60 min was used to investigate the transportation characteristics of tryptophan enantiomer through the SiO2 nanochannels membrane and β-CD functionalized SiO2 nanochannels membrane, respectively. Under the optimal experimental conditions, the amounts of D-Trp and L-Trp
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Y. Liu et al. / Journal of Membrane Science 453 (2014) 12–17
with β-CD. In order to express the separation efficiency of the membrane for D-Trp and L-Trp one can adopt S to express the separation efficiency: S ¼ K LTrp =K DTrp where KL-Trp, KD-Trp are the transport rates of L-Trp and D-Trp through the membrane respectively. The separation efficiency of the tryptophan enantiomers calculated according to Fig. 9b was 5.98, indicating that the tryptophan enantiomers were separated successfully.
4. Conclusions
Fig. 8. Influence of the deposition time of SiO2 nanochannel on the tryptophan enantiomers separation.
10
D-Trp L-Trp
Concentration/10-6 M
8
Silica nanochannels were successfully prepared using sol–gel template synthesis method, and the properties the nanochannels were investigated. Tryptophan enantiomers were successfully separated with the prepared silica nanochannel membranes. This method provides a certain idea and an alternative method for the separation of complex structure of chiral substances. Silica nanochannels will have broad application prospects in the field of nanotechnology and nanomaterials, biomedical, chiral separation, molecular biology and so on.
6
Acknowledgments
4 2 0 0
2
4
6
8
10
This work was supported by the Project of the National Science Foundation of People's Republic of China (21275100), Shanghai Leading Academic Discipline Project (S30406) and Key Laboratory of Resource Chemistry of Ministry of Education.
Time/h
Appendix. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.10.064. References
Fig. 9. Separation efficiency of tryptophan enantiomer through the SiO2 nanochannels membrane (a) and β-CD functionalized SiO2 nanochannels membrane (b)
transported through SiO2 nanochannels membranes was determined using polarimeter. Two cases were considered. At first, the transportation of 2.50 10 5 M D-Trp and L-Trp at the same concentration with D-Trp single component through SiO2 nanotubules membrane was observed. The results showed that the transport rate of D-Trp and L-Trp through SiO2 nanochannels was almost similar (Fig. 9a), indicating that tryptophan enantiomers could not be separated by SiO2 nanotubules membrane because SiO2 nanotube membrane does not have chiral recognition layer. When the SiO2 nanochannel was modified with β-CD, 2.50 10 5 M tryptophan racemic mixture was put into the feed cell, the determination of D-Trp and L-Trp in the permeating cell showed that the difference of transport rate between D-Trp and L-Trp increased due to the chiral recognition in the inner wall of nanotubules (Fig. 9b), i.e. the separation efficiency (S) for D-Trp and L-Trp can be improved by modifying the SiO2 nanochannels
[1] L. Cademartiri, G.A. Ozin, Ultrathin nanowires-a materials chemistry perspectiveAdv. Mater. 21 (9) (2009) 1013–1020. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (5) (1999) 435–445. [3] Y. Xia, P. Yang, Y. Sun, et al., One-dimensional nanostructures: synthesis, characterization, and applicationsAdv. Mater. 15 (5) (2003) 353–389. [4] M. Law, J. Goldberger, P.D. Yang, Semiconductor nanowires and nanotubes, Annu. Rev. Mater. Res. 34 (2004) 83–122. [5] C.R. Martin, P. Kohli, The emerging field of nanotube biotechnology, Nat. Rev. Drug Discov. 2 (1) (2003) 29–37. [6] H. Hillebrenner, F. Buyukserin, J.D. Stewart, et al., Template synthesized nanotubes for biomedical delivery applications, Nanomedicine 1 (1) (2006) 39–50. [7] S.J. Son, X. Bai, S.B. Lee, Inorganic hollow nanoparticles and nanotubes in nanomedicine: Part 1. Drug/gene delivery applications, Drug Discov. Today 12 (15–16) (2007) 650–656. [8] D.T. Mitchell, S.B. Lee, L. Trofin, et al., Smart nanotubes for bioseparations and biocatalysis, J. Am. Chem. Soc. 124 (40) (2002) 11864–11865. [9] C.R. Martin, Nanomaterials: a membrane-based synthetic approach, Science 266 (5193) (1994) 1961–1966. [10] V.M. Cepak, C.R. Martin, Preparation of polymeric micro-and nanostructures using a template-based deposition method, Chem. Mater. 11 (5) (1999) 1363–1367. [11] C.C. Chen, Y.C. Liu, C.H. Wu, et al., Preparation of fluorescent silica nanotubes and their application in gene deliveryAdv. Mater. 17 (4) (2005) 404–407. [12] B. He, S.J. Son, S.B. Lee, Suspension array with shape-coded silica nanotubes for multiplexed immunoassaysAnal. Chem. 79 (14) (2007) 5257–5263. [13] X. Bai, S.J. Son, S.X. Zhang, et al., Synthesis of superparamagnetic nanotubes as MRI contrast agents and for cell labeling, Nanomedicine 3 (2) (2008) 163–174. [14] F. Buyukserin, C.D. Medley, M.O. Mota, et al., Antibody-functionalized nano test tubes target breast cancer cells, Nanomedicine 3 (3) (2008) 283–292. [15] C.J. Welch, Crawling out of the chiral pool: the evolution of pirkle-type chiral stationary phases, Adv. Chromatogr. 35 (1995) 171–197.
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[16] V. Sundaresan, R. Abrol, Biological chiral recognition: the substrate's perspective, Chirality 17 (S1) (2005) S30–S39. [17] S. Lam, G. Malikin, Strategy combining chiral chromatography with βcyclodextrin and stereospecific enzyme reaction detection with hydroxysteroid dehydrogenases for resolving steroid epimers, Chirality 4 (6) (1992) 395–399. [18] R. Gasparac, P. Kohli, M.O. Mota, et al., Template synthesis of nano test tubes, Nano Lett. 4 (3) (2004) 513–516.
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[19] S.S. Huang, C. Sheng, Z.F. Yin, J. Shen, R.N. LiBin Peng, Immunoreaction-based separation of antibodies using gold nanochannels membrane, J. Membr. Sci. 305 (2007) 257. [20] I.H. Huisman, G.T. ÈgaÊrdh, C.T. ÈgaÊrdh, A.P. Èki, Determining the zetapotential of ceramic microsltration membranes using the electroviscous effect, J. Membr. Sci. 147 (1998) 187.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
β-cyclodextrin modified silica nanochannel membrane for chiral separation Yue Liu, Ping Li, Li Xie, Dingyan Fan, Shasheng Huang n Life and Environmental Science College, Shanghai Normal University, Shanghai 200234, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2013 Received in revised form 16 October 2013 Accepted 28 October 2013 Available online 4 November 2013
The silica nanochannel membrane were prepared by sol–gel template synthesis method. The morphology of the nanochannels, characterized by field emission scanning electron microscope, showed that the SiO2 nanochannel wall was smooth and uniform. The characteristics of the nanochannels were investigated by the Zeta potential measurement, XPS and other methods. The inner wall of the silica nanochannels was modified with β-cyclodextrin (β-CD) and characterized by cyclic voltammetry and AC impedance method. The modification of β-CD provided a chiral environment for the separation of tryptophan enantiomer. The effects of pH and the pore size of the nanotubules on the separation of tryptophan enantiomers were investigated. The β-CD-modified SiO2 nanochannels was verified by the separation of chiral tryptophan. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sol–gel template synthesis method Silica nanochannel Chiral separation
1. Introduction The application of the nanomaterials of one-dimensional structure with unique thermodynamic properties, electronic properties, mechanical properties, and optical properties [1–3] has received intensively attention in many basic disciplines. It has been confirmed that such one-dimensional structure can improve the photoelectric characteristics of nanomaterials [4]. At present, the researchers try to explore the biological characteristics of the one-dimensional structure of nanomaterials. Silica nanochannels [5–8], a kind of one-dimensional structure of inorganic nanomaterials with low cytotoxicity, can be prepared by the template synthesis method [9,10]. These materials show many advantages in the field of biological applications because they are easily dispersed and their surface is easily functionalized. The versatile silica nanochannels has been successfully applied to the separation of biological molecules [10], gene delivery [11], biosensing [12], labeling of magnetic cell [13] and selective identification of cancer cell [14]. Chiral is a universal phenomenon in nature. Amino acids, sugars, proteins and nucleic acids are chiral molecules, they compose the basic substance units of the organism [15–17]. For example, the sugar in nature is the D-configuration, the amino acid is L-configuration, and the protein and the helical conformation of DNA are dextrose. Generally, chiral drugs have the same physical and chemical properties besides different rotary polarization,
n
Corresponding author. Tel./fax: þ 86 021 64321828. E-mail address: [email protected] (S. Huang).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.064
physiological activity, metabolism and pharmacokinetics. The separation of chiral drugs has an important significance for the life science. The amino acid enantiomers are important compounds in the fields of medicinal chemistry, agricultural chemistry, food chemistry and biochemistry. It plays an important role in the exploitation of life nature. In the present work, we deposited silica onto the inner wall of the alumina membrane nanopore to prepare silica nanotubes by sol–gel template synthesis method [18]. In our experiment, it was found that the inner wall of silica nanochannels modified with β-cyclodextrin showed a chiral environment. The separation of tryptophan enantiomer based on modified silica nanochannels membrane was discussed. 2. Experimental 2.1. Reagents and apparatus The alumina membranes with pore diameters labeled 100 nm, thickness of 10 μm, were purchased from Whatman company (GE). Tetraethyl orthosilicate (TEOS) was purchased from Sigma-Aldrich (St Louis, MO, USA). Ethanol, acetone, sodium dihydrogen phosphate and disodium hydrogen phosphate were obtained from Shanghai Runjie Chemical Reagent Co. D-tryptophan, L-tryptophan and DLtryptophan was purchased from Shanghai Yuanju biological Technology Co., Ltd. β-cyclodextrin (β-CD) was purchased from Sinopharm. All reagents used in the experiments were of analytical grade. Milli-Q 18.2 MΩ water was used throughout all experiments.
Y. Liu et al. / Journal of Membrane Science 453 (2014) 12–17
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After the tryptophan racemic mixture in the feed cell was separated, the concentrations of L-tryptophan and D-tryptophan in the permeating cell were detected by a polarimeter (Autopol IV/IV, USA). The electrochemical measurements were carried out at the electrochemical workstation (CHI 760C, Shanghai Chenhua). The field emission scanning electron microscope (FESEM, S4800, HITACHI), transmission electron microscopy (TEM, JEM-2010F, JEOL) and X-ray photoelectron spectroscopy (XPS, 5000C ESCA, PHI) were used to characterize the nanotubules. Zeta potential was measured by a Zeta potential analyzer (ZEN3600, Malvern). 2.2. Procedure 2.2.1. Preparation of silica nanochannels The silica nanotubules were prepared by depositing silica onto the inner wall of nanopore of the alumina membrane using sol–gel template synthesis method [18]. A sol precursor solution was prepared by mixing ethanol, TEOS and 1 M HCl in accordance with the volume ratio at a proportion of 50:5:1. After the mixture solution was set for 30 min, the alumina membrane was immersed in the sol solution and sonicated for 1 min, then sonicated in pure water to remove impurity adsorbed on the surface of nanotubules membrane for 3 min. The alumina film impregnated with the sol solution was set in air for 10 min prior to place alumina membrane in a vacuum oven at 150 1C overnight.
Fig. 1. Relationship between the inner diameter of SiO2 nanochannel and the deposition time.
2.2.2. Functionalization of silica nanochannels The SiO2 nanochannels membrane, after being immersed in acetone for 5 min and then rinsed with water, was immersed in a phosphate buffer saline (PBS, 0.10 M pH 5.90) containing 1.85% β-CD solution for 12 h. In this paper, β-CD-SiO2 was used to denote the SiO2 film modified with β-CD. 2.2.3. Separation and determination of tryptophan enantiomer The apparatus used for the separation of tryptophan enantiomer based on nanochannels membrane was the same as that described before [19]. The membrane was mounted between two halves of a U-tube permeate cell, 10 mM PBS solution (pH 5.9) containing 2.5 10 5 M L-tryptophan, 2.5 10 5 M D-tryptophan or DL-tryptophan to be tested were put into the feed cell while PBS of the same volume as to-be-tested solution was put into permeate cell to keep the same height of liquid surface in both cells. In other words, the equal liquid level between feed cell and permeate cell was maintained during the experiment. The permeate half-cell was sampled, and the UV absorbance was used to determine the flux of the molecules transported periodically at 279 nm when the single component of tryptophan was separated. When a mixture containing 2.5 10 5 M L-tryptophan and 2.5 10 5 M D-tryptophan enantiomer was separated, the tryptophan was detected using a Polarimeter.
3. Results and discussion 3.1. Effect of deposition time to the pore size of SiO2 nanochannel The sol–gel template synthesis method was used to prepare silica nanotubes. Generally, under the same conditions, when a deposition method is used to deposit SiO2 onto the inner walls of nanotubules, the diameter of the nanotubules may be controlled by the deposition time. The longer the time of deposition, the smaller the diameter was (Fig. 1). From Fig. 1, we can see a faster deposition rate in the beginning of deposition. However, the deposition rate of SiO2 begins to slow down when the deposition time was over 40 min. The scanning electron micrographs of the Al2O3 nanoporous membrane with pore diameter of 100 nm after
Fig. 2. FESEM scheme of a SiO2 nanotube array (depositing time, 4 min).
depositing for 0 min (a), 4 min (b), 10 min (c), 20 min (d), 40 min (e) and 60 min (f), respectively showed that the pore size of membrane gradually decreases with increasing deposition time (Fig. S1). From Fig. 1 and Fig. S1, it can be seen that the silica nanochannels membrane has been successfully prepared. 3.2. Characterization of Al2O3 and SiO2 membrane 3.2.1. Morphology of Al2O3 and SiO2 membrane Field emission scanning electron microscope (FESEM) was used to characterize the surface and cross-section of Al2O3 film and SiO2 film. In order to investigate the SiO2 nanotubules, the alumina template membrane was put on the slides surface, and the silica was deposited in the nanopore of alumina membrane. After removing the template, a SiO2 nanotubules array standing on the slide surface was obtained (Fig. 2). The FESEM of a SiO2 nanotubules array after deposition of 40 min showed SiO2 nanotubules were arranged with a smooth wall and a uniform size. The outer diameter of the SiO2 nanotube was consistent with the diameter of the pore in template. In addition, it can be seen that some SiO2 nanochannels are enclosed branch tubes. Perhaps, this phenomenon is caused by the alumina template membrane itself. After plating for 40 min, the silica nanotubules prepared above was dispersed into ethanol. The TEM of ethanol solution shows that the silica nanopipes in good shape can be obtained in this way (Fig. 3).
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3.2.2. X-ray photoelectron spectroscopy analysis of silica nanochannels X-ray photoelectron spectroscopy (XPS) was used to analyze the elements of SiO2 nanochannels (Fig. 4). From Fig. 4, it can be seen that the peaks at 980 eV can be considered to correspond to the Auger peak of oxygen and the peaks at 534 eV, 122 eV and 77 eV belong to O 1s peak, Al 2s peak and Al 2p peak, respectively, for Al2O3 membrane (the curve below). After SiO2 (the curve above) was deposited, the peaks at 978 eV, 532 eV, 285 eV, 155 eV, 120 eV, 104 eV and 75 eV are O Auger peak, O 1s peak, C 1s peak, Si 2s peak, Al 2s peak, Si 2p peak and Al 2p peak, respectively. In Fig. 4, a small peak at 285 eV corresponding to the C 1s peak can be observed, it may be caused by the pollutant of C-containing organics in the sample. Fig. 4 illustrates that the nanochannels prepared here were composed of Si element and O element, i.e. the nanochannels prepared in this way were SiO2 nanochannels. 3.2.3. Cyclic voltammetry (CV) of nanotubules film Two Pt wires, where one was used as the working electrode and another as auxiliary electrode, were inset into the feed cell of the separation apparatus mounted with SiO2 nanochannels film and a saturated calomel electrode used as reference electrode was inset in the permeate cell. The cyclic voltammogram of the SiO2 nanochannels film obtained by different deposition time in 0.1 M K3[Fe(CN)6] solution showed that the peak current decreased gradually when increasing the depositing time (Fig. S2). The reason may be that the pore size of the nanotubules reduced gradually with increasing the coating time, resulting in an increase of the electron transfer resistance through the film. 3.2.4. Zeta potential study of silica nanochannels The zeta-potential is an important measure of particle–particle and particle–surface interactions, and these interactions have been
shown to be of importance to membrane performance [20]. Fig. 5 shows the Zeta potential of SiO2 nanochannels. The Zeta potential of SiO2 nanochannel was 4.01 mV, indicating that the surface of SiO2 nanochannel has a certain amount of negative charge due to a lot of –OH group on the surface of SiO2 nanochannels. 3.3. Modification of nanochannels
β-cyclodextrin in the inner wall of the silica
β-cyclodextrin containing a large amount of hydroxyl group is easy to be modified at the surface of the silica nanochannel with a lot of hydroxyl group (Fig. S3). The cyclic voltammograms (Fig. S4) of SiO2 nanochannel membrane showed that after the SiO2 nanochannel was modified with β-CD, the peak current of the system (The apparatus used for this measurements was the same as described in Section 3.2.3) decreased. The reason may be that upon being modified with β-CD, the inside diameter of the SiO2 nanotubules was decreased to a certain degree, and the modification of β-CD could block the transport of redox through the nanotubules. On the other hand, the electron transfer capacity of β-CD itself is relatively weak, resulting in the decrease of peak current of modified nanochannels. The AC impedance spectra (Fig. S5) of the film showed that the resistance of electron transfer through the nanotubules increased after the SiO2 nanochannel was modified with β-CD. The reason for the impedance changes of the SiO2 nanochannel was the same as the results of peak current of that membrane, indicating that the β-CD was modified onto the inner wall of the silica nanochannels successfully. β-CD modified on the inner wall of the silica nanochannels can be called chiral layer, which can be used to separate chiral enantiomer. Then, the separation of tryptophan enantiomer based on modified silica nanochannels membrane was investigated (Fig. 6). 3.4. Optimization of separation of tryptophan enantiomer 3.4.1. Effect of pH of buffer solution on the tryptophan enantiomers separation The impact of pH value of PBS solution on the separation of tryptophan enantiomers was investigated (Fig. 7). From Fig. 7, it can be seen that at the isoelectric point of tryptophan (pH 5.90), a largest separation efficiency was obtained. The separation efficiency (The definition of the separation efficiency was showed in Section 3.5) decreased whether pH of the buffer solution is less than or greater than the isoelectric point of the tryptophan. 3.4.2. Effect of the pore size on tryptophan enantiomers separation Generally, the size of SiO2 nanotubules can be controlled by the deposition time, and the pore size of the silica nanochannels decreased with increasing the deposition time. The separation
Fig. 3. TEM of a SiO2 nanotube.
Si 2p
O 1s
Intensity/a.u.
Intensity/a.u.
Si 2s
O Auger
Al 2s Al 2p
C 1s
0
200
400
600
Eb/eV
800
1000
1200
0
50
100
150
Eb/eV
Fig. 4. XPS spectra of Al2O3 (the curve below) and after depositing SiO2 (the curve above).
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Fig. 5. Zeta potential of SiO2 nanochannels.
Fig. 6. Schematic diagram of D-Trp and L-Trp transport through the nanotubules membrane.
2.0
efficiency of tryptophan enantiomers through functionized SiO2 nanochannels obtained at different deposition time was investigated in pH 5.90 PBS solution (Fig. 8). The results showed that the separation efficiency of tryptophan enantiomer increased with the increasing the deposition time. However, when the deposition time of SiO2 was over 60 min (Fig. 1), the decrease of diameter of nanotubules was very slow. In the following experiments, the deposition time was controlled at 60 min.
Separation efficiency
1.8 1.6 1.4 1.2
3.5. A preliminary application of nanotubules membrane, separation of tryptophan enantiomers
1.0 0.8 0.6
5.0
5.5
6.0
6.5
7.0
7.5
pH Fig. 7. Effect of the pH of PBS on the tryptophan enantiomers separation.
According to the procedure described in Section 2.2.3, a ∅ 100 nm Al2O3 film deposited SiO2 for 60 min was used to investigate the transportation characteristics of tryptophan enantiomer through the SiO2 nanochannels membrane and β-CD functionalized SiO2 nanochannels membrane, respectively. Under the optimal experimental conditions, the amounts of D-Trp and L-Trp
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with β-CD. In order to express the separation efficiency of the membrane for D-Trp and L-Trp one can adopt S to express the separation efficiency: S ¼ K LTrp =K DTrp where KL-Trp, KD-Trp are the transport rates of L-Trp and D-Trp through the membrane respectively. The separation efficiency of the tryptophan enantiomers calculated according to Fig. 9b was 5.98, indicating that the tryptophan enantiomers were separated successfully.
4. Conclusions
Fig. 8. Influence of the deposition time of SiO2 nanochannel on the tryptophan enantiomers separation.
10
D-Trp L-Trp
Concentration/10-6 M
8
Silica nanochannels were successfully prepared using sol–gel template synthesis method, and the properties the nanochannels were investigated. Tryptophan enantiomers were successfully separated with the prepared silica nanochannel membranes. This method provides a certain idea and an alternative method for the separation of complex structure of chiral substances. Silica nanochannels will have broad application prospects in the field of nanotechnology and nanomaterials, biomedical, chiral separation, molecular biology and so on.
6
Acknowledgments
4 2 0 0
2
4
6
8
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This work was supported by the Project of the National Science Foundation of People's Republic of China (21275100), Shanghai Leading Academic Discipline Project (S30406) and Key Laboratory of Resource Chemistry of Ministry of Education.
Time/h
Appendix. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.10.064. References
Fig. 9. Separation efficiency of tryptophan enantiomer through the SiO2 nanochannels membrane (a) and β-CD functionalized SiO2 nanochannels membrane (b)
transported through SiO2 nanochannels membranes was determined using polarimeter. Two cases were considered. At first, the transportation of 2.50 10 5 M D-Trp and L-Trp at the same concentration with D-Trp single component through SiO2 nanotubules membrane was observed. The results showed that the transport rate of D-Trp and L-Trp through SiO2 nanochannels was almost similar (Fig. 9a), indicating that tryptophan enantiomers could not be separated by SiO2 nanotubules membrane because SiO2 nanotube membrane does not have chiral recognition layer. When the SiO2 nanochannel was modified with β-CD, 2.50 10 5 M tryptophan racemic mixture was put into the feed cell, the determination of D-Trp and L-Trp in the permeating cell showed that the difference of transport rate between D-Trp and L-Trp increased due to the chiral recognition in the inner wall of nanotubules (Fig. 9b), i.e. the separation efficiency (S) for D-Trp and L-Trp can be improved by modifying the SiO2 nanochannels
[1] L. Cademartiri, G.A. Ozin, Ultrathin nanowires-a materials chemistry perspectiveAdv. Mater. 21 (9) (2009) 1013–1020. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (5) (1999) 435–445. [3] Y. Xia, P. Yang, Y. Sun, et al., One-dimensional nanostructures: synthesis, characterization, and applicationsAdv. Mater. 15 (5) (2003) 353–389. [4] M. Law, J. Goldberger, P.D. Yang, Semiconductor nanowires and nanotubes, Annu. Rev. Mater. Res. 34 (2004) 83–122. [5] C.R. Martin, P. Kohli, The emerging field of nanotube biotechnology, Nat. Rev. Drug Discov. 2 (1) (2003) 29–37. [6] H. Hillebrenner, F. Buyukserin, J.D. Stewart, et al., Template synthesized nanotubes for biomedical delivery applications, Nanomedicine 1 (1) (2006) 39–50. [7] S.J. Son, X. Bai, S.B. Lee, Inorganic hollow nanoparticles and nanotubes in nanomedicine: Part 1. Drug/gene delivery applications, Drug Discov. Today 12 (15–16) (2007) 650–656. [8] D.T. Mitchell, S.B. Lee, L. Trofin, et al., Smart nanotubes for bioseparations and biocatalysis, J. Am. Chem. Soc. 124 (40) (2002) 11864–11865. [9] C.R. Martin, Nanomaterials: a membrane-based synthetic approach, Science 266 (5193) (1994) 1961–1966. [10] V.M. Cepak, C.R. Martin, Preparation of polymeric micro-and nanostructures using a template-based deposition method, Chem. Mater. 11 (5) (1999) 1363–1367. [11] C.C. Chen, Y.C. Liu, C.H. Wu, et al., Preparation of fluorescent silica nanotubes and their application in gene deliveryAdv. Mater. 17 (4) (2005) 404–407. [12] B. He, S.J. Son, S.B. Lee, Suspension array with shape-coded silica nanotubes for multiplexed immunoassaysAnal. Chem. 79 (14) (2007) 5257–5263. [13] X. Bai, S.J. Son, S.X. Zhang, et al., Synthesis of superparamagnetic nanotubes as MRI contrast agents and for cell labeling, Nanomedicine 3 (2) (2008) 163–174. [14] F. Buyukserin, C.D. Medley, M.O. Mota, et al., Antibody-functionalized nano test tubes target breast cancer cells, Nanomedicine 3 (3) (2008) 283–292. [15] C.J. Welch, Crawling out of the chiral pool: the evolution of pirkle-type chiral stationary phases, Adv. Chromatogr. 35 (1995) 171–197.
Y. Liu et al. / Journal of Membrane Science 453 (2014) 12–17
[16] V. Sundaresan, R. Abrol, Biological chiral recognition: the substrate's perspective, Chirality 17 (S1) (2005) S30–S39. [17] S. Lam, G. Malikin, Strategy combining chiral chromatography with βcyclodextrin and stereospecific enzyme reaction detection with hydroxysteroid dehydrogenases for resolving steroid epimers, Chirality 4 (6) (1992) 395–399. [18] R. Gasparac, P. Kohli, M.O. Mota, et al., Template synthesis of nano test tubes, Nano Lett. 4 (3) (2004) 513–516.
17
[19] S.S. Huang, C. Sheng, Z.F. Yin, J. Shen, R.N. LiBin Peng, Immunoreaction-based separation of antibodies using gold nanochannels membrane, J. Membr. Sci. 305 (2007) 257. [20] I.H. Huisman, G.T. ÈgaÊrdh, C.T. ÈgaÊrdh, A.P. Èki, Determining the zetapotential of ceramic microsltration membranes using the electroviscous effect, J. Membr. Sci. 147 (1998) 187.