pubs.acs.org/Langmuir © 2010 American Chemical Society
Anionic-Cationic Switchable Amphoteric Monodisperse Mesoporous Silica Nanoparticles Yanhang Ma, Lei Xing, Haoquan Zheng, and Shunai Che* School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China Received October 3, 2010. Revised Manuscript Received November 29, 2010 Anionic-cationic switchable monodisperse mesoporous silica nanoparticles were synthesized by one-pot amino and carboxylic acid bifunctionalization based on the self-assembly of the surfactant, two types of co-structure-directing agents containing amino and carboxylic groups, and silica sources. These nanoparticles revealed properties of dispersity and reversibility, with the advantage of the pH-responsive anionic-cationic/acid-base switchability. It was demonstrated that the extracted materials achieved reutilization and controllable dispersity in aqueous solution by adjusting the static electric power among the particles during the switching process.
Introduction Organic-functionalized mesoporous silica materials with a high surface area and a large pore volume have attracted a great amount of attention because of their potential applications in separation, adsorption, catalysis, sensor design, drug delivery, and nanotechnology. To obtain diversified functional materials, three general methods (postsynthesis, co-condensation, and PMOs (i.e., periodic mesoporous organosilicas)) were developed.1-12 Among these functional materials, monodisperse silica nanoparticles show promising applications because of their size effect on the nanometer scale. Various organic-group-functionalized monodisperse mesoporous silica nanoparticles have been synthesized by co-condensation and postsynthesis routes.13-19 It is worth noting that functional groups could be well distributed on the pore surface in the co-condensation method. To date, research interests are focused on choosing desirable modification groups and exploiting their properties. Amino acid, as one of the essential biomolecules, is of central importance in biochemistry and pharmacology. There is great (1) Asefa, T.; MacLachlan, M.; Coombs, N.; Ozin, G. Nature 1999, 402, 867. (2) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367. (3) Gao, C.; Che, S. Adv. Funct. Mater. 2010, 20, 2750. (4) Hoffmann, F.; Cornelius, M.; Morell, J.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (5) Liu, Y.; Lin, H.; Mou, C. Langmuir 2004, 20, 3231. (6) Mal, N.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350. (7) Basallote, M.; Blanco, E.; Blazquez, M.; Fernandez-Trujillo, M.; Litran, R.; Manez, M.; del Solar, M. Chem. Mater. 2003, 15, 2025. (8) Crudden, C.; Sateesh, M.; Lewis, R. J. Am. Chem. Soc. 2005, 127, 10045. (9) Victor, S.; Lai, C.; Huang, J.; Song, S.; Xu, S. J. Am. Chem. Soc. 2001, 123, 11510. (10) Yang, Q.; Wang, S.; Fan, P.; Wang, L.; Di, Y.; Lin, K.; Xiao, F. Chem. Mater. 2005, 17, 5999. (11) Ying, J.; Mehnert, C.; Wong, M. Angew. Chem., Int. Ed. 1999, 38, 56. (12) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2003, 15, 1713. (13) Grun, G.; Kumar, B. D.; Schumacher, K.; Bidingmaier, K.; Unger, K. Stud. Surf. Sci. Catal. 2000, 128, 155. (14) Wang, Y.; Price, A. D.; Caruso, F. J. Mater. Chem. 2009, 19, 6451. (15) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (16) Suzuki, T.; Nakamura, T.; Sudo, E.; Akimoto, Y.; Yano, K. Microporous Mesoporous Mater. 2008, 111, 350. (17) Mizutani, M.; Yamada, Y.; Nakamura, T.; Yano, K. Chem. Mater. 2008, 20, 4777. (18) Gao, F.; Botella, P.; Corma, A.; Blesa, j.; Dong, L. J. Phys. Chem. B 2009, 113, 1796. (19) Hoshikawa, Y.; Yabe, H.; Momura, A.; Yamaki, T.; Shimojima, A.; Okubo., T. Chem. Mater. 2010, 22, 12.
Langmuir 2011, 27(2), 517–520
interest in techniques for immobilizing the amino acids on both pores and particle surfaces of porous materials,20-22 which would provide two points of attachment for biomolecules over a wide range of pH because pKa = 1-5 for the carboxylic acid moiety and pKb = 3.4 for the amino group. The amino acid functionality can be varied from the cationic ammonium-carboxylic acid form to the zwitterionic form and then to the anionic aminocarboxylate form along with changing from low to high pH. Therefore, the modification of amino and carboxyl groups on the mesoporous silica would make sense. The two functional organic groups preserved the properties of acid and base and achieved the acid-base switch in aqueous solution by changing the pH. These amphoteric and acid-base-switchable bifunctional properties make silica mesoporous nanoparticles more valuable in biosensor, catalyst, and separation media and biochemicals.23-25 In our previous work, we synthesized amphoteric amino acid bifunctional mesoporous silica with irregular morphology.26 Herein, we report monodisperse amphoteric mesoporous silica nanoparticles with pH-responsive anionic-cationic/acid-base switchability and controllable dispersity. Our approach is based on the amphoteric properties of the amino acid introduced into the mesoporous silica nanoparticles. The anionic-cationic switchability and controllable dispersity of the mesoporous nanoparticles have been achieved through changing the pH of the solution to convert the charge between cationic ammonium and anionic carboxylate and to shift the electrostatic force between the nanoparticles. The ionized functional groups in acid or basic media provide an efficient impetus for the nanoparticles’ mutual repulsion.
Experimental Section Amino acid bifunctional mesoporous silica nanoparticles were synthesized on the basis of the self-assembly of the surfactant and (20) Fu, L.; Weckhuysen, B.; Verberckmoes, A.; Schoonheydt, R. Clay Miner. 1996, 31, 491. (21) Weckhuysen, B.; Verberckmoes, A.; Fu, L.; Schoonheydt, R. J. Phys. Chem. 1996, 100, 9456. (22) Weckhuysen, B.; Verberckmoes, A.; Vannijvel, I.; Pelgrims, J.; Buskens, P.; Jacobs, P.; Schoonheydt, R. Angew. Chem., Int. Ed. 1996, 34, 2652. (23) Lei, C.; Shin, Y.; Liu, J.; Ackerman, E. J. Am. Chem. Soc. 2002, 124, 11242. (24) Luechinger, M.; Kienh fer, A.; Pirngruber, G. Chem. Mater. 2006, 18, 1330. (25) Han, L.; Sakamoto, Y.; Terasaki, O.; Li, Y.; Che, S. J. Mater. Chem. 2007, 17, 1216. (26) Han, L.; Ruan, J.; Li, Y.; Terasaki, O.; Che, S. Chem. Mater. 2007, 19, 2860.
Published on Web 12/17/2010
DOI: 10.1021/la103979c
517
Letter
Ma et al.
Figure 1. Schematic representation of the synthesis of amphoteric mesoporous nanoparticles with anionic-cationic switchability and controllable dispersity.
Figure 2. XRD pattern (a), SEM (b), and HRTEM (c) images of the extracted amino acid bifunctional mesoporous silica nanoparticles. The synthesis molar composition is 1:1:1:15:500:30 000 C18-3-1/CES/APS/TEOS/ethanol/H2O. cationic and anionic forms, the nanoparticles would be well dispersed with the help of a repulsive electrostatic force. However, when the pH is close to the isoelectric point of the particle surfaces, particles aggregate simply because of the reduction of electrostatic repulsion between neighboring particles.
the subsequent co-condensation of the costructure directing agent (CSDA) and silica sources (Figure 1). The cationic gemini surfactant [C18H33Nþ(CH3)2(CH2)3Nþ(CH3)3]Br2 (C18-3-1) and tetraethyl orthosilicate (TEOS) have been used as the structuredirecting agent and silica source, respectively. As amino and carboxylic acid group sources, 3-aminopropyltrimethoxysilane (APS) and carboxyethylsilanetriol sodium salt (CES) have been used, respectively. The positively charged headgroup of the surfactant interacts electrostatically with the negatively charged carboxylate site of CES. The triol sites of CES and APS were co-condensed with the TEOS silica source to form the wall framework, and ethylene groups regularly extended to the inner pore and outside of the particle surface. The hydrophobic alkoxysilane part of APS can play a role in its uniform dispersion in the hydrophobic part of the surfactant in the beginning of the reaction, which would be distributed on the pore and particle surfaces uniformly during the synthesis via co-condensation with the silica source. To obtain the monosized nanoparticles, ethanol was added as a retarding agent to control the reaction rate of both the hydrolysis and condensation of CSDA and TEOS.27,28 In the obtained solution with pH ∼8, the as-made nanoparticles were well dispersed without being pH-responsive, likely because of the particle surface being surrounded by superfluous surfactant. After the removal of the surfactant by extraction with tetrahydrofuran (THF) and HCl, the carboxylic acid groups and protonated ammonium groups remain on the pore and particle surfaces. In a manner similar to that for amino acids, these “amino acid nanoparticles” can be varied from a cationic ammoniumcarboxylic acid form to a zwitterionic form and then to an anionic carboxylate-amino form with increasing solution pH. In both
Results and Discussion Figure 2 shows the powder X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) images of the extracted amino acid bifunctional mesoporous silica nanoparticles. The presence of a broad peak in the region of 2θ = 1.5-1.8° indicates the presence of less-ordered mesostructure. The TEM images demonstrate the presence of a wormlike, radially oriented mesopore within the spherical particles. The further observation of the edge of the nanoparticles shows that most of the channels are open to the outside. N2 adsorption-desorption analysis (Figure S1 in SI) shows that the mesoporous nanoparticles possess a high surface area of 654 m2/g, a large volume of 0.89 cm3/g, and pore size of 2.95 nm. The mesoporous nanoparticles are monodisperse with an average diameter of about 100 nm. The removal of the surfactant and the existence of amino and carboxyl groups were confirmed by the solid-state 13C MAS NMR spectrum. CI, CII, and CIII of APS and CIV, CV, and CVI of CES show resonance signals at 8.3, 20.5, and 41.8 ppm and at 6.5, 26.2, and 176.5 ppm, respectively, in the 13C NMR spectrum.29,30 As shown in Figure 3, strong resonance peaks assigned to CH2
(27) Nakamura, T.; Mizutani, M.; Nozaki, H.; Suzuki, N.; Yano, K. J. Phys. Chem. C 2006, 111, 1093. (28) Schumacher, K.; Gr€un, M.; Unger, K. K. Microporous Mesoporous Mater. 1999, 27, 201.
(29) Gao, C.; Qiu, H.; Zeng, W.; Sakamoto, Y.; Terasaki, O.; Sakamoto, K.; Chen, Q.; Che, S. Chem. Mater. 2006, 18, 3904. (30) Huh, S.; Wiench, J.; Yoo, J.; Pruski, M.; Victor, S. Chem. Mater. 2003, 15, 4247.
518 DOI: 10.1021/la103979c
Langmuir 2011, 27(2), 517–520
Ma et al.
Letter
Figure 5. Photographs of the first switched cationic form at pH 3.0 (a), the first zwitterionic form at pH 6.0 (b), the first switched anionic form at pH 8.5 (c), the tenth switched cationic form at pH 3.0 (d), the tenth zwitterionic form at pH 6.0 (e), and the tenth switched anionic form at pH 8.5 (f) of amino acid bifunctional mesoporous silica nanoparticles.
Figure 3. 13C MAS NMR of the as-synthesized and extracted sample shown in Figure 2.
Figure 4. Isoelectric point curve of bifunctional mesoporous nanoparticles shown in Figure 2.
groups of the C18-3-1 surfactant ranged from 0 to 70 ppm in the spectrum of the as-synthesized sample. These strong peaks disappeared after extraction, but resonance peaks assigned to CH2 groups of CES and APS remained, which indicated that the surfactant was extracted clearly and amino groups and carboxyl groups were modified successfully in the mesoporous silica nanoparticles. This result could be further supported by FTIR (Figure S2 in SI). After the extraction, the resonances at approximately 2925 and 2854 cm-1 assigned to CH2 were weakened, and the resonance at 1718 cm-1 arose distinctly and would be assigned to COOH of CES.26 The adjoining presence of amino acid pairs has been confirmed by the formation of amide. It is reasonable that amino groups would react with neighboring carboxyl groups in the presence of N,N0 -dicyclohexylcarbodiimide (DCC) as a peptide coupling reagent under basic conditions. The resonance at 1718 cm-1 corresponding to carboxyl groups disappeared, and a new resonance was seen at 1548 cm-1 (Figure S3 in SI), which indicated that two reactions (i.e., the neutralization with triethylamine or amide formation) may happen. The formation of amide has been confirmed by neutralization and hydrolysis. There was no distinct resonance at 1548 cm-1 without the addition of a coupling agent. Langmuir 2011, 27(2), 517–520
However, the COOH resonance at 1718 cm-1 appeared again and the resonance at 1548 cm-1 disappeared in the hydrolysis sample, indicating that the two functional groups were uniformly distributed in pairs. These amino acid bifunctional nanoparticles can behave as an acid or base depending on the different conditions. After removal of the surfactant by extraction, ammonium groups and carboxyl groups still exist and are covalently connected to the silica wall, which can exhibit positive and negative electricity at different pH values. The isoelectric point curve of the mesoporous materials functionalized with amino and carboxyl groups have been obtained by measuring the pH changes with the titration of a strong base as shown in Figure 4. The quantitative determination of the functional groups was determined by CHN elemental chemical analysis. The loading level of amino groups and the carbon content from APS were calculated by the nitrogen content of the analytical result, and the loading level of the carboxyl groups was calculated by the residual carbon content. The results show that the loading amounts of the amino and carboxyl organic groups were ca. 0.58 and 0.69 mmol/g SiO2, respectively. The theoretical loading of both functional groups is 1.0 mmol/g SiO2. The isoelectric point of the material could be controlled by adjusting the loading amounts of two organic groups. As expected, the amphoteric anionic-cationic/acid-base switching occurred over several minutes, which has been tested more than 10 times. In the process, hydrochloric acid and sodium hydroxide were used as basifying and acidifying agents, respectively. Figure 5 shows photographs of the extracted, the first, and the tenth switched anionic and cationic forms of bifunctional mesoporous silicas nanoparticles. The nanoparticles displayed good dispersity in both cationic and anionic forms in low- and high-pH aqueous solutions, even if switched 10 times. This reversible switchability would confer striking benefits, implying the possibility of recuperation and recycling of the nanoparticles. The pH-responsive dispersity of nanoparticles could be owed to the static electrical repulsion between the nanoparticles because of the existence of ionized functional groups in cationic and anionic forms. However, when the pH is close to the isoelectric point, the ammonium and carboxylate on the particle surface would interact electrostatically, which leads to the immediate aggregation and precipitation of the particles. It has been found that the good dispersity of these nanoparticles requires pH beyond the range of 5.5-7.5. Typically, with regard to nanoparticles, there are requirements for the zeta potential of solution to maintain its dispersity.31,32 It is well known that the zeta value should be higher than 30 mV or lower than -30 mV to maintaining the good (31) Ravi Kumar, M.; Bakowsky, U.; Lehr, C. Biomaterials 2004, 25, 1771. (32) Yin Win, K.; Feng, S. Biomaterials 2005, 26, 2713.
DOI: 10.1021/la103979c
519
Letter
dispersity. The zeta potentials of the amino acid bifunctional nanoparticles were 43 and -41 mV at pH 5.0 and 8.0, respectively, and 31 and -19 mV at pH 5.5 and 7.5, respectively (Figure S6 in SI). The above results show that two kinds of organic functional groups not only endow mesoporous nanoparticles with amphoteric properties but also act as agents to provide a repulsive force for protecting nanoparticles from aggregation. The nanoparticles could maintain the good dispersity for at least 5 days. From the XRD patterns and N2 adsorption-desorption results of the first and tenth switched cationic and anionic forms of bifunctional mesoporous silica nanoparticles, it has been considered that the ordering of the pore arrangement was retained even after the switching repeated 20 times (Figure S4 in SI). The changes between carboxyl and carboxylate in the cationic and anionic forms were confirmed by the FTIR spectrum of the switched samples (Figure S5 in SI). The band appearing at 1718 cm-1 corresponding to the CdO stretching vibration demonstrates the existence of COOH in the acidic samples, which disappeared when the samples were switched to their base forms. The band corresponding to carboxylate would appear at 1548 cm-1, which overlapped with the vibration of the N-H amino group band.
520 DOI: 10.1021/la103979c
Ma et al.
Conclusions We have successfully synthesized amino acid bifunctional monosized mesoporous silica nanoparticles by a one-pot method. These mesoporous nanoparticles have been demonstrated to achieve anionic-cationic switching and controllable dispersity in aqueous solution by simply adjusting the pH value. Acknowledgment. We acknowledge the support of the National Natural Science Foundation of China (grant no. 20821140537), the 973 project (2009CB930403), and the Grand New Drug Development Program (no. 2009ZX09310-007) of China. Supporting Information Available: Experimental details, N2 adsorption/desorption isotherms and pore size distribution of the amphoteric mesoporous nanoparticles, FT-IR of the as-made and extracted materials, the peptide coupling reaction, and the pH-responsive dispersity and stability of the material after 10 cationic-anionic switches. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2011, 27(2), 517–520
