Structure Modulation of Silica Microspheres in Bio‐Inspired Silicification: Effects of TEOS Concentration

Abstract

Silica nanoand microspheres have applications in various areas, such as in photonic crystals, catalysis, biosensors, bioassay, and drug delivery. Numerous synthetic methods have been developed and been modified to meet the demands of the applications mentioned above. In particular, the Stcber method is considered as a basic platform for the chemical synthesis of silica spheres: it generally uses ammonia as a catalyst and silicon alkoxide as a precursor in the water/ethanol co-solvent system. Although the conventional Stcber method is quite useful for sizeand shape-control of silica structures, the reaction conditions are harsh owing to the use of ammonia (pH>12) and cannot be applied to biological systems, such as living cells. In this respect, biomimetic (or bio-inspired) silicification, inspired by diatom and glass sponges, was suggested as an alternative approach for the mild and biocompatible formation of silica structures, because it proceeds under physiologically mild conditions (i.e., neutral pH, room temperature, and ambient pressure). The bio-inspired silicification has successfully been applied to the coating of individual living cells without deterioration of cell viability by us. We also have recently reported that individually separated silica microspheres were formed under relatively mild conditions in the presence of cetyltrimethylammonium bromide (CTAB) by using cysteamine (HSCH2CH2NH2) as a biomimetic hydrolysis catalyst, inspired by silicatein, a silica-forming protein, found in glass sponge. It is noteworthy that the diameter of the formed silica spheres was on the micrometer-scale, because microspheres had barely been found in both Stcber and modified Stcber methods. Understanding how the bioinspired silicification was affected by reaction parameters in the cysteamine/CTAB system, such as reactant concentrations and solvents, was required for the detailed elucidation of mechanisms and the morphological control of silica. However, there have been few reports on the effects of the concetrations of silica precursors on silica morphogenesis. Herein, we systematically investigated the effects of the concentration of tetraethyl orthosilicate (TEOS) in detail, along with the ratio of water and ethanol. The synthetic procedure was as follows (Figure 1). The final concentrations of cysteamine and CTAB were fixed to be 50 mm and 5 mm, respectively, after optimization for silica formation. We varied the water/ethanol ratio from 0.6:1 to 1:1 (v/v), and the concentration of TEOS from 80 to 140 mm in 20 mm-intervals. Although the observable changes in the reaction could be seen after 45 minutes, the silicification was performed for 3 hours for comparative studies. The resulting silica precipitates were washed with ethanol several times using centrifugation, dispersed in ethanol, and characterized by attenuated total reflectance infrared (ATR-IR) spectroscopy and field-emission scanning electron microscopy (FE-SEM). The IR spectra showed the characteristic peaks at 1049 (Si O Si asymmetric stretching), 955 (Si O stretching), and 784 cm 1 (Si O Si symmetric stretching) after silicification (for the representative IR spectrum, see the Supporting Information, Figure S1). To investigate the effects of TEOS concentration, we first varied only the concentration of TEOS in the 0.65:1 water/ ethanol system, while keeping the other parameters (the concentrations of cysteamine and CTAB) the same. Of interest, the silica morphology was found to be affected greatly by the concentration of TEOS, as shown in the FE-SEM micrographs (Figure 2a). Specifically, interconnected aggre[a] J. H. Park, J. Y. Choi, T. Park, S. H. Yang, S. Kwon, Prof. Dr. H.-S. Lee, Prof. Dr. I. S. Choi Molecular-Level Interface Research Center Department of Chemistry KAIST Daejeon 305-701 (Korea) Fax: (+82)42-350-2810 E-mail : ischoi@kaist.ac.kr hee-seung_lee@kaist.ac.kr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100265.