Molecular aspects on the amino acid-mediated sol–gel process of tetramethoxysilane in water

Abstract

The aqueous sol–gel process of (TMOS) in conjunction with several natural amino acids (AAs) is employed for synthesis of amino acid/silica hybrid materials. These hybrid materials contain the amino acid physically bonded within the matrix. The sol–gel process occurs readily in unbuffered aqueous amino acid solution. The pH value during the processing is only determined by the dissolved amino acid. It ranges between pH 3–11 as function of the isoelectrical point (IEP) of the AAs. Systematic study of the influence of molecular structure of the AAs L-alanine, e-aminocaproic acid, L-arginine, L-asparagine, L-aspartic acid, L-glutamic acid, L-glutamine, glycine, L-histidine, L-hydroxyproline, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, and L-valine on the gelation time and dissolution behavior of TMOS in water is discussed. The IEP of the AA determines both dissolution and gelation of TMOS in water. Glycine was used as a model compound to investigate how concentration affects dissolution and gelation time of TMOS as well as porosity of resulting silica. As a result, the hydrolysis rate of TMOS in water and therewith dissolution is decelerated and gelation process is accelerated by increasing glycine concentration. Glycine-mediated sol–gel processes of TMOS were additionally examined by 1H NMR spectroscopy, dynamic light scattering, and ATR-FTIR spectroscopy as a function of reaction time to support the visual observed results. Furthermore, various glycine derivatives such as N-acetylglycine, N-benzoylglycine, glycine ethylester, glycine ethylester·HCl, N-methylglycine, N,N-dimethylglycine, N,N-dimethylglycine methylester, and N-phenylglycine are used for studying the influence of amino acid structure functionalities on dissolution and gelation time of TMOS.