Synthesis and characterization of the amphoteric amino acid bifunctional mesoporous silica

New amphoteric, amino acid bifunctional mesoporous silicas with highly ordered mesopores were synthesized via the cooperative self-assembly of surfactants, co-structure-directing agents (CSDA), and a silica source. Uniform distributions of acid and base organic groups on the mesopore surfaces were formed by interactions between the counter charged surfactant head groups and ionic parts of CSDAs, that is, between the cationic and anionic surfactant head groups and the carboxyl and amino groups of CSDAs, respectively. For structural determination, the materials were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and N₂ sorption measurements. It has been demonstrated that organic (NH₂ and COOH) pairs incorporated in the mesopore walls behave as amino acids, collectively exhibiting an isoelectric point, pI, of ∼6.0, a value close to that of a neutral amino acid. Further, the amphoteric amino acid moiety can be switched readily in a moment from net cationic and acidic to net anionic and basic by simply increasing the pH of the solution. Loading of the NH₂ and COOH groups was controlled at an almost equal basis of ca. 1.0 and 0.9 mmol/g SiO₂, respectively. Close adjacency of organic NH₂ and COOH groups was confirmed by the formation of amide moieties. The basic idea employed here will be easily and widely applicable to the design of relevant mesoporous materials.