Modification of hollow core-shell particles with poly (amino) acids to induce chirality

Abstract

Due to the potential environmental applications, photocatalytic reactions occurring on the surface of photoirradiated titanium (IV) oxide (TiO2) have garnered a wide interest [1, 2]. Despite displaying high photocatalytic activity, the utilization of TiO2 is limited by its lack of selectivity. To address this problem, Ikeda et al. [3, 4] conducted a study, in which they reported the fabrication of a novel core-shell composite photocatalyst which consisted of commercially available TiO2 particles incorporated into a hollow silica shell (SiO2/void/TiO2). The SiO2/void/TiO2 showed decomposition of small substrates but negligible activity for larger molecules. This composite also exhibited the ability to retain the intrinsic activity of original TiO2 for small substrates due to the presence of a void space between the TiO2 core and the hollow silica shell. In this study, platinized TiO2 core and hollow silica shell (SiO2/void/Pt-TiO2) were synthesized, and the silica pores were modified with chiral agents, namely poly (amino acid)s. The fabrication of the composite is firstly done by modifying the surface of TiO2 powder with amino groups using 3-amino-propyltrimethoxysilane (APS). To obtain carbon-coated TiO2, the amino-functionalized TiO2 was treated with aqueous glucose (carbon layer). Then n-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPS) and tetraethyl orthosilicate (TEOS) (silica layer) was used to yield Si/C/TiO2, followed by heat treatment to remove the carbon layer to obtain SiO2/void/TiO2. As the synthesis of the SiO2/void/TiO2 is still on-going and yet to be successful, poly-L-lysine and poly (L-lactic acid) were impregnated on platinized TiO2 (ST-41) in order to study the effect of poly (amino) acids in inducing chirality. The resulting material and platinized TiO2 was tested out in the photocatalytic redox-combined synthesis of L-PCA (pipecolinic acid) from (racemic) DL-lysine in order to obtain optically active PCA [5]. The results show that platinized TiO2 modified with poly (L-lactic acid) displayed a higher optical purity of L-PCA as compared to normal platinized TiO2, although the conversion was slightly lower. Platinized TiO2 modified with poly-L-lysine on the other hand, demonstrated a much lower conversion percentage among the three samples prepared. This shows that poly (L-lactic acid) have potential in inducing chirality in SiO2/void/TiO2 in order to produce optically active PCA.