Effect of surface modification with silane coupling agent on enhancing pzc value and enzyme loading capacity of mesoporous TiO₂ whiskers

Mesoporous TiO₂ whiskers(MTiO₂_ws) were chemically modified with 3-aminopropyltriethyloxy silane(APTES). As a result, the point of zero charge(pzc)for the modified material(MTiO₂_ws-APTES)rose from 5.3 to 6.8. Also the specific surface area of MTiO₂_ws-APTES slightly increased, but the pore structure had little change compared to that of MTiO₂_ws. Then, MTiO₂_ws-APTES was used as the support for γ-glutamyltranspeptidase(GGT) immobilization. MTiO₂_ws-APTES showed better loading capacity than MTiO₂_ws. While the enzyme concentration was less than 150 U·g^-1, the recovery of activity was above 99% and the highest specific activity for immobilized GGT(MTiO₂_ws-APTES-GGT)was 184.0 U·g^-1. Although both optimal temperature and thermal stability of MTiO₂_ws-APTES-GGT slightly decreased compared with the native enzyme, pH stability was greatly improved. The affinity constant(K_m)of MTiO₂_ws-APTES-GGT toward γ-glutamyl-p-nitroaniline(GpNA)was also determined to be 0.889 mmol·L^-1, which was higher than that of the native enzyme, but less than that of MTiO₂_ws-GGT. After storage at 4°C for 60 days and reused for 21 batches, MTiO₂_ws-APTES-GGT maintained 80.07% of its initial activity.