Large-pore periodic mesoporous organosilicas (PMO and PMA) for lipase and laccase immobilization

Abstract

Periodic mesoporous organosilicas (PMO) have burst into the field of enzyme immobilization as excellent supports where the organic moiety needed to interact with the enzyme is integrated in the surface and not anchored on it, giving rise to nanostructured biocatalysts with improved properties. Initially, research on PMO had been focused on the incorporation of functional species into the walls of the channellike structures obtaining different morphologies. To synthesize these materials, organic-bridged alkoxysilane compounds were used as precursors. While the framework composition of PMO was found highly tailorable, their pore size turned out to be much less adjustable than expected on the basis of the studies of their pure-silica counterparts. Currently, there is a quest for large-pore PMO from the point of view of immobilization and encapsulation of large molecules; therefore, the development of the synthesis method is closely related to their specific applicability. Based on our previous works on the synthesis of PMO containing ethylene groups for lipase immobilization1 and on the synthesis of ordered mesoporous materials (OMM) with expanded pore size2, we propose here to obtain large-pore PMO containing amine groups. There have been few attempts in the literature to introduce nitrogencontaining groups into the PMO walls and the obtained pore sizes were always smaller than those attainable for pure silica, not exceeding 6 nm. This factor limits the potential applications of PMO as supports for the immobilization of large enzymes. We report the synthesis of periodic mesoporous aminosilica (PMA) at low temperature and in the presence of an inorganic salt, using 1,3,5-triisopropylbenzene as micelle expander and bis[(3-trimethoxysilyl)propyl] amine and 1,2- bis(trimethoxysilyl)ethane as co-precursors for producing ultra large-pore (> 10 nm) PMA for the immobilization of laccase. The properties of this multifunctional PMA having hydrophilic amino groups and hydrophobic ethylene/propylene groups within the framework were studied. In this work we also compare the confinement of two model enzymes in the tailored pores of different PMO materials and its effect on immobilization and stabilization parameters. Laccase immobilized on PMA and lipase immobilized on PMO exhibited higher stability in solvents (ethanol and methanol, respectively) compared to functionalized silica materials with pending organic groups on the surface. We achieved also high retention and low leaching of the enzymes from the inner surfaces. These results can be attributed to the different interactions (hydrophobic, electrostatic and hydrogen bonding) between the enzymes and the surface of PMO/PMA and to the tailored design of optimum pore size.