Spatial Distribution of Enzymes Immobilized in Mesoporous Silicas for Biocatalysis

Journal article, 2019

Mesoporous silica (MPS) particles are promising materials for the immobilization of a variety of enzymes due to the possibility of tailoring their morphology and surface properties. A better understanding of the spatial distribution of enzymes immobilized in these materials is important for improved biocatalytic applications. In this work three types of MPS materials are compared for the immobilization of alcohol dehydrogenase (ADH): two ordered mesoporous silica type SBA-15 particles with 6.8 and 10 nm pore diameters and one mesostructured cellular foam (MCF) with a pore diameter of 26.8 nm and window diameter of 10.7 nm. Nitrogen sorption analysis and transmission electron microscopy (TEM) using immunogold staining (IGS) are used to study the immobilized amount and spatial distribution of ADH in the MPS. In the MCF the mesopore volume occupied by the enzymes measured by nitrogen sorption analysis agrees well with the calculated pore filling, which indicates that the amount of enzyme bound to the external surface is small as supported by TEM imaging showing an essentially even distribution in the MCF particles at least up to 2000 nm from the particle surface. In the case of SBA-15 with 6.8 nm pore size the ADH can enter the pores even though the pore size distribution is smaller than the hydrodynamic size of the protein, but the enzymes only penetrate to about 500 nm from the particle surface. With 10 nm pores the ADH penetrates further into SBA-15 but is still not evenly distributed. MCF can accommodate the largest amount of enzyme per gram of particles and still has a substantial fraction of its mesopore volume available, showing that MCF is a good host for the immobilization of large enzymes such as ADH.