Protein Immobilization in Surface-Functionalized SBA-15: Predicting the Uptake Capacity from the Pore Structure

Immobilization of enzymes in ordered nanoporous materials has been studied for two decades, but little attention has been paid to a quantitative assessment of the protein uptake capacity. We propose a pore-filling model for globular proteins that takes into account not only the primary pore channels but also secondary and disordered pores, which can make a significant contribution to the pore volume of polymer-templated silica materials like SBA-15. On the basis of this model we present a comparative study of protein adsorption in SBA-15 materials with native and chemically functionalized pore walls, using lysozyme as a model protein. At low pH, functionalization with sulfonic acid enhances the adsorption affinity while functionalization with a zwitterionic sulfobetaine causes no significant effect. The maximum uptake of lysozyme near its isoelectric point at pH 11 is determined by the accessible pore volume and can be predicted by the new model. We propose that the new model can form a rational basis for predicting and comparing protein immobilization in mesoporous materials.