Protein Adsorption in Functionalized SBA-15 Silica: Influence of pH and Ionic Strength

Ordered mesoporous silica materials have favorable properties as hosts for the immobilization and thermal stabilization of proteins. The ability to control and modify the interaction of protein molecules with the pore walls is of importance in many biotechnical and biomedical application fields, including ultrafiltration, drug delivery and biosensing.

In this work we studied the adsorption of lysozyme, a “hard” globular protein with a high isoelectric point (pI = 11), in the nanochannels of native and surface-functionalized SBA-15. Functionalization with sulfonic acid may help to increase the adsorption of the protein at pH < 7, where the native silica surface is only weakly charged. We also studied the influence of a “protein-repellant” zwitterionic sulfobetaine on the protein uptake. At pH 4, lysozyme adsorption is indeed suppressed in the zwitterionized material and enhanced in the sulfonated material, relative to native SBA-15. At pH 11, on the other hand, a dense protein monolayer is formed in the pores of all materials. We also find that an increase in ionic strength (100 mM) causes an increase in adsorption of lysozyme at low pH but a decrease at high pH. All these findings are consistent with the assumed dominant role of electrostatic interactions in the adsorption of lysozyme in the nanopores of the SBA-15 materials.