Specific Ion Effects on Adsorption of Lysozyme on Functionalized SBA-15 Mesoporous Silica

Ordered mesoporous materials (OMMs) have a pore size suitable to host proteins. Previous works have shown how to tune the amount of adsorbed protein by changing pH or ionic strength of the adsorbing solution. Here we investigated the adsorption of lysozyme on a functionalized SBA-15 (SBA-15-NH2) as a function of added salts. For the first time, it was ascertained that the amount of adsorbed protein follows a reversed Hofmeister series for anions (sodium salts), SCN- > ClO4- > Br- > NO3- > Cl- > SO42-, whereas for cations (chloride salts) the sequence was Na+ > Li+ > K+ > Cs+. These findings not only demonstrate a specific effect of the Na+ SCN- ion pair in favoring the adsorption at a solid surface but confirm also the role of the biologically important sodium ions. In addition, the process was found to be more effective at 0.2 M than at 0.8 M, thus indicating that adsorption also depends on the added salt concentration.