Control of interfacial pH in mesoporous silica nanoparticles via surface functionalization

The pH at silica-water interfaces (pH_int) was measured by grafting a dual emission fluorescent probe (SNARF) onto the surface of mesoporous silica nanoparticles (MSN). The values of pH_int of SNARF-MSN suspended in water were different from the pH of the bulk solution (pH_bulk). The addition of acid or base to aqueous suspensions of SNARF-MSN induced much larger changes in pH_bulk than pH_int, indicating that the interface has buffering capacity. Grafting additional organic functional groups onto the surface of SNARF-MSN controls the pH_int of its buffering region. The responses of pH_int to variations in pH_bulk are consistent with the acid/base properties of the surface groups as determined by their pK_a and are affected by electrostatic interactions between charged interfacial species as evidenced by the dependence of ζ-potential on pH_bulk. Finally, as a proof of principle, we demonstrate that the hydrolysis rate of an acid-sensitive acetal can be controlled by adjusting pH_int via suitable functionalization of the MSN surface. Our findings can lead to the development of nanoreactors that protect sensitive species from adverse conditions and tune their chemical reactivity.