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 zeta-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.