Determination of the surface pK of carboxylic- and amine-terminated alkanethiols using surface plasmon resonance spectroscopy

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pK(d)) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pK(d) of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pK(d) values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pK(d) values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pK(d). The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pK(d) of the surface.