SERS titration of 4-mercaptopyridine self-assembled monolayers at aqueous buffer/gold interfaces

Determination of the surface pK(a) of self-assembled monolayers (SAMs) at a queous buffer/gold interfaces using surface-enhanced Raman scattering (SERS ) measurements, namely, SERS titration, is reported for the first time. Fro m the analysis of pa-dependent SERS spectra of 4-mercaptopyridine monolayer s on gold, the pK(1/2) (3.9 +/- 0.2) was determined. By further correlating the surface pH value with die bulk pH value in terms of the Gouy-Chapmann model, the surface pK(a) (5.3 +/- 0.3) was evaluated accordingly. Compared to the conventional contact angle titration, SERS titration has the advanta ge of giving further insight into the microscopic and dynamic information o f the surface-confined functional groups. In addition, such a technique has high sensitivity, micrometer spatial resolution, and chemical selectivity.