Measuring Local pKa and pH Using Surface Enhanced Raman Spectroscopy of 4-Mercaptobenzoic Acid

We report spectroscopic measurements of the local pH and pK_a at an electrode/electrolyte interface using surface enhanced Raman scattering (SERS) spectroscopy of 4-mercaptobenzoic acid (4-MBA). In acidic and basic solutions, the protonated and deprotonated carboxyl functional groups at the electrode surface exist in the solution as -COOH and -COO^-, which have different Raman active vibrational features at around 1697 and 1414 cm^-1, respectively. In pH neutral water, as the applied electrochemical potential is varied from negative to positive, the acidic form of the 4-MBA (i.e., -COOH) decreases in Raman intensity and the basic form (i.e., -COO^-) increases in Raman intensity. The change in local ion concentration is due to the application of electrochemical potentials and the accumulation of ions near the electrode surface. Under various applied potentials, the ratio of 1697 and 1587 cm^-1 (pH-independent) peak areas spans the range between 0.7 and 0, and the ratio of the 1414 and 1587 cm^-1 peak areas ranges from 0 to 0.3. By fitting these data to a normalized sigmoid function, we obtain the percentage of surface protonation/deprotonation, which can be related to the pK_a and pH of the system. Thus, we can measure the local pK_a at the electrode surface using the surface enhanced Raman signal of the 4-MBA.