Ionization and Reprotonation of Self-Assembled Mercaptopropionic Acid Monolayers Investigated by Surface Plasmon Resonance Measurements

We have investigated self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) on gold substrates by using angle and time-resolved surface plasmon resonance (SPR) measurements. The angle-resolved SPR measurements, carried out in two solvents (ethanol and water) and at two wavelengths (632.8 and 850.0 nm), lead to a value of 0.50 ± 0.02 nm for the thickness of the MPA SAM. Employing a high-resolution bicell photodetector for time-resolved SPR, we have determined the rate constant for the formation of MPA SAM in water to be 7.54 × 10 - 3 min - 1 . By combining the results of the two types of SPR experiments, we determined the change of refractive index (∼10 - 3 refractive index units) as a function of MPA ionization and reprotonation.