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 x 10(-3) min(-1). By combining the results of the two types of SPR experiments, we determined the change of refractive index (similar to10(-3) refractive index units) as a function of MPA ionization and reprotonation.