Monolayers of homocysteine on Au(111)-surfaces have been investigated by voltammetry, in situ scanning tunnelling microscopy (STM) and subtractively normalised interfacial Fourier transform spectroscopy (SNIFTIRS). A pair of sharp voltammetric peaks build up in the potential range 0 to -0.1 V (vs. SCE) in phosphate buffer pH 7.7. The peak half-widths are about 25 mV at a scan rate of 10 mV s(-1). This is much smaller than for a one-electron Faradaic process (90.6 mV) under similar conditions. The coverage of homocysteine is 6.1 (+/- 0.2) x 10(-10) Mol cm(-2), or 5.9 x 10(-5) C cm(-2), from Au-S reductive desorption at -0.8 V (SCE) in 0.1 M NaOH, while the charge is only about 8 x 10(-6) C cm(-2) (pH 7.7) for the 0 to -0.1 V peak. This suggests a capacitive origin. The peak potential and shape depend on pH. At pH 7.7 both cathodic and anodic peak currents reach a maximum, but drop at both higher and lower pH. The midpoint potential shows biphasic behaviour, decreasing linearly with increasing pH until pH 10.4 towards a constant value at higher pH. The cathodic and anodic peak charges decay at pH both higher and lower than 7.7. The homocysteine monolayer was investigated by in situ STM at different potentials at pH 7.7. The molecules pack into highly ordered domains around the peak potential. High-resolution in situ STM reveals a (root 3 x 5) R30 degrees lattice with three homocysteine molecules in each unit cell. The adlayer changes into disordered structures on either side of the peak potential. This process is reversible. We propose that the voltammetric peaks are capacitive. The ordered domains are formed only around the potential of zero charge (pzc) and dissipate at potentials on either side of the peak, inducing mirror charge flow in the metallic electrode as the charged -COO- and -NH3+ groups approach the surface. No bands for carboxylate coordinated to the surface were observed in SNIFTIRS implying more subtle orientation changes of the charged groups on transcending the voltammetric peak. This scenario is incorporated in a simple phenomenological model.
Chemical Physics, 2005, Vol 319, Issue 1-3, p. 210-221