The electrochemical characteristics of self-assembled monolayers (SAMs) of 6-mercaptonicotinic acid (MNA) on gold electrode have been studied. The ionization of carboxyl groups of MNA monolayer has been investigated by voltammetric and Faradic impedance analysis using Fe(CN)(6)(3-/4-) redox marker. The surface pK(a) of MNA was determined to be 5.5 and it does not vary with the ionic strength of supporting electrolyte and surface coverage of monolayer. The carboxyl groups of the monolayer are far apart and the in-plane interaction does not exist between the carboxyl groups. The surface pK(a) of MNA shifts to positive side when it is co-assembled with MPY on the Au surface due to the increase in the hydrophobicity of the monolayer. The negatively charged MNA monolayer favors the electrode reaction of positively charged hydrophilic Ru(NH3)(2+) whereas it shows a sluggish voltammetric response for monopositive hydrophobic dopamine (DA). The electrostatic and hydrophobic effects control the selectivity of NINA monolayer towards DA and dihydroxyphenylacetic acid (DOPAC). (c) 2005 Elsevier B.V. All rights reserved.