Synthesis and characterization of a catechol-terminated alkanethiolate monolayer adsorbed on gold

Structural, redox, and metal ion binding characterizations of the monolayer formed by the chemisorption of 3,4-dihydroxyphenethyl mercaptan (DHPM) on gold are described. This system was explored as a model for investigations of surface-immobilized molecules that contain redox-transformable coordination sites for binding metal ions from solution. The characterizations were carried out using infrared reflection spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and long optical pathlength thin-layer spectroscopy. The XPS data show that this monolayer adsorbs as a thiolate, and the IRS results indicate that the average orientation of the aromatic ring plane is close to the surface normal. Cyclic voltammetry reveals that the pendant catechol/quinone undergoes the expected two-electron, two-proton redox transformation; however, the oxidized form of the couple is unstable, particularly in alkaline aqueous solutions. A determination of Cu(II) binding ability to the surface-immobilized catechol moieties, using a long optical pathlength thin-layer cell to follow the depletion of Cu(II) from solution, indicated that the complexation of the metal was not substantially different from that for the solution form of catechol. The instability of the oxidized form of the ligand, nevertheless, precluded an assessment of its metal ion binding ability. An approach using these data for estimating the conditional formation constant for the surface complexation reaction is also developed.