Adsorption characteristics of anthraquinone-2-carboxylic acid on gold

The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) on a gold surface has been investigated by reflection-absorption infrared (RAIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), surface-enhanced Raman scattering (SERS), quartz crystal microbalance (QCM), and ab initio quantum mechanical calculation. The RAIR spectral data clearly showed that the molecule was chemisorbed on gold as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the gold substrate; the molecular plane was determined to be tilted by 30 degrees from the surface normal. The XPS and SERS spectral features were also consonant with the RAIR data. On the other hand, in CV, symmetrical redox peaks arising from a surface-confined species were reversibly identified at -647.5 mV, and the surface coverage determined from CV, that is, 1.8 x 10(-10) mol/cm(2), was consistent with that estimated from QCM. The adsorption rate of AQ-2-COOH on gold was, however, much slower than that on silver, as well as the adsorption strength to gold being considerably weaker than to silver. Control experiments showed that ordinary aromatic and aliphatic carboxylic acids hardly adsorb by self-assembly on the gold surface, in contrast to AQ-2-COOH. Theoretical considerations based on a frontier orbital approach, however, failed in the elucidation of the higher adsorptivity of AQ-2-COOH to gold over ordinary aromatic and aliphatic carboxylic acids.