Grafting of thin organic films by electrooxidation of arylhydrazines

Anodic oxidation of arylhydrazines in aqueous solution leads to formation of a chemisorbed monolayer on the electrode surface of both glassy carbon and gold. The grafting process occurs in a three-electron process, in which the arylhydrazine is oxidized to the actual grafting agent, that is, the corresponding aryl radical, via the formation of the aryldiazene as intermediate. Analysis of the modified surface by X-ray photoelectron spectroscopy and polarization modulation infrared reflection absorption spectroscopy shows conclusively that the bonding to the surface does not occur through the hydrazine group but rather through the aryl group after expulsion of nitrogen. In general, the method is found to be tolerant toward variations in the experimental conditions, but at high pH (>7), the grafting rate increases considerably and multilayers may be formed. The more efficient grafting at high pH can be attributed to an increase of the rate of the deprotonation steps in the oxidation mechanism, the result of which will be that the aryl radical formation takes place close to the surface to be modified. In general, the monolayer exhibits a low susceptibility toward further grafting due to a lowering of the electron transfer rate and the occurrence of physisorption. In particular, at high concentrations of the arylhydrazine, physisorbed products formed during the grafting process are layered on top of the chemisorbed film, hence enhancing the blocking effect and precluding further growth