In situ photopolymerization of methacrylic acid at a self-assembled xanthate monolayer surface on gold. Formation of poly(methacrylic acid) brushes and their interaction with cytochrome c

Mixed monolayers of photoinitiatable xanthate-carrying, double-chained disulfide compound 1 and the corresponding xanthate-free compound 2 were spontaneously formed on a gold-coated quartz crystal microbalance (QCM). Self-assembling processes of these mixtures could be monitored as a frequency shift of QCM, and the composition of the resultant monolayers was found to be comparable to that of feed in solutions (f(1), mole fraction of 1 in the mixture). Photopolymerizations of methacrylic acid in aqueous solution were carried out upon UV irradiation initiated with xanthate groups of the monolayer-modified QCMs and immediately traced by measuring the frequency change. The polymerization rate was strongly dependent upon f(1) and gave a maximum at around f(1) = 0.2. In addition, the polymerization rate showed a marked pH dependence. Finally, an adsorption of cytochrome c to the poly(methacrylic acid) brushes thus prepared was examined by means of QCM and spectroscopy. As a result, cytochrome c was found to adsorb successfully to the poly(methacrylic acid) brushes through electrostatic interaction, depending upon the composition of the poly(methacrylic acid) chain grown at the monolayer surface.