Comparative study of electropolymerization versus adsorption of tyrosine and the decyl ester of tyrosine on platinum electrodes

We compared adsorption versus electropolymerization via cyclic voltammetry (CV) of tyrosine, an amino acid, to that of the decyl ester Of D-tyrosine (DEDT), on Pt electrodes. DEDT is an amphiphilic compound, capable of self-assembly into long rod like or tubular aggregate structures. Tyrosine and DEDT differ fundamentally in their interactions with the Pt electrode, affecting their electropolymerization behavior. The DEDT exhibits a significant level of adsorption to the Pt surface, resisting desorption via washing to allow a significant peak current I-p to be measured. At concentrations 30 times higher than DEDT, tyrosine exhibits no equivalent behavior. The sweep rate dependence of I-p measured during the first CV cycle of film formation for the two species indicates very different behavior as well. Tyrosine has a square root dependence indicating a diffusion limiting mechanism, while DEDT has a linear sweep rate dependence indicating that it is a surface confined species and follows an adsorption limiting mechanism. Electrochemical quartz crystal microbalance (EQCM) studies show that in the absence of an applied potential, tyrosine does not adsorb to the Pt electrode since no Deltaf shift is observed. On the other hand, DEDT does adsorb to a significant extent, displaying a sizable Deltaf shift. When these same samples are then electropolymerized, tyrosine forms an electropolymerized film stepwise with each potential cycling step, while DEDT, already adsorbed to the Pt, exhibits no net mass gain for any of the potential cycles. Therefore, little electropolymerization of DEDT can take place in the presence of a significant adsorption level of this monomer. We ascribe these differences between tyrosine and DEDT largely to the presence of the decyl chain on DEDT, which can bind hydrophobically to the Pt surface and allow DEDT monomers to self-assemble with one another. (C) 2002 Published by Elsevier Science B.V.