Cyclic voltammetric studies with gold wire electrodes covered with lipoyl gamma-cyclodextrins: positional isomerism effect of the modified cyclodextrins on monolayer quality and electrochemical responses on soluble probes

Four positional isomers of gamma -cyclodextrins (gamma -CyD) having two lip oyl residues were prepared for investigating the effect of the positional i somerism of the lipoyl residues on the quality of the resulting monolayers deposited on gold wire electrodes. Charges associated with reductive desorp tion of lipoyl residues from the gold surface revealed that the 6(A),6(E)-i somer (4) formed a nearly defect-free monolayer from its 1 mM solution, whe reas the 6(A),6(C)-, 6(A) 6(D) -isomers (2 and 3, respectively) covered 86% and 89%, respectively, of the gold surface, and 6(A),6(B)-isomer (1) cover ed only 60% of the gold surface. The cyclic voltammetric responses of Fe(CN )(6)(3-) on the 1-4-modified electrodes was partly blocked by the 1-4 monol ayers, and the largest inhibition was achieved by 1 monolayer. The cyclic v oltammetric responses of ferrocenecarboxylic acid (FCA) was slightly affect ed by the presence of 1-4 monolayers. Ursodeoxycholic acid, which is an exc ellent guest for gamma -CyD, reduced the anodic current of FCA at the 1-4-m odified electrodes, and the degree in the reduction caused by the monolayer s was in the order of 4 > 3 > 2 = 1. This order indicated that the cavity o f 4 was the most active ones to bind a guest in a solution phase, while the cavities of 1 and 2 was less active. These results indicate that in monola yer film, the positional isomerism of disubstituted-CyDs actually influence d the electrode responses. (C) 2001 Elsevier Science B.V. All rights reserv ed.