Cyclic voltammetric and scanning electrochemical microscopic study of thiolated beta-cyclodextrin adsorbed on a gold
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Potassium ferricyanide ([Fe(CN)(6)](3-)) reduction and ferrocenemethanol (Fe-OH) oxidation at a beta-cyclodextrin (beta-CD)-modified gold electrode were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) in phosphate buffer pH 7. CV and SECM experiments demonstrated that the surface of the modified electrode represents an insulating substrate for ferricyanide and a conductor substrate for Fc-OH. This difference is explained by the fact that Fc-OH can enter into the beta-CD cavity forming an inclusion complex; on the other hand, ferricyanide is larger than the cavity of P-CD and so it cannot form an inclusion complex. Using the high sensitivity of the electron transfer of ferricyanide to the modification of the gold surface with beta-CD, we selected this reaction as a probe to study the different modification stages at this modified electrode. When the electrode was partially modified, it was possible to obtain an adequate microscopic discrimination by using approach curves at different tip positions showing a nonhomogeneous surface. SECM images obtained from bare and partially and totally modified electrodes show very good resolution with different topography or null according to the extent of modification. When the modification was total, a homogeneous current with a regular topography was found, but when the modification was intermediate the topography was very irregular. To our knowledge, this is the first electrochemical study of a thiolated beta-CD layer adsorbed on a gold electrode by SECM, and taken together, these results demonstrate that while the CV experiments give overall information about the studied surface, the SECM experiments permit microscopic characterization of the layer.