Scanning electrochemical microscopy (SECM) study of superoxide generation and its reactivity with 1,4-dihydropyridines

A scanning electrochemical microscope (SECM), in the tip generation substrate collection and feedback modes, was used in a method to characterize the electrode mechanism of the O-2/O-2*(-) couple in dimethylsulphoxide (DMSO) containing 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. Also the quantification of the interaction between O-2*(-) and different 1,4-dihydropyridine compounds is reported. The SECM results demonstrated that the O-2/O-2*(-) couple follows an E mechanism, in contrast with the EC2 mechanism (DISPs) that was previously reported by cyclic voltammetry. This result implies that in the time scale of SECM measurements there is no time for a homogeneous chemical reaction to be coupled to the electron transfer, i.e., the superoxide is a stable radical. Also we have determined the heterogeneous standard rate constant, k(0) of the quasi-reversible reduction of oxygen to superoxide anion. Taking advantage of the fact that the superoxide suffers no chemical decay during the SECM experiment, a method to obtain the direct interaction of different 1,4-dihydropyridine molecules with superoxide was developed. The study revealed that all the 1,4-DHPs scavenged superoxide with sufficiently high interaction constants (similar to 10(5) M-1 s(-1)). No significant difference between the different molecules was found. This paper shows that the SECM feedback mode is a sensitive technique, giving an accurate determination of the homogeneous interaction constant and allowing the determination of faster rate constants than those found from cyclic voltammetry. (c) 2004 Elsevier B.V. All rights reserved.