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