Spectroelectrochemical Study of N‐Substituted Phenoxazines as Electrochemical Labels of Biomolecules

The electrochemical conversion of N‐substituted phenoxazines (NSP's) bearing a CH₂CH₂? X substitute (where X?OH, COOH, CH₂NH₂, CH₂SO₃H, CH₂NHCOR) was investigated using cyclic voltammetry on a bulk gold electrode and a thin‐layer spectroelectrochemical cell. The electrochemical oxidation of NSP's on the gold electrode was quasi‐reversible and proceeded in a diffusion‐controlled regime. The formal redox potential of NSP's covered the range from 0.39 to 0.45 V vs. SCE. The electrochemical oxidation of NSP's in the thin‐layer spectroelectrochemical cell produced radical cations that showed absorbance at 385, 410 and 530 nm. Electrochemical conversion fitted the general voltammetric current‐potential equation of a reversible wave, whereas electrolysis at constant potential showed a typical Cottrell behavior. Combining of NSP's with a biologically‐relevant theophylline molecule did not change electrochemical and spectral properties of the phenoxazine core. Theophylline enlarged with NSP's demonstrated electrochemical and biocatalytic behavior similar to that of NSP's. The investigated NSP's possess electrochemical and spectral properties that are useful as biomolecular labels for electroanalysis.