Metal substrate effects on pH response of tetracyanoquinodimethane modified electrodes

Abstract

In recent work the voltammetric and spectral response of tetracyanoquinodimethane (TCNQ) modified electrodes have been described. The effect of pH of the cyclic voltammetry of these modified electrodes can be understood on the basis of the classical 3 x 3 nine-membered square scheme characteristic of quinone-hydroquinone couples. As for other compounds with quinoid structures, TCNQ exhibits quasi-reversible behavior in protic solvents with an apparent direct 2e/sup -/, 2H/sup +/ exchange in acidic pH region. This scheme is complicated further by dimer formation, disproportionation of the anion radical in strongly acidic solutions, and the apparent reduction of neutral TCNQ by hydroxide ions in very basic media (pH > 12). In basic solution the formation of TCNQ/sup -/ is clearly signaled by its electron spin resonance and visible absorption spectra. While quinone-hydroquinone couples do exhibit subtle electrochemical behavior, they are the basis for a widely used and reliable redox pH electrode. Thus an attempt to utilize TCNQ modified electrodes as a pH sensor seemed to be a logical step based on the kinship between TCNQ and benzoquinone. However, the results support the idea that the electrode substrate plays an important role in determining the response to the pH of the bulk solution. more » 20 references, 1 figure. « less