KINETIC SEPARATION OF AMPEROMETRIC RESPONSES OF COMPOSITE REDOX-ACTIVE MONOLAYERS ASSEMBLED ONTO AU ELECTRODES - IMPLICATIONS TO THE MONOLAYERS STRUCTURE AND COMPOSITION

Chronoamperometry provides a sensitive method to elucidate the composi tion and properties of complex redox-active monolayers assembled onto Au electrodes. Chronoamperometry is applied to characterize a redox-ac tive monolayer composed of a bisdiaminobenzoquinone dyad. The differen t distances of the quinone units in respect to the electrode yield dif ferent electron-transfer rate constants, k(1) = 25.3 s(-1) and k(2) = 1.05 s(-1), and the surface coverage of the two quinone units is estim ated to be Gamma(Q1) = 6.5 x 10(-11) mol cm(-2) and Gamma(Q2) = 6.5 x 10(-11) mol cm(-2). The surface densities of the quinone units coincid e nicely with those obtained by voltammetric analysis of the monolayer during the steps of its assembly. Similarly, a binary monolayer consi sting of bipyridinium and anthraquinone redox-active units was charact erized by chronoamperometry. The electron-transfer rate constant to th e bipyridinium and anthraquinone components corresponds to k(1) = 503 s(-1) and k(2) = 62 s(-1) respectively, and the surface coverage of th e redox-active groups is Gamma(V2+) = 1.44 x 10(-10) mol cm(-2) and Ga mma(Q) = 0.46 x 10(-10) mol cm(-2). Kinetic separation of the amperome tric response of a redox-active monolayer formed by the coupling of N, N'-bis(7-carboxyheptyl)-4,4'-bipyridinium, V2+-(CO2H)(2), to a cystami ne-modified monolayer on an Au electrode, reveals the formation of two redox-active groups within the monolayer assembly: one redox-active f orm consists of a single-point attachment of V2+-(CO2H)(2) to the elec trode, whereas the second form includes the bifunctional, two-point li nkage of V2+-(CO2H)(2) to the monolayer. The surface coverage of the t wo redox-active forms corresponds to Gamma(1) = 1.11 x 10(-10) and Gam ma(2) = 1.13 x 10(-10) mol cm(-2), respectively. The electron-transfer rate constants to the single-point-associated bipyridinium sites and to the two-point linkage bipyridinium units are k(1) = 508 s(-1) and k (2) = 896 s(-1), respectively. Chronoamperometric analysis of microper oxidase-ll assembled as a monolayer on an Au electrode reveals that th e heme-substituted oligopeptide binds to the surface by two different modes. These include the covalent Linkage of carboxylic functions asso ciated with the porphyrin ligand or the peptide backbone to the primar y cystamine monolayer assembled onto the Au electrode.