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 composition and properties of complex redox-active monolayers assembled onto Au electrodes. Chronoamperometry is applied to characterize a redox-active monolayer composed of a bisdiaminobenzoquinone dyad. The different distances of the quinone units in respect to the electrode yield different 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 estimated 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 coincide nicely with those obtained by voltammetric analysis of the monolayer during the steps of its assembly. Similarly, a binary monolayer consisting of bipyridinium and anthraquinone redox-active units was characterized by chronoamperometry. The electron-transfer rate constant to the bipyridinium and anthraquinone components corresponds to k(1) = 503 s(-1) and k(2) = 62 s(-1) respectively, and the surface coverage of the redox-active groups is Gamma(V2+) = 1.44 x 10(-10) mol cm(-2) and Gamma(Q) = 0.46 x 10(-10) mol cm(-2). Kinetic separation of the amperometric response of a redox-active monolayer formed by the coupling of N,N'-bis(7-carboxyheptyl)-4,4'-bipyridinium, V2+-(CO2H)(2), to a cystamine-modified monolayer on an Au electrode, reveals the formation of two redox-active groups within the monolayer assembly: one redox-active form consists of a single-point attachment of V2+-(CO2H)(2) to the electrode, whereas the second form includes the bifunctional, two-point linkage of V2+-(CO2H)(2) to the monolayer. The surface coverage of the two redox-active forms corresponds to Gamma(1) = 1.11 x 10(-10) and Gamma(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 microperoxidase-ll assembled as a monolayer on an Au electrode reveals that the heme-substituted oligopeptide binds to the surface by two different modes. These include the covalent Linkage of carboxylic functions associated with the porphyrin ligand or the peptide backbone to the primary cystamine monolayer assembled onto the Au electrode.