Heterogeneous electron-transfer kinetics for PQQ covalently attached to aminoalkanethiol monolayers on gold

Pyrroloquinoline quinone (PQQ) was covalently attached to a gold electrode surface via the self-assembled monolayers of aminoalkanethiols (NH2)(CH2)nSH) with different alkyl chain lengths (n=2, 6, 8, and 11). The voltammetric behaviors of PQQ-SAMs both at full and at partial monolayers were examined. It was found that the redox response of PQQ became more reversible with increasing dilution by alkanethiols owing to the less lateral interaction between PQQ heads. Voltammograms were transformed from quasi-reversibility to irreversibility as the alkyl chain spacer became longer. The electron tunneling barrier constant (beta) and distance dependence of the long range electron-transfer parameters have been studied in acidic solution using cyclic voltammetry. A logarithmic dependence of the heterogeneous electron-transfer rate constant on the alkyl chain length was found for full and partial PQQ monolayers. The beta value was close to 1.0 per CH2 unit regardless of dilution effect.