Electrochemical characteristics of hydroquinone-terminated self-assembled monolayers on gold

A series of thiol-functionalized hydroquinone derivatives with different alkyl chain lengths, 2-(n-mercaptoalkyl)hydroquinone (abbreviated as H(2)Q(CH2)(n)SH, where n = 0, 1, 4, 6, 8, 10, and 12), has been synthesized and used for preparation of self assembled monolayers (SAMs) on gold electrodes. The compounds spontaneously adsorb onto a gold surface to form stable and reproducible monolayers. The voltammetric behavior of the hydroquinone (H(2)Q) SAMs has been investigated by cyclic voltammetry to study the effect of alkyl chain length on the electrochemical characteristics for H(2)Q-SAMs. The H(2)Q-SAMs show well-developed voltammetric responses in aqueous solution and pH dependence with a slope of 58.5 mV/pH of surface formal potential in the broad pH range from 1.3 to 12.1 corresponding to the two-electron two-proton redox reaction of a H(2)Q subunit. The redox response of SAMs at the same time scale was transformed from reversibility to irreversibility as the alkyl chain length became larger. The monolayers obtained from a 0.1 muM or lower thiol concentration solution show the most ideal current-potential features. From the dependence of logarithmic apparent electron-transfer rate constant on the alkyl chain length between the hydroquinone and the electrode, kinetic parameters and electron tunneling constants (beta) were evaluated for H(2)Q(CH2)(n)SH SAMs. The obtained experimental value of beta = (1.04 +/- 0.06)/CH2 unit is in good agreement with the values reported for reversible redox centers in solutions or tethered monolayer systems.