Hg. Hong et W. Park, Electrochemical characteristics of hydroquinone-terminated self-assembled monolayers on gold, LANGMUIR, 17(8), 2001, pp. 2485-2492
A series of thiol-functionalized hydroquinone derivatives with different al
kyl chain lengths, 2-(n-mercaptoalkyl)hydroquinone (abbreviated as H(2)Q(CH
2)(n)SH, where n = 0, 1, 4, 6, 8, 10, and 12), has been synthesized and use
d for preparation of self assembled monolayers (SAMs) on gold electrodes. T
he compounds spontaneously adsorb onto a gold surface to form stable and re
producible monolayers. The voltammetric behavior of the hydroquinone (H(2)Q
) SAMs has been investigated by cyclic voltammetry to study the effect of a
lkyl chain length on the electrochemical characteristics for H(2)Q-SAMs. Th
e 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 i
n 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 th
e same time scale was transformed from reversibility to irreversibility as
the alkyl chain length became larger. The monolayers obtained from a 0.1 mu
M or lower thiol concentration solution show the most ideal current-potenti
al features. From the dependence of logarithmic apparent electron-transfer
rate constant on the alkyl chain length between the hydroquinone and the el
ectrode, kinetic parameters and electron tunneling constants (beta) were ev
aluated 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.