Ion transport and electron transfer at self-assembled alkylthiol/gold monolayers

The electrical and electrochemical properties of self-assembled n-alkylthiol monolayers (SAMs) on gold are important if SAMs are to be used as molecular building blocks in biomimetic membranes and in micro- or nano-electronics. Ion transport and electron transfer at SAM/electrolyte interfaces are two important processes which have been characterized by cyclic voltammetry and a.c. impedance spectroscopy. Ion transport from an aqueous phase to the hydrophobic SAM region has been addressed by investigating the insulating properties of a wide variety of X(CH₂)_nS/Au SAMs (X = CH₃, OH, CO₂H and CF ₃, and n = 7, 9, 11, 15). It was established that when the phase angle at a frequency characteristic of ion diffusion processes ( i.e. 1 Hz) is ≥88°, the SAM is defect-free and obeys the Helmholtz ideal capacitor model. However, when ϕ_1HZ < 88°, the SAM is no longer an ionic insulator and ion/water penetration from the electrolyte into the SAM hydrophobic region is observed. The behavior of the phase angle with frequency was used to characterize the permeability of SAMs to electrolyte ions (K+, H₂PO₄ −, and HPO₄2−) as a function of the applied d.c. potential. A critical potential, V_c, was identified for each type of SAM corresponding to a transition from an insulating state to a more permeable state. When X = CH₃, V _c becomes more cathodic with increasing chainlength, i.e. V_c = −0.15 V (vs. Ag/AgCl) for n = 7, −0.25 V for n = 9, 11, and −0.35 V for n = 15. The SAM ionic permeability can also be modulated by maintaining n constant (15) and by varying the terminal group X. V_c is considerably more anodic for hydrophilic SAM/electrolyte interfaces (+0.25 V vs . Ag/AgCl for X = OH and + 0.15 V for X = CO₂H) than for hydrophobic interfaces (−0.35 V for X = CH₃). The kinetics of electron transfer at CH₃(CH₂)₁₅CH₃ SAMs have been investigated by a.c. impedance spectroscopy at various d.c. overpotentials with three redox couples, Ru(NH₃)₆3+/2+, Fe(CN)₆3−/4−, and Co(bpy)₃ 3+/2+. Fits of σ - E (Warburg coefficient as a function of applied d.c. potential) curves to theoretical models reveal that electron transfer at SAM-coated electrodes is irreversible with Co(bpy) ₃3+/2+, and operates via a CE (chemical reaction followed by electron transfer) mechanism with Ru(NH₃) ₆3+/2+. The kinetics of Fe(CN)₆3− reduction at SAM-coated electrodes is complex and cannot be resolved in terms of σ - E plots. Finally, measurements of the surface potential Δ&phis; of C_nS/Au (n = CH₃(CH₂)_n−1, 4 ≤ n ≤ 20) SAMs show that Δ&phis; increases linearly with n for n > 6. Moreover, the Δ&phis; value is systematically greater by 122 mV for odd-numbered SAMs than for the corresponding even-numbered SAMs. The net dipole orientation of the methyl terminal group is suggested to be at the origin of this odd/even effect.