Temperature effects on the voltammetry of ferrocene terminated self-assembled monolayers

The voltammetry of self-assembled monolayers (SAMs) of 11-mercaptoundecyl-ferrocenecarboxylate (FcCOO(CH2)11SH) has been studied as a function of temperature. Such SAMs are, when oxidised, susceptible to loss of ferrocene via nucleophilic attack, but at temperatures only just above room temperature, there is an accelerated loss of ferrocene from the chain termini, an increase in the capacitance of the layer in its reduced state and a positive shift in the E1/2 value. A comparison between these data and the effects of multiple scans recorded at room temperature suggests that there is a re-orientation, induced at slightly elevated temperatures, which is associated with the ferrocene ester linkage at the chain terminus and which apparently renders ferrocene more susceptible to nucleophilic attack. Accelerated loss of ferrocene with temperature increases the permeability of the layer and hence its capacitance. The positive shift of the E1/2 value is harder to interpret but may result because the ferrocene is in more intimate contact with the layer and is placed in a more hydrophobic, less polar environment. Other possible influences on the shift of E1/2 are discussed. This work confirms that electroactive terminal groups can provide information on the micro-environment at the SAM ∣ electrolyte interface through variations in current and potential.