Self-Assembled Monolayers on Gold of Ferrocene-Terminated Thiols and Hydroxyalkanethiols

Abstract

In this paper, a study on the adsorption of mixed self-assembled monolayers (SAMs) for two different combinations of thiols (Fc(CH2)6SH/HO(CH2)2SH and Fc(CH2)16SH/HO(CH2)11SH (Fc = ferrocene)) is presented, to obtain surfaces with single isolated ferrocenylalkanethiols embedded in shorter hydroxyalkanethiols. These hydrophilic substrates with very low surface concentration of ferrocene moieties are required to perform force spectroscopy experiments on host−guest supramolecular complexes, by using SAMs of β-cyclodextrin heptathioether adsorbed on gold-coated AFM tips. Several SAMs have been prepared on polycrystalline gold electrodes from 1 mM thiol solutions, changing the ferrocenylalkanethiols/hydroxyalkanethiols ratio. The amount of electroactive component immobilized on the electrode was determined by cyclic voltammetry, and it has been related to the solution composition. The general trend is that the longer chain component is preferentially adsorbed, suggesting a thermodynamic control of the adsorption. However, relevant differences in the layer formation and composition can be observed for the two systems on the basis of a different balance of the driving forces that govern the adsorption process. Fc(CH2)16SH is adsorbed to a larger extent, compared to the Fc(CH2)6SH for the same solution composition, as shown by higher charge densities values. Furthermore for the Fc(CH2)16SH/HO(CH2)11SH system upon increase of the percentage of ferrocenylalkanethiols in solution, a shift in the redox peak position to more positive values is observed, indicating that phase segregation occurs. The differences between the two systems can be related to chain length effects from which arise favorable enthalpic contributions to the adsorption of the longer chain component.