Ferrocene-terminated Self-assembled Monolayers Research Articles

Mediated Oxidation of Ascorbic Acid on a Homologous Series of Ferrocene-Terminated Self-Assembled Monolayers

The kinetics of electrocatalytic oxidation of ascorbate was studied on a series of redox self-assembled monolayers (SAMs) of the general formula Fc(CH2)4COO(CH2)nSH as electron-transfer mediators, where Fc is the ferrocenyl group and n = 3, 6, 9, and 11. We show that the rate of electron transfer from ascorbate to the surface-confined Fc+ decreases with increasing n. The rationale for the dependence of the rate of electrocatalytic activity and n, in the presence of ClO4, is obtained from Fourier-transform surface-enhanced Raman spectroscopy (FT-SERS), cyclic voltammetry, and electrochemical quartz crystal microbalance (EQCM) data. In particular, FT-SERS shows decreasing amounts of surface-bound ClO4- upon oxidation of the ferrocene with decreasing n, while EQCM data show the effective electrode mass increase was consistently higher on the shorter chain SAMs. This mass increase is likely due to increasing ferricinium cation hydration. As n decreases, the SAMs become less ordered (FT-SERS data), as is widely known from previous literature. Disorder favors water penetration into the SAM, which, in turn, increases the hydration of the Fc+ (EQCM data). Increased hydration of the Fc+ impedes the formation of Fc+-ClO4- ion pairs (EQCM and FT-SERS data), which, consequently, accelerates the electrocatalytic electron transfer from the solution-dissolved ascorbate.

Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers.

Inorganic anions strongly influence the electron transfer rate from the ascorbate to the ferrocene-terminated self-assembled monolayer (SAM) composed of 9-mercaptononyl-5'-ferrocenylpentanoate (Fc(CH2)4COO(CH2)9SH, MNFcP). At the 1 M concentration level of the supporting anion (sodium salt electrolyte), a more than 10-fold increase in the electrocatalytic oxidation rate constant of the ascorbate is observed in the following sequence: PF6-, ClO4-, BF4-, NO3-, Cl-, SO4(2-), NH2SO3- (sulfamate), and F-. The sequence corresponds to the direction of increasing hydration energy of the corresponding anion, suggesting that highly hydrated ions promote electrocatalytic electron transfer to the ferrocene-terminated SAMs, while poorly hydrated ions inhibit it. Fourier transform surface-enhanced Raman spectroscopy (FT-SERS), in combination with cyclic voltammetry, indicates the formation of surface ion pairs between the ferricinium cation (Fc+) and low hydration energy anions, while, on the contrary, no ion pairs were observed in the electrolytes dominated by the high hydration energy anions. Though it is evident that the ion-pairing ability of hydrophobic anions is directly responsible for the electrocatalytic electron transfer inhibition, an estimate of the free, ion-unpaired Fc+ surface concentration shows that it cannot be directly related to the electron transfer rate. This suggests that the principal reason of the anion-induced electron transfer rate modulation might be related to the molecular level changes of the physical and chemical properties as well as the structure of the self-assembled monolayer.

Characterization of Charge Transfer Processes in Self-Assembled Monolayers by High-Pressure Electrochemical Techniques

Here we report the first high-pressure investigation of redox processes in surface-confined monolayers. We have explored the electrochemical behavior of ferrocene-terminated self-assembled monolayers (SAMs) on gold electrodes immersed in aqueous solutions containing 1M NaClO{sub 4}. Electron-transfer reaction for ferrocene in the monolayer is restricted with the application of pressure, whereas the same reaction for ferrocene in solution is not. The dependence of the cyclic voltammetric peak redox potentials on pressure reveals that the oxidation of the ferrocene within the monolayer becomes thermodynamically and kinetically more difficult at high pressures. At pressures above 1-2 kbar, positive volumes of reaction are associated with the oxidation process, indicating that the oxidation step involves an increase in volume. Different monolayer samples, exhibiting different voltammetric responses, appear to impose different volume constraints on the charge transfer reaction and, therefore, present different pressure responses within a general common trend. These results point out the importance of structural and environmental effects, via steric constraints, on electron transfer processes in surface-confined monolayer assemblies. 36 refs., 7 figs., 6 tabs.