Electrochemical characterization of the ultrathin polypeptide film/1,2-dichloroethane liquid ∣ liquid interface

A model liquid ∣ liquid interface formed by an ultrathin hydrophilic polypeptide film at a chemically modified gold surface in contact with a 1,2-dichloroethane (DCE) electrolyte solution is characterized with a combination of electrochemical and in situ polarization modulation FTIR (PM-FTIR) measurements. The hydrophilic films are prepared by the sequential layer by layer electrostatic adsorption of the polypeptides poly-l-lysine and poly-l-glutamic acid onto gold thin films that have been derivatized with an ω-carboxylic acid functionalized alkanethiol monolayer. The polypeptide film thickness can be varied from 5 to 30 nm, and ionic electroactive species such as ferri/ferrocyanide can be incorporated into the film. The oxidation–reduction reactions of this electrochemical system are characterized with cyclic voltammetry and potential step measurements, and require both an electron transfer step at the gold ∣ film interface and an ion transfer step at the film ∣ DCE interface. No diffusion processes are observed in these ultrathin film systems, but the voltammetric response is asymmetric and broader than that observed in reversible monolayer systems. Based on digital simulations, the standard rate constant is estimated to be ca. 0.24 s−1, and the standard deviation of the formal potential distribution is found to be approximately 130 mV. When the film is brought in contact with an organic solution containing decamethylferrocene, an electrocatalytic reaction is observed in the cyclic voltammetry corresponding to an electron transfer reaction across the film ∣ DCE interface. In situ PM-FTIR experiments show that the cyclic voltammetry samples only a fraction of the electroactive species in the polypeptide film.