Electrochemical impedance spectroscopy and quartz crystal nanobalance (EQCN) studies of insulin adsorption on Pt

The interfacial behaviour of insulin at a Pt surface was studied over the temperature range from 273 to 353 K in a phosphate buffer solution, pH 7.0, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and at 298 K using electrochemical quartz crystal nanobalance (EQCN) measurements. It was shown that the surface charge density and corresponding charge transfer resistance is directly proportional to the amount of adsorbed insulin (surface concentration), indicating that adsorption at positive potentials is accompanied by the transfer of charge through carboxylate groups on the acidic amino acid residues. The adsorption process for insulin was described with the Langmuir adsorption isotherm, which revealed a very high affinity of the protein towards adsorption onto a Pt surface. At positive potentials the adsorption of insulin is a competitive process with the surface oxidation. This was verified at 298 K by EQCN measurements where the mass changes calculated from the recorded frequency changes corresponded to the adsorption and reduction of adsorbed oxide species in the presence of insulin. Chain A and Chain B peptide units of insulin showed similar behaviour. However, the frequency change in the double layer (DL) region gave a measure of the extent of solvent displacement by the adsorbed protein. The ΔGADS values obtained from the |Δm| values calculated from the frequency measurements gave excellent agreement within experimental uncertainty with the ΔGADS values calculated from the QADS values from the CV measurements and charge transfer resistance, Rct, from EIS measurements. The observed trends in ΔGADS showed that the smaller molecules, Chain A and Chain B of insulin, were found to have a smaller affinity for the platinum surface as indicated by their smaller negative ΔGADS values.