Accurate evaluation of diffusion coefficient for electroactive analytes in human serum samples using nitrogen-terminated sputtered carbon film electrode.

We have developed an N-terminated carbon film electrode that allows accurate determination of the diffusion coefficient of electroactive molecules dissolved in a highly concentrated serum protein solution. The carbon film electrode was formed by the unbalanced magnetron sputtering (UBM) method. Then, nitrogen functional groups were introduced by employing NH3 or H2O plasma treatment. Cyclic voltammetry measurements with ferricyanide ion ([Fe(CN)6]3-) showed that the N-terminated carbon film electrode exhibited great anti-fouling property against simulated serum proteins (50 mg/mL human serum albumin and 15 mg/mL γ-globulin dissolved in 1 M KCl solution). In contrast, glassy carbon, H2O plasma-treated, and especially untreated carbon film electrodes were subject to severe electrode fouling, making it difficult to electrochemically determine the diffusion coefficient of the [Fe(CN)6]3- ion. The control experiment using less adsorptive ethylene glycol as a viscosity modifier showed that the increase in viscosity is a main factor of the decrease in diffusion coefficient for nitrogen plasma treated electrode, which is not significantly affected by the possible interaction between [Fe(CN)6]3- ions and serum proteins. Finally, we applied the electrode for the electrochemical analysis of acetaminophen dissolved in phosphate buffer (0.1 M, pH = 7.0), which suggests that NH3 plasma-treated carbon film exhibits the lowest ΔE increase when we compare ΔE with and without proteins and also a more stable peak current in continuous voltametric measurements compared with other carbon electrodes.