Accurate evaluation of diffusion coefficient for electroactive analytes in human serum samples using...

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.