Insulin oxidation and determination at carbon electrodes.

The redox chemistry of insulin was investigated at glassy carbon (GC) electrodes that were coated with films of chitosan (CHIT) and multiwalled carbon nanotubes (CNT). While bare electrodes deactivated quickly during insulin oxidation, the GC electrodes coated with CHIT and CHIT-CNT films generated stable insulin currents. The GC/CHIT-CNT electrodes were used for investigating the electrooxidation process of insulin and amperometric determination of insulin. The mass spectrometric, electron paramagnetic resonance, and separation studies of electrolyzed insulin solutions suggested that the loss of 4 mass units upon insulin oxidation at CNT could be accounted for by the formation of two dityrosine cross-links intramolecularly. At a potential of 0.700 V and physiological pH 7.40, the GC/CHIT-CNT electrodes displayed a detection limit of approximately 30 nM insulin (S/N = 3), sensitivity of 135 mA M(-1) cm(-2), linear dynamic range from 0.10 to 3.0 microM (R2 = 0.995), and superior operational and long-term stability. The CNT-based electrodes are promising new insulin detectors for diabetes-related studies such as fast chromatographic analysis of therapeutic insulin formulations or evaluation of quality of pancreatic islets prior to their transplantation.