Iridium-Based Electrocatalytic Systems for the Determination of Insulin

Abstract

Two electrochemical catalytic systems for the determination of insulin were developed. The homogeneous system was based on the oxidation of insulin by chloro complexes of iridium(IV). Kinetic studies revealed that the aquation of iridium complexes activated them toward the oxidation of insulin in acidic solutions; e.g., the rate constant was equal to 25, 900, and 8400 L mol -1 s -1 for the oxidation of insulin by the IrCl 6 2-, Ir(H 2 O)Cl 5 -, and Ir(H 2 O) 2 Cl 4 complexes, respectively. The inertness of the iridium complexes argued for the outer-sphere mechanism of the homogeneous oxidation reaction. Electroplating of aquated iridium complexes on the glassy carbon electrode resulted in the formation of the iridium oxide (IrOx) surface film, which was used in the heterogeneous detection system for insulin. The catalytic activity of the IrOx film toward insulin oxidation was ascribed to a combination of electron-transfer mediation and oxygen transfer which was related to the acid/base chemistry of the film. The IrOx film electrode was used as an amperometric detector for flow injection analysis of insulin in pH 7.40 phosphate buffer. Linear least-squares calibration curves over the range 0.05-0.50 µM (five points) had slopes of 35.2 ( 0.4 nA µM -1 and correlation coefficients of 0.999. The detection limit for insulin was 20 nM using the criterion of a signal of 3 times the peak-to-peak noise. The advantageous properties of the detector based on the IrOx film are its inherent stability at physiological pH, high catalytic activity toward insulin oxidation, and simplicity of preparation.

The Ins and Outs of Secretion from Pancreatic β-Cells: Control of Single-Vesicle Exo- and Endocytosis

“…The amperometric detection of insulin itself has not been widely adopted, however, largely due to the difficulty in producing insulin-oxidizing electrodes. Improved methods for producing these will undoubtedly prove useful (69,74,85,107).…”

A multiwalled carbon nanotube/dihydropyran composite film electrode for insulin detection in a microphysiometer chamber

“…A ruthenium dioxide/cyanoruthenate film on a carbon fiber microelectrode was later utilized at pH 7.4 to monitor chemical secretion by single β cells in response to stimulation with glucose, tolbutamide, and potassium [21]. Redox mediators and electrocatalysts such as ruthenium oxide [22], ruthenium metallodendrimer [23], and iridium oxide [24] layers have been shown to promote the oxidation of insulin at physiological pH and have been used in amperometric detection and quantization of insulin. However, iridium oxide sensors have the disadvantages of pH sensitivity resulting from the proton mediated nature of electron transfer and also the slow nature of proton transport within the film that delays the electrode response [25]; ruthenium oxide and ruthenium metallodendrimer films have mixed redox states leading to complex electron transfer kinetics, with the electrochemical behavior of ruthenium oxide varying between crystal faces [26].…”

Reductive H2O2 Detection at Nanoparticle Iridium/Carbon Film Electrode and Its Application as L‐Glutamate Enzyme Sensor

“…For decades considerable interest has focused on iridium (Ir) and iridium oxide (IrOx) electrodes due to their unique electrochemical properties, which include high conductivity [1], an excellent pH response [2 ± 6], and an excellent electrocatalytic ability with regard to O 2 evolution [7], Cl 2 evolution [8 ± 9], nitrite and nitrous oxide reduction [10 ± 11], insulin oxidation, [12], methanol oxidation [13], and H 2 O 2 reduction [14 ± 15]. Several methods have been developed for preparing Ir or IrO 2 films.…”