Electrochemical and Spectroelectrochemical Determination of Stress Biomarker Isatin on Optically Transparent Boron-Doped Diamond Electrodes
Authors: Sobaszek Michal , Niedzialkowski Pawel , Bogdanowicz Robert , Swain Greg M 
Published: February 2020

Isatin (1H-indole-2,3-dione) is versatile heterocyclic compound and is well known as one of the most important indole present in mammalian tissues and brain.1 The function of Isatin is a strong endogenous neurochemical regulator in humans as it is a metabolic derivative of adrenaline. Next, the control of Isatin level in humans and rats urine can be used as marker for Parkinson’s disease.2,3 Furthermore, Isatin and its derivatives like Schiff and Mannich bases are widely used as antibacterial, antifungal and anti-HIV component.4

In 1998 group of Glover et al.5 showed that it is an important inhibitor of endogenous monoamine oxides (MAO), what could be useful for marking stress and anxiety. Isatin has been shown to inhibit MAO in the range of 3−20 μM.6 Tozawa et al.7 have shown that isatin output in rat urine is increased after stress. But as neuroactive compound is present in brain, blood, urine, and tissue. The highest level of isatin per gram of brain is found in the hippocampus: ∼0.13 μg/g equal to 0.7 μM assuming 80% of water. Outside the brain, the mean concentration varieties in the range from 0.2 to 11 μg/g (1−55 μM) depending the organs.
Survey of the literature displays that till date, the complete redox study of isatin was just reported at glassy carbon electrode (GCE). Diculescu et al.8 showed redox properties of isatin by means of cyclic, square wave and differential pulse voltammetry. Their study reveals that the redox process of isatin is a pH dependent and irreversible with the one electron and one proton transferred. Oxidation results in an electroactive oxidation by-products that strongly adsorb on the GCE surface saturating its sensitivity. They achieved limit of detection 0.194 µM in pH 7.0 0.2M phosphate buffer.
Unfortunately, EC techniques using GCE or CPE are limited by the oxidation potential and require routine maintenance due to adsorption problems. Additionally, they both suffer from fouling during voltammetric analysis. To enhance detection limits and minimize the effect of fouling, alternative carbon based electrodes have to be considered. Remarkably, the boron doped diamond (BDD) electrodes not only exhibit a very wide potential window and resistance to corrosion, but also show reduced background currents and fouling compared to other carbon based electrodes.
To the best of our knowledge there is still lack of spectroelectrochemical or redox investigations of Isatin at other carbon based electrodes like boron-doped diamond. In view of such important pharmacological role of isatin, this molecule needs to have a simple and sensitive method for its quality control like e.g. stable BDD-based sensing device. Furthermore, this electrode material brings also wide wavelength range of high optical transparency from UV to far IR. This feature allows for usage spectroeletrochemistry that is beneficial for investigation reaction mechanisms.
Thus, we report here the successful application of BDD electrode for spectroelectrochemical studies of redox processes of Isatin as a potential stress biomarker. The cyclic voltammetry and simultaneous UV absorbance measurements were applied for quantitate determination of Isatin concentration in pH 7.2 0.1 M phosphate buffer. The synchronous EC and optical investigations allow for deep insight into reduction and oxidation processes of this specific indole compound.
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