Electrochemical synthesis and characterization of poly(thionine)-deep eutectic solvent/carbon nanotube-modified electrodes and application to electrochemical sensing

Electropolymerization of thionine (TH) on multiwalled carbon nanotube (MWCNT)-modified glassy carbon electrodes (GCE) in ethaline deep eutectic solvent (DES) was carried out for the first time, to prepare poly(thionine) (PTH) films with different nanostructured morphologies. PTH films were formed on MWCNT/GCE by potential cycling electropolymerization in ethaline with the addition of different acid dopants CH₃COOH, HClO₄, HNO₃, H₂SO₄ and HCl, acetic acid being the best. The electropolymerization process was monitored with an electrochemical quartz crystal microbalance. The polymerization scan rate was a key factor affecting the electrochemical and morphological properties of the PTH_Ethaline-CH₃COOH/MWCNT/GCE; electrodeposition at 200 mV s^-1 showing the best performance. The PTH/MWCNT/GCE platform was characterized using cyclic and differential pulse voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy. The analytical characteristics of the PTH films were evaluated for sensing of ascorbic acid and biosensing of uric acid. The developed sensor exhibited a low detection limit (1.1 μM), wide linear range (2.8-3010 μM) and high sensitivity (1134 μA cm^-2 mM^-1) for ascorbic acid. After immobilization of uricase, UOx, on PTH/MWCNT/GCE, the biosensor was successfully applied to the determination of uric acid, with fast response (˂ 7 s), good sensitivity (450 μA cm^-2 mM^-1, wide linear range (0.48-279 μM) and low detection limit (58.9 nM), better than in the literature and than with PTH prepared in aqueous solution. The determination of uric acid in synthetic urine samples was successfully tested and the mean analytical recovery was 100.8 ± 1.4%. This is a promising approach for the determination of uric acid in real samples. Graphical abstract.

Caveolin proteins electrochemical oxidation and interaction with cholesterol

Caveolae consist in lipid raft domains composed of caveolin proteins, cholesterol, glycosphingolipids, and GPI-anchored proteins. Caveolin proteins present three different types, caveolin 1 (CAV-1), caveolin 2 (CAV-2) and caveolin 3 (CAV-3), with a very similar structure and amino acid composition. The native caveolin proteins oxidation mechanism was investigated for the first time, at a glassy carbon electrode, using cyclic, square wave and differential pulse voltammetry. The three native caveolin proteins oxidation mechanism presented only one tyrosine and tryptophan amino acid residues oxidation peak. Denatured caveolin proteins presented also the tyrosine, tryptophan and cysteine amino acid residues oxidation peaks. The reverse cholesterol transport is related to caveolae and caveolin proteins, and CAV-1 is directly connected to cholesterol transport. The influence of cholesterol on the three caveolin proteins electrochemical behaviour was evaluated. In the absence and in the presence of cholesterol, significant differences in the CAV-1 oxidation peak current were observed.