[en] We have shown that quercetin forms 2:1 complexes (M:Q = 2:1) with Fe²⁺, Fe³⁺, Cu²⁺, and Pb²⁺. Fe³⁺ can oxidize quercetin to quercetin quinone in the pH region of 2 to 4, whose characteristic absorption peak appears at ca. 300 nm. Above pH 5, Fe³⁺ forms a complex with quercetin with an absorption peak at 425 nm. It is also possible to electrochemically produce quercetin quinone. In strongly acidic solutions, none of studied ions can form a complex with quercetin. The slow quercetin oxidation by hydrogen peroxide is accelerated in the presence of Fe³⁺. Polyphenols having a benzo-γ-pyrone structure constitute a family known as flavonoids. Flavonoids are widely distributed among higher plants. By virtue of astringent taste, they are believed to serve as defensive molecules, protecting plant tissues from herbivorous attack. Depending on a multiplicity of phenolic groups, flavonoids are classified into several groups, such as flavones, flavonols (quercetin, kaempferol), flavanones (naringenin), and flavanes (catechin), displaying functional diversities and complexities. Flavonoids have been best known as antioxidants and radical scavengers. While this beneficial effect mainly comes from their ability to accept free radicals, complexation properties with metal ions have also been recognized to contribute to the total biological activity

[en] Electrochemical characteristics of nitrite ion were investigated at a poly(methylene blue)-modified glassy carbon electrode by cyclic voltammetry and differential pulse voltammetry. The poly(methylene blue)- modified glassy carbon electrode exhibited enhanced anodic signals for nitrite. The effects of key parameters on the detection of nitrite were evaluated at the modified electrode, such as pH, accumulation time, and scan rate. Under optimum condition, the chemically modified electrode can detect nitrite in the concentration range 2.0 x 10^-6 to 5.0 x 10^-4 M with the detection limit of 2.0 x 10^-6 M and a correlation coefficient of 0.999. The detection of nitrite using the chemically modified electrode was not affected by common ions such as Na⁺, K⁺, Ca²⁺, Cl^-, HPO₄^2- and H₂PO₄^-. The modified electrode showed good stability and reproducibility. The practical application of the present method was successfully applied to the determination of nitrite ion in cabbage samples

[en] Electrochemical characteristics of nitrite ion were investigated at a poly(methylene blue)-modified glassy carbon electrode by cyclic voltammetry and differential pulse voltammetry. The poly(methylene blue)- modified glassy carbon electrode exhibited enhanced anodic signals for nitrite. The effects of key parameters on the detection of nitrite were evaluated at the modified electrode, such as pH, accumulation time, and scan rate. Under optimum condition, the chemically modified electrode can detect nitrite in the concentration range 2.0 x 10^-6 to 5.0 x 10^-4 M with the detection limit of 2.0 x 10^-6 M and a correlation coefficient of 0.999. The detection of nitrite using the chemically modified electrode was not affected by common ions such as Na⁺, K⁺, Ca²⁺, Cl^-, HPO₄^2- and H₂PO₄^-. The modified electrode showed good stability and reproducibility. The practical application of the present method was successfully applied to the determination of nitrite ion in cabbage samples

[en] A highly sensitive and selective electrochemical method based on a poly(chromotrope 2B)-modified anodized glassy carbon electrode (PCHAGCE) was developed for the determination of dopamine (DA) in the presence of uric acid (UA) and ascorbic acid (AA). The PCHAGCE sensor exhibited excellent electron-mediating behavior towards the oxidation of DA in 0.1 M phosphate buffer solution (PBS) (pH 7.0). It was found that the electrocatalytic activity was significantly dependent on the charge status and molecular structure of the target molecules. Differential pulse voltammetry (DPV) measurements revealed oxidation signals for DA, UA, and AA that were well-resolved into three distinct peaks with AA–DA, DA–UA, and AA–UA peak potential separations (ΔE_p) of 172, 132, and 304 mV, respectively. A detection limit of 0.04 ± 0.001 μM (S/N = 3) and a quantification limit (S/N = 10) of 0.149 ± 0.03 μM were obtained for DA sensing in a linear range of 1 to 40 μM in PBS (pH 7.0) with a very high sensitivity of 1.522 ± 0.032 μA·μM"−"1. The DA concentrations in human urine samples were also successfully determined with recoveries of 94.0–98.0%. This approach provides a simple, easy, sensitive, and selective method to detect DA in the presence of AA and UA