A sensitive colorimetric method for the detection of polyphenols (i.e., flavonoids, simple phenolic, and hydroxycinnamic acids) was proposed in this research based on the reduction of Ag(+) ions by polyphenols in the presence of citrate-stabilized silver seeds. The color of the stable suspension was controlled by varying the concentration of trisodium citrate, silver nitrate, and silver seeds. The reduction of Ag(+) to spherical silver nanoparticles (SNPs) by polyphenols in the presence of trisodium citrate and silver seeds produced a very intense surface plasmon resonance (SPR) absorption band of SNPs at 423 nm. The plasmon absorbance of SNPs allows the quantitative spectrophotometric detection of the polyphenols, and the developed method gave a linear response over a wide concentration range of standard polyphenolic compounds. In contrast to other reported NP-based antioxidant assays, it was established in this work that growth but not nucleation of SNPs gave a linear concentration-dependent response. The trolox equivalent antioxidant capacity (TEAC) values of various (hydrophilic and lipophilic) antioxidants using the developed method were comparable to those of the CUPRAC assay. Common food ingredients like oxalate, citrate, fruit acids, amino acids, and reducing sugars did not interfere with the proposed sensing method. This assay was validated through linearity, additivity, precision and recovery, demonstrating that the assay is reliable and robust. The developed method was used to screen total antioxidant capacity (TAC) of some commercial fruit juices and herbal teas without preliminary treatment, and showed a promising potential for the preparation of antioxidant inventories of a wide range of food plants.

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A novel differential pulse voltammetric (DPV) method is presented, using a chromogenic oxidizing reagent, cupric neocuproine complex (Cu(Nc)2(2+)), for the assessment of antioxidant capacity of polyphenolic compounds (i.e., flavonoids, simple phenolic acids, and hydroxycinnamic acids), ascorbic acid, and real samples for the first time. The electrochemical behavior of the Cu(Nc)2(2+) complex was studied by cyclic voltammetry at a glassy carbon (GC) electrode. The electroanalytical method was based on the reduction of Cu(Nc)2(2+) to Cu(Nc)2(+) by antioxidants and electrochemical detection of the remaining Cu(II)-Nc (unreacted complex), the difference being correlated to antioxidant capacity of the analytes. The calibration curves of individual compounds comprising polyphenolics and vitamin C were constructed, and their response sensitivities and linear concentration ranges were determined. The reagent on the GC electrode retained its reactivity toward antioxidants, and the measured trolox equivalent antioxidant capacity (TEAC) values of various antioxidants suggested that the reactivity of the Cu(II)-Nc reagent is comparable to that of the solution-based spectrophotometric cupric ion reducing antioxidant capacity (CUPRAC) assay. This electroanalytical method better tolerated sample turbidity and provided higher sensitivity (i.e., lower detection limits) in antioxidant determination than the spectrophotometric assay. The proposed method was successfully applied to the measurement of total antioxidant capacity (TAC) in some herbal tea samples such as green tea, sage, marjoram, and alchemilla. Results demonstrated that the proposed voltammetric method has precision and accuracy comparable to those of the spectrophotometric CUPRAC assay.

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