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Oleuropein (Ole), naturally occurring phenolic compound found in olive products, is well known for its benefits for human health. In the present work, a simple, sensitive and rapid determination of Ole was achieved using a label-free electrochemical DNA biosensor. The application was related to the molecular interaction between Ole and double-stranded DNA (dsDNA). So, the voltammetric behavior of Ole at the surface of a DNA-immobilized chitosan-modified carbon paste electrode was studied using differential pulse voltammetry (DPV) where the oxidation peak current of Ole was measured as an analytical signal. A considerable increase was observed in the oxidation signal of Ole at the DNA-coated electrode compared with the DNA-free electrode, indicating the pre-concentration of Ole due to the interaction with the surface-confined DNA layer. In order to use the proposed sensor for real samples, different parameters affecting Ole signal such as, immobilization time and potential, accumulation time and pH, and stripping pH were optimized. Under optimized experimental conditions, a linear concentration range of 0.30–12 μmol L − 1 with a detection limit of 0.090 μmol L − 1 was obtained for Ole determination. The proposed biosensor was successfully applied to the determination of Ole in olive leaf extract and human serum samples. [ABSTRACT FROM AUTHOR]

A simple and sensitive label-free electrochemical DNA biosensor was proposed for the rapid determination of uric acid (UA) using a carbon nano tube paste electrode (CNTPE) modified with salmon sperm dsDNA. At first, the interaction between UA and the DNA was studied using differential pulse voltammetry (DPV). The addition of the DNA to UA solution resulted in a decrease in the peak current of UA and at the same time, a positive shift in the peak potential indicating an intercalative interaction. Then, the voltammetric response of a DNA-immobilized CNTPE was investigated for the determination of UA. The immobilization of the DNA was carried out using acid-functionalized carbon nanotubes and studied using Fe(CN)6 3−/Fe(CN)6 4− redox indicator. Compared with unmodified CNTPE, the oxidation signal of UA showed a significant increase at the DNA-coated electrode, and shifted to more positive potentials attributed to the pre-concentration of UA at the electrode surface due to interaction with the surface-confined DNA layer. This interaction was used for the fabrication of a simple and sensitive biosensor for determining UA. After the optimization of operational parameters, a linear dependence of the peak current on the UA concentration was observed in the range of 7.0×10−7 to 1.1×10−4 molL−1, with the detection and quantification limits of 1.8×10−7 and 5.8×10−7 molL−1, respectively. The proposed biosensor was successfully applied to validate its capability for the analysis of UA in human serum and urine samples. [ABSTRACT FROM AUTHOR]