Norepinephrine-modified glassy carbon electrode for the simultaneous determination of ascorbic acid and uric acid

The oxidation of norepinephrine (NE) on a preactivated glassy carbon electrode leads to the formation of a deposited layer of about 4.2 x 10(-10) mol cm(-2) at the surface of the electrode. The electron transfer rate constant, k(s), and charge transfer coefficient, alpha, for electron transfer between the electrode and immobilized NE film were calculated as 44 s(-1) and 0.46, respectively. The NE-modified glassy carbon electrode exhibited good electrocatalytic properties towards ascorbic acid (AA) oxidation in phosphate buffer (pH 7.0) with an overpotential of about 475 mV lower than that of the bare electrode. The electrocatalytic response was evaluated by cyclic voltammetry, chronoamperometry, amperometry and rotating disk voltammetry. The overall number of electrons involved in the catalytic oxidation of AA and the number of electrons involved in the rate-determining step are 2 and 1, respectively. The rate constant for the catalytic oxidation of AA was evaluated by RDE voltammetry and an average value of k(h) was found to be 8.42 x 10(3) W s(-1). Amperometric determination of AA in stirred solution exhibits a linear range of 2.0-1300.0 mu M (correlation coefficient 0.9999) and a detection limit of 0.076 mu M. The precision of amperometry was found to be 1.9% for replicate determination of a 49.0 mu M solution of AA (n = 6). In differential pulse voltammetric measurements, the NE-modified glassy carbon electrode can separate the AA and uric acid (UA) signals. Ascorbic acid oxidizes at more negative potential than UA. Also, the simultaneous determination of UA and AA is achieved at the NE-modified electrode. (c) 2005 Elsevier Ltd. All rights reserved.