Ultrasensitive Determination of Vitamin B12 Using Disposable Graphite Screen-Printed Electrodes and Anodic Adsorptive Voltammetry

A facile, rapid and ultra-sensitive method for the determination of vitamin B-12 (cyanocobalamin) at the sub-nanomolar concentration range by using low-cost, disposable graphite screen-printed electrodes is described. The method is based on the cathodic preconcentration of square planar vitamin B-12s, as occurred due to the electro reduction of Co(III) center in vitamin B-12a to Co(I), at -1.3 V versus Ag/AgCl/3M KCl for 40 s. Then, an anodic square wave scan was applied and the height of the peak appeared at ca. -0.73 V versus Ag/AgCl/3 M KCl, due to the oxidation of Co(I) to Co(II) in the adsorbed molecule, was related to the concentration of the vitamin B-12 in the sample. EDTA was found to serve as a key-component of the electrolyte by eliminating the background signal caused by metal cations impurities contained in the electrolyte (0.1 M phosphate buffer in 0.1 M KCl, pH 3). It also blocks trace metals contained in real samples, thus eliminating their interference effect. The method was optimized to various working parameters and under the selected conditions the calibration curve was linear over the range 1 x 10(-10)-8 x 10(-9) mol L-1 vitamin B-12 (R-2 = 0.994), while the limit of detection for a signal-to-noise ratio of 3 (7 x 10(-11) mol L-1 vitamin B-12) is the lowest value of any reported in the literature for the electrochemical determination of vitamin B-12. The sensors were successfully applied to the determination of vitamin B-12 in pharmaceutical products.