Amperometric microsensor for direct probing of ascorbic acid in human gastric juice

This article reports on a novel microsensor for amperometric measurement of ascorbic acid (AA) under acidic conditions (pH 2) based on a carbon fiber microelectrode (OWE) modified with nickel oxide and ruthenium hexacyanoferrate (NiO-RuHCF). This sensing layer was deposited electrochemically in a two-step procedure involving an initial galvanostatic NiO deposition followed by a potentiodynamic RuHCF deposition from solutions containing the precursor salts Several important parameters were examined to characterize and optimize the NiO-RuHCF sensing layer with respect to its current response to AA by using cyclic voltammetry, and scanning electron microscopy-energy dispersive X-ray spectroscopy methods With the NiO-RuHCF coated CFME, the AA oxidation potential under acidic conditions was shifted to a less positive value for about 0 2 V (E-p of ca 0.23 V vs Ag/AgCl) as compared to a bare CFME, which greatly improves the electrochemical selectivity Using the hydrodynamic amperometry mode, the current vs AA concentration in 001 M HCl, at a selected operating potential of 0 30 V, was found lobe linear over a wide range of 10-1610 mu M (n = 22, r = 0.999) with a calculated limit of detection of 1.0 mu M. The measurement repeatability was satisfactory with a relative standard deviation (r s.d) ranging from 4% to 5% (n = 6). depending on the AA concentration, and with a sensor-to-sensor reproducibility (r s.d) of 6 9% at 100 mu M AA The long-term reproducibility, using the same microsensor for 112 consecutive measurements of 20 mu M AA over 11 h of periodic probing sets over 4 days, was 16.1% r s.d., thus showing very good stability at low AA levels and suitability for use over a prolonged period of time. Moreover, using the proposed microsensor, additionally coated with a protective cellulose acetate membrane, the calibration plot obtained in the extremely complex matrix of real undiluted gastric juice was linear from 10 to 520 mu M (n = 14, r = 0 998) These results demonstrated the unique featuring of the proposed NiO-RuHCF microsensor under acidic conditions with enhanced sensitivity and stability and proved its promising potentiality for direct amperometric probing of AA at physiological levels in real gastric juice environments. (C) 2010 Elsevier B.V All rights reserved