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13.
Amperometric microsensor for direct probing of ascorbic acid in human gastric juice
Hutton, Emily A. ...
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 (CFME) 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 toa less positive value for about 0.2 V (Ep 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 0.01 M HCl, at a selected operating potential of 0.30 V, was found to be linear over a wide range of 10-1610 M (n = 22, r = 0.999) with a calculated limit of detection of 1.0 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 M AA. The long-term reproducibility, using the same microsensor for 112 consecutive measurements of 20 M AA over 11 h of periodic probing sets over 4 days, was 16.1% r.s.d., thus showing verygood 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 inthe extremely complex matrix of real undiluted gastric juice was linear from10 to 520 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.
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