title: Amperometric microsensor for direct probing of ascorbic acid in human gastric juice.
abstract: 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 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 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 μM (n=22, r=0.999) with a calculated limit of detection of 1.0 μM. 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 μM AA. The long-term reproducibility, using the same microsensor for 112 consecutive measurements of 20 μM 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 μM (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.
authors.affiliation: Analytical Chemistry Laboratory, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia.
authors.forename: Emily A
authors.initials: EA
authors.lastname: Hutton
authors.forename: Rasa
authors.initials: R
authors.lastname: Pauliukaitė
authors.forename: Samo B
authors.initials: SB
authors.lastname: Hocevar
authors.forename: Božidar
authors.initials: B
authors.lastname: Ogorevc
authors.forename: Malcolm R
authors.initials: MR
authors.lastname: Smyth
mesh_terms: D001205:Ascorbic Acid / Q000032:analysis* / Q000737:chemistry; D015374:Biosensing Techniques / Q000295:instrumentation / Q000379:methods*; D002138:Calibration; D002244:Carbon / Q000737:chemistry; D000077482:Carbon Fiber; D002482:Cellulose / Q000031:analogs & derivatives / Q000737:chemistry; D055664:Electrochemical Techniques / Q000295:instrumentation / Q000379:methods*; D005295:Ferrocyanides / Q000737:chemistry; D005750:Gastric Juice / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D057230:Limit of Detection; D008839:Microelectrodes; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry; D015203:Reproducibility of Results; D017975:Ruthenium Compounds / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy
chemical_list: D000077482:Carbon Fiber; D005295:Ferrocyanides; D017975:Ruthenium Compounds; C005062:acetylcellulose; D002244:Carbon; D009532:Nickel; D002482:Cellulose; C028007:nickel monoxide; D001205:Ascorbic Acid