Rapid Determination of Oxalate by an Amperometric Oxalate Oxidase-Based Electrode

Abstract

An enzymatic biosensor for determination of oxalate in different real food samples (spinach, sesame seed, tea leaves, and strawberries) as well as in human urine is described and evaluated. Amperometric oxalate biosensor is made from a graphite (spectroscopic purity) electrode modified with chromium(III) hexacyanoferrate(II) film (CrHCF). Oxalate oxidase was immobilized using bovine serum albumin and glutaraldehyde cross-linking procedure. This enzyme biocatalyzes the oxidation of oxalate to hydrogen peroxide and carbon dioxide. CrHCF film enables one to measure a current resulting in the reduction of hydrogen peroxide at low potential (0 V vs. Hg/Hg2Cl2/3 M KCl electrode), proportional to oxalate concentration. The electrocatalytic properties of CrHCF electrode were well pronounced in 0.05 M succinic buffer, pH 3.8, containing 0.1 M KCl and 5.4 mM EDTA. For determination of oxalate in some food matrices (tea leaves and strawberries) dialysis membrane as an outer coat was necessary to prevent electrochemical interferents. The sensitivity of biosensor was 31 µA/(mM cm2) in synthetic oxalic acid solutions. The biosensor has shown good linearity (R2=0.9984) in the oxalate concentration range between 2.5 and 400 µM and its half-lifetime was longer than one month. The performance of the method was compared with the reference enzymatic spectrophotometric method and a very good correlation was obtained between these results.