Use of a ruthenium(III), iron(II), and nickel(II) hexacyanometallate-modified graphite electrode with immobilized oxalate oxidase for the determination of urinary oxalate

This paper describes the performance of a biosensor with an Ru(III), Ni(II) , and Fe(II) hexacyanometallate-modified graphite electrode and immobilized oxalate oxidase for the determination of urinary oxalate. The addition of ruthenium enhances the electrochemical reversibility and chemical stability of the electrocrystallized layer and improves the sensitivity of the biose nsor. Hydrogen peroxide, produced by the enzyme-catalyzed oxidation of oxal ate, was measured at -50 mV vs an Hg\Hg2Cl2\3M KCl electrode in a solution of pH 3.6 succinic buffer, 0.1M KCl, and 5.4mM ethylenediaminetetraacetic a cid. The linear concentration range for the determination of oxalate was 0. 18-280 muM. The recoveries of added oxalate (10-35 muM) from aqueous soluti on ranged from 99.5 to 101.7%, whereas from urine samples without oxalate ( or with a concentration of oxalate below the detection limit) the recoverie s of added oxalate ranged from 91.4 to 106.6%. The oxalate in 24 h urine sa mples, taken during their daily routine from 35 infants and children, was m easured and found to range from 0.6 to 121.7 mg/L. There were no interferen ces from uric acid, acetylsalicylic acid, and urea in the concentration ran ge investigated, but paracetamol and ascorbic acid did interfere. A good co rrelation (R-2 = 0.9242) was found between values obtained for oxalate in r eal urine samples by 2 laboratories, with the proposed biosensor and ion ch romatography, respectively.