Selective membranes for the construction and optimization of an amperometric oxalate enzyme electrode

Oxalate oxidase has been immobilized within a membrane laminate. Integration of the laminate within an amperometric electrode afforded a sensor for determining oxalate via electrochemical oxidation of H2O2 and subsequent current generation. A study of novel poly(vinyl chloride) (PVC) and cellulose acetate (CA) membranes with various outer/inner membrane configurations made it possible to eliminate direct electrochemical interference from species present in urine, typically ascorbate (60-500 mumol l-1), homovanillic acid (40-60 mumol l-1) and oxalate itself (50-500 mumol l-1). Of the membranes studied, only CA and plasticized PVC were found to screen out the direct oxalate electrochemical signal effectively. Cellulose acetate exhibited the best selectivity ratio for H2O2 over urine interference. An outer 0.05 micron pore radius polycarbonate membrane imparted sensor linearity in the narrow range 2-200 mumol l-1 oxalate, which fell below the normal range of oxalate in urine. The application of unplasticized PVC as an outer membrane allowed the sensor to exhibit linear characteristics which extended beyond the clinically relevant range (> 700 mumol l-1). The Michaelis-Menten constant of the immobilized enzyme was measured and found to be 10(-3) mol l-1.