Nitrate Biosensor Based on the Ultrathin-Film Composite Membrane Concept

This paper describes the construction and characterization of an electrochemical nitrate biosensor based on the ultrathin-film composite membrane concept. The composite membrane is prepared by electropolymerization of a thin anion-permselective coating of 1-methyl-3-(pyrrol-1-ylmethyl)pyridinium across the surface of a microporous support membrane. This film separates the analyte solution from an internal sensing solution which contains the enzyme nitrate reductase and an electrocatalyst (methyl viologen). The ultrathin anion-permselective film prevents loss of these components from the internal sensing solution, yet allows the analyte, NO3-, to enter the internal solution. A glassy carbon working electrode is immersed into the internal solution. The nitrate is reduced enzymatically, yielding the oxidized form of the enzyme, which is reduced again by electrogenerated methyl viologen radical cation. The nitrate concentration in the analyte solution is proportional to the catalytic methyl viologen reduction current. This sensor shows good sensitivity to nitrate, with a detection limit of 5.4 ?M and a dynamic range which extends up to 100 ?M NO3-.