Development of a disposable amperometric NH4+ biosensor based on a chemically modified screen-printed carbon electrode coated with glutamate dehydrogenase, 2-oxoglutarate, and NADH

A screen-printed carbon electrode (SPCE), impregnated with the electrocatal yst Meldola's Blue (MB), has been investigated as the base transducer in a disposable amperometric NH4+ biosensor; The MB-SPCE detects the cofactor NA DH when it is polarized at a potential of only +0.05 V (vs. Ag/AgCl); elect rocatalytic oxidation of the cofactor readily occurs at this potential. The device was converted into an NH4+ biosensor by coating the surface of the MB-SPCE with glutamate dehydrogenase, 2-oxoglutarate and NADH. When ammoniu m ions are present in the sample solution, a decrease in the anodic current occurs as a result of the enzymatic conversion of 2-oxoglutarate to glutam ate which requires NADH. Chronoamperometry was performed on 40 mu L aliquot s of solutions containing various concentrations of NH4+. A 30 s incubation period was used, then the potential was stepped from open circuit to +0.05 V (vs. Ag/AgCl); response currents were measured from the resulting chrono amperograms at a time of 120s (t(120s)). The detection limit was found to b e about 2 mu M with biosensors containing 4.6 U of enzyme. The stability of these biosensors was examined after storage at 4 degrees C in a desiccator containing silica gel, the response was found to be constant for a period of about 29 days. The proposed biosensors were evaluated on samples of unsp iked, and spiked river water; the recovery and precision data indicated tha t the devices could be expected to give reliable results for the low levers of NH4+ normaly expected in rivers.