A microsensor for nitrate based on immobilized denitrifying bacteria

A biosensor for NO3(-) was constructed by attaching a 30- to 70-micrometer-wide capillary with immobilized denitrifying bacteria in front of an N2O microsensor. These bacteria reduced O2 so that only bacteria in the very tip of the sensor were exposed to O2 whereas bacteria at a greater depth could carry out the anaerobic process of denitrification. In the presence of acetylene, which inhibits nitrous oxide reductase, bacteria reduced NO3(-) (or NO2(-)) from the surrounding medium to N2O and the concentration sensed by the N2O microsensor was directly proportional to the concentration of NO3(-) in the medium. By applying a 250-micrometer-long capillary in front of the N2O microsensor, the 90% response time of the biosensor was 50 s. Biosensors may also be made with nitrous oxide-deficient strains so that acetylene inhibition can be omitted