A Microscale NO(3)(-) Biosensor for Environmental Applications.

A biosensor for NO(3)(-) containing immobilized dentrifying bacteria and a reservoir of liquid growth medium for the bacteria was constructed. The bacteria did not have a N(2)O reductase and therefore reduced NO(3)(-) to N(2)O, which was then subsequently quantified by a built-in electrochemical transducer for N(2)O. The only agents interfering with the determination of NO(3)(-) were NO(2)(-) and N(2)O. The sensitivity for NO(2)(-) was identical to the one for NO(3)(-) whereas the sensitivity for N(2)O was 2.4 times higher than for NO(3)(-). Diffusive supply of electron donors to the bacteria from the built-in reservoir of growth medium ensured that the biosensor could work for 2-4 days. The tip diameter was down to 20 μm, and the sensors exhibited perfectly linear responses to nitrate in both freshwater and seawater. The detection limit was ∼1 μM. The 90% response time to changes in NO(3)(-) concentration was from 15 to 60 s at room temperature and about twice that at 6 °C, which was the lowest temperature for successful operation. The new NO(3)(-) biosensor is a very useful tool for the study of nitrogen metabolism in nature.