A Microscale NO3- Biosensor for Environmental Applications

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Abstract
A biosensor for NO3- containing immobilized dentrifying bacteria and a reservoir of liquid growth medium for the bacteria was constructed. The bacteria did not have a N2O reductase and therefore reduced NO3- to N2O, which was then subsequently quantified by a built-in electrochemical transducer for N2O. The only agents interfering with the determination of NO3- were NO2- and N2O. The sensitivity for NO2- was identical to the one for NO3- whereas the sensitivity for N2O was 2.4 times higher than for NO3-. 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 NO3- 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 NO3- biosensor is a very useful tool for the study of nitrogen metabolism in nature.