BIOSENSORS FOR DETECTING NITRATE OR NITRITE IONS

A biosensor for detecting nitrate or nitrite ions is provided containing a nitrate reductase or nitrite reductase for nitrate or nitrite ions, respectively, a co-factor such as NADPH, a photoluminescent ingredient and a transducer having a photodetector. The NADPH is oxidized to NADP.sup.+ by reduction of nitrate or nitrite ions which directly or indirectly causes the photoluminescent ingredient to produce photons which are detected by the photodetector. The reductase and co-factor may be provided in an immobilized enzyme composition containing a co-enzyme such as alcohol dehydrogenase which produces the photons by inducing an oxidation reaction that converts NADP.sup.+ back to NADPH to maintain a supply of NADPH to permit further reduction by the reductase. In the oxidation reaction, an alcohol such as hexanol or octanol is oxidized to its aldehyde with alcohol dehydrogenase from Thermoanaerobium brockii, and the aldehyde is detected by oxidizing FMNH.sub.2 using luciferase. Nitrate reductase and nitrite reductase may both be present so that nitrite reductase can further reduce nitrite produced by nitrate reduction. The photons may be produced by one or more additional co-enzymes acting in a co-enzyme chain reaction.