Graphite electrodes fabricated by screen-printing have been used as amperometric detectors in biosensors based on NAD(+)-dependent dehydrogenases, tyrosinase, or genetically modified acetylcholinesterases. The mono-enzyme sensors have been optimized as disposable or reusable devices for detection of a variety of substrates important in the food industry ( D-lactic acid, L-lactic acid, acetaldehyde) or in environmental pollution control (phenols and dithiocarbamate, carbamate and organophosphorus pesticides). The sensors were prepared in four configurations differing in enzyme confinement, enzyme immobilization and location of the immobilization agent in the biosensor assembly. Tests on real samples have been performed with the biosensors; D-lactic acid and acetaldehyde have been detected in wine and phenols in air.

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Two-enzyme systems based on acetylcholinesterase (AChE) - a mono-enzyme system based on AChE, with p-aminophenyl acetate as substrate, and a bi-enzyme system based on AChE and tyrosinase, with phenyl acetate as substrate - have been studied for detection of organophosphate insecticides. The analytical performance and detection limits for determination of the pesticides were compared for the two AChE configurations. The enzyme loading, pH, and applied potential of the bi-enzyme system were optimised. When phenyl acetate was used as substrate for AChE activity the phenol generated by enzymatic hydrolysis was determined with a second enzyme, tyrosinase. Amperometric measurements were performed at 100 mV and -150 mV relative to the Ag/AgCl reference electrode for the mono-enzyme and bi-enzyme systems. Screen-printed sensors were used to detect the organophosphorus pesticides paraoxon and chlorpyrifos ethyl oxon; the detection limits achieved with phenyl acetate as substrate were 5.2x10(-3) mg L(-1) and 0.56x10(-3) mg L(-1), respectively.