Peroxidase (POD) is one of the most commonly used enzymes for biosensor construction. Due to their presence in many plants, a variety of biosensors have been described.Wang et al. (1992) used horseradish root tissue as PODsource (the most commonly found) and immobilised it onto a rough graphite disk. They detected submillimolar concentrations of 2-butanone peroxide in 20 s. The main advantage of their approach was that the biosensor operated in organic phase.Vieira and Fatibello-Filho (2000) also developed a biosensor able to work in organic media. They modified a paraffin/graphite mixture with sweet potato tissue (Ipomea batatas) rich in POD for the determination of hydroquinone in cosmetic creams. In the presence of hydrogen peroxide, its natural substrate, POD oxidises hydroquinone to p-quinone, which is reduced back to hydroquinone at –0.22 V vs. Ag/AgCl (Fig. 1). The detection limit was 8.1 μM, comparable to that