A paper-based biosensor was developed for the detection of the degradation products of organophosphorus pesticides. The biosensor quantifies acetylcholine esterase inhibitors in a fast, disposable, cheap, and accurate format. We specifically focused on the use of sugar or protein stabilizer to achieve a biosensor with long shelf-life. The new biosensor detected malathion with a detection limit of 2.5？ppm in 5？min incubation time. The operational stability was confirmed by testing 60 days storage at 4°C when glucose was used as stabilizer. 1. Introduction Detection of pesticide traces in food and water is an important safety issue due to intensive agricultural applications and their consequent toxicity. Pesticides, such as organophosphates (OP) and carbamates (CM), have inhibitory effects on cholinesterases which are enzymes essential for the proper functioning of the nervous system of vertebrates and insects. The toxic action of organophosphate and carbamates arise from the inhibition of acetylcholinesterase activity leading to accumulation of acetylcholine at the nerve endings and therefore causing cholinergic overstimulation characterized by severe consequences in humans including abdominal cramps, muscular tremor, hypotension, breathing difficulty, diarrhea, slowing heartbeat (bradycardia), muscular fasciculation, and paralysis [1]. Therefore, portable and accurate quantification of pesticides is essential for public health. The detection of pesticides or nerve agents has been traditionally carried out in laboratory settings with large and expensive instruments such as gas chromatography coupled with mass spectroscopy (GC-MS) [2], spray mass spectroscopy [3], or high performance liquid chromatography (HPLC) [4]. Recent research efforts focused on developing biosensors platforms that can be incorporated into mobile detection devices. In that respect, paper attracts considerable attention as a matrix for developing low-cost analytical devices [5]. Paper is affordable, abundant, disposable, and has high volume to surface ratio. Paper-based biosensors are usually fast-responding and low-cost diagnostic tools in health and environmental applications. Bioactive papers are obtained by modification of paper matrix with biomolecules in order to add sensor functionality. One of the major advantages of bioactive paper sensors is that they are designed to operate without sophisticated equipment [6]. Generally the sensitivity is tolerated at the expense of simple applications compared to other analytical methods. In bioactive paper biosensors, enzyme-immobilized
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