Changjun Hou of Chongqing University and Yu Lei of the University of Connecticut and their colleagues presented a prototype biosensor that could detect a common class of toxic pesticides on food and in the environment (organophosphats, which represent about the 40% of the pesticides used on crops) at a small cost.

The portable biosensors currently used  can detect the reaction occurring between organophosphates and acetylcholinesterase (which is inhibithed byb the reaction). Unfortunately, this enzyme is expensive because it needs to be extracted from animal tissue. So some researchers have considered using a cheaper alternative, plant esterase.

designed a plant-esterase-based biosensor for organophosphates that boosts performance by including gold nanoparticles to ease electron transfer, graphene nanosheets to improve conductivity, and chitosan, a hydrophilic polysaccharide derived from shrimp and insect shells, to stabilize the enzyme.

The team used a solution of gold nanoparticles and graphene nanosheets, which has been deposited on the surface of glassy carbon electrodes. Then they extracted and purified plant esterase from wheat flour, and mixed it with chitosan. They applied the solution of plant esterase and chitosan onto the surface of the nanocomponents and let it dry. If the system is immersed in a buffer solution containing 1-naphthyl acetate, a carboxylic ester, plant esterase hydrolyzes the ester to produce an electroactive product, 1-naphthol. Since organophosphate pesticides inhibit this reaction, by monitoring the decrease in electrochemical current in the electrodes researchers can misure their concentration in a solution.

The team used the biosensor to measure concentrations of two organophosphate pesticides, methyl parathion and malathion, in solution at up to 500 ppb. It has detection limits of 50 ppt and 0.5 ppb for these compounds, respectively, comparable to or more sensitive than most previous acetylcholinesterase-based methods and below the maximum limits set by the EPA (2 ppb for methyl parathion). It also accurately quantified spikes of these pesticides on ground up samples of carrots and apples with an average error of 4%.

Dr Lei says the team hopes to develop a portable device that uses test strips and a simple electrochemical readout, similar to blood glucose meters.