Sol-gel cholinesterase biosensor for organophosphorus pesticide fluorimetric analysis

Sol-gel crystals derived from tetramethyl orthosilicate (TMOS) doped with cholinesterase using microencapsulation have been prepared. Organophosphorus pesticides (ORPs) are determined by fluorimetric detection based on enzymatic inhibition of immobilized cholinesterase; the percentage inhibition is correlated to pesticide concentration. The working substrate concentration (indoxyl-acetate) for ORP determination, 1.63 X 10(-4) M, is optimized by selecting a linear range in a plot of substrate concentration against signal. Precision as a relative standard deviation (RSD) is 10.2% (n=7) for the working substrate concentration. Fenitrothion, azinphos-ethyl, methidation, naled and mecarbam are analysed. Calibration plots have been established by measuring in a cuvette and through an optical fibre modified at its end with the immobilized cholinesterase gel. The biosensor performance is evaluated by measuring the percentage inhibition covering concentration ranges from 1.21-11.99 mu g ml(-1) (naled) up to 4.9-328.9 mu g ml(-1) (mecarbam). RSDs from 1.5 to 10.3% and detection limits from 0.12 (naled) to 57.6 mu g ml(-1) (methidation) have been achieved. Mass production and good storage for one month of the immobilized enzyme render the gel disposable after a single use. ORP determination is fast (inhibition time of 5 min) and the enzyme immobilization directly onto the end of the optical fibre enhances the sensitivity.