Rapid detection of organophosphorous (OP) compounds such as paraoxon would allow taking immediate decision on efficient decontamination procedures and could prevent further damage and potential casualties. In the present study, a biosensor based on nanomagnet-silica core-shell conjugated to organophosphorous hydrolase (OPH) enzyme was designed for detection of paraoxon. Coumarin1, a competitive inhibitor of the OPH enzyme, was used as a fluorescence-generating molecule. Upon excitation of cumarin1 located at the active site of the enzyme, i. e., OPH, the emitted radiations were intensified due to the mirroring effect of the nanomagnet-silica core-shell conjugated to the enzyme. In presence of paraoxon and consequent competition with the fluorophore in occupying enzyme's active site, a significant reduction in emitted radiations was observed. This reduction was proportional to paraoxon concentration in the sample. The method worked in the 10-to 250-nM concentration range had a low standard deviation (with a coefficient of variation (CV) of 6-10 %), and the detection limit was as low as 5x10(-6) mu M.