In this paper, we first report a novel biosensor for the detection of paraoxon based on (CdSe)ZnS core-shell quantum dots (QDs) and an organophosphorus hydrolase (OPH) bioconjugate. The OPH was coupled to (CdSe)ZnS core-shell QDs through electrostatic interaction between negatively charged QDs surfaces and the positively charged protein side chain and ending groups (-NH2). Circular dichroism (CD) spectroscopy showed no significant change in the secondary structure of OPH after the bioconjugation, which indicates that the activity of OPH was preserved. Detectable secondary structure changes were observed by CD spectroscopy when the OPH/QDs bioconjugate was exposed to organophosphorus compounds such as paraoxon. Photoluminescence (PL) spectroscopic study showed that the PL intensity of the OPH/QDs bioconjugate was quenched in the presence of paraoxon. The overall quenching percentage as a function of paraoxon concentration matched very well with the Michaelis -Menten equation. This result indicated that the quenching of PL intensity was caused by the conformational change in the enzyme, which is confirmed by CD measurements. The detection limit of paraoxon concentration using OPH/QDs bioconjugate was about 10(-8) M. Although increasing the OPH molar ratio in the bioconjugates will slightly increase the sensitivity of biosensor, no further increase of sensitivity was achieved when the molar ratio of OPH to QDs was greater than 20 because the surface of QDs was saturated by OPH. These properties make the OPH/QDs bioconjugate a promising biosensor for the detection of organ opho sphorus compounds.