In this work, the results on using biosensor composed from quantum dots as transducer and acetylcholinesterase enzymes (AChE) to detect pesticides optically are presented. The used quantum dots were CdTe, CdSe/ZnS 10 monolayer (ML) and CdSe/ZnSe2ML/ZnS 8 ML – the brand new thick-shell quantum dots (QD). The study results pointed out that the CdSe/ZnS 10 ML and CdSe/ZnSe 2ML/ZnS 8ML quantum dots best fit for the role of transducers in biosensors. In the biosensor, acetylthiocholine (ATCh) is used as indicator for the AChE enzymes to work, since it is a very powerful hydrolyte with the presence of AChE enzymes. Moreover, the organophosphorus (OP) pesticides are the inhibitors for the AChE enzymes, thus, by the biosensors that we designed, we can detect pesticides by the change in photoluminescence (PL) intensity of QDs, with the detection of OP like parathion methyl is 0.05 ppm, and acetamiprid is 2.5 ppm.