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The development of a simple and on-site assay for the detection of organophosphonis pesliced residues is veiy important for food safety and exosystem protection. This paper reports the surface coordination-originated fluorescence resonance energr transfer (FRE1) of CdTe quantum dots (QDs) and a simple ligand-replacement turn-on mechanism for the highly sensitive and selective detection of organophosphorethioate pesticides. It has been demonstrated that coordination of dithizone at the surface of CdTe QDs in basic media can strongly quench the green emission of CdTe QUs by a FREF mechanism. Upon the addition of organophosphorothioate pesticides, the dithizone ligands at the CdTe QD surface are replaced by the hydrolyzate of the organophosphorothioate, and hence the fluorescence is turned on. The fluorescence turn on is immediate, and the limit of detection for chlorpyrifos is as low as ∼0.1 nM. Two consecutive linear ranges allow a wide determination of chiorpyrifos concentrations from 0.1 nM to 10 μM. importantly, the fluorescence turn-on chemosensor can directly detect chlorpyrifos residues in apples at a limit of 5.5 ppb, which is under the maximum residue limit allowed by the U.S. Environmental Protection Agency. The veiy simple strategy reported here should fadlitate the desvlopment of fluorescence turn-on chemosensois for chemo/biodetection. [ABSTRACT FROM AUTHOR]