This communication describes a quantum dot probe that can be activated by a reporter enzyme,beta-lactamase. Our design is based on the principle Of fluorescence resonance energy transfer (FRET). A biotinylated beta-lactamase substrate was labeled with a carbocyanine dye. Cy5. and immobilized on the Surface of quantum dots through the binding of biotin to streptavidin pre-coated oil the quantum dots. In assembling this nanoprobe, we have found that both the distance between Substrates and the quantum dot surface, and the density of substrates are important for its function. The fluorescence emission from quantum dots can be efficiently quenched (tip to 95%) by Cy5 due to FRET. Our final quantum dot probe, assembled with QD605 and 1: 1 Mixture of biotin and a Cy5-labeled lactam, can be activated by 32 mu g/mL of beta-lactaniase with 4-fold increase in the fluorescence emission. (c) 2006 Elsevier Inc. All rights reserved.