We identified a mechanism to detect chemical changes with a modified semiconductor nanoparticle (e.g., an oxazine-adsorbed CdSe--ZnS core-shell quantum dot). Our strategy is based on the chemical transformation of chromo-genie ligands adsorbed on the surface of a quantum dot. This activates an energy transfer pathway from the quantum dot to the adsorbed chromogenic ligands, which causes a change (e.g., increase or decrease) in a characteristic of fluorescent emission (e.g., intensity or lifetime). Thus, modified quantum dots acting through this mechanism can efficiently transduce a chemical event or occurrence into a change in optical signal. Our design can be adapted to signal chemical changes by a diversity of target analytes and, thus, it can be used to develop other fluorescent chemosensors based on the unique properties of quantum dots.