Aggregation of two different sizes of semiconductor quantum clots is successfully applied to a novel ratiometric fluorescent sensor, in which Forster type energy transfer between quantum dots is induced by the interaction with 2,4,6-trinitrotoluene (TNT) as a binder of quantum dots. The detection limit for TNT achieved is 5 pM, and the system shows relatively high sensitivity against TNT in comparison to 2,4-dinitrotoluene and 2-nitrotoluene at 5 pM-500 nM range.