We report the trace vapor detection of the molecule 1,4-dinitrobenzene, a model analyte for the explosive substance 2,4,6-trinitrotoluene, via fluoresence quenching of a first generation conjugated dendrimer containing a 2,2^′ -bis[9,9-di- n -hexylfluorene] core. We show that much greater sensitivity can be obtained by using the material as a surface emitting distributed feedback laser. We find that the slope efficiency of the laser is a convenient and sensitive indicator of the presence of the analyte. The slope efficiency decreases by a factor 50 in the presence of 1,4-dinitrobenzene.