Diffusion of nitroaromatic vapours into fluorescent dendrimer films for explosives detection

Fluorescence-based sensing with organic semiconductors is a powerful method for the detection of a broad range of analytes including explosives, chemical weapons and drugs. Diffusion of an analyte into an organic semiconductor thin film, and its subsequent interaction with the chromophore are key factors that govern the sensing performance of a chemosensor. In this study the diffusion behaviour of an explosive analyte analogue into a sensing film of a conjugated dendrimer was investigated using a quartz crystal microbalance (QCM) and correlated with neutron reflectivity measurements. The mechanistic insights of para-nitrotoluene (pNT) sorption in the films of different thicknesses of a first generation dendrimer with fluorenyl surface groups, carbazole dendrons and a spirobifluorene core were studied and interpreted in terms of the underlying kinetics and thermodynamics. Sorption measurements suggest that the process of diffusion of pNT vapour into the dendrimer films is Super Case II, which involves swelling of the film. Swelling of the film was confirmed by neutron reflectometry, which also showed uniform distribution of the pNT molecules throughout the film. The activation energy barrier and change in Gibbs free energy in the sorption process were calculated from the QCM responses. The sorption process was found to be thermodynamically (not kinetically) controlled and independent of film thickness. This work sheds insight into the structure–property relationships that govern the performance of organic semiconductor fluorescence-based chemosensors.