Detection of Explosive Vapours: The Roles of Exciton and Molecular Diffusion in Real-Time Sensing

Dr. Paul Shaw (University of Queensland)

The performance of fluorescence-based chemosensors depends critically on the interactions between the analyte vapours and the sensing film. The sensing mechanism is relatively simple: in the presence of explosive analyte molecules that have sufficiently high electron affinity, electron transfer from the photoexcited sensing material to the analyte occurs. This results in non-radiative relaxation of the excited state via the analyte molecule and a loss of fluorescence intensity from the sensing material. As most excitons will not be photo-generated in contact with a bound analyte, exciton diffusion has been believed to play a critical role, particularly with low vapour pressure analytes. The high sensitivity of conjugated polymers to explosive vapours has been widely ascribed to efficient long range exciton diffusion over distances as large as ~90 nm. However, comparable sensitivity has been observed in fluorescent materials with exciton diffusion lengths of less than 10 nm. To explain this contradiction, we have used a range of techniques to probe the diffusion mechanism of the analyte vapour (neutron reflectometry, quartz crystal microbalance with insitu fluorescence) and the photo physics of the the quenching process (transient absorption spectroscopy). Our results show that, contrary to prior assertions in the literature, the high sensitivities of conjugated polymers to nitrated analyte vapors are not the result of an “amplified” response arising from long range exciton diffusion but the analyte diffusion process, which results in a high concentration front propagating though the film. I will conclude by discussing the implications of these results for the design of materials with high sensing performance in the solid state.