Poly(dendrimer)s for explosives sensing

Abstract

Trace detection of explosives and explosive related compounds is crucial to countering terrorism and providing homeland security. There are a variety of technologies for this purpose, including ion mobility spectrometry (IMS), mass spectrometry, colourimetric detectors, electrochemical sensing, and surface acoustic wave (SAW) devices. However, the best way to detect explosives is to use a stand-off detection method. One method of achieving this goal is to use a luminescence quenching based system to detect explosive vapours. An example of this type of system is FIDO®, a small, lightweight handheld device. FIDO® uses luminescent conjugated polymers but the detection materials used are not very selective and can give rise to false positives. More recently Arborescent2 Ltd has developed Arbsense, which is similar to FIDO®, but utilises dendrimers as the sensing material. Arbsense sensing elements that contain triarylamine units have been shown to have good selectivity for nitro-containing explosives and taggants. This thesis investigates poly(dendrimer)s and their corresponding monomers, as new materials for the detection of trace quantities of explosives. The poly(dendrimer) structures consist of conjugated triphenylamine based chromophores linked together by a non-conjugated backbone. The chromophores for these novel compounds are based on the material used in the Arbsense device. Such a structural motif combines the versatile processability of polymeric materials as well as an alternative route to controlling the packing of the luminescent chromophores and thus reducing intermolecular π-π interactions that can cause quenching of the luminescence. Furthermore, the poly(dendrimer) can in principle give rise to a less dense film structure, which would facilitate diffusion of analytes into the sensing film. The synthesis and characterization of the monomers and poly(dendrimer)s is described. The chromophores of the materials have variations in conjugation and surface groups to explore the effects of varying the chromophore structure on sensing performance. Solution-based Stern-Volmer measurements were conducted on the materials to investigate the quenching responses to the nitro-aliphatic taggant DMNB and the nitroaromatic analyte DNT. The steady-state measurements indicated the over-all quenching response and it was discovered that there was a general trend for an increase in the solution quenching response observed (for both analytes) when going from the monomer to the poly(dendrimer). Time resolved Stern-Volmer measurements allowed for discrimination between dynamic and static components of the quenching and revealed that for most materials the predominant quenching mechanism was dynamic. The exception to this was the M1/P1 monomer-poly(dendrimer) pair, which had the smallest chromophore. The materials were cast into thin films and solid state quenching measurements were also performed in sub-saturated environments of DMNB and DNT vapours. The solid state measurements were not in direct accordance with solution-based measurements, exemplifying that solution-based measurements alone are not adequate for assessing the sensing performance of materials in a ‘real-world’ environment. Neutron reflectometry measurements of the poly(dendrimer)s P1-4 with deuterated DNT revealed analyte uptake occurred in a uniform manner for all the films. The analyte recovery process did not result in a significant change in SLD for P1, P2 and P4 films, suggesting strong interactions between the analyte and film material. However, P3 showed partial recovery, indicating that the analyte may not bind as strongly to the film and/or the film structure readily accommodates analyte diffusion into and out of the film.