The binding and fluorescence quenching efficiency of nitroaromatic (explosive) vapors in fluorescent carbazole dendrimer thin films

Abstract

We present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 °C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of ~0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.

Diffusion of nitroaromatic vapours into fluorescent dendrimer films for explosives detection

“…More recently, fluorescent dendrimers have shown substantial potential as explosive sensing materials [12][13][14][15][16][17][18] owing to their high photoluminescence quantum yields, monodispersity and the ability to control and tailor the physical properties through tuning of the chemical structure. For the photoluminescence (PL) to be quenched, the fluorophore and the quencher have to be close enough to allow electron transfer to occur.…”

“…However, while neutron reflectometry can be used to follow the analyte desorption for a thermally activated process it does not have the sensitivity or time resolution to measure the A c c e p t e d M a n u s c r i p t 3 sorption of the analyte, which is when the initial response of the sensor is triggered [14,18,19]. In this study, we have utilized a custom-built experimental setup enabling time-resolved quartz crystal microbalance (QCM) measurements to monitor the analyte uptake in dendrimer sensing films, thus providing important information about the diffusion behaviour for such materials.…”

“…The sensing film was a layer of a first-generation dendrimer comprised of a 9,9′-spirobifluorene core, carbazole dendrons and 9,9-di-n-propylfluorene surface groups ( Fig. 1) [13,14]. This sensing material was chosen because of its high affinity for nitroaromatic vapours, which are known to rapidly quench the fluorescence of thin films [14].…”

“…1) [13,14]. This sensing material was chosen because of its high affinity for nitroaromatic vapours, which are known to rapidly quench the fluorescence of thin films [14]. As mentioned above, QCM measurements were used to quantify the analyte uptake (sorption) with the results correlated with neutron reflectometry.…”

Assessing the sensing limits of fluorescent dendrimer thin films for the detection of explosive vapors

“…This suggests that the interactions and binding between the analyte and the dendrimer molecules are different between solution and the solid-state. [23] It could also be a consequence of how the analyte vapors interact with and diffuse into the film, a process that will only occur in the solid-state but appears to play a first order role in the sensing performance. [22] Additional quenching measurements for all three dendrimers with varying vapor concentrations of all three analytes were performed (the exposure time remained constant at 20 seconds) with the results summarized in Figure 3a.…”