Innovative compounds for optical sensors
Among the various methods implemented so far for the detection of nitro-aromatic compounds such as explosives and herbicides, fluorescence quenching seems to be the most promising for the next generation of optical sensors. Quenching refers to the decrease in the fluorescence intensity of specific materials containing emissive and charge-transporting compounds. Numerous inorganic materials can be used to identify the presence of nitroaromatic compounds by a clearly distinguishable change in their photoluminescence. However, the interest in replacing inorganic materials in optical sensors with organic molecular materials has been increasing because of their ease in fabrication and low cost. Within the framework of the OPAMD project, researchers at the laboratories of the Thales Research & Technology in France synthesised a molecularly functionalised polymer from the aromatic monomer styrene, PST-NI. With the introduction of a moiety of naphthalimide chromophores in polystyrene, they achieved fluorescence quantum yields in thin solid films, which can reach up to 60\;%. Similar spectroscopic properties were observed for solutions, indicating the absence of interactions between polymer side chains. The absorption and fluorescence spectra of PST-NI were also studied after the introduction of different organic pigments. Addition of quinacridone and perylene imide chromophores resulted in the fluorescence intensity peak being displaced towards the green and red portion of the visible spectrum, respectively. Moreover, the fluorescence lifetimes of PST-NI were measured both in the presence and absence of nitro-aromatic compounds, such as 2,4-dinitrotoluene (DNT). The drop in the fluorescence intensity verified upon several minutes of exposure to DNT did not exceed 50\;% which makes these polymeric materials very attractive for sensing applications.
