Dendrimer sensors

Abstract

In the present global political climate, the ability to rapidly and reliably detect traces of explosive compounds is of paramount importance for armed forces and security services alike. Despite the development of a plethora of techniques for detecting explosives, few have been engineered into portable sensors for stand-off detection. Oxidative fluorescence quenching has been identified as a promising mechanism for the detection of high electron affinity analytes, such as nitroaromatics found in military explosive compositions and landmines. Commercial explosives sensors with fluorescent conjugated polymer films as the active sensing layer have been deployed in conflict zones throughout the world. However, these sensors are insensitive to lower electron affinity nitroaliphatic analytes and are prone to false alarms towards non-explosive vapours. The aims of this work were twofold: to enhance sensitivity towards low electron affinity analytes through material design and to improve selectivity by employing microfluidic channels as sensing architectures. It is known that fluorescent bifluorene-based dendrimer thin films display different binding strengths towards a range of analytes and this is the basis of affinity chromatography. In principle, such materials could act as both a chromatographic stationary phase and a fluorescent sensing element. Due to their micron-sized dimensions, long microfluidic channels for analyte separation can be prepared on relatively small device substrates. Monitoring the retention of fluorescence quenching bands within the sensing channel would allow for the detection and identification of quenching vapours, reducing false alarms. Microfluidic devices are typically prototyped in poly(dimethylsiloxane) (PDMS) and this substrate is incompatible with hydrophobic solvents typically used for solution-processing of organic thin films. The first step in this work was therefore to prepare bifluorene-based dendrimers (1a/b) bearing hydrophilic 2-(2-methoxyethoxy)ethoxy surface groups that were soluble in carbonate solvents for depositing thin films in PDMS-based microchannels. The 9-positions of the bifluorene core units were substituted with either n-propyl (1a) or 2-(2-methoxyethoxy)ethyl (1b) groups to examine the effect of increased core hydrophilicity on solubility and analyte sensitivity. Nitroaliphatics used as tagging agents and accelerants in explosive compositions are traditionally hard to sense by oxidative fluorescence quenching but carbazole-based sensing materials have shown promise for their detection. In this work, two families of carbazole-based dendrimers for nitroaliphatic sensing were designed and synthesised. The dendrimers possessed first-generation biphenyl-based dendrons and 2-ethylhexyloxy surface groups with either carbazole (9a/b) or 3,6-diacetylenylcarbazole (10a/b) cores. The 9-position of the carbazole-based cores were