Date of Award
Doctor of Philosophy in Chemistry
William B. Euler
With increasing public concern for possible future terrorist attacks involving novel explosives, there is a demand for advanced early detection technology. While trained canines are effective at detecting minute quantities of explosive vapors, canines also suffer from false positives, short attention spans, stress, expensive training, and require the assistance of an accompanying handler. Even with these disadvantageous, canines are currently more common in explosive detection due to conventional sensors. The extremely low vapor pressures at room temperature of most explosives limit the number of explosives molecules to be collected in a reasonable detection time pushes limits for most conventional sensors. Most of these sensor devices are big, as they require a vapor collection and pre-concentration system, and require time-consuming procedures. In addition, the concentration of explosive vapors decreases exponentially as a function of distance from the source, and as the function of time the explosive material is present in a location. The detection of trace quantities of explosives in the gas phase is very important in countering terrorist threats. Nanotechnology-enabled sensors could offer significant advantages over conventional sensors, such as better sensitivity and selectivity, lower production costs, reduced power consumption as well as improved stability. The purpose of this research is to provide a fundamental understanding of the materials and mechanisms required and aid in the development of small, inexpensive, effective portable sensor, which is capable for real time detecting explosives vapor at room temperature using only nature vapor pressure.
A new array sensing system for explosive gas phase is proposed in this study. This sensor is based on a layered structure of fluorophore deposited onto a few hundred nanometers of a transparent polymer, supported by a glass slide. The fluorophores selected, Rhodamine 6G (Rh6G) and related species, are inexpensive xanthene laser dyes which are widely used as a fluorescence tracers because of their strong absorption properties in the visible light region and a high fluorescence quantum yield.
The adsorption of these dyes into solid host systems can alter the photophysical properties of the dye and protect it from thermal decomposition and photobleaching, thus improving the operating and lifetime of the dye. The incorporation of fluorescent dyes into a solid host system becomes of great interest for design of photonic devices with potential applications such as solid tunable lasers and sensors. The type and concentration of aggregates depends on the conditions used for preparation of the hybrid materials.
Formation of aggregates can affect the photophysical characteristics by inducing spectral shifts and band splitting. The presence of aggregates also leads to strong fluorescence quenching at high concentrations. Therefore, controlling the state of aggregation of the dye molecules in a solid matrix is a critical condition for efficiency of fluorescence sensor. The solvent system also plays a role in aggregation formation and a variety of solvents with differing polarities will be used to optimize this technology.
When this fluorophore layer is applied on an appropriate substrate structure before putting on the glass substrate, such as transparent layer of polymer, a huge emission enhancement will occur. This emission enhancement can be explained by internal reflection: When the light hits on the fluorophore layer, some of the light is reflected at each interface, which allows the light to bounce along the polymer layer, this internal reflection can provide more opportunities for the incident light to be absorbed, as a result, the emission enhancement could be over a factor of 1000. This huge emission enhancement shows the potential to be used as fluorescence-based sensor with improved sensitivity, and it is sensitive enough to detect explosives with low vapor pressures under room temperature.
With this new sensing system, selectivity can be improved by using a variety of fluorophores with high quantum yield to create a sensor array. Each array of fluorophores will give a distinctive response to an analyte, resulting in different response pattern for each analyte. A standard pattern for each known analytes can be used on identifying unknowns by this array response pattern.
The photophysical properties of the fluorophores structure and polymer layer effect on the fluorophores needs to be further understood in order to develop this array sensor with improved sensitivity and selectivity.
Chapman, Mingyu, "Development of Rhodamine 6G Thin Filmn as a Fluorescent Sensor for Explosive Vapor Detection" (2017). Open Access Dissertations. Paper 588.