Polymer-Based Trinitrotoluene Sensor Using Initiated Chemical Vapor Deposition

Lucas D. McIntosh, Department of Chemical Engineering, Missouri University of Science & Technology, 1011 West 14th Street, Rolla, MO 65401

Chemical sensors designed to rapidly detect the presence of explosives are in high demand for their potential application in military and humanitarian de-mining, remediation of explosives manufacturing sites, and forensic and criminal investigations. Current techniques for the detection of nitroaromatic high explosives such as 2,4,6-trinitrotoluene (TNT) are considered inadequate for their expense and/or inconvenience in field operations. For example, modern land mines commonly employ plastic casing undetectable to traditionally used metal detectors, and canines—though considered the most reliable tool for explosive vapor detection—are expensive to train and are easily fatigued.

The goal of our project was to develop a mechanically robust, low power, highly sensitive and selective polymer-based TNT sensor that can be incorporated into a field-ready detection system. Such a system would rely on a thin polymer film swelling as a result of absorption or reaction with TNT, thus closing a DC electrical circuit. A radio frequency signal would then alert the user to the presence of TNT.

To fabricate the TNT detection device, various homo- and co-polymers were deposited on silicon trench wafers using a novel polymerization technique – initiated chemical vapor deposition (iCVD). Initiated CVD enables the fabrication of chemically well-defined thin polymeric films on complex objects with micro- and nano-scale features. By depositing polymers from the vapor phase, many wetting and solution effects are avoided, and conformal films can be created. In iCVD, a variant of hot filament CVD, the deposition rate is enhanced and chemical functionalities of the polymers' constituents are maintained by including a thermally labile initiator in the feed stream. Due to the low energy required when using an initiator, delicate substrates can be coated.

Monomers were chosen based on their potential to interact exclusively with the aromatic ring and nitro groups of TNT. Homo- and co-polymers were designed to be rubbery at room temperature (i.e., with glass transition temperatures below room temperature). Rubbery polymers facilitate diffusion of vapor phase solutes into the bulk polymer matrix. Further, polymer films were synthesized with low molecular weight chains and cross linking was avoided.

Polymer films—typically on the order of 50 – 200 nanometers thick—were deposited on silicon trench wafers. Trench wafers are thicker than traditional silicon wafers and have trenches of varying width cut into the surface. Polymer films deposited on trench wafers using iCVD conform to the wafer surface, resulting in a break in the film. Ultimately, electrical leads would be attached on both sides of the trench and the polymer film coated with an electrically conductive material. A polymer film absorbing an analyte such as TNT would swell shut, closing the circuit.

Samples were tested for their interaction with TNT analogues such as 4-nitrotoluene and nitrobenzene using in situ interferometry. The interferometry signal was calibrated for various thicknesses of polymer film. The polymer's swelling was monitored while bubbling nitrogen through liquid phase TNT analogues and flowing the mixture over samples in a temperature controlled chamber. A certain homopolymer (patent pending) was found to swell by approximately 800% in the presence of 4-nitrotoluene at a concentration of 187 ppm. The sample did not interact with controls such as water, nitrogen, and cyclohexane. The sample did not return to its original thickness, possibly due to morphology degradation. In situ measurement using TNT analogues will be used to confirm that the polymer swells shut and closes the circuit.