Sensitive and selective detection of adsorbed explosive molecules using opto-calorimetric infrared spectroscopy and micro-differential thermal analysis

Abstract

Multi-modal explosive sensors based on microelectromechanical systems (MEMS) have been developed. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with the use of microheater/thermometer devices and a widely tunable quantum cascade laser. These devices respond to the heat generated by non-radiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the device for the quantitative analysis. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same devices to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. We have demonstrated successful detection, differentiation, and quantification of trace amounts of explosive molecules (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) and their mixtures using three orthogonal sensing signals.