Jennifer L. Stepnowski

Nerve agent detection using networks of single-walled carbon nanotubes

We report the use of carbon nanotubes as a sensor for chemical nerve agents. Thin-film transistors constructed from random networks of single-walled carbon nanotubes were used to detect dimethyl methylphosphonate (DMMP), a simulant for the nerve agent sarin. These sensors are reversible and capable of detecting DMMP at sub-ppb concentration levels, and they are intrinsically selective against interferent signals from hydrocarbon vapors and humidity. We provide additional chemical specificity by the use of filters coated with chemoselective polymer films. These results indicate that the electronic detection of sub-ppb concentrations of nerve agents and potentially other chemical warfare agents is possible with simple-to-fabricate carbon nanotube devices.

Stand-off detection of trace explosives via resonant infrared photothermal imaging

We describe a technique for rapid stand-off detection of trace explosives and other analytes of interest. An infrared (IR) laser is directed to a surface of interest, which is viewed using a thermal imager. Resonant absorption by the analyte at specific IR wavelengths selectively heats the analyte, providing a thermal contrast with the substrate. The concept is demonstrated using trinitrotoluene and cyclotrimethylenetrinitramine on transparent, absorbing, and reflecting substrates. Trace explosives have been detected from particles as small as 10 μm.

The design of functionalized silicone polymers for chemical sensor detection of nitroaromatic compounds

Abstract The solubility properties of a series of nitroaromatic compounds have been determined and utilized with known linear solvation energy relationships to calculate their sorption properties in a series of chemoselective polymers. These measurements and results were used to design a series of novel chemoselective polymers to target polynitroaromatic compounds. The polymers have been evaluated as thin sorbent coatings on surface acoustic wave (SAW) devices for their vapor sorption and selectivity properties. The most promising materials tested, include siloxane polymers functionalized with acidic pendant groups that are complimentary in their solubility properties for nitroaromatic compounds. The most sensitive of the new polymers exhibit SAW sensor detection limits for nitrobenzene (NB) and 2,4-dinitrotoluene in the low parts per billion (ppb) and low parts per trillion (ppt) concentration range, respectively. Polymers with favorable physicochemical properties exhibit low water vapor sorption, and rapid signal kinetics for NB, reaching 90% of signal response in 4 s. Studies with an in situ infrared spectroscopy technique are used to determine the mechanism of interaction between nitroaromatic compounds and the chemoselective polymer.

The “NRL-SAWRHINO”: a nose for toxic gases

Abstract At the Naval Research Laboratory (NRL), surface acoustic wave (SAW) chemical sensor systems have been in development since 1981. The primary focus has been the detection and identification of chemical agents and other toxic gases or vapors. In the recently developed “NRL-SAWRHINO” system (Rhino, Gr. Nose), a self-contained unit has been developed capable of autonomous field operation. An automated dual gas sampling system is included, for immediate and periodic detection capability. The latter, utilizes a trap-and-purge miniature gas chromatographic column, which serves to collect, concentrate, and separate vapor or gas mixtures prior to SAW analysis. The SAWRHINO includes all the necessary electronic and microprocessor control, SAW sensor temperature control, onboard neural net pattern recognition capability, and visual/audible alarm features for field deployment. The SAWRHINO has been trained to detect and identify a range of nerve and blister agents, and related simulants, and to discriminate against a wide range of interferent vapors and gases.

Performance optimization of surface acoustic wave chemical sensors

Acoustic wave devices coated with a thin layer of chemoselective material provide highly sensitive chemical sensors for the detection and monitoring of vapors and gases. In this work, a variety of coating materials and coating deposition techniques have been evaluated on surface acoustic wave (SAW) devices. A novel thin film deposition technique, matrix assisted pulsed laser evaporation (MAPLE), is utilized to coat high quality polymer films on SAW devices, and conventional pulsed laser deposition is used to deposit a passivation layer of diamond-like-carbon on a SAW device surface to prevent water adsorption. In addition, chemoselective coatings are formed by covalent attachment of functionalized species to the silica surface of SAW devices. The self-assembled monolayer or near monolayer structures are designed to populate the SAW device surface with the desirable hexafluoroisopropanol moeity. The rapid kinetic signals achievable with the various coated SAW sensors during vapor tests are examined as a function of the coating material and the quality of the thin films. In parallel to the thin film deposition, growth, and vapor testing, the electrical characteristics of the SAW sensor have been characterized. The quality factor and residual phase noise of polymer coated SAW devices are examined, and a prediction of the theoretical limit of the phase noise performance of the loop oscillator is made.

Photonic microharp chemical sensors

We describe a new class of micro-opto-mechanical chemical sensors: A photonic microharp chemical sensor is an array of closely spaced microbridges, each differing slightly in length and coated with a different sorbent polymer. They are optically interrogated using microcavity interferometry and photothermal actuation, and are coupled directly to an optical fiber. Simultaneous measurements of the fundamental flexural resonant frequency of each microbridge allow the real-time detection and discrimination of a variety of vapor-phase analytes, including DMMP at concentrations as low as 17 ppb.

All-optical micromechanical chemical sensors

The authors describe experimental results from micromechanical resonators coated with a chemoselective polymer that detect chemical vapors from volatile organic compounds using all-optical interrogation. The shift in the resonant frequency of the gold microbeam is read out using photothermal actuation and microcavity interferometry. Response times of less than 5s are achieved for vapor concentrations as low as 60ppm using optical powers of a few megawatts.

Stand-off detection of trace explosives by infrared photothermal imaging

We have developed a technique for the stand-off detection of trace explosives using infrared photothermal imaging. In this approach, infrared quantum cascade lasers tuned to strong vibrational absorption bands of the explosive particles illuminate a surface of interest, preferentially heating the explosives material. An infrared focal plane array is used to image the surface and detect a small increase in the thermal intensity upon laser illumination. We have demonstrated the technique using TNT and RDX residues at several meters of stand-off distance under laboratory conditions, while operating the lasers below the eye-safe intensity limit. Sensitivity to explosives traces as small as a single grain (~100 ng) of TNT has been demonstrated using an uncooled bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light and have a range of thermal conductivities. By varying the incident wavelength slightly, we demonstrate selectivity between TNT and RDX. Using a sequence of lasers at different wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. At higher energy levels we also show it is possible to generate vapor from solid materials with inherently low vapor pressures.

Advances in standoff detection of trace explosives by infrared photo-thermal imaging

A technique for stand-off detection of trace explosives using infrared (IR) photo-thermal (PT) imaging, remote explosives detection (RED), is under development at the Naval Research Laboratory. In this approach, compact IR quantum cascade lasers (QCLs) tuned to strong absorption bands of trace explosives illuminate a surface of interest. An IR focal plane array is used to image the surface and detect any small increase in the thermal emission upon laser illumination. The technique has been previously demonstrated at several meters of stand-off distance indoors and in field tests with sensitivity to explosive traces as small as a single grain (~1 ng), while operating the lasers below the eye-safe intensity limit (100 mW/cm2) at the tested wavelengths. By varying the incident wavelength slightly, selectivity between TNT and RDX has been achieved. A complete test and analysis can be performed in less than 1 second. This manuscript critically examines components used with RED and demonstrates several improvements. These include QCL drive electronics for narrower spectral emission linewidth, fixed wavelength QCL packaging that optimizes spectral and spatial output, fiber-optic coupling for QCL beam steering and spatial filtering, cooled IR sensors that increase sensitivity and speed, tunable QCL sources that increase selectivity and extend the library of possible analytes, and dynamic PT signal processing that can increase sensitivity and speed. When considered in combination with the capabilities previously demonstrated for RED, and its capability to operate within eye-safety limits, this technology offers the potential for a wide area of applications relating to the detection of trace explosives on surfaces in both non-contact and stand-off configurations.

Stand-off detection of trace explosives by infrared photo-thermal spectroscopy

We have developed a technique for stand-off detection of trace explosives using infrared photo-thermal imaging. Compact infrared quantum cascade lasers tuned to strong absorption bands in the explosive traces illuminate a surface of interest while an infrared camera detects the small increase in thermal signal. We have demonstrated the technique at several meters of stand-off distance under laboratory conditions using TNT and RDX traces, while operating the lasers below the eye-safe limit (100 mW/cm2). Sensitivity to explosive traces as small as 1ng has been demonstrated, using a micro-bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light. By varying the incident wavelength slightly, we show selectivity between TNT and RDX. Using several laser wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. We have developed a prototype system for outdoor testing at longer stand-offs.