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Hydrogen sulfide, Nitrogen Dioxide, and sulfur dioxide are highly toxic gases with health effects at ppb and ppm levels in air. These compounds occur in industrial operations as well as in environmental atmospheric polluted air. Traditionally, microfabricated electrochemical gas sensors use photolithography processes for patterning of the electrodes and defining the electrode geometry for high surface area to increase the sensor gas sensitivity. We present herein a series of gas sensor designs fabricated via laser ablation of sputtered Au films. For these sensors, the laser ablation step itself takes less than two hours to pattern up to 60 sensors. The sensors are fabricated on a porous hydrophobic substrate made of layers of a Teflon woven mesh, and the 400 nm gold film was sputtered onto a 100 nm tungsten adhesion layer. A total of 60 sensors was patterned in a 6 by 10 grid per substrate with each sensor having a footprint of 15 mm x 15 mm. An image of some of the sensors and their corresponding designs are illustrated in figure 1. Laser ablation was performed by an Optec WS-Flex USP femtosecond 1030 nm laser. The pulse frequency was set to 400 kHz and both the speed and jump speed were set to 200 mm/s. The laser power is nominally 4 W and the power percentage setting was determined for each substrate by patterning a test array onto the substrate in increments of 2%. From this test array, the lowest power that completely ablated the gold could be determined and that was used for the remaining sensor ablation. Images of the etched surface area shown in the figure 1. Surface analysis with EDS using SEM microscope indicated the composition of the layers and effective removal of the gold from the Teflon sheet without damage to the Teflon porosity in the woven mesh, figure 2. The sensor electrodes were assembled by lamination between several plastic layers and then an electrolyte was added. The assembled sensors were diced from the assembled 10x6 sensor wafer and individual sensors were connected to a potentiostat and tested via exposure to the various pollutant gases. The current response of these amperometric sensors was measured and found to be linear with respect to concentration in the low ppm range. This work illustrates an alternative to photolithography for the preparation of thin film gas porous electrodes for use in amperometric gas sensors. Figure 1

The thermal decomposition of 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and other explosives over heated noble metal surfaces generates characteristic products that can be efficiently detected by amperometric gas sensors. This observation was exploited to develop a method for the selective in situ detection and quantitation of explosives contamination in soils. Amperometric sensing of the compounds generated from the thermal decomposition of soil-bound TNT was selective and sensitive over a broad range. Laboratory and field-usable probes that accommodate the electrochemical sensors, the thermal desorption/catalyzer unit or “heater”, and pneumatic components were designed, fabricated, and deployed. A field test demonstrated that this method is viable for in situ analysis of explosives.

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