Thermal-ratcheting molecular concentrator for trace-level vapor detector

Technology overcomes diffusion limits and enables nanosensors (with potential advantages for few-molecule sensitivity, selectivity, power consumption) to be used at low concentrations without prohibitively long collection times


When thinking about vapor sensing devices such as those used for the detection of explosives or biologicals, it’s obvious to focus on the type of sensor employed. But upstream from the sensor are the equally important components as well as processes involved with collecting, concentrating, and delivering the analytes to the sensor. In fact, a sensor is only as good as the corresponding components’ ability to deliver the molecules of interest. One of the critical concerns in the pre-detection process is that of diffusion limits associated with carrier gasses that inflate the concentrations and times required for conventional point sensing systems to perform at the sub-part-per-billion concentration level.

The U.S. military uses hand-held devices that automatically detect, identify and alarm to chemical warfare agents and toxic industrial vapors. (DoD photo)

The key to overcoming the diffusion limits and enabling efficient collection, concentration, and delivery of molecules to a sensor thus appears to involve having a way of moving the molecules by means other than a carrier gas such as air. Until now, no method or device has existed for doing this and thereby for surmounting the diffusion limitation.

Navy researchers have solved the diffusion limit problem and incorporated this in a device for transporting desired analyte molecules in a vapor to a sensor. This molecular concentrator simultaneously concentrates the selected analyte and filters it from interferents so that it can be easily sensed and analyzed.

The device takes advantage of the concept of a thermal ratchet for driving molecules from one place to another. In this case, heater wires are arranged above a substrate – each wire configured to strongly sorb the vapor of interest at room temperature and to rapidly desorb it at an elevated temperature. By selectively heating one or more of the wires, a concentration of vapor molecules can be directed in a desired manner – from one wire to its neighbor or ultimately from the wires closest to the vapor-containing environment to a sensing device.

In one envisioned form, the device is a micro-fabricated, two-dimensional concentric ring configuration of closely-spaced wire heating elements that herds molecules into a very small region for purposes of transduction and detection. The wires can be situated directly on a substrate or they can be suspended above the substrate to improve the thermal isolation of each wire from the others.

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