Detecting homemade explosives, not toothpaste

Sandia National Laboratories researchers want airports, border checkpoints and others to detect homemade explosives made with hydrogen peroxide without nabbing people whose toothpaste happens to contain peroxide.

Hydrogen peroxide is found in everyday products ranging from soap, toothpaste and hair colour to laundry bleach, carpet cleaners and stain removers.

Part of the challenge faced in developing a portable sensor is to detect a common homemade explosive called a FOx (fuel/oxidiser) mixture, made by mixing hydrogen peroxide with fuels, said Chris Brotherton, principal investigator for a Sandia research project on chemiresponsive sensors. The detector must be able to spot hydrogen peroxide in concentrations that don’t also raise suspicions about common peroxide-containing products.

Brotherton’s Early Career Laboratory Directed Research and Development (LDRD) project proved a sensor could identify relatively high concentrations of hydrogen peroxide and differentiate that from a common interfering substance such as water, he said. The next step would be to work with an industrial partner to design an overall system that works faster and can be mass produced.

Sandia National Laboratories researcher Chris Brotherton checks tiny sensors in a test fixture, where he exposes them to different environments and measures their response to see how they perform. Brotherton is principal investigator on a project aimed at detecting a common type of homemade explosive made with hydrogen peroxide. (Photo by Randy Montoya)

A major challenge was distinguishing between hydrogen peroxide and water, which exhibit similar behaviour in chemiresistors. The key was choosing certain molecules in a polymer matrix, suggested by Brotherton’s Sandia technical mentor, polymer chemistry expert David Wheeler. When exposed to peroxide, those molecules react in a different way than when exposed to water.

The idea is to engineer the polymer to be as similar to the target material as possible, relying on the undergraduate rule that like dissolves like. The tiny sensor incorporates the polymer and chains of miniscule conductive metal beads. The polymer reacts when it’s exposed to the substance being analysed.

Exposure to water also changes the polymer, but it returns to its previous state once the water is removed. Exposing the polymer to concentrated hydrogen peroxide, however, is irreversible. It’s also a detector that doesn’t react to toothpaste and other common peroxide products, he said.

The sensor has other potential uses, such as monitoring underground water, looking for plumes of contamination or monitoring industrial processes. Researchers need to reduce the chemical reaction time so the sensor doesn’t take too long to be useful at a checkpoint, he said. The detector also must be incorporated into a larger unit that includes equipment to gather a sample for analysis. It wouldn’t have to be a large unit. Various detectors on the market today are about the size of a small, handheld vacuum cleaner, Smith said.

The support equipment would suck up a sample of air and the detector would test it. Although a detector package could target a single type of vapour, a manufacturer could add it to a unit that detects several substances. That way, a checkpoint could have one sensing system rather than separate units for every material of concern, Brotherton suggested.