At airport security check points, officials need quick, inexpensive tests for explosives. Now researchers in Australia report a paper-based microfluidic device that changes color in seconds when exposed to nanogram quantities of TNT and similar explosive compounds (Anal. Chem. 2014, DOI: 10.1021/ac403062y).
TNT and other trinitro aromatic explosives produce colorful anions when they react with hydroxide or methoxide ions. Lucas Blanes of the University of Technology, Sydney, and his team wanted to use this color change as a quick way to detect these explosives on a microfluidic paper-based analytical device (µPAD). The team printed 5- to 10-mm-diameter rings of wax onto filter paper to define the sample regions and treated the inner circles with various concentrations of sodium and potassium hydroxide. They also left some circles untreated. Each device costs a penny, or even less, to make, Blanes says.
To test explosive-treated surfaces, they sprayed a mix of methanol and water on the surface and then dabbed the paper device on it. In the presence of tens of nanograms of explosives, the color change began within seconds in the spots treated with hydroxide. The device did not change color in the presence of interfering compounds such as cleansers, detergents, or other types of explosives.
To confirm the results, the team used a hole punch to remove the untreated paper circles and extracted organic compounds with a solvent. They then tested the solution with a lab-on-a-chip device made by Agilent. Using this more sensitive technique, they could detect as little as 7 ng of TNT.
The researchers have been developing similar methods for other types of explosives. Blanes believes that these types of detection devices also could help police in developing countries by providing them with low-cost tools for forensic analysis.