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Emerald Group Publishing Limited
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Researchers develop miniature sensor for detecting key ingredient of improvised explosives
Article Type: Mini features From: Sensor Review, Volume 29, Issue 2
Hydrogen peroxide (H2O2) can easily be combined with acetone to create acetone peroxide, an explosive, and another explosive material, hexamethylene-triperoxidediamine, is produced by combining hydrogen peroxide, hexamine and citric acid. Being readily available, hydrogen peroxide is widely used by terrorists to manufacture improvised explosives. The bombs that killed more than 50 people in London on July 7, 2005, used acetone peroxide and more than 1,500 pounds of a hydrogen peroxide-based mixture was discovered after an alleged bomb plot in Germany that resulted in the widely publicised arrest last September of three people. Techniques that can rapidly detect trace levels of hydrogen peroxide are therefore of interest to the security services, first responders and forensic teams.
Now, a team of chemists and physicists at the University of California, San Diego (UCDS), has developed a hydrogen peroxide sensor based on organometallic compounds termed metal phthalocyanines. When exposed to most oxidising agents, such as chlorine, these compounds show an increase in the electrical current that will flow through a film of the material, whilst reducing agents have the opposite effect – a decrease in the current. However, when exposed to hydrogen peroxide, an oxidant, the phthalocyanine films behave differently depending on the metal used. Chemiresistive films made of cobalt phthalocyanine (CoPc) show a decrease in current, while those based on copper or nickel (CuPc, NiPc) show an increase. The UCSD team exploited this unusual feature in miniature hydrogen peroxide sensors (Figure 1) which consist of 50 nm-thick films of both CoPc and CuPc, produced by organic molecular beam epitaxy on a single substrate. Whilst the individual sensor outputs are not specific to hydrogen peroxide, when they are considered jointly, they create a signature which is unique to this compound. The sensor is exceedingly sensitive and tests have shown that it can detect hydrogen peroxide vapour from peroxide-based explosives in the parts-per-billion concentration range.
In addition to detecting explosives, the UCSD team believe that the sensor could have industrial applications, for example inexpensively monitoring hydrogen peroxide vapours arising from bleached pulp to which workers may be exposed. According to William Trogler, a professor of chemistry and biochemistry at UCSD “The detection capability of this tiny electronic sensor is comparable to current instruments, which are large, bulky and cost thousands of dollars each. If this device were mass produced, it’s not inconceivable that it could be made for less than a dollar”. Funding for this research was provided by the Air Force Office of Scientific Research and the UCDS team has applied for patents. Full details can be found in Bohrer et al., 2008 (“Selective detection of vapour phase hydrogen peroxide with phthalocyanine chemiresistors”, Journal of the American Chemical Society, Vol. 130, pp. 3712-3).