Emerging detection technology offers improved security screening

Scientists have been developing a range of nanotechnology-based equipment that may significantly enhance airport security. The key area of development is in the area of screening luggage and passengers between check-in and boarding the aircraft. A number of companies are independently devising procedures that enhance the detection of compounds related to explosives and improvised explosive devices (IEDs). One potential tool is being developed by Stirling-based Cascade Technologies, which has concentrated its efforts on mid-infrared (MIR) spectroscopy
techniques for trace vapour detection of IED precursors. The technology is based on the use of MIR laser sources called quantum cascade (QC) lasers. These are semiconductor lasers developed by a number of companies and research units that emit in the mid- to far-infrared
portion of the electromagnetic (EM) spectrum.

Extreme sensitivity

Richard Cooper, operations director of Cascade Technologies, says the detection ability of the
new QC laser-based optical absorption spectroscopy platform is extremely sensitive. “We are
talking about instrument sensitivity in sub parts per billion,” he comments. “To put it into perspective, imagine detecting a single drop of contaminated water in an Olympic-sized swimming
pool.”

The technology would aid throughput by dramatically speeding up the screening of airport
passengers. According to Erwan Normand, chief scientific officer at Cascade Technologies, QC-based systems have the capacity to screen one person per second, giving almost instantaneous results, and could be deployed in a portal or multi-portal configuration screening up to 100 per cent of passengers at walking speed.

The technology works by using an infrared (IR) laser light from a QC laser unit to seek out the chemical fingerprint of specific molecules in gas phase, such as for example those that are linked to IEDs. “It works like a sniffer dog,” explains Normand. “All solid or liquid substances have
a tendency to evaporate to a gaseous form.” The QC laser uses nano-sized crystals to
harness the power of a laser source spanning the full spectrum of the technologically
significant mid-IR wavelengths (3-25 μm). This important spectral region is where the majority
of chemical fingerprints or absorption features lie. These absorption features allow unambiguous
identification fingerprinting results from rotational vibrational transitions of atoms within
the molecules. The benefit of highresolution spectroscopy coupled to the QC laser wavelength selectivity is that it can significantly decrease false-positive detection results.

Cascade Technologies’ work has been supported by the UK Home Office Scientific Development Branch and the UK Ministry of Defence. The company has invested “in excess of a few millions” in overall development of the technology, covering its civilian and security applications, and has tested the technology in a new IED detection programme. In tests, the researchers focused on explosives compounds such as nitroglycerine and ethylene glycol dinitrate, as well as IEDs and their precursors in either liquid or solid form.

The detection of explosives, IEDs and their precusors traditionally relies on a suite of different
technologies, including ion mobility spectroscopy, gas chromatograph mass spectroscopy and Fourier transform infrared. However, a lack of sensitivity and selectivity, combined with concerns over cross-interference and measurement accuracy, prevent them from carrying out 100 per cent
screening of individuals or packages. Of more concern is that, after several consecutive false positives, security officials may by-pass protocols to relieve congestion.

However, the QC laser can also detect IED precursors in pre-defined levels. This would help, for
example, in avoiding false positives from passengers who carry commonly available items that
can contain precursor traces. The laser technology is being trialled by an undisclosed
large security company, and Normand foresees that if tests are positive it could soon be
commercially available. The price of the technology is not being disclosed but clients are expected to include those security companies that specialise in airport screening procedures.
In practical terms, Normand envisages the core technology, which resembles a humble
black box and requires no higher scientific skills to operate, sitting alongside conventional security
items such as x-ray machines or metal detectors. “It could be installed at the end of an xray
machine or by a portal that people walk through,” he explains. Meanwhile, Dr Dmitri Gramotnev, co-leader of Australia’s Queensland University of Technology applied optics programme, is developing a project that he believes will also deliver in terms of detecting residual vapours of chemicals, explosives and biological agents. “All going well with the newly developed nano-focusing technology, luggage could be monitored and any vapours from suspect agents detected as people pass through airports,” he says. Dr Gramotnev is focusing on plasmonic waveguides, which are metallic nanostructures capable of guiding EM waves. “These could be a thin metal strip or a nano-gap in a metal film,” says Dr Gramotnev. “In such structures, electromagnetic waves are
coupled to electron plasma in the metal, which gives rise to their special name — plasmons.”

New sensor technology

Unlike normal waves in dielectric media, such as air, plasmons can be focused or concentrated
into spatial regions that are much smaller than the wavelength (which is called sub-wavelength
localisation). The proposed new sensor technology is based on a non-linear interaction between
tightly focused plasmons and tested molecules, which is called surface-enhanced Raman
spectroscopy. This type of spectroscopy can potentially identify single molecules of particular
substances — most pertinently, molecules of explosives in the air. This would be achieved in a system in which passengers and baggage pass through a gate or a tube in which air samples are continuously taken. This air is then taken by tubes into the analyser that continuously monitors it. If residual vapours of such substances/molecules are detected, then the system will produce an alarm.
Dr Gramotnev expects that the technology could be designed in the form of either portable or stationary equipment. “Hopefully, it will be capable of operating in an automatic or semi-automatic regime,” he says. “I do not think that significant special training would be required.”
He adds: “It will hopefully enable mass routine screening, which is generally not available at this
stage. We are not exactly sure how many passengers it will be possible to screen per hour, but
it is anticipated that this could probably be done at a similar pace as the current screening
by metal-detector systems.” So far, the Australian Federal Police and National Institute of Forensic
Services have committed approximately AUD120,000 (USD108,000) towards the research.
As with any security technology, both Normand and Dr Gramontev take pains not to claim
that their systems are 100 per cent foolproof. “Unfortunately, there are no bulletproof technologies,”
says Dr Gramotnev. “It is always possible, in principle, to circumvent them, for example,
by developing new substances that may not be detectable by the proposed technology,
or by following special procedures that would prevent residual vapours from being released
into the surrounding air.”