Developing Sensors to Detect Deadly Explosives

October 09, 2016 by Dr. Steve Arar

Researchers are trying to commercialize TATP detection sensors as soon as possible.

Sensor development is important for security around the world. Learn how scientists are developing sensors for triaceton triperoxide (TATP), one of the most notorious explosives in the world.

After the terrorist bombings in Brussels early this year, Senator Chuck Schumer asked the American government’s anti-terrorism agency to expedite the testing of a recent technology to detect the explosive used in the attack.

The bombing, which killed at least 31 people, utilized triaceton triperoxide. The explosive was also the main ingredient of the Paris attack last year.

Triaceton Triperoxide (TATP)

Triacetone triperoxide, a peroxide explosive, has been increasingly utilized by terrorists during the last decades. This is mainly due to the fact that the starting materials of the explosive—i.e., acetone, hydrogen peroxide, and acid—are all commercially available in pharmacies and hardware stores. Moreover, the synthesis process of triacetone triperoxide, or TATP for short, is simple and available on the internet.

TATP is almost as strong as TNT, which is the most commonly employed military explosive. However, unlike TNT, TATP is so sensitive to heat, shock, and friction that it has no military use. Because TATP could explode during manufacturing, it presents as much danger to the maker as it does to the target, explaining the other name of the explosive, “Mother of Satan”.

Interestingly, the explosion of TATP involves a rare phenomenon known as entropic explosion. This means that the reaction does not generally produce any heat or flame. Instead, it causes a large change in volume by producing four gas-phase molecules from each molecule of TATP in the solid state. This reminds us of the reaction which occurs in the safety airbags and rapidly produces a large amount of gas during accidents.

A vial containing TATP filter paper of a URI laboratory. Image courtesy of Phys.

Challenges of TATP Detection

Since TATP does not contain any nitro groups or metallic elements, it does not have significant absorption in the ultraviolet region and does not fluoresce. As a result, the conventional methods of explosive detections, such as spectroscopy methods, are not successful in the case of TATP.

The previous studies on TATP detection, such as ion mobility spectrometry, mass spectrometry, fluorescence spectroscopy, and absorption spectroscopy, are not fast enough and/or cannot provide sufficient accuracy. These methods generally require expensive and non-portable instrumentation and rely on extensive sampling.

Vapor Phase Detection of TATP

The explosive has a vapor pressure of approximately 0.03 torr at room temperature. The vapor pressure, which is the pressure exerted by the vapor of the compound in thermodynamic equilibrium with its solid (or liquid) form, is an indication of the compound’s sublimation (evaporation) rate.

Having a high vapor pressure, TATP can easily sublimate at room temperature. This makes storage of the explosive difficult and dangerous. However, researchers have employed this feature to perform vapor phase detection of TATP.

Research published in 2009 by a group of chemical engineers utilizes an array of sensors where each element of the array is a field effect-like transistor. The gate of each of these devices is replaced by a monolayer of receptor molecules. When exposed to the TATP vapor, the current passing the device varies. However, the limit of detection for this scheme was reported to be around 100ppb (parts per billion).

The scheme of the GaAs-based device and a die containing 20 devices. Image courtesy of ScienceDirect.

In 2010, research done at the University of Illinois at Urbana-Champaign employed a colorimetric sensor array to detect the vapor of TATP with limits of detection below 2ppb.

A colorimetric sensor is based on a gas-sensitive material which experiences a change in the color as it is exposed to the target gas. These sensors are capable of detecting gasses such as carbon monoxide, ammonia, nitrogen dioxide, and ethylene.

The researchers at the University of Illinois pretreated the TATP vapor stream with a solid acid catalyst, Amberlyst-15, and exposed the sensor array to the acid decomposition product. Depending on the concentration of the TATP vapor, the experiment led to different color maps as shown in the following figure.

Color difference maps for various concentrations of TATP vapor. Image courtesy of JACS.

Using these patterns, it is possible to detect different concentrations of TATP vapor. According to the study, the choice of the appropriate catalyst acid plays a significant role in the minimum concentration which can be detected. Unfortunately, this method is based on a one-time use and cannot be employed in continuous real time detection.

The Detection Method Developed by URI

The research team of chemical engineering of the University of Rhode Island examined the complex vapor which is obtained from TATP sublimation. They designed a sensor which is sensitive to both the peroxide bonds in TATP and its organic by-products.

The study led to a thermodynamic-based gas sensor which could detect TATP with high selectivity and high sensitivity. The sensor measured the heat absorbed or generated by the catalyst in presence of TATP and its decomposition products. An array of different catalysts was employed to perform real-time detection with a minimum number of false alarms.   

Before the Brussels attacks, the technology was to be tested in the fall at facilities in Atlantic City, New Jersey and in Savannah, Georgia. However, after the attack, Schumer said that the detectors could save countless lives and that the testing must be done as soon as possible without wasting a moment.

In 2008, the U.S. Department of Homeland Security began funding the university’s work through a center for explosives research.

Schumer added that the lab experiments had shown that the detector can continuously monitor the air and sniff out a very tiny amount of the explosive. In the view of Schumer, only the real life and in situ operation of the system needs to be verified. 

Otto Gregory, a University of Rhode Island professor of chemical engineering behind the technology, noted that clearing the research-related hurdles and giving the technology government priority would make it possible to expedite the testing by several months. He added that the detector can compete with a police bomb-sniffing dog and, unlike a dog, it does not need training or breaks!

He expected that the first prototype would cost about $1,000 to $2,000 but estimated that his final device would be a several-hundred-dollar handheld product.

Professor Otto Gregory holding a silicon wafer which contains the sensors to detect TATP. Image courtesy of Phys.

The Brussels attacks, once again, attracted attention to how dangerous TATP is. It also reminded us all that fast, sensitive, and reliable detection of the explosive is of paramount importance.