University of Connecticut scientists have developed a novel buried explosive detection system using a nanofiberous film and ultraviolet light. Image, top, shows a Petri dish (left) with buried trace levels of 2,4-DNT explosive in soil and a Petri dish (right) without DNT. Image, bottom, shows each Petri dish after application of the chemical sensing film and following 30 minutes exposure under ultraviolet light. The location of the buried DNT appears in the Petri dish (left) as a dark blot on the film.

A
chemical sensing system developed by engineers at the University of
Connecticut is believed to be the first of its kind capable of detecting
vapors from buried landmines and other explosive devices with the naked
eye rather than advanced scientific instrumentation.

The research was first reported in the May 11, 2012 online edition of Advanced Functional Materials.

The
key to the system is a fluorescent nanofiberous film that can detect
ultra-trace levels of explosive vapors and buried explosives when
applied to an area where explosives are suspected. A chemical reaction
marking the location of the explosive device occurs when the film is
exposed to handheld ultraviolet light.

The
system can detect nitroaromatics such as those found in TNT and 2,4-DNT
(the military’s primary explosive and the principle components in
landmines) as well as the elements used in harder to detect plastic
explosives such as HMX, RDX, Tetryl, and PETN. The ultra-sensitive
system can detect elements at levels as low as 10 parts per billion
(TNT), 74 parts per trillion (Tetryl), 5 ppt (RDX), 7 ppt (PETN) and 0.1
ppt (HMX) released from one billionth of a gram of explosive residue.

If
there is no explosive vapor present, the recyclable film retains a
bright fluorescent cyan blue color when exposed to ultraviolet light. If
explosive molecules are present, the fluorescence is quenched and a
dark circle identifying the threat forms on the film within minutes.

“Our
initial results have been very promising,” says UConn Dr. Ying Wang,
who developed the system as a chemical engineering doctoral student
working under the supervision of UConn Associate Engineering Professor
Yu Lei. “We are now in the process of arranging a large-scale field test
in Sweden.”

Rather
than using sophisticated chemical modifications or costly synthetic
polymers in preparing the sensing material, UConn scientists prepared
their ultra-thin film by simply electrospinning pyrene with polystyrene
in the presence of an organic salt (tetrabutylammonium
hexafluorophosphate or TBAH). This resulted in a highly porous
nanofiberous membrane that absorbs explosive vapors at ultra-trace
levels quickly and reliably. The film also has excellent sensitivity
against common interferences such as ammonium nitrate and inorganic
nitrates. Initial vapor detection took place within seconds with more
than 90% fluorescent quenching efficiency within six minutes.

According
to the United Nations, there are an estimated 110 million active
landmines hidden underground in 64 countries around the world. It is
estimated that as many as 25,000 people are maimed or killed by
landmines each year across the globe. The mines not only threaten
people’s lives, they can paralyze communities by limiting the use of
land for farming or roads for trade.

Clearing
mines is a slow and deliberative process often involving
specially-trained dogs and metal detectors, but each method has its
shortcomings. Dogs, considered the gold standard in detection,
eventually tire and can experience difficulty differentiating in dense
minefields. Metal detectors are prone to false positive readings that
can be triggered by buried pieces of metal unrelated to a mine or
unexploded ordinance.

While
explosive material can be concealed within landmines and IEDs, the seal
is often not airtight and small amounts of vapors escape allowing for
detection.

The
film developed by Wang and Lei is very light weight, similar to paper,
and can be rolled out over a suspect area like a sheet. The
electrospinning process makes it both easy and affordable to produce.

“We
would be very interested in following up on any kind of research that
looks at chemical detection systems,” says Erik Tollefsen, advisor for
stockpile destruction, EOD and technology for the Geneva International
Centre for Humanitarian Demining (GICHD). “This is something we might
use as a quality control tool for animal detection. There are some cost
benefits here.”

“The
general observation is that chemical detection systems work on a
nano-level and our animal-based systems are on the pico-level, which is
1,000 times more sensitive,” Tollefsen says. “But obviously with
animals, you can’t switch them on and off like a machine and they are
sometimes difficult to work with.”

Wang
and Lei have also developed a novel chemical test for detecting TNT in
water and other liquids. The application could be used to detect
potential terrorist threats in airports as well as groundwater
contamination in areas where explosives were used in construction.

The
ultra-sensitive, real-time sensor can detect TNT concentrations ranging
from about 33 parts per trillion (the equivalent of one drop in 20
Olympic-sized swimming pools) to 225 parts per million.

“Our
new sensor based on a recently developed fluorescent polymer for
explosives in aqueous samples has two sensing mechanisms in one sensing
material, which is very unique,” says Lei. “The sensor can easily be
incorporated into a paper test strip similar to those used for pregnancy
tests, which means it can be produced and used at a very low cost.”

Source: University of Connecticut