Researchers develop paper sensors to detect explosives

Electrochemical and colorimetric devices can detect some of the compounds most often used in terrorist attacks, and they also have medical and environmental applications.

By Elton Alisson

Agência FAPESP – Law enforcement investigators and airport security agents may soon have within their arsenal cheaper and portable devices that provide faster results than the methods currently in use to detect explosives in luggage or in locations deemed potential targets for terrorist attacks.

Researchers from the Chemistry Institute at the University of São Paulo (IQ/USP) have developed an electrochemical sensor (based on chemical reactions that produce energy) and a colorimetric sensor (based on the quantification of a substance through perception of its color) out of office paper and filter paper (similar to that used for coffee), that are capable of detecting some of the explosives most often used in terrorist attacks.

The devices were developed under the FAPESP-funded project entitled “Development of intelligent devices (electronic noses and tongues) using electrochemical, colorimetric and mass measurements aiming to the discrimination of forensic and food samples.”

The electrochemical sensor was described in an article published March 6, 2014 in the online edition of the journal Analyst of the Royal Society of Chemistry (RSC). The colorimetric sensor was described last November in the online edition of the journal Analytical Methods and will be the cover story of the April printed version of the publication, also edited by the RSC.

“We came up with the idea of developing devices using office paper or other types of conventional printer paper because these materials are easy to find anywhere in the world,” Thiago Regis Longo Cesar da Paixão, professor at IQ/USP and the project coordinator, told Agência FAPESP.

“Our goal is to make the devices for detecting explosives and other chemical compounds less expensive, readily accessible and able to be used in remote locations, without the need of a laboratory infrastructure for analysis and trained people to use them,” said Paixão.

According to the researcher, the large-scale use of explosives by terrorist groups in recent years has led to the development of new commercial devices that are able to identify and quantify such explosives.

However, the equipment that is currently available, such as electron capture detectors, mass spectrometers and X-ray readers, is highly sophisticated and expensive, and trained individuals are required to conduct the analyses.

In addition, says Paixão, not all of the techniques are appropriate for detecting explosive peroxides, such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD), whose use by terrorists has increased in recent years owing to the relative ease of their synthesis and of obtaining raw materials such as hydrogen peroxide, acids and acetone to create them.

That is why electrochemical and chromatographic sensors (the latter based on the separation of mixtures and the identification of their components) are being developed to detect this type of explosive, which has gained notoriety after the London Underground attacks in 2005.

“The problem is that most of these new sensors use chromatographic paper, which is more expensive than office paper,” said Paixão.

In an effort to find a less expensive option, the researcher developed an electrochemical sensor using office paper in partnership with the doctoral student he advises, William Reis de Araujo, a FAPESP fellowship recipient.

By using office paper instead of chromatographic paper, the cost of fabricating the sensor has decreased 97%, said Paixão. “We’ve demonstrated that it’s possible to develop electrochemical sensors using office paper and that by replacing the chromatographic paper, there is a huge cost reduction with these devices,” said the researcher.

Combining simple materials

The researchers used a wax printer to print a series of individual, 1.6-cm-diameter white circles on office paper on which to place solutions or samples of material for analysis.

The printed sheets are placed in a laboratory oven or thermal press for three minutes at a temperature of 120ºC. The heating process causes the wax to melt, penetrating all layers of the paper and forming a hydrophobic (water-impermeable) barrier that allows the solution to penetrate and confine itself within the white circles that received no wax impression.

By using a transparency that serves as a template, the researchers paint electrodes on the printed office sheet, using electrically conductive silver ink.

After the ink dries, scissors are used to cut out each electrochemical cell, resulting in a disposable electrochemical device with three electrodes.

When connected to a potentiostat (equipment used to apply an electrode potential and measure the electric current of the conductive solution), the electrochemical sensor made out of paper is able to detect the explosive, picric acid, and lead, which is a component of gunpowder residue, Paixão explained.

“The idea is for these devices to have forensic and security applications for the detection of explosives,” said Paixão. “But they also have sensitivity to chloride ions and heavy metals, which also makes their use in environmental monitoring a possibility,” he explained.

Colorimetric sensor

During the post-doctoral fellowship of Maiara Oliveira Salles, also funded by FAPESP, the researchers applied the same principle as that used with the electrochemical sensor to develop a colorimetric sensor that changes color when exposed to the explosives TATP, HMTD, 4-amino-2-nitrophenol (4A2P), nitrobenzene and picric acid.

To produce the sensor, they placed small amounts of potassium iodide (KI), creatinine and aniline in the white circles – this time printed on filter paper.

Upon contact with each of the five types of explosives, these chemical reagents produce a unique pattern of colors that varies according to the concentration of the compound.

Each color variation of the colorimetric sensor in response to different concentrations of the five explosives was captured and stored using a smartphone application, also developed by the researchers during the project, in collaboration with Gabriel Negrão Meloni, a doctoral candidate at IQ/USP.

The application uses mathematics to analyze the photograph – also taken by smartphone in a device the researchers call a “stool pigeon” – of a colorimetric paper exposed to one of the five types of explosives and indicates which explosive and what amount of it is present in the sample, based on its color pattern.

“The colorimetric sensor allows an airport security agent, for example, to pass the paper over a piece of luggage, then take a photo using a cell phone and get the results from a software analysis that indicates the presence of an explosive,” Paixão explained.

“It was able to identify very low levels of explosives, on the order of 0.2 micrograms,” he said.

According to the researcher, the two devices have not yet been used in the field, in criminal investigations or for detecting security risks at airports.

Other applications

The idea, however, is to make these applications viable and to enable the devices to be used for other purposes, such as detecting illegal drugs like cocaine.

“We have been studying how to use the colorimetric sensor to detect chemical substances used to adulterate cocaine, such as caffeine, acetaminophen and phenacetin, to help police identify the origin of the drug,” explained Paixão.

In addition to their potential crime detection applications, the goal of the researchers is that the technological principle of the sensors will be able to generate a cheaper alternative to the glucose test strips that are used to monitor blood glucose levels in patients suffering from diabetes.

“The strip used in the glucose monitoring test could be replaced with a piece of paper,” said Paixão.

The article “Fabrication of disposable electrochemical devices using silver ink and office paper” (doi: 10.1039/C4AN00097H), by Araujo and Paixão, is available at the journal Analyst at: pubs.rsc.org/en/content/articlelanding/2014/an/c4an00097h#!divAbstract.

The article “Explosive colorimetric discrimination using a smartphone, paper device and chemometrical approach” (doi: 10.1039/c3ay41727a), by Paixão and others, is available at the journal Analytical Methods at: pubs.rsc.org/EN/content/articlelanding/2013/ay/c3ay41727a#!divAbstract.