Voltammetric Detection of Explosives in Wet Environments with Ionic Liquids and Ionogels
Authors: Lee Junqiao , Silvester Debbie S. 
Published: May 2020

2,4,6-Trinitrotoluene (TNT) is an explosive compound commonly used for industrial, military, as well as mining applications. It is also used by terrorists in homemade explosives, due to its insensitivity to shock and friction compared to other high explosives such as nitroglycerin. At mining sites where TNT is used for land clearing, if not completely removed after detonation, TNT and its reduction products may contaminate ground water and rivers — which is an environmental issue since they are known to be toxic and carcinogenic to humans. The development of sensors for explosives such as TNT is therefore of huge interest due to the wide range of security and environmental concerns. In recent years, room temperature ionic liquids (RTILs) have been investigated as a replacement solvent in sensors1 due to their favourable properties including wide electrochemical windows, intrinsic conductivity, high chemical and physical stability, and the ability to dissolve a wide range of analytes. However, RTILs are liquid and tend to flow, which makes the sensor not sufficiently robust for portable applications due to electrolyte spillage. To overcome this, we have mixed a RTIL with a cheap and widely available methacrylate polymer to produce a non-flowing ionogel, which we refer to as a gel polymer electrolyte (GPE).2

It was observed that the electrochemical signal in the GPE was significantly less affected in humid environments compared to the neat RTIL, where water molecules can act as proton donors, altering the mechanism of electrochemical reactions leading to sensor instability.2 Notably, the voltammetry of TNT in a hydrophobic RTIL was found to be unaffected by the presence of oxygen, which allows for selectivity over oxygen and moisture for the detection of TNT in real environments.4 With the choice of a hydrophobic RTIL/polymer mixture and the use of a cheap commercially available thin-film electrode, we have developed a new electrochemical system that enables direct detection and quantification of TNT in aqueous solutions (see Figure 1).3 The RTIL acts to preconcentrate TNT into the GPE and provides ionic conductivity, while the hydrophobic polymer serves to increase viscosity to ensure mechanical stability of the GPE while immersed in water, in addition to blocking water from reaching the electrode surface. Using square wave voltammetry, linear plots of peak current vs. concentration of TNT were obtained, and the sensor device was able to quickly and easily quantify TNT concentrations at typical ground water contamination levels via. liquid/liquid extraction mechanism. The low-cost and portability of the sensor device, along with the minimal amounts of GPE materials required, makes this a viable platform for the onsite monitoring of explosives, which is currently a significant operational challenge. Our GPEs may further allow for rapid and easy forensic surveying and detection of explosives in real-environments with simple and portable equipment — which we are presently developing; negating the need for the samples to be transferred to a lab to be analysed with expensive bulky instruments, and the need for careful pre-processing of the samples. The ability to immediately detect explosives onsite also reduces the chances of residue decomposition and possible contamination of samples during transport.
Our polymer/RTIL based materials can readily be extended to the sensing of other redox active explosive materials, and other analyte species as well. The ease of mass production and low cost of our electrode/GPE system, along with the minimal amounts of RTIL and polymer required for the GPE, make this a viable electrochemical sensing device to be applied on a bulk scale, in a wide range of environmental conditions.

References

1. Silvester, D. S., Recent advances in the use of ionic liquids for electrochemical sensing. Analyst, 2011, 136, 4871.
2. Lee, J., Du Plessis, G., Arrigan, D.W.M., and Silvester D.S., Towards improving the robustness of electrochemical gas sensors: impact of PMMA addition on the sensing of oxygen in an ionic liquid. Analytical Methods, 2015, 7, 7327.
3. Yu, H.A., Lee, J., Lewis, S.W. and Silvester, D.S., Detection of 2,4,6-trinitrotoluene using a miniaturized, disposable electrochemical sensor with an ionic liquid gel-polymer electrolyte film, Analytical Chemistry, 2017, 89, 4729.
4. Lee, J., Silvester, D.S, Electrochemical Detection of Explosive Compounds in an Ionic Liquid in Mixed Environments: Influence of Oxygen, Moisture, and Other Nitroaromatics on the Sensing Response, Australian Journal of Chemistry, 2018, 72(2), 122.

Figure 1