Ionic liquid thin layer EQCM explosives sensor

An integrated sensor that combines electrochemical and piezoelectric transduction mechanisms into a single miniaturized platform was developed and validated for the detection of nitro aromatic compounds such as ethyl nitrobenzene (ENB) and dinitrotoluene that are analogues of redox active explosives. An ionic liquid (IL) BMIBF₄ was used as both the electrolyte and the sorption solvent for the two-dimensional electrochemical and piezoelectric gas sensors. The electrochemical behaviors of these nitro compounds in BMIBF₄ were studied by cyclic voltammetry, differential pulse voltammetry and square wave voltammetry, in parallel. The electrochemical properties of these compounds resembled the electrochemical reduction processes in their aprotic solutions, showing first a reversible reduction process and then subsequently an irreversible reduction processes. The redox properties of these compounds also depend on the number of nitro groups and the position of the nitro groups on the benzene ring. Square wave voltammetry was used to quantitatively analyze the ENB in BMIBF₄. Reduction peaks in the square wave voltammetric curves could be obtained when the concentrations were at ppm level. A small amount of moisture in the IL electrolyte did not significantly affect the redox behaviors. Piezoelectric quartz crystal microbalance (QCM) electrodes and the electrodes for amperometry were fabricated on a single piece of quartz plate. Detection of the volatile ENB vapor with this integrated EQCM chip was tested with both QCM and amperometric methods. The sensor's signal was related quantitatively to the ENB vapors adsorbed in BMIBF₄ from air. Combining amperometric and QCM detection simultaneously can cross-validate the detection technology, reduce false positives and false negatives and increase the accuracy of the detection.