Sensitive detection of high explosives using electrogenerated chemiluminescence

Suman Parajuli

Abstract

The core of this dissertation lies in the search of analytical tools which can be used to detect and quantify the high explosives confiscated from suspects in transportation hubs and from soil and water bodies where these explosives pose a greater threat to public health and safety. High explosives, namely, hexamethylene triperoxide diamine (HMTD), triacetone triperoxide (TATP), trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN), were detected and quantified by electrochemical methods such as electrogenerated chemiluminescence (ECL) and cyclic voltammetry (CV). Sensitive detection and quantification of HMTD, one commonly used explosive by terrorists, was presented first in this dissertation on the basis of ECL technology coupled with silver nitrate (AgNO3 ) enhancement in acetonitrile (MeCN) at a Pt electrode. Upon the anodic potential scanning, HMTD irreversibly oxidized at ∼1.70 V vs Ag/Ag+ (10 mM) at a scan rate of 50 mV/s, and the ECL profile was coincident with the oxidation potential of HMTD in the presence of tris(2,2'-bipyridine)ruthenium(II) cation (Ru(bpy) 32+ ) luminophore species which showed a half-wave potential of 0.96 V vs Ag/Ag+ . The addition of small amounts of AgNO 3 (0.50 to 7.0 mM) into the HMTD/Ru(bpy)32+ system resulted in significant enhancement in HMTD ECL production (up to 27 times). This enhancement was found to be largely associated with NO3 - and was linearly proportional to the concentrations of NO3- and Ag+ in solution. Detection of TATP and the differentiation of TATP from HMTD were accomplished subsequently with ECL at glassy carbon electrode in water-MeCN mixture solvents. In the presence of Ru(bpy)32+ , TATP or hydrogen peroxide (H2 O2 ) derived from TATP via UV irradiation or acid treatment produced ECL emissions upon cathodic potential scanning. Interference of H2 O2 on TATP detection was eliminated by pre-treatment of the analyte with catalase enzyme. Selective detection of TATP from HMTD was realized by scanning the electrode potential positively as well as negatively; HMTD showed ECL emissions at both directions. ECL quenching method can also be used to detect explosive compounds where the explosive of interest can quench ECL response either by excited state quenching or quenching by depletion of the precursors of the excited state species in the test solution. In this investigation, ECL quenching behavior of the Ru(bpy) 32+ /tri-n -propylamine (TPrA) system with TNT at a Pt electrode in MeCN was explored. Effective ECL quenching of the system upon the addition of TNT was observed, with a Stern-Volmer constant of 2×104 M-1 . The apparent ECL quenching constant calculated from the Stern-Volmer plot was found to be 3.5×10 10 M-1 s-1 , which suggests the efficient quenching of ECL by TNT. The consumption of the TPrA· free radicals and Ru(bpy)3+ species (produced as a result of reduction of Ru(bpy)32+ by TPrA · ) by TNT could be the main reason of this quenching, as both TPrA· and Ru(bpy)3+ species are precursors of the excited state Ru(bpy)32+ * species. The present technique can sensitively detect TNT as low as 4.4 μM. Electrochemical detection of PETN was studied in MeCN with a Ag wire as the working electrode, where an irreversible reduction wave at -0.9 V vs Ag/Ag+ was observed. The reduction of PETN probably involved the formation of alcohol and nitrite ion with the trace amount of water present in the solvent. The limit of detection of PETN by simple electrochemical CV method was 25 μM. (Abstract shortened by UMI.)