Sensitive Determination of Hexamethylene Triperoxide Diamine Explosives, Using Electrogenerated Chemiluminescence Enhanced by Silver Nitrate Suman Parajuli and Wujian Miao* Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Sensitive detection and quantification of hexamethylene triperoxide diamine (HMTD), which is one of commonly used explosives by terrorists, was presented on the basis of electrogenerated chemiluminescence (ECL) technology coupled with silver nitrate (AgNO 3 ) enhancement in acetonitrile at a platinum electrode. Upon 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 oxida- tion potential of HMTD in the presence of ruthe- nium(II) tris(bipyridine) (Ru(bpy) 3 2+ ) luminophore species, which showed a half-wave potential of 0.96 V vs Ag/Ag + . The addition of small amounts of AgNO 3 (0.50-7.0 mM) into the HMTD/Ru(bpy) 3 2+ system resulted in significant enhancement in HMTD ECL production (up to 27 times). This enhancement was determined to be largely associated with NO 3 - and was linearly proportional to the concentrations of NO 3 - and Ag + in solution. Homogeneous chemical oxidations of HMTD by electrogenerated NO 3 • and Ag(II) species proximity to the electrode were proposed to be respon- sible for the ECL enhancement. On the basis of cyclic voltammetry (CV) and CV digital simulations, standard potential values of 1.79 V vs Ag/Ag + (or 1.98 V vs NHE) and 1.82 V vs Ag/Ag + (or 2.01 V vs NHE) were estimated for Ag(II)/Ag(I) and NO 3 • /NO 3 - couples, respectively. A detection limit of 50 μM of HMTD was achieved with the current technique, which was 10 times more sensitive than that reported previously, which was based on a high-performance liquid chro- matography/Fourier transform infrared (HPLC/FT-IR) detection method. Legal authorities have witnessed an increased number of threats of illegal use of peroxide-based explosive materials by terrorists. These compounds are very popular among terrorists because they can be synthesized readily from commercially available chemicals. 1 These explosives, which are also known as “unconventional explosives” (for the reason that they have no use for military purposes, because of their high instability and powerful initiating explosive capability), are basically linked to terrorists. 2 There is an urgent need for detection of these “home-made” explosives, especially in the checkpoints of the mass-transit facilities and other government and public facilities. 3-6 Hexamethylene triperoxide diamine (HMTD) is a representa- tive of the aforementioned peroxide-based explosives. It is a white solid with cyclic structure 7 (see Scheme 1), which was first synthesized by Legler in 1885, 8 that is sensitive to friction, impact, and electrical discharge. 9 The friction sensitivity of HMTD is comparable to other well-known explosives, such as triacetone triperoxide (TATP) and trinitrotoluene (TNT), although its impact sensitivity is approximately half of TATP. 10-12 Several analytical techniques have been used to detect peroxide explosives, including HMTD. These techniques include separation- based gas chromatography coupled with mass spectrometry (GC/MS), 13-15 liquid chromatography (LC)/MS, 9 time-of-flight (TOF)/MS, 16 desorption electrospray ionization (DESI)/MS, 6,17 liquid chromatography/Fourier transform infrared (LC/FT-IR), 10 and ultraviolet-visible (UV-vis) spectrometric-based methods. 1 Mass spectroscopy (MS)-based methods are very sensitive and can detect peroxide explosives at nanogram to picogram levels; however, the instruments are generally very expensive and not portable for field tests. Chromatographic, UV-vis, and infrared (IR) detection techniques are often time-consuming in sample * To whom correspondence should be addressed. Tel.: +1 601-266 4716. Fax: +1 601-266 6075. E-mail: wujian.miao@usm.edu. (1) Schulte-Ladbeck, R.; Kolla, P.; Karst, U. Analyst 2002, 127, 1152–1154. (2) Pumera, M. Electrophoresis 2008, 29, 269–273. (3) Munoz, R. A. A.; Lu, D.; Cagan, A.; Wang, J. Analyst 2007, 132, 560–565. 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