Colorimetric Sensing of the Insensitive Energetic Material 3-Nitro-1,2,4-triazol-5-one (NTO) Using l-Cysteine Stabilized Gold Nanoparticles and Copper(II)

Abstract

Since conventional sensitive explosives have given rise to unforeseen accidents during storage and transport, the demand of modern armies for insensitive energetic materials is on the rise. There are very few determination methods for the most widely used insensitive energetic materials such as 3-nitro-1,2,4-triazole-5-one (NTO). Thus, the aim of this work is the development of a rapid and practical nanoparticle-based colorimetric sensor for determination of NTO. The detection principle of the sensor involved electrostatic attraction of NTO anion to the ammonium group of l-cysteine functionalized gold nanoparticles (AuNP-Cys), followed by the formation of a Cu²⁺-coordination complex between particles to result in AuNPs agglomeration. The concomitant color change was from red to violet. The surface plasmon resonance band of AuNPs at 520?nm shifted to 650?nm upon chemical reaction and agglomeration. Spectroscopic evaluation was made by taking the ratio of 650?nm absorbance to that of 520?nm, and correlating this ratio to NTO concentration. The analytical performance characteristics of this ratiometric sensor for NTO as the molar absorptivity (ε); limits of detection (LOD) and quantification (LOQ) were: ε = (8.62?±?0.29) × 10³ L mol^?1 cm^?1, LOD = 0.25?mg L^?1, and LOQ = 0.85?mg L^?1. The sensor was applied to various energetic material mixtures containing 2,4,6-trinitrotoluene, hexahydro-1,3,5-trinitro-1,3,5-triazine, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, and tetryl. Additionally, the possible interference effects of commonly found soil ions such as Cl^–, NO₃^–, SO₄^2–, PO₄^3–, Mg²⁺, Ca²⁺, Na⁺, and K⁺ were studied. The proposed method was statistically validated against a literature liquid chromatography–tandem mass spectrometry (LC/MS-MS) method.