"All-in-a-tube" detection of RDX and TNT: old silver mirror reaction revived for nitro-explosive quantification

Inspired by analyte-induced in situ formation/growth of silver nanoparticles (AgNPs), the traditional Tollens' reagent of diamminesilver(I) complex cation [Ag(NH₃)₂]⁺ was shown to be capable of detecting explosives. Thus, a color test based on in situ formation of AgNPs was developed for 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT). The resultant yellow-colored AgNPs were characterized using UV-visible spectrophotometry, scanning transmission electron microscopy, and dynamic light scattering measurements. The system displays turn-on absorbance behavior with RDX and TNT, yielding detection limits of 50.3 and 67.2 nmol L^-1, respectively. The assay could distinguish between RDX and TNT using a simple extraction-based recovery procedure. Good recoveries for RDX and TNT were obtained from real (Composite B, Composite A5, and Octol) and synthetically prepared formulations. Method validation was performed by statistically comparing the analytical results obtained by the reference liquid chromatography-tandem mass spectrometric method applied to RDX standards and RDX-contaminated soil samples. As a novelty of this system, direct spectrophotometric detection of RDX through its decomposition product, formaldehyde, was successfully performed for the first time. Compared with the widely used indirect spectrophotometric methods based on nitrite formation from RDX degradation, the system is superior in that it does not require pre-hydrolysis of RDX and does not respond to 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX). It is also noteworthy that the system responds to TNT in a way that constitutes an innovative alternative to the existing mechanism of Meisenheimer/Janowsky complex formation in the literature.