SERS and DFT studies of highly sensitive detection of trace nitro-explosives assisted by alkali ions†

In this study, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), 2,4,6-trinitrochlorobenzene (TNCB), and 2,4,6-trinitrotoluene (TNT) were sensitively detected with the aid of alkali ions (Li⁺, Na⁺ and K⁺ from the corresponding nitrate salts) using surface-enhanced Raman scattering (SERS), through an extremely simple process of adding explosive compounds and alkali ions to negatively charged silver colloids. The results show that the quantitative SERS detection limit of CL-20 is 1 nM with the addition of Na⁺, whereas for TNCB and TNT, the corresponding detection limit can reach 0.01 nM adjusted using Na⁺ and K⁺, respectively. The possible mechanisms of the sensitive SERS detection of trace nitro-explosives (CL-20, TNCB, and TNT) with the aid of alkali ions (Li⁺, Na⁺ and K⁺) have been investigated by Density Functional Theory (DFT) calculations. Natural Bond Orbital (NBO) analysis of the optimized structures of CL-20, TNCB and TNT revealed that the O atoms on the nitro group of these explosives carry a relatively greater negative charge, which can be successfully coordinated with the alkali ions accompanied by an obvious energy release to form positively charged complexes. Subsequently, these complexes spontaneously approach the negatively charged silver colloid, creating more SERS active sites, thereby enhancing the Raman detection sensitivity.