Detection of Multiple Nitroaromatic Explosives via Formation of a Janowsky Complex and SERS

Military-grade explosives such as 2,4,6-trinitroluene (TNT) are still a major worldwide concern in terms of terror threat and environmental impact. The most common methods currently employed for the detection of explosives involve colorimetric tests, which are known to be rapid and portable; however, they often display false positives and lack sensitivity. Other methods used include ion mobility mass spectrometry, gas chromatography-mass spectrometry (GC-MS), and liquid chromatography-mass spectrometry (LC-MS), which despite producing more reliable results often require large, expensive instrumentation and specially trained staff. Here we demonstrate an alternative approach that utilizes the formation of a colored Janowsky complex with nitroaromatic explosives through reaction of the enolate ion of 3-mercapto-2-butanone. The colored complex is formed rapidly and can then be detected sensitively using surface-enhanced Raman scattering (SERS). We demonstrate that SERS can be used as a quick, sensitive, and selective technique for the detection of 2,4,6-trinitrotoluene (TNT), hexanitrostillbene (HNS), and 2,4,6-trinitrophenylmethylnitramine (tetryl) with a detection limit of 6.81 ng mL–1 achieved for TNT, 17.2 ng mL–1 for tetryl, and 135.1 ng mL–1 for HNS. This method of detection also requires minimal sample preparation, can be done in a solution-based format, and utilizes the same precursor reagents for complex formation with each of the explosives which can then be identified due to the specificity of the unique SERS response obtained. We demonstrate the ability to simultaneously identify three explosive compounds within a total analysis time of 10 min. This method of detection shows promise for the development of rapid and portable SERS-based assays which can be utilized in the field in order to achieve reliable and quantitative detection.


S-2
It can be seen from the SERS spectra of the control samples in Figure S1 that 3-mercapto-2butanone added directly to silver nanoparticles produces a very strong SERS response, subsequent to addition of DBU, the response obtained from 3M2B drastically decreases and very few peaks can be observed. This result further supports the assumption that addition of DBU successfully produces a 3M2B anion as this would have a much lower affinity for the nanoparticle surface and hence the SERS response obtained would be significantly lower.
It is also evident from these results that the control sample containing RDX as opposed to TNT does not form a complex with the 3M2B as no SERS response was obtained. This would also be expected as Janowsky type complexes require an electron deficient arene ring in order to form and this is not present in RDX. The pseudo-Janowsky type complex thought to be formed from the addition of TNT alone also produced no SERS response when added to silver nanoparticles, this is expected as this type of complex would lack the thiol functionality to facilitate attachment to the nanoparticle surface and hence the same response is achieved as that of TNT alone added to silver nanoparticles. Figure S2 shows the absorption spectra associated with (a) the complex formed between HNS and 3M2B and (b) tetryl and 3M2B.

UV-Vis absorption spectroscopy of tetryl and HNS complexes
Similar to TNT, each complex also appeared to form a pseudo-Janowsky complex when only DBU was added to the explosive. For tetryl this complex displayed a doublet peak with a λmax of 435 nm and 510 nm. For HNS, this complex displayed a broad peak with a λmax of 680 nm.
However, this complex was short lived, suggesting instability and after 5 minutes the absorbance spectrum had disappeared entirely. When 3M2B was introduced into each reaction, the effect observed was very similar to that of the TNT complex, wherein tetryl produced a compound with an absorbance maximum of 445 nm and HNS also displayed a hypsochromic shift to a complex with absorbance maxima of 470 nm and 560 nm.

S-4
Upon addition of 3M2B, the tetryl complex exhibited a shift in absorbance maximum of only 10 nm, therefore it was not possible to measure the rate of formation of the 3M2B tetryl complex over time using absorbance spectrometry. However, very little difference was observed in the absorbance spectrum immediately after the addition of 3M2B to tetryl/DBU compared with the spectrum obtained 5 minutes after addition of 3M2B. This suggests the 3M2B tetryl complex likely formed immediately after addition of 3M2B and that this complex produces a very similar absorbance spectrum to the pseudo Janowsky complex formed when only tetryl and DBU were present in the reaction. This was not true of the HNS complex, wherein an increase in absorbance at 470 nm was observed over a period of 15 minutes, suggesting a slower rate of formation than that observed for the tetryl and TNT complexes, the increase in absorbance at 470 nm is shown over a period of 15 minutes in figure S3.

Principal Component Analysis
Principal component analysis was carried out using the spectral data shown in Figure S4. All spectra were collected using a Renishaw plate reader (532 nm, 100 mW) and an acquisition time of 1s. Spectra have been baseline corrected, scaled and offset for clarity.