The threat of chemical warfare agents being released upon civilian and military personnel continues to escalate. One aspect of chemical preparedness is to analyze and protect the portable water supply for the military. Chemical nerve, blister, and choking agents, as well as biological threats must all be analyzed and low limits of detection must be verified. For chemical agents, this generally means detection down to the low ppb levels. Surface-Enhanced Raman Spectroscopy (SERS) is a spectroscopic technique that can detect trace levels of contaminants directly in the aqueous environment. In this paper, results are presented on the use of SERS to detect chemical and biological agent simulants with an end goal of creating a Joint Service Agent Water Monitor. Detection of cyanide, 2-chloroethyl ethyl sulfide, phosphonates, Gram-positive and Gram-negative bacteria using SERS has been performed and is discussed herein. Aspects of transferring laboratory results to an unattended field instrument are also discussed.
Through its several orders of magnitude signal enhancement over normal Raman, surface-enhanced Raman spectroscopy (SERS) provides an opportunity to extend the benefits of vibrational spectroscopy to trace level detection. SERS in particular holds great potential for biological sensing due to the weak Raman bands of water and the reduction in fluorescence backgrounds from interactions of the analyte with the metal SERS substrate. This work examines the trace level detection of biological molecules and oligomers such as amino acids, peptides, and oligonucleotides as well as the detection of whole cell bacteria. The SERS substrates employed are electrochemically roughened gold. The biological molecules show well-resolved and intense bands that are an effective spectral signature; these bands also persist in corresponding oligomeric compounds. Spectra from whole cell bacteria have been obtained for several species, including gram-positive and gram-negative strains. Viable and nonviable cells have also been examined and significant spectral differences are observed. The results show the potential for using SERS as an analytical tool for the identification of biological molecules and microorganisms with applications in biological agent detection, food and water monitoring, and the search for signs of extraterrestrial life.
The sensitivity demonstrated for other nitro-aromatic explosives through SERS has been applied to triaminotrinitro-benzene (TATB). Gas phase and solution phase in strong acids and bases, as well as organic solvents SERS spectra have been collected. For each method of TATB sample introduction on electrochemically roughened gold substrates or gold colloids, different bands and sensitivities were observed. These bands likely result from the three possible adsorption sites in the molecule and its reaction with the gold surface. In some cases, the SERS spectra closely overlapped the carbonaceous background and indicate TATB degradation. Although the mechanisms of the reaction of TATB with the surface are not understood, important aspects of optimized TATB SERS detection have been observed. Para-nitroaniline (p-NA) was also studied due to its similarity with TATB and its greater solubility in water.
We report surface-enhanced Raman scattering (SERS) for vapors of 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene, 4-amino-2, 6-dinitrotoluene and trinitrotoluene (TNT) adsorbed onto gold metal foils. Detection of 2,4-DNT down to approximately 1 ppb has been demonstrated. A compact field portable Raman unit with fiber optic SERS attachment has been fabricated and field tested for landmine detection. Preliminary results showed little environmental interference to the SERS measurement and detection of a buried landmine. The results demonstrate that SERS can detect buried landmines and, with further improvements, has the potential to be a man-portable field unit for landmine detection.
The characterization of the type and quanity of explosives residue left behind as fingerprints is critical for the problem of trace explosives detection as well as forensic investigation. A nondestructive analytical technique has to be used to identify the energetic component of the explosive from the plasticizers, dyes, and fingerprint oils that make up the background. Raman microspectroscopy has been demonstrated in the past to separate explosive particulate from other residue in a microscopic image by filtering out other spectra except the region of the strong bands displayed by PETN and RDX using He-Ne excitation. In addition, gray level/measurements have been done on features of the sample, captured under white light onto a CCD, to obtain quantitative data about size and volume distribution. The objective of this paper will be to show how integrated line images of the sample, captured with high spectral resolution using a scanning Raman spectrometer, can be used to separate out components in the image scene captured by the CCD. This paper will also show how confocal scanning through the depth of the sample, while taking an image, can be used to come up with a quantitative measure of the concentration of chosen components in the entire image. The special instrumentation used for the work will be shown as well as any modifications done to it to obtain a protocol for analysis. The image analysis results will be presented of actual fingerprint samples containing plastic explosives. The variance between the Raman imaging method and other more traditional destructive methods for doing quantitative analysis will be presented. And the probability of doing direct Raman microspectroscopy in the UV region without any background subtraction will be determined for its potential for doing in-situ analysis for explosives detection.
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