Samuel P. Hernández-Rivera

Surface Enhanced Raman Scattering (SERS) Studies of Gold and Silver Nanoparticles Prepared by Laser Ablation

Gold and silver nanoparticles (NPs) were prepared in water, acetonitrile and isopropanol by laser ablation methodologies. The average characteristic (longer) size of the NPs obtained ranged from 3 to 70 nm. 4-Aminobenzebethiol (4-ABT) was chosen as the surface enhanced Raman scattering (SERS) probe molecule to determine the optimum irradiation time and the pH of aqueous synthesis of the laser ablation-based synthesis of metallic NPs. The synthesized NPs were used to evaluate their capacity as substrates for developing more analytical applications based on SERS measurements. A highly energetic material, TNT, was used as the target compound in the SERS experiments. The Raman spectra were measured with a Raman microspectrometer. The results demonstrate that gold and silver NP substrates fabricated by the methods developed show promising results for SERS-based studies and could lead to the development of micro sensors.

Highly Sensitive Filter Paper Substrate for SERS Trace Explosives Detection

We report on a novel and extremely low-cost surface-enhanced Raman spectroscopy (SERS) substrate fabricated depositing gold nanoparticles on common lab filter paper using thermal inkjet technology. The paper-based substrate combines all advantages of other plasmonic structures fabricated by more elaborate techniques with the dynamic flexibility given by the inherent nature of the paper for an efficient sample collection, robustness, and stability. We describe the fabrication, characterization, and SERS activity of our substrate using 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, and 1,3,5-trinitrobenzene as analytes. The paper-based SERS substrates presented a high sensitivity and excellent reproducibility for analytes employed, demonstrating a direct application in forensic science and homeland security.

Vibrational spectra and structure of RDX and its 13C- and 15N-labeled derivatives: A theoretical and experimental study

Unambiguous vibrational band assignments have been made to cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine, commonly known as the alpha-phase of RDX or alpha-RDX, with the use of (13)C and (15)N (on ring) enriched isotopic RDX analogues. Vibrational spectra were collected using Raman and IR spectroscopy in solid state and ab initio normal mode calculations were performed using density functional theory (DFT) and a 6-311G++** basis set. The calculated isotopic frequency shifts, induced by (13)C and (15)N labeling, are in very good accordance with measures ones. The changes in vibrational modes associated with the isotopic substitutions are well modeled by the calculation and previous assignments of the vibrational spectra have been revised, especially where the exact nature of the vibrational modes had been either vague or contradictory.

Remote Detection of Hazardous Liquids Concealed in Glass and Plastic Containers

Raman-based standoff detection systems have the potential to characterize materials through various transparent glass and plastic containers. This allows the analysis of the content without opening the container. In the Defense and Security arena, this implies to minimize exposure of operators to potentially harmful substances and helping to speed a screening process. The objective of this work is to study, in real field conditions, the remote detection of hazardous liquids concealed in commercial products bottles. A telescope assisted Raman system with a 488.0 nm excitation wavelength was used for noninvasive, nondestructive interrogation of the content of common liquid containers and determining if the liquid is the intended one or a concealed hazardous liquid. Commercial products such as: sodas, juices and liquors were mixed with hazardous liquids such as liquid explosives or chemical warfare agent simulants (CWAS) and characterized by remote Raman spectroscopy. Spectroscopic signatures of hazardous liquids were used to discriminate between consumer products liquids at 22 feet target-collector distance. The effect of the container type and color was also studied providing good understanding of the technique strengths and challenges. This technique has potential to be used as a detector for hazardous liquids at a check point or to inspect the contents of suspicious bottles remotely.

Characterization and differentiation of high energy amine peroxides by direct analysis in real time TOF/MS

Characterization of hexamethelene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD) and tetramethylene diperoxide acetamide (TMDA) has been carried out using Direct Analysis in Real Time/Time of Flight Mass Spectrometry (DART-TOF/MS). The study also centered in the detection of their precursors such as hexamine and formaldehyde. Analysis of the compounds by GC-MS was also conducted. HMTD shows a clear peak at 209 m/z that allowed its detection in standard solutions and lab made standards. TATP samples with deuterium enrichment are being analyzed to compare results that could differentiate from HMTD and similar substances. All samples were characterized by Raman and FT-IR to confirm the DART results. Some of the vibrations observed were in the ν(O-O), ν(N-C), ν(N-H), ν(C-O), δ(CH3-C) and δ(C-O). Development methodology for trace detection was compared with GC/MS and HPLC-MS results previously presented for HMTD and TATP.

Detection of Nitroaromatic and Peroxide Explosives in Air Using Infrared Spectroscopy: QCL and FTIR

A methodology for processing spectroscopic information using a chemometrics-based analysis was designed and implemented in the detection of highly energetic materials (HEMs) in the gas phase at trace levels. The presence of the nitroaromatic HEM 2,4-dinitrotoluene (2,4-DNT) and the cyclic organic peroxide triacetone triperoxide (TATP) in air was detected by chemometrics-enhanced vibrational spectroscopy. Several infrared experimental setups were tested using traditional heated sources (globar), modulated and nonmodulated FT-IR, and quantum cascade laser- (QCL-) based dispersive IR spectroscopy. The data obtained from the gas phase absorption experiments in the midinfrared (MIR) region were used for building the chemometrics models. Partial least-squares discriminant analysis (PLS-DA) was used to generate pattern recognition schemes for trace amounts of explosives in air. The QCL-based methodology exhibited a better capacity of discrimination for the detected presence of HEM in air compared to other methodologies.

Cyclic Organic Peroxides Characterization by Mass Spectrometry and Raman Microscopy

Triacetone triperoxide and hexamethylene triperoxide diamine are among the most used and most studied non-nitro based high explosives. Still, there are other peroxides which do not have reported methods for their detection. Direct Analysis in Real Time is an emerging ambient pressure ionization technique in mass spectrometry (MS). This contribution presents the analysis, characterization and detection of triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) using DART. Although typical experiments are done using helium, adding a small amount of ammonium hydroxide improved the detection limit to 30 ppb. This study also presents the detection and identification of benzoyl peroxide, tricyclopentanone triperoxide and tetramethylene diperoxide dicarbamide (TMDD) by MS and Raman microscopy. TATP showed a single peak at m/z 240.144 [M+NH4]+ with the peaks at m/z 223.118 [M+H]+ or 222.110 [M]+ completely absent. Deuterium enriched (TATP-d18) was analyzed for comparison. These presented a similar peak at m/z 258.267 [TATP-d18 +NH4]+. HMTD showed a peak at m/z 209.078 [M+H]+ and small adduct peak at m/z 226.103 [M+NH4]+ that allowed its detection in standard solution and lab made samples. TMDD showed several peaks with a base peak at m/z 101.033, molecular peak at m/z 237.081 [M+H]+ and a strong ammonium adduct at m/z 254.108 [M+NH4]+. All samples were analyzed by Raman Microscopy in order to characterize them and to confirm the MS results.

Theoretical and experimental vibrational and NMR studies of RDX

The cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine, commonly known as RDX, is a powerful secondary explosive that has attracted considerable attention due to its polymorphism. The conformers of RDX are distinguished mainly by the arrangement of the nitro groups relative to the ring atoms of the RDX molecule. FTIR and Raman spectra of solid RDX and 13C enriched RDX are presented and assigned with the help of the density functional theory calculations. Chemical shielding constants were computed by gauge independent atomic orbital method (GIAO) for β-RDX conformer. The data are compared with the experimental solid and solution data, focusing on the agreement the spectral patterns and spectral trends. The results of harmonic vibrational frequencies and absolute shielding (GIAO) calculations using density functional theory approximation supported by experimental studies are reported. The qualitative agreement between the experimental and calculated values of the chemical shifts shed more light on the understanding the basic shift-molecular structure relationship of RDX conformers.

FT-IR Standoff Detection of Thermally Excited Emissions of Trinitrotoluene (TNT) Deposited on Aluminum Substrates

A standoff detection system was assembled by coupling a reflecting telescope to a Fourier transform infrared spectrometer equipped with a cryo-cooled mercury cadmium telluride detector and used for detection of solid-phase samples deposited on substrates. Samples of highly energetic materials were deposited on aluminum substrates and detected at several collector-target distances by performing passive-mode, remote, infrared detection measurements on the heated analytes. Aluminum plates were used as support material, and 2,4,6-Trinitrotoluene (TNT) was used as the target. For standoff detection experiments, the samples were placed at different distances (4 to 55 m). Several target surface temperatures were investigated. Partial least squares regression analysis was applied to the analysis of the intensities of the spectra obtained. Overall, standoff detection in passive mode was useful for quantifying TNT deposited on the aluminum plates with high confidence up to target–collector distances of 55 m.

Improving SERS Detection of Bacillus thuringiensis Using Silver Nanoparticles Reduced with Hydroxylamine and with Citrate Capped Borohydride

The development of techniques that could be useful in fields other than biological warfare agents countermeasures such as medical diagnostics, industrial microbiology, and environmental applications have become a very important subject of research. Raman spectroscopy can be used in near field or at long distances from the sample to obtain fingerprinting information of chemical composition of microorganisms. In this research, biochemical components of the cell wall and endospores of Bacillus thuringiensis (Bt) were identified by surface-enhanced Raman scattering (SERS) spectroscopy using silver (Ag) nanoparticles (NPs) reduced by hydroxylamine and borohydride capped with sodium citrate. Activation of “hot spots”, aggregation and surface charge modification of the NPs, was studied and optimized to obtain signal enhancements from Bt by SERS. Slight aggregation of the NPs as well as surface charge modification to a more acidic ambient was induced using small-size borohydride-reduced NPs in the form of metallic suspensions aimed at increasing the Ag NP-Bt interactions. Hydroxylamine-reduced NPs required slight aggregation and no pH modifications in order to obtain high spectral quality results in bringing out SERS signatures of Bt.