Oliva M. Primera-Pedrozo

Nanotechnology-Based Detection of Explosives and Biological Agents Simulants

Nanotechnology based detection of threat agents, such as explosives and biological agents, has been a top research priority at the Center for Chemical Sensors Development at the Department of Chemistry of the University of Puerto Rico-Mayaguez (UPRM). Nanoparticles are of fundamental interest since they possess unique size-dependent properties are quite different from the bulk state. When a bulk metal is reduced in size, its properties begin to change dramatically because the constituent electrons begin to suffer the effects of quantum confinement. One of these important properties deals with the extraordinary enhancement of the intensities of Raman scattering events in chemical systems called surface enhanced Raman scattering (SERS). Until very recently, only aromatic moieties containing strong chromophores or highly delocalized pi electrons would experience such an enhancement, when in close proximity to a silver or gold nanometallic assembly. In other cases, this SERS condition was not sufficient to satisfy the enhanced Raman scattering requirements because of Coulombic repulsions do not allow an intimate contact with the colloidal suspension of nanoparticles. Recent work in the research group includes optimization of particle size, agglomeration rate and ionic strength of the SERS active aqueous colloidal metallic suspensions. Results have led to extend existing benchmarks limits of detection of 10-7 M to 10-8 M (10-15 g) in DNT and to 10-12 M (10-19 g) in the case of TNT. Other works include preparation and testing of bimetallic nano-interalloys: Au/Ag and metallic-semiconductor SERS active colloidal substrates: Ag/TiO2. Group members have prepared silver and gold nanorods and nanolayers in an effort to change the sensing platform: from aqueous media to solventless detection.

Enhanced Raman Scattering of 2,4,6-TNT Using Metallic Colloids

Surface-enhanced Raman scattering (SERS) combines extremely high sensitivity, due to enhanced Raman cross sections comparable or even better than fluorescence emission. The observation of vibrational spectra of adsorbed species on nanoparticles, provides one of the most incisive analytical methods for chemical and biochemical detection and analysis. Nanoparticles are of fundamental interest since they possess unique size-dependent properties (optical, electrical, mechanical, chemical, magnetic, etc.), which are quite different from the bulk and the atomic state. Bimetallic nanoparticles are of particular interest since they combine the advantages of the individual monometallic counterparts. Metal colloids have become the most commonly used nanostructures for SERS. The present study focuses on the use of metallic nanoparticles, with a particle size of 35-80 nm for detecting TNT in solution. Gold, silver, and Au/Ag colloids were synthesized by chemical reduction methods, and used for detecting TNT in solution with high sensitivity and molecular specificity. The nanoparticles were characterized with UV-VIS spectroscopy, Scanning and Transmission Electron Microscopies and Raman Spectroscopy. The detection of TNT was conducted via an indirect method that involved the alkaline hydrolysis of TNT in the presence of a strong base. This method offered the advantage of generating reaction products that provided enhanced detection in the SERS experiments. The spectra were obtained in the 100-3500 cm-1 range. The results revealed an increase in the intensity of the vibrational signals, attributed to the SERS spectra of TNT degradation products. Bands associated with out-of-plane bending modes centered at 820 and 850 cm-1 and symmetric and asymmetric stretching modes were detected.

Use of fiber optic coupled FT-IR in detection of explosives on surfaces

Explosives detection is a very important task for National Security. The formidable task includes development of new probes and methods for detection of concealed explosives which is of utmost priority to Homeland Security and other security enforcing federal agencies. Here we report on the detection of triacetone triperoxide (TATP) on metallic surfaces using a Fiber Optic Coupled FTIR method. FT-IR spectroscopy is well suited to be used outside the confinement of the sample compartment, provided the excitation source and the reflected light can be transported to the interferometer. Fiber optic cables that transmit in the Mid-IR range have made this possible by allowing the development of a range of spectroscopic probes for in situ analysis. In our study we used a specially designed sampling probe that operates at the grazing-angle to detect and to quantify μg/cm2 levels of explosives on stainless steel. Calibration curves were prepared using stainless steel plates, 3 inches wide x 6 inches long. The samples were deposited on the surface using a smearing method. To carry out the experiments, TATP was synthesized in the laboratory. For the calibration curves TATP was dissolved in dichloromethane. The standard solutions (20) μL were transferred on the plates resulting in surface mass concentrations of TATP that ranged from 8 to 150 μg/cm2. The data was analyzed using Chemometrics routines and Discriminant Analysis algorithms. In particular, multivariate Partial Least Squares (PLS) was used to determine the most significant peak for the analysis. In other experiments done with stainless steel plates coated with 150 μg/cm2 TATP, spectra were recorded every 27 seconds. For this concentration TATP sublimates to surface concentrations below detection in 800 s.

Characterization of thermal inkjet technology TNT deposits by fiber optic-grazing angle probe FTIR spectroscopy

Fiber Optic Coupled/Grazing Angle Probe Fourier Transform Infrared Spectroscopy has made possible to develop new methods for detection of traces of chemical compounds on surfaces. Thermal Inkjet Technology is able to deposit very small amounts of chemical compounds, including energetic materials, in a specific location on a surface. Aliquots of TNT solutions were deposited on stainless steel film. A thin coating of TNT can be produced by controlling the concentration of TNT, the number of drops dispensed and the distribution of drops over the surface. A Vector 22, a Bruker Optics FTIR fiber coupled to a Remspec Corp. grazing angle head was used for the experiments. The spectra were recorded at 4 cm-1 resolution and 50 scans. The results of the experiments gave intense absorption bands in the fingerprint region of the infrared spectra that were used for quantification. Chemometrics routines were applied in the enhancement of the quantitative analysis.

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.

Temperature dependence of detection limits of TNT on metallic surfaces using fiber optic coupled FTIR

Fourier transform infrared (FTIR) spectroscopy has been established as well suited for work outside the confinement of the sample compartment, provided the excitation source and the reflected light can be transported to the interferometer. Fiber optic cables that transmit in the Mid-IR range have made this possible by allowing the development of a series of spectroscopic probes for in situ analysis. In previous work it was established that surface concentrations of TNT as low as 0.3 μg/cm2 (300 ng/cm2) could be detected with high confidence level. This detection limit varies according to macro properties. Properties such vapor pressure, physical adsorption, sublimation rate and surface-adsorbate thermodynamics can influence the detection limit. A close relation between vapor pressure and limit detection is shown for nitroexplosives. The amount of explosive and the residence time on stainless steel depends on this property, because at low surface concentrations the explosive goes to the vapor phase fast. Loading concentrations near the limits of detection were prepared and spectra were recorded at different temperatures in the range of 14-30°C. The nitro band was monitored for the experiments and the data was analyzed by using peak areas and Chemometrics. A close relationship between the detection limit and temperature was observed.

Raman microspectroscopy crystallization studies of 2,4,6-TNT in different solvents

2,4,6-Trinitrotoluene is a high explosive that has been used for military purposes since 1902. Ammunition manufacturing facilities where TNT is made as well as sites across the world used to test military explosives in diverse ways, such as landmines and unexploded ordnance that have been buried in soil; grenades, etc are concerned with the health hazard and environmental problem of TNT. Since TNT is a contaminant that remains in the soil and produces various carcinogenic compounds as a result of photodecomposition and biodegradation, large amounts of the nitroaromatic compounds represent both a threat and a problem. Vibrational spectroscopy is a powerful tool that can be used to characterize TNT in its diverse condensed forms: droplets and crystals of polymorphs. Crystallization of TNT from different solvents: water, methanol, chloroform, acetone, and acetonitrile, was carried out and the vibrational spectra were obtained during crystallization. Crystals produced from evaporation of the mentioned solvent showed a similar crystallization pattern, and their spectroscopic information obtained was found to depend on the physical form of TNT. The nitroaromatic compound exhibits a series of unique characteristic bands that allow its detection and spectroscopic characterization. The spectroscopic signatures of neat TNT samples were determined with Raman Microspectroscopy and used as comparison standards. Strong bands about 1365 and 2956 cm-1 dominate the Raman spectrum of neat TNT. The intensity and even the presence of these bands are found to be remarkably dependent on TNT form and source.

Detection of 2,4,6-trinitrotoluene on non-traditional surfaces using fiber optic coupled grazing angle probe: FTIR

With heightened awareness of Homeland Security issues, the detection of explosive has become a pressing priority. Explosives detection is a very important task for National Security: threat compounds need to be detected on a variety of surfaces. Every surface will interact with the target compounds in a very unique manner and the degree of adhesion will vary from surface to surface. The formidable task includes development of new probes and methods for detection of concealed explosives. Fiber Optic Coupled Infrared Spectroscopy has been used as a potential technique to develop new methodologies for detection of explosives on surfaces. On one of such proposed methodologies involves a Grazing Angle Probe rendering the latter as a remote sensed, in situ and capability of detecting nanograms/cm2 of the compounds. In this research a smearing technique was used for transferring the target analytes onto the substrates to be used as standards. Smearing was also used as a sample transfer method of the threat agents to target surfaces. One of the most relevant areas of investigation is to analyze 2,4,6-trinitrotoluene (TNT) on various non traditional surfaces such as plastics. The work also centered in to obtaining an optimization method where a more accurate spectrum could be obtained and a better spectroscopic preprocessing routine could be applied. A series of statistical methods can be used for quantification of TNT on plastic surfaces, among these are: peak height analysis and peak areas integration. Both of these can be coupled to Partial least squares regression, which is an extension of multiple linear regression models. Using peak areas in the range from 1380 to 1273 cm-1, the method was found to be linear for loading concentration lower than 5.0 μg/cm2. A loading concentration of 0.62 μg/cm2 (620 ng/cm2) was considered as limit of quantification and 0.16 μg/cm2 (160 ng/cm2) as limit of detection.

Standoff infrared detection of explosives at laboratory scale

An actively operated standoff infrared detection system was designed from commercial infrared equipment: VECTOR 22 FTIR (Bruker Optics), an external mirror and an external MCT detector. One type of experiment was done for IR detection of high explosives RDX and TNT on reflective surfaces. In the detection on surface, the samples were move to different distances and a beam of infrared light was reflect on surface in angle of ~ 0° (backward collection from surface normal). First the samples: 2 to 30 μg/cm2 of high explosives TNT and RDX were characterized after depositing on stainless steel reflective surfaces using Reflection-Absorption Infrared Spectroscopy (RAIS). Then targets were moved to increasing distances: 3 to 12 feet and remote-sensed spectra were collected in active reflectance mode. The limits of detection were determined for all distances measured in both nitroexplosives. Limit of detection of 18 and 20 μg/cm2 for TNT and RDX respectively in the longest distances measured.

SERS and Density Functional Theory Study of o -Dinitrobenzene on Cu Nanoparticles

Cu colloidal suspensions were prepared by wet chemistry methods: chemical reduction using trisodium citrate. Steadiness varied under conditions such as stirring rate, rate of addition of reducing agent, and temperature control. Sizes of nanoparticles prepared ranged from 2 to 20 nm. Morphology and sizes were characterized using high-resolution transmission electron microscopy. Enhanced Raman spectra of o -dinitrobenzene on prepared nanoparticles suspensions of Cu colloids were obtained with visible excitation at 488 and 514.5 nm. It was found that enhanced Raman signal intensities resulted from the interaction of the analytes with the metallic surface resonance plasmon of Cu nanoparticles excited at the laser frequency. The adsorption behavior of the analytes on the Cu nanoparticles was modeled with Gaussian 03 density functional theory software package. The simplified models used work reasonably well in describing some enhancement results for the Raman experiments on the noble metal colloidal suspensions.