Coordination of Quantum Dots in a Polar Solvent by Small-Molecule Imidazole Ligands.

Colloidal quantum dots (QDs) are attractive fluorophores for bioimaging and biomedical applications because of their favorable and tunable optoelectronic properties. In this study, the native hydrophobic ligand environment of oleate-capped sphalerite CdSe/ZnS core/shell QDs was quantitatively exchanged with a set of imidazole-bearing small-molecule ligands. Inductively coupled plasma-optical emission spectroscopy and 1H NMR were used to identify and quantify three different ligand exchange processes: Z-type dissociation of the Zn(oleate)2, L-type association of the imidazole, and X-type anionic exchange of oleate with Cl-, all of which contributed to the overall ligand exchange.

Effect of substituents on surface equilibria of thiophenols and isoquinolines on gold substrates studied using surface-enhanced Raman spectroscopy.

The effect of substituents on the surface adsorption equilibria of thiophenols and isoquinolines on gold substrates was studied using surface-enhanced Raman spectroscopy (SERS) in order to determine the effects of the localized dipole moments and charge donating/withdrawing properties on the binding affinity. Two common classes of molecules used in SERS studies were examined, which included substituted aromatic thiols and nitrogen heterocyclic aromatic molecules (azaarenes), due to their strong affinity for gold surfaces. Unsubstituted thiophenol in aqueous solution binds strongly to gold surfaces. Therefore, it is difficult to measure an equilibrium constant, since even at concentrations of 10-8 M nearly a complete self-assembled monolayer (SAM) forms. In contrast, substituted thiophenols with electron-withdrawing groups, such as halogenated thiophenols, bind much less strongly, allowing equilibrium constants to be obtained. It is believed that the substituent withdrawing charge away from the sulfur atom affects the adsorption/binding between the analyte and surface. Thiophenols substituted with electron donating groups behaved similar to unsubstituted thiophenol, where a SAM was observed at concentrations as low as 10-8 M. These functional groups did not hinder the ability of the sulfur groups to bind with gold. In addition, a series of bromine-substituted isoquinolines, a group of azaarene compounds, were measured to determine the effects that the bromine substituent has when it is bound to the two different rings and if position on the rings has an effect. The azaarene class of molecules, including isoquinoline, adsorbs less strongly than thiophenols, and a dual Langmuir isotherm phenomenon is observed where protonated and neutral bromoisoquinoline molecules occupy two different types of sites on Klarite substrates, which consist of inverted micro-pyramids on Si wafers with rough/nanostructured Au coatings. Protonated isoquinolines bind to nucleophilic sites on the substrates which tend to occur on flatter regions of the substrate. By contrast, neutral isoquinolines bind to electrophilic sites which are predominant near microscopic edges on the substrate. The presence of the bromine substituent and its position in the fused ring structure changes the Gibbs free energies of adsorption, depending on which ring the substituent is in. These results can help to guide the development of SERS for analytical applications by demonstrating how changes in functional groups can affect the equilibrium constants, which are critical for determining the effectiveness of SERS as a tool for trace detection of analytes.

Quantitative surface-enhanced Raman spectroscopy chemical analysis using citrate as an in situ calibrant.

Direct detection, or inferring the presence of illicit substances, is of great forensic and toxicological value. Surface-enhanced Raman spectroscopy (SERS) has been shown capable of detecting such molecules in a quick and sensitive manner. Herein we describe an analysis strategy for quantitation of low concentrations of three analytes (methamphetamine, cocaine, and papaverine) by SERS analysis using the citrate capping agent that initially saturates the silver nanoparticles' surface as an in situ standard. The citrate is subsequently displaced by the analyte to an extent dependent on the analyte's concentration in the analyte solution. A general model for the competitive adsorption of citrate and a target analyte was developed and used to determine the relative concentrations of the two species coexisting on the surface of the silver nanoparticles. To apply this model, classical least squares (CLS) was used to extract the relative SERS contribution of each of the two species in a given SERS spectrum, thereby accurately determining the analyte concentration in the sample solution. This approach, in essence, transforms citrate into a local standard against which the concentration of an analyte can be reliably determined.

Dielectrophoretic Nanoparticle Aggregation for On-Demand Surface Enhanced Raman Spectroscopy Analysis.

Rapid chemical identification of drugs of abuse in biological fluids such as saliva is of growing interest in healthcare and law enforcement. Accordingly, a label-free detection platform that accepts biological fluid samples is of great practical value. We report a microfluidics-based dielectrophoresis-induced surface enhanced Raman spectroscopy (SERS) device, which is capable of detecting physiologically relevant concentrations of methamphetamine in saliva in under 2 min. In this device, iodide-modified silver nanoparticles are trapped and released on-demand using electrodes integrated in a microfluidic channel. Principal component analysis (PCA) is used to reliably distinguish methamphetamine-positive samples from the negative control samples. Passivation of the electrodes and flow channels minimizes microchannel fouling by nanoparticles, which allows the device to be cleared and reused multiple times.

A Standard Method To Inactivate Bacillus anthracis Spores to Sterility via Gamma Irradiation.

In 2015, a laboratory of the United States Department of Defense (DoD) inadvertently shipped preparations of gamma-irradiated spores of Bacillus anthracis that contained live spores. In response, a systematic evidence-based method for preparing, concentrating, irradiating, and verifying the inactivation of spore materials was developed. We demonstrate the consistency of spore preparations across multiple biological replicates and show that two different DoD institutions independently obtained comparable dose-inactivation curves for a monodisperse suspension of B. anthracis spores containing 3 × 1010 CFU. Spore preparations from three different institutions and three strain backgrounds yielded similar decimal reduction (D10) values and irradiation doses required to ensure sterility (DSAL) to the point at which the probability of detecting a viable spore is 10-6 Furthermore, spores of a genetically tagged strain of B. anthracis strain Sterne were used to show that high densities of dead spores suppress the recovery of viable spores. Together, we present an integrated method for preparing, irradiating, and verifying the inactivation of spores of B. anthracis for use as standard reagents for testing and evaluating detection and diagnostic devices and techniques.IMPORTANCE The inadvertent shipment by a U.S. Department of Defense (DoD) laboratory of live Bacillus anthracis (anthrax) spores to U.S. and international destinations revealed the need to standardize inactivation methods for materials derived from biological select agents and toxins (BSAT) and for the development of evidence-based methods to prevent the recurrence of such an event. Following a retrospective analysis of the procedures previously employed to generate inactivated B. anthracis spores, a study was commissioned by the DoD to provide data required to support the production of inactivated spores for the biodefense community. The results of this work are presented in this publication, which details the method by which spores can be prepared, irradiated, and tested, such that the chance of finding residual living spores in any given preparation is 1/1,000,000. These irradiated spores are used to test equipment and methods for the detection of agents of biological warfare and bioterrorism.

Reassessing SERS enhancement factors: using thermodynamics to drive substrate design.

Over the past 40 years fundamental and application research into Surface-Enhanced Raman Scattering (SERS) has been explored by academia, industry, and government laboratories. To date however, SERS has achieved little commercial success as an analytical technique. Researchers are tackling a variety of paths to help break through the commercial barrier by addressing the reproducibility in both the SERS substrates and SERS signals as well as continuing to explore the underlying mechanisms. To this end, investigators use a variety of methodologies, typically studying strongly binding analytes such as aromatic thiols and azarenes, and report SERS enhancement factor calculations. However a drawback of the traditional SERS enhancement factor calculation is that it does not yield enough information to understand substrate reproducibility, application potential with another analyte, or the driving factors behind the molecule-metal interaction. Our work at the US Army Edgewood Chemical Biological Center has focused on these questions and we have shown that thermodynamic principles play a key role in the SERS response and are an essential factor in future designs of substrates and applications. This work will discuss the advantages and disadvantages of various experimental techniques used to report SERS enhancement with planar SERS substrates and present our alternative SERS enhancement value. We will report on three types of analysis scenarios that all yield different information concerning the effectiveness of the SERS substrate, practical application of the substrate, and finally the thermodynamic properties of the substrate. We believe that through this work a greater understanding for designing substrates will be achieved, one that is based on both thermodynamic and plasmonic properties as opposed to just plasmonic properties. This new understanding and potential change in substrate design will enable more applications for SERS based methodologies including targeting molecules that are traditionally not easily detected with SERS due to the perceived weak molecule-metal interaction of substrates.

Sensing of NO2 with zirconium hydroxide via frequency-dependent electrical impedance spectroscopy.

Zirconium hydroxide has been investigated as a candidate nitrogen dioxide dielectric sensor using impedance spectroscopy analysis. Significant changes in electronic and physical properties down to our dosage minimum of 2 ppm h have been observed. Using disc-shaped pressed pellets of Zr(OH)4 in parallel plate geometry, we observe a maximum signal shift of 35% at 2 ppm h dosage, which increases six orders of magnitude as the dosage reaches 1000 ppm h. Changes in impedance correlate with nitrogen and oxygen atomic ratio increases observed via X-ray photoelectron spectroscopy (XPS) at higher NO2 dosages. In contrast to the sharp frequency-dependent features and net impedance decreases during NO2 exposures, Zr(OH)4 exhibits a large and broad impedance increase after exposure to humid air (water vapor). The results indicate that Zr(OH)4 could be used as a selective low-cost impedance-based NO2 detector by applying frequency-dependent impedance fingerprinting.

Detection of Papaverine for the Possible Identification of Illicit Opium Cultivation.

Papaverine is a non-narcotic alkaloid found endemically and uniquely in the latex of the opium poppy. It is normally refined out of the opioids that the latex is typically collected for, hence its presence in a sample is strong prima facie evidence that the carrier from whom the sample was collected is implicated in the mass cultivation of poppies or the collection and handling of their latex. We describe an analysis technique combining surface-enhanced Raman spectroscopy (SERS) with microfluidics for detecting papaverine at low concentrations and show that its SERS spectrum has unique spectroscopic features that allows its detection at low concentrations among typical opioids. The analysis requires approximately 2.5 min from sample loading to results, which is compatible with field use. The weak acid properties of papaverine hydrochloride were investigated, and Raman bands belonging to the protonated and unprotonated forms of the isoquinoline ring of papaverine were identified.

Optimization of Surface-Enhanced Raman Spectroscopy Conditions for Implementation into a Microfluidic Device for Drug Detection.

A microfluidic device is being developed by University of California-Santa Barbara as part of a joint effort with the United States Army to develop a portable, rapid drug detection device. Surface-enhanced Raman spectroscopy (SERS) is used to provide a sensitive, selective detection technique within the microfluidic platform employing metallic nanoparticles as the SERS medium. Using several illicit drugs as analytes, the work presented here describes the efforts of the Edgewood Chemical Biological Center to optimize the microfluidic platform by investigating the role of nanoparticle material, nanoparticle size, excitation wavelength, and capping agents on the performance, and drug concentration detection limits achievable with Ag and Au nanoparticles that will ultimately be incorporated into the final design. This study is particularly important as it lays out a systematic comparison of limits of detection and potential interferences from working with several nanoparticle capping agents-such as tannate, citrate, and borate-which does not seem to have been done previously as the majority of studies only concentrate on citrate as the capping agent. Morphine, cocaine, and methamphetamine were chosen as test analytes for this study and were observed to have limits of detection (LOD) in the range of (1.5-4.7) × 10-8 M (4.5-13 ng/mL), with the borate capping agent having the best performance.

Coherent plasmon-exciton coupling in silver platelet-J-aggregate nanocomposites.

Hybrid nanostructures that couple plasmon and exciton resonances generate hybridized energy states, called plexcitons, which may result in unusual light-matter interactions. We report the formation of a transparency dip in the visible spectra of colloidal suspensions containing silver nanoplatelets and a cyanine dye, 1,1'-diethyl-2,2'-cyanine iodide (PIC). PIC was electrostatically adsorbed onto the surface of silver nanoplatelet core particles, forming an outer J-aggregate shell. This core-shell architecture provided a framework for coupling the plasmon resonance of the silver nanoplatelet core with the exciton resonance of the J-aggregate shell. The sizes and aspect ratios of the silver nanoplatelets were controlled to ensure the overlap of the plasmon and exciton resonances. As a measure of the plasmon-exciton coupling strength in the system, the experimentally observed transparency dips correspond to a Rabi splitting energy of 207 meV, among the highest reported for colloidal nanoparticles. The optical properties of the silver platelet-J-aggregate nanocomposites were supported numerically and analytically by the boundary-element method and temporal coupled-mode theory, respectively. Our theoretical predictions and experimental results confirm the presence of a transparency dip for the silver nanoplatelet core J-aggregate shell structures. Additionally, the numerical and analytical calculations indicate that the observed transparencies are dominated by the coupling of absorptive resonances, as opposed to the coupling of scattering resonances. Hence, we describe the suppressed extinction in this study as an induced transparency rather than a Fano resonance.

Critical role of adsorption equilibria on the determination of surface-enhanced Raman enhancement.

Surface-enhanced Raman spectroscopy (SERS) is a useful technique for probing analyte-noble metal interactions and determining thermodynamic properties such as their surface reaction equilibrium constants and binding energies. In this study, we measure the binding equilibrium constants and Gibbs free energy of binding for a series of nitrogen-containing aromatic molecules adsorbed on Klarite substrates. A dual Langmuir dependence of the SERS intensity on concentration was observed for the six species studied, indicating the presence of at least two different binding energies. We relate the measured binding energies to the previously described SERS enhancement value (SEV) and show that the SEV is proportional to the traditional SERS enhancement factor G, with a constant of proportionality that is critically dependent on the adsorption equilibrium constant determined from the dual Langmuir isotherm. We believe the approach described is generally applicable to many SERS substrates, both as a prescriptive approach to determining their relative performance and as a probe of the substrate's affinity for a target adsorbate.

Recent advances and remaining challenges for the spectroscopic detection of explosive threats.

In 2010, the U.S. Army initiated a program through the Edgewood Chemical Biological Center to identify viable spectroscopic signatures of explosives and initiate environmental persistence, fate, and transport studies for trace residues. These studies were ultimately designed to integrate these signatures into algorithms and experimentally evaluate sensor performance for explosives and precursor materials in existing chemical point and standoff detection systems. Accurate and validated optical cross sections and signatures are critical in benchmarking spectroscopic-based sensors. This program has provided important information for the scientists and engineers currently developing trace-detection solutions to the homemade explosive problem. With this information, the sensitivity of spectroscopic methods for explosives detection can now be quantitatively evaluated before the sensor is deployed and tested.

Ultraviolet resonance Raman spectroscopy of explosives in solution and the solid state.

Resonance Raman cross sections of common explosives have been measured by use of excitation wavelengths in the deep-UV from 229 to 262 nm. These measurements were performed both in solution and in the native solid state for comparison. While measurements of UV Raman cross sections in solution with an internal standard are straightforward and commonly found in the literature, measurements on the solid phase are rare. This is due to the difficulty in preparing a solid sample in which the molecules of the internal standard and absorbing analyte/explosive experience the same laser intensity. This requires producing solid samples that are mixtures of strongly absorbing explosives and an internal standard transparent at the UV wavelengths used. For the solid-state measurements, it is necessary to use nanostructured mixtures of the explosive and the internal standard in order to avoid this bias due to the strong UV absorption of the explosive. In this study we used a facile spray-drying technique where the analyte of interest was codeposited with the nonresonant standard onto an aluminum-coated microscope slide. The generated resonance enhancement profiles and quantitative UV-vis absorption spectra were then used to plot the relative Raman return as a function of excitation wavelength and particle size.

Surface-enhanced Raman scattering (SERS) evaluation protocol for nanometallic surfaces.

We present the results of a three-year collaboration between the U.S. Army Edgewood Chemical Biological Center and the U.S. Army Research Laboratory-Aldelphi Laboratory Center on the evaluation of selected nanometallic surfaces developed for the Defense Advanced Research Projects Agency Surface-Enhanced Raman Scattering (SERS) Science and Technology Fundamentals program. The primary role of the two Army labs was to develop the analytical and spectroscopic figures of merit to unambiguously compare the sensitivity and reproducibility of various SERS substrates submitted by the program participants. We present the design and implementation of an evaluation protocol for SERS active surfaces enabling an enhancement value calculation from which different substrates can be directly compared. This method was established to: (1) collect physical and spectral characterization data from the small number of substrates (performer supplied) typically encountered, and (2) account for the complex fabrication technique and varying nature of the substrate platforms encountered within this program.

Comparison of visible and near-infrared Raman cross-sections of explosives in solution and in the solid state.

Raman cross-sections of explosives in solution and in the solid state have been measured using visible and near-infrared excitation via secondary calibration. These measurements are valuable for both fundamental scientific purposes and applications in the standoff detection of explosives. The explosive compounds RDX, HMX, TNT, 2,4-DNT, 2,6-DNT, and ammonium nitrate were measured using discrete excitation wavelengths ranging from 532 nm to 785 nm. A comparison of the spectral features and cross-sections between the solid state and solution was performed. Comparison is also made to cross-sections measured with deep ultraviolet excitation.

Characterization of polymorphic states in energetic samples of 1,3,5-trinitro-1,3,5-triazine (RDX) fabricated using drop-on-demand inkjet technology.

The United States Army and the first responder community are evaluating optical detection systems for the trace detection of hazardous energetic materials. Fielded detection systems must be evaluated with the appropriate material concentrations to accurately identify the residue in theater. Trace levels of energetic materials have been observed in mutable polymorphic phases and, therefore, the systems being evaluated must be able to detect and accurately identify variant sample phases observed in spectral data. In this work, we report on the novel application of drop-on-demand technology for the fabrication of standardized trace 1,3,5-trinitro-1,3,5-triazine (RDX) samples. The drop-on-demand sample fabrication technique is compared both visually and spectrally to the more commonly used drop-and-dry technique. As the drop-on-demand technique allows for the fabrication of trace level hazard materials, concerted efforts focused on characterization of the polymorphic phase changes observed with low concentrations of RDX commonly used in drop-on-demand processing. This information is important when evaluating optical detection technologies using samples prepared with a drop-on-demand inkjet system, as the technology may be "trained" to detect the common bulk α phase of the explosive based on its spectral features but fall short in positively detecting a trace quantity of RDX (ß-phase). We report the polymorphic shifts observed between α- and ß-phases of this energetic material and discuss the conditions leading to the favoring of one phase over the other.

Fate dynamics of environmentally exposed explosive traces.

The chemical and physical fates of trace amounts (<50 µg) of explosives containing 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) were determined for the purpose of informing the capabilities of tactical trace explosive detection systems. From these measurements, it was found that the mass decreases and the chemical composition changes on a time scale of hours, with the loss mechanism due to a combination of sublimation and photodegradation. The rates for these processes were dependent on the explosive composition, as well as on both the ambient temperature and the size distribution of the explosive particulates. From these results, a persistence model was developed and applied to model the time dependence of both the mass and areal coverage of the fingerprints, resulting in a predictive capability for determining fingerprint fate. Chemical analysis confirmed that sublimation rates for TNT were depressed by UV (330-400 nm) exposure due to photochemically driven increases in the molecular weight, whereas the opposite was observed for RDX. No changes were observed for PETN upon exposure to UV radiation, and this was attributed to its low UV absorbance.

Semi-automated detection of trace explosives in fingerprints on strongly interfering surfaces with Raman chemical imaging.

We have previously demonstrated the use of wide-field Raman chemical imaging (RCI) to detect and identify the presence of trace explosives in contaminated fingerprints. In this current work we demonstrate the detection of trace explosives in contaminated fingerprints on strongly Raman scattering surfaces such as plastics and painted metals using an automated background subtraction routine. We demonstrate the use of partial least squares subtraction to minimize the interfering surface spectral signatures, allowing the detection and identification of explosive materials in the corrected Raman images. The resulting analyses are then visually superimposed on the corresponding bright field images to physically locate traces of explosives. Additionally, we attempt to address the question of whether a complete RCI of a fingerprint is required for trace explosive detection or whether a simple non-imaging Raman spectrum is sufficient. This investigation further demonstrates the ability to nondestructively identify explosives on fingerprints present on commonly found surfaces such that the fingerprint remains intact for further biometric analysis.

Review of chemical signature databases.

This review assesses the current state of chemical signature databases, the primary characteristics that determine their applicability, characterization of their capability to support spectral identifications, and the target audience to which they are directed. Database file formats, spectrometer operating conditions, and spectral matching tools are found to be primary characteristics that determine the applicability of databases and their ability to support spectral identifications. Chemical signature databases have evolved in two very different directions. One movement offers a single portal for chemical signature determinations by multiple analytical techniques. The other movement is toward highly specialized databases that address narrow scientific disciplines. Both movements are necessary, and serve distinctly different needs in the analytical community.

Bioaerosol analysis with Raman chemical imaging microspectroscopy.

Raman chemical imaging microspectroscopy is evaluated as a technology for waterborne pathogen and bioaerosol detection. Raman imaging produces a three-dimensional data cube consisting of a Raman spectrum at every pixel in a microscope field of view. Binary and ternary mixtures including combinations of polystyrene beads, gram-positive Bacillus anthracis, B. thuringiensis, and B. atrophaeus spores, and B. cereus vegetative cells were investigated by Raman imaging for differentiation and characterization purposes. Bacillus spore aerosol sizes were varied to provide visual proof for corroboration of spectral assignments. Conventional applications of Raman imaging consist of differentiating relatively broad areas of a sample in a microscope field of view. The spectral angle mapping data analysis algorithm was used to compare a library spectrum with experimental spectra from pixels in the microscope field of view. This direct one-to-one matching is straightforward, does not require a training set, is independent of absolute spectral intensity, and only requires univariate statistics. Raman imaging is expanded in its capabilities to differentiate and distinguish between discrete 1-6 microm size bacterial species in single particles, clusters of mixed species, and bioaerosols with interference background particles.