Noninvasive Detection of Concealed Explosives: Depth Profiling through Opaque Plastics by Time-Resolved Raman Spectroscopy

A versatile Raman setup with time-gating and fast wide-field imaging capabilities

Raman spectroscopy is a well-established method for chemical identification, with a wide variety of applications. The two major limitations are that fluorescence can hamper detection, and that Raman imaging is slow; it typically takes multiple hours to measure even a small surface area. We have developed a multimodal setup that mitigates these limitations. The setup has a point-scanning mode that allows for time-gated as well as continuous Raman spectroscopy, and both modes use an 80 MHz, 532 nm excitation laser with up to 20 W of power. The fluorescence suppression capabilities of the setup were demonstrated by comparing time-gated to continuous detection of a Dracaena leaf. Raman bands showed a 4-8 times improvement in signal-to-background ratio, and one band that was invisible in the continuous measurement, became visible in the time-gated measurement. The setup also has a 4-band simultaneously detected wide-field mode. Using a set of beam splitters, the Raman signal from the sample is split. This signal is imaged onto four separate cameras, each with a specific band-pass filter. The wide-field data were processed using principal component analysis with k-means clustering. To illustrate the wide-field capabilities of the setup, a 1mm2 sample containing aspirin, caffeine and paracetamol was measured using 10 W excitation power. A 10-second measurement enabled identification of the compounds, and a 1-second measurement showed promising results. This brings the setup close to real-time imaging, showing great potential for applications in quality control or for measuring samples that change over time.

Neglected acidity pitfall: boric acid-anchoring hole-selective contact for perovskite solar cells

The spontaneous formation of self-assembly monolayer (SAM) on various substrates represents an effective strategy for interfacial engineering of optoelectronic devices. Hole-selective SAM is becoming popular among high-performance inverted perovskite solar cells (PSCs), but the presence of strong acidic anchors (such as -PO3H2) in state-of-the-art SAM is detrimental to device stability. Herein, we report for the first time that acidity-weakened boric acid can function as an alternative anchor to construct efficient SAM-based hole-selective contact (HSC) for PSCs. Theoretical calculations reveal that boric acid spontaneously chemisorbs onto indium tin oxide (ITO) surface with oxygen vacancies facilitating the adsorption progress. Spectroscopy and electrical measurements indicate that boric acid anchor significantly mitigates ITO corrosion. The excess boric acid containing molecules improves perovskite deposition and results in a coherent and well-passivated bottom interface, which boosts the fill factor (FF) performance for a variety of perovskite compositions. The optimal boric acid-anchoring HSC (MTPA-BA) can achieve power conversion efficiency close to 23% with a high FF of 85.2%. More importantly, the devices show improved stability: 90% of their initial efficiency is retained after 2400 h of storage (ISOS-D-1) or 400 h of operation (ISOS-L-1), which are 5-fold higher than those of phosphonic acid SAM-based devices. Acidity-weakened boric acid SAMs, which are friendly to ITO, exhibits well the great potential to improve the stability of the interface as well as the device.

Detection Limits of Trinitrotoluene and Ammonium Nitrate in Soil by Raman Spectroscopy

The detection limit of 2,4,6-trinitrotoluene (TNT) and ammonium nitrate (AN) in mixtures of Ottawa sand (OS) was studied using a Raman microscope applying conventional calibration curves, Pearson correlation coefficients, and two-sample t-tests. By constructing calibration curves, the conventionally defined detection limits were estimated to be 1.9 ± 0.4% by mass in OS and 1.9 ± 0.3% by mass in OS for TNT and AN. Both TNT and AN were detectable in concentrations as low as 1% by mass when Pearson correlation coefficients were used to compare averaged spectra to a library containing spectra from a range of soil types. AN was detectable in concentrations as low as 1% by mass when a test sample of spectra was compared to the same library using two-sample t-tests. TNT was not detectable at a concentration of 1% by mass when using two-sample t-tests.

In situ real-time identification of packaged chemicals using a dual-offset optical probe

In situ real-time and nondestructive identification of packaged chemicals is essential for applications such as homeland security and terrorism prevention. Although various Raman spectroscopic methods such as spatially offset Raman spectroscopy (SORS) and time-resolved Raman spectroscopy have been investigated for real-time detection, the background interference originating from packaging materials limits the accuracy of the analysis. In principle, the Raman background from the packaging cannot be removed completely. To overcome this limitation, we developed a SORS-based dual-offset optical probe (DOOP) system that offers real-time prediction of 20 chemicals concealed in various containers by completely removing the background signal. The DOOP system selectively acquires the Raman photons generated from both the outer packaging and the inner contents, whose intensities are dependent on the penetration depth of the laser. The Raman spectra obtained at two remote offsets are automatically subtracted after normalization. We demonstrate that the DOOP method provides the pure component spectra by completely removing background interference from three plastic containers for a total of 20 samples in three different containers. In addition, an artificial neural network (ANN) was applied to evaluate the accuracy of the real-time chemical identification system; our system led to drastic improvements of the ANN prediction accuracy.

Challenges in application of Raman spectroscopy to biology and materials

Raman spectroscopy has become an essential tool for chemists, physicists, biologists and materials scientists. In this article, we present the challenges in unravelling the molecule-specific Raman spectral signatures of different biomolecules like proteins, nucleic acids, lipids and carbohydrates based on the review of our work and the current trends in these areas. We also show how Raman spectroscopy can be used to probe the secondary and tertiary structural changes occurring during thermal denaturation of protein and lysozyme as well as more complex biological systems like bacteria. Complex biological systems like tissues, cells, blood serum etc. are also made up of such biomolecules. Using mice liver and blood serum, it is shown that different tissues yield their unique signature Raman spectra, owing to a difference in the relative composition of the biomolecules. Additionally, recent progress in Raman spectroscopy for diagnosing a multitude of diseases ranging from cancer to infection is also presented. The second part of this article focuses on applications of Raman spectroscopy to materials. As a first example, Raman spectroscopy of a melt cast explosives formulation was carried out to monitor the changes in the peaks which indicates the potential of this technique for remote process monitoring. The second example presents various modern methods of Raman spectroscopy such as spatially offset Raman spectroscopy (SORS), reflection, transmission and universal multiple angle Raman spectroscopy (UMARS) to study layered materials. Studies on chemicals/layered materials hidden in non-metallic containers using the above variants are presented. Using suitable examples, it is shown how a specific excitation or collection geometry can yield different information about the location of materials. Additionally, it is shown that UMARS imaging can also be used as an effective tool to obtain layer specific information of materials located at depths beyond a few centimeters.

This paper reviews various facets of Raman spectroscopy. This encompasses biomolecule fingerprinting and conformational analysis, discrimination of healthy vs. diseased states, depth-specific information of materials and 3D Raman imaging.

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

Raman Spectroscopic Techniques for Planetary Exploration: Detecting Microorganisms through Minerals

Raman spectroscopy can provide highly specific chemical fingerprints of inorganic and organic materials and is therefore expected to play a significant role in interplanetary missions, especially for the search for life elsewhere in our solar system. A major challenge will be the unambiguous detection of low levels of biomarkers on a mineral background. In addition, these biomarkers may not be present at the surface but rather inside or underneath minerals. Strong scattering may prevent focusing deeper into the sample. In this paper, we report the detection of carotenoid-containing microorganisms behind mineral layers using time-resolved Raman spectroscopy (TRRS). Two extremophiles, the bacterium Deinococcus radiodurans and the cyanobacterium Chroococcidiopsis, were detected through translucent and transparent minerals using 440 nm excitation under resonance conditions to selectively enhance the detection of carotenoids. Using 3 ps laser pulses and a 250 ps gated intensified CCD camera provided depth selectivity for the subsurface microorganisms over the mineral surface layer and in addition lowered the contribution of the fluorescent background. Raman spectra of both organisms could be detected through 5 mm of translucent calcite or 20 mm of transparent halite. Multilayered mineral samples were used to further test the applied method. A separate tunable laser setup for resonance Raman and a commercial confocal Raman microscope, both with continuous (non-gated) detection, were used for comparison. This study demonstrates the capabilities of TRRS for the depth-selective analysis through scattering samples, which could be used in future planetary exploration to detect microorganisms or biomarkers within or behind minerals.

Looking inside catalyst extrudates with time-resolved surface-enhanced Raman spectroscopy (TR-SERS)

Raman spectroscopy is one of the major characterization methods employed over the last few decades as a nondestructive technique for the study of heterogeneous catalysts and related catalytic reactions. However, the promise of practical applicability on millimeter-sized catalyst bodies, such as extrudates, has not been fulfilled completely. Large fluorescence signals and the highly scattering nature of the extrudates often hamper its practical usage. Different approaches to overcome this problem were examined, including the use of time-resolved Raman spectroscopy (TRRS), spatially offset Raman spectroscopy (SORS), surface-enhanced Raman spectroscopy (SERS), and combinations of these techniques. This paper demonstrates that especially TRRS can provide chemical information at depth within catalyst bodies, overcoming fluorescence background signals and allowing for visualization of analytes at different depths. It also examines the application of time-resolved SERS within catalyst bodies to gain insight into localized activity. With these options a wider applicability of Raman spectroscopy for industrial catalysis research becomes within reach.

Depth profiling for the identification of unknown substances and concealed content at remote distances using time-resolved stand-off Raman spectroscopy

Time-resolved stand-off Raman spectroscopy was used to determine both the position and identity of substances relative to each other at remote distances (up to tens of meters). Spectral information of three xylene isomers, toluene, and sodium chlorate was obtained at a distance of 12 m from the setup. Pairs and triplets of these samples were placed at varying distances (10-60 cm) relative to each other. Via the photon time of flight the distance between the individual samples was determined to an accuracy of 7% (corresponding to a few cm) of the physically measured distance. Furthermore, at a distance of 40 m, time-resolved Raman depth profiling was used to detect sodium chlorate in a white plastic container that was non-transparent to the human eye. The combination of the ranging capabilities of Raman LIDAR (sample location usually determined using prior knowledge of the analyte of interest) with stand-off Raman spectroscopy (analyte detection at remote distances) provides the capability for depth profile identification of unknown substances and analysis of concealed content in distant objects. To achieve these results, a 532 nm laser with a pulse length of 4.4 ns was synchronized to an intensified charge-coupled device camera with a minimum gate width of 500 ps. For automated data analysis a multivariate curve resolution algorithm was employed.