Chiral Purity of Crystals Using Low-Frequency 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.

Precise Construction of Chiral Plasmonic Nanoparticles for Enantioselective Discrimination

Chiral plasmonic nanostructures exhibit potential in the advanced manufacturing industry, due to their fascinating characteristics. However, the limitation of existing fabrication methods as difficulty to precisely manipulate chiral nanostructures at the nanoscale restricts their application and optimization of performance. In this work, we report a simple and robust route for the precise construction of chiral Au nanoparticles (NPs), employing star-like block copolymers with well-defined structures as chiral templates. The globular unimolecular micelles as nanoreactors enabled control over the size, shape, and chirality of in situ grown nanocrystals. Utilizing the chiral anisotropy property of surface-enhanced Raman scattering (SERS), the enantioselective discrimination on various substrates was accomplished with an enhancement factor over 9.3 × 106. NPs with a smaller size exhibited strengthened Raman enhancement and chiral recognition. Furthermore, these chiral unimolecular-micelle-based templates with high efficiency and strong controllability could pave the way for tailor-made chiral nanomaterials.

Low-Frequency Raman Spectroscopy: An Exceptional Tool for Exploring Metastability Driven States Induced by Dehydration

The use of low-frequency Raman spectroscopy (LFRS; ω < 150 cm^-1) is booming in the pharmaceutical industry. Specific processing of spectra is required to use the wealth of information contained in this spectral region. Spectra processing and the use of LFRS for analyzing phase transformations in molecular materials are detailed herein from investigations on the devitrification of ibuprofen. LFRS was used to analyze the dehydration mechanism of two hydrates (theophylline and caffeine) of the xanthine family. Two mechanisms of solid-state transformation in theophylline were determined depending on the relative humidity (RH) and temperature. At room temperature and 1% RH, dehydration is driven by the diffusion mechanism, while under high RH (>30%), kinetic laws are typical of nucleation and growth mechanism. By increasing the RH, various metastability driven crystalline forms were obtained mimicking successive intermediate states between hydrate form and anhydrous form achieved under high RH. In contrast, the dehydration kinetics of caffeine hydrate under various RH levels can be described by only one master curve corresponding to a nucleation mechanism. Various metastability driven states were achieved depending on the RH, which can be described as intermediate between forms I and II of anhydrous caffeine.

Terahertz spectroscopy of temperature-induced transformation between glutamic acid, pyroglutamic acid and racemic pyroglutamic acid

Under heating conditions, L-Glutamic acid (L-Glu) can be dehydrated to form L-pyroglutamic acid (L-PGA), and L-PGA can racemize to form DL-PGA. Here, we characterized this transformation at different temperatures and times by terahertz time domain spectroscopy (THz-TDS). By Powder X-ray diffraction (PXRD), the validity of THz spectroscopy is verified. The results prove that the reaction rate of dehydration and racemization is significantly affected by temperature. The THz spectra divided the reactions into three stages. At 150-155 °C, the reaction changes drastically. Furthermore, we found that the absorption intensity at 0.97 and 1.55 THz has a good dependence on the reaction temperature and time, showing a non-linear relationship (R² > 0.98). Our findings suggest that the chemical transformation and reaction rate can be sensitively probed by terahertz spectroscopy, which provides a potential method for the quantitative analysis of reaction products.

Identification of Enantiomers Using Low-Frequency Raman Spectroscopy

Distinguishing between d and l enantiomers is of important scientific interest, especially for the pharmaceutical industry. Enantiomeric differentiation in the solid form is repeatedly presented as a challenge in the research community. Raman spectroscopy is a nondestructive tool, widely used for the characterization of different materials by probing their vibrational modes. The low-frequency region of the Raman spectrum reveals lattice-level interactions and global fluctuations in the molecule. Lower frequencies correspond to vibrations arising from weaker bonds and long-range interactions and hence are very susceptible to polarization changes. This work presents low-frequency Raman (LFR) spectroscopy as a facile technique to identify enantiomers. The optical setup of conventional Raman spectroscopy is engineered such that the excitation and collection geometries use an asymmetrical focal cone. In addition, a half-wave retarder is added to the excitation path and a Glan-Taylor polarizer is added to the collection path, and these modifications allow us to select the polarization plane for both excitation and collection geometries. The asymmetry in the foci when using a polarized beam for excitation provides different intensities of the collected signal for each polarization plane. In a calibrated system, one can define the chirality of an analyte by comparing the intensity of the LFR signal along orthogonal sets of polarization planes. For nonchiral molecules, the spectral intensity is always higher in the co-polarized plane when compared to the orthogonally depolarized plane, as expected. This contrast in the intensity of Raman spectra serves as a distinct tool for identifying enantiomers.

Photoacoustic measurement of localized optical dichroism in chiral crystals

In this communication, we present a novel method to measure local optical dichroism (OD) in opaque crystal powder suspensions using photoacoustic (PA) effect. Our method is based upon the novel laser speckle contrast technique, in combination with a simple statistical approach, we were able to measure the OD of chiral crystals suspensions under completely random orientation.

Chiral discrimination of amino acids using phosphorene assisted graphene-enhanced Raman spectroscopy

Discrimination of enantiomers poses a scientific challenge as the chemical and physical properties of enantiomers are nearly identical. The chiral analysis is usually performed by separation techniques, including chromatography, electrophoresis, or optical instrumentation based on an interaction of the analyzed sample with a polarized beam of light. Here we present a novel method for a chiral screening based on a combination of the black phosphorus@Graphene nanocomposite and Raman spectroscopy. The nanocomposite allows to enhance the Raman signal with factors higher than 100 asymmetrically and provide altered signals for mixtures containing varying enantiomeric ratios of target compounds. Tryptophan, Phenylalanine, DOPA, Isoleucine, and Leucine were selected as model compounds; the method allows us to discriminate between mixtures with 10, 25, 50, 75, and 100% enantiomeric purity.

Polydopamine/Silver Substrates Stemmed from Chiral Silica for SERS Differentiation of Amino Acid Enantiomers

Polydopamine (PDA) and silver (Ag) nanoparticles were first generated on chiral silica nanofibers and then detached from silica to form PDA/Ag composites. The as-obtained PDA/Ag showed surface-enhanced Raman scattering (SERS) activity but very weak circular dichroism optical activity. Interestingly, the PDA/Ag substrates could make a pair of tyrosine (or phenylalanine) enantiomers show different Raman scattering signal intensities, where the differences could reach 3 times. In contrast, PDA/Ag prepared by using racemic or achiral silica did not exhibit such discrimination performance. Therefore, this research offered a novel SERS-based enantiomeric differentiation method with the assistance of plasmonic metal-containing substrates stemmed from intrinsically chiral inorganic silica.

Polarization Dependence of Low-Frequency Vibrations from Multiple Faces in an Organic Single Crystal

Recent developments in optical filters have enabled the facile use of Raman spectroscopy for detection of low frequency (LF) vibrational modes. LF-Raman spectroscopy offers fast and sensitive characterization of LF vibrations, and enables the measurement of single microcrystals and detection of defects. It is useful for probing intermolecular interactions in crystals, which are lower in energy, such as hydrogen bonds, shear modes, and breathing modes. Crystal excitation from multiple faces allows learning the orientation of intermolecular interactions, as polarization dependence varies with the polarizability of the interactions along the planes. Elucidating the orientations of the intermolecular interactions in organic crystals is essential for guiding the reactions or adsorption to a specific crystal face. In this study, we investigated the dependence of the LF-Raman signal intensity on the orientation of an organic single microcrystal of L-alanine. Three incident beam directions provided the orientations of the intermolecular interactions by analyzing the corresponding LF-Raman spectra. The signal intensity correlated well with the proximity between the incident beam’s direction and the orientations of the intermolecular interactions. Excellent compatibility was found between the spectra and simulated orientations based on structural information.