Terahertz time-domain and low-frequency Raman spectroscopy of organic materials

Using terahertz spectroscopy to quantify bioactive flavonoids in Moxa Wool as predictor of rheumatoid arthritis treatment outcomes

BACKGROUND

Moxibustion, a traditional Chinese medicine practice, employs Moxa Wool, derived from Artemisia argyi. Flavonoids, the key pharmacological constituents in Moxa Wool, are known for their anti-inflammatory and analgesic properties. The purity of Moxa Wool, particularly its flavonoid content, directly influences the efficacy of moxibustion treatments. However, quantifying these bioactive flavonoids accurately and non-destructively has been a challenge.

PURPOSE

This study introduces terahertz spectroscopy as a non-destructive optical detection method for qualitative detection and quantitative analysis of flavonoids in Moxa Wool. By establishing a mathematical model between spectral signals and clinical efficacy, a reliable correlation between flavonoid concentration and the therapeutic effect of moxibustion can be established, providing a potential predictive model for the treatment outcomes of rheumatoid arthritis.

STUDY DESIGN

We adopted terahertz spectroscopy technology and combined it with terahertz metamaterial biosensors to achieve rapid, efficient, and non-destructive testing of the quality of Moxa Wool. This method reduces the detection time from hours to minutes while lowering the sample detection limit, overcoming the limitations of traditional detection methods in pharmacological research.

METHODS

Through terahertz metamaterial biosensors, rapid detection of the purity of Moxa Wool has been achieved. A combination of molecular simulation and terahertz spectroscopy was used to quantitatively analyze the flavonoid content in different purities of Moxa Wool. To ensure accuracy, the quantitative results of flavonoids obtained by terahertz spectroscopy were validated using high-performance liquid chromatography (HPLC). In addition, moxibustion treatment was performed on rats with rheumatoid arthritis using Moxa Wool, and medical indicator information was recorded. A mathematical analysis model was established to evaluate the correlation between flavonoid content and analgesic and anti-inflammatory effects.

RESULTS

Terahertz spectroscopy analysis shows that there is a direct correlation between the flavonoid content in moxibustion and the absorption peak intensity. The maximum R2 in the model analysis is 0.98, indicating a high accuracy in predicting the purity of Moxa Wool. These results were also validated by HPLC. In a rat model, the purity of 30:1 Moxa Wool samples showed a 50 % decrease in TNF-α, IL-1β, and IL-6 levels during treatment compared to low-purity samples, significantly reducing inflammation markers and pain symptoms. Meanwhile, The PLS prediction model established a correlation between terahertz-detected flavonoid levels and treatment outcomes (PWL and IL-1β). The maximum R2 in the model is 0.91, indicating a high correlation between flavonoid levels and the anti-inflammatory and analgesic effects of moxibustion treatment.

CONCLUSION

This study not only demonstrates the effectiveness of terahertz spectroscopy in the pharmacological quantification of bioactive compounds but also establishes a novel predictive model for the efficacy of moxibustion in rheumatoid arthritis treatment. It underscores the potential of integrating traditional medicine insights with advanced technology to enhance therapeutic strategies in pharmacology.

Uncovering low-frequency vibrations in surface-enhanced Raman of organic molecules

Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm-1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum. Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration. Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening, further supporting the key role of local environment.

Spectroscopic and imaging considerations of THz-TDS and ULF-Raman techniques towards practical security applications

Nitrogen-containing high-energy organic compounds represent a class of materials with critical implications in various fields, including military, aerospace, and chemical industries. The precise characterization and analysis of these compounds are essential for both safety and performance considerations. Spectroscopic characterization in the far-infrared region has great potential for non-destructive investigation of high energetic and related compounds. This research article presents a comprehensive study of common organic energetic materials in the far-infrared region (5-200 cm-1), aiming to enhance security measures through the utilization of cutting-edge spectroscopic techniques. Broadband terahertz time-domain spectroscopy and ultra-low frequency Raman spectroscopy are employed as powerful tools to probe the vibrational and rotational modes of various explosive materials. One of the key objectives of this present work is unveiling the characteristic spectral features and optical parameters of five common nitrogen based high energy organic compounds towards rapid and accurate identification. Further, we have explored the potential of terahertz reflection imaging for non-contact through barrier sensing, a critical requirement in security applications. Based on the spectral features obtained from the spectroscopic studies and using advanced imaging algorithms we have been able to detect these compounds under various barriers including paper, cloth, backpack, etc. Subsequently, this study highlights the capabilities of the two techniques offering a pathway to enhance their utility over a wide range of practical security applications.

Drug-Coformer Loaded-Mesoporous Silica Nanoparticles: A Review of the Preparation, Characterization, and Mechanism of Drug Release

Drug-coformer systems, such as coamorphous and cocrystal, are gaining recognition as highly effective strategies for enhancing the stability, solubility, and dissolution of drugs. These systems depend on the interactions between drug and coformer to prevent the conversion of amorphous drugs into the crystalline form and improve the solubility. Furthermore, mesoporous silica (MPS) is also a promising carrier commonly used for stabilization, leading to solubility improvement of poorly water-soluble drugs. The surface interaction of drug-MPS and the nanoconfinement effect prevent amorphous drugs from crystallizing. A novel method has been developed recently, which entails the loading of drug-coformer into MPS to improve the solubility, dissolution, and physical stability of the amorphous drug. This method uses the synergistic effects of drug-coformer interactions and the nanoconfinement effect within MPS. Several studies have reported successful incorporation of drug-coformer into MPS, indicating the potential for significant improvement in dissolution characteristics and physical stability of the drug. Therefore, this study aimed to discuss the preparation and characterization of drug-coformer within MPS, particularly the interaction in the nanoconfinement, as well as the impact on drug release and physical stability.

Probing Complex Chemical Processes at the Molecular Level with Vibrational Spectroscopy and X-ray Tools

Understanding the origins of structure and bonding at the molecular level in complex chemical systems spanning magnitudes in length and time is of paramount interest in physical chemistry. We have coupled vibrational spectroscopy and X-ray based techniques with a series of microreactors and aerosol beams to tease out intricate and sometimes subtle interactions, such as hydrogen bonding, proton transfer, and noncovalent interactions. This allows for unraveling the self-assembly of arginine-oleic acid complexes in an aqueous solution and growth processes in a metal-organic framework. Terahertz and infrared spectroscopy provide an intimate view of the hydrogen-bond network and associated phase changes with temperature in neopentyl glycol. The hydrogen-bond network in aqueous glycerol aerosols and levels of protonation of nicotine in aqueous aerosols are visualized. Future directions in probing the hydrogen-bond networks in deep eutectic solvents and organic frameworks are described, and we suggest how X-ray scattering coupled to X-ray spectroscopy can offer insight into the reactivity of organic aerosols.

Old Polymorph, New Technique: Assessing Ritonavir Crystallinity Using Low-Frequency Raman Spectroscopy

Two decades ago, postmarket discovery of a second crystal form of ritonavir with lower solubility had major implications for drug manufacturers and patients. Since then, ritonavir has been reformulated via the hot-melt-extrusion process in an amorphous form. Here, quantitative low- and mid-frequency Raman spectroscopy methods were developed to characterize polymorphs, form I and form II, in commercial ritonavir 100 mg oral tablets as an alternate analysis approach compared to X-ray powder diffraction (XRPD). Crystallization in three lots of ritonavir products obtained from four separate manufacturers was assessed after storage under accelerated conditions at 40 °C and 75% relative humidity (RH). Results were compared with quantitative XRPD methods developed and validated according to ICH Q2 (R1) guidelines. In a four-week open-dish study, form I crystallization occurred in two of the four products and form II crystallization was detected in another ritonavir product. The limits of detection for XRPD, low-frequency Raman (LFR), and mid-frequency Raman (MFR) were determined to be 0.7, 0.8, and 0.5% for form I and 0.6, 0.6, and 1% for form II, respectively. Root-mean-squared-error of predictions were 0.6-1.0 and 0.6-2.5% for LFR- and MFR-based partial least-squares models. Further, ritonavir polymorphs could also be identified and detected directly from ritonavir tablets using transmission LFR. In summary, LFR was applied for the assessment of polymorphism in real-world samples. While providing analytical performance similar to conventional techniques, LFR reduced the single measurement time from 66 min (XRPD) to 10 s (LFR) without the need for tedious sample preparation procedures.

Investigating the function and design of molecular materials through terahertz vibrational spectroscopy

Terahertz spectroscopy has proved to be an essential tool for the study of condensed phase materials. Terahertz spectroscopy probes the low-frequency vibrational dynamics of atoms and molecules, usually in the condensed phase. These nuclear dynamics, which typically involve displacements of entire molecules, have been linked to bulk phenomena ranging from phase transformations to semiconducting efficiency. The terahertz region of the electromagnetic spectrum has historically been referred to as the 'terahertz gap', but this is a misnomer, as there exist a multitude of methods for accessing terahertz frequencies, and now there are cost-effective instruments that have made terahertz studies much more user-friendly. This Review highlights some of the most exciting applications of terahertz vibrational spectroscopy so far, and provides an in-depth overview of the methods of this technique and its utility to the study of the chemical sciences.

Mechanistic View on the Order-Disorder Phase Transition in Amphidynamic Crystals

We combine temperature-dependent low-frequency Raman measurements and first-principles calculations to obtain a mechanistic understanding of the order-disorder phase transition of 2,7-di-tert-butylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBu-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconducting amphidynamic crystals. We identify the lattice normal modes associated with the phase transition by following the position and width of the Raman peaks with temperature and identifying peaks that exhibit nonlinear dependence toward the phase transition temperature. Our findings are interpreted according to the "hardcore mode" model previously used to describe order-disorder phase transitions in inorganic and hybrid crystals with a Brownian sublattice. Within the framework of this model, ditBu-BTBT exhibits an ideal behavior where only one lattice mode is associated with the phase transition. TIPS-pentacene deviates strongly from the model due to strong interactions between lattice modes. We discuss the origin of the different behaviors and suggest side-chain engineering as a tool to control polymorphism in amphidynamic crystals.

High-Mobility Naphthalene Diimide Derivatives Revealed by Raman-Based In Silico Screening

Charge transport in crystalline organic semiconductors (OSCs) is considerably hindered by low-frequency vibrations introducing dynamic disorder in the charge transfer integrals. Recently, we have shown that the contributions of various vibrational modes to the dynamic disorder correlate with their Raman intensities and suggested a Raman-based approach for estimation of the dynamic disorder and search for potentially high-mobility OSCs. In the present paper, we showcase this approach by revealing the highest-mobility OSC(s) in two series of crystalline naphthalene diimide derivatives bearing alkyl or cycloalkyl substituents. In contrast to our previous studies, Raman spectra are not measured, but are instead calculated using periodic DFT. As a result, an OSC with a potentially high charge mobility is revealed in each of the two series, and further mobility calculations corroborate this choice. Namely, for the naphthalene diimide derivatives with butyl and cyclopentyl substituents, the estimated room-temperature isotropic electron mobilities are as high as 6 and 15 cm2 V-1 s-1, respectively, in the latter case even exceeding 20 cm2 V-1 s-1 in a two-dimensional plane. Thus, our results highlight the potential of using the calculated Raman spectra to search for high-mobility crystalline OSCs and reveal two promising OSCs, which were previously overlooked.

Dissociation and Recombination Processes in Lactose Monohydrate Detected by THz Time-Domain Spectroscopy

The terahertz (THz) region has much appeal for differentiating between hydrate systems and for physically characterizing pharmaceutical drug materials. The present study employs THz absorption spectroscopy to investigate the effect of heating on dehydration and hydration in α-lactose monohydrate. Distinctive THz absorption spectra were observed following various heating durations. The THz absorption spectra for α-lactose monohydrate and anhydrous α-lactose exhibit clear differences. Pure α-lactose monohydrate displays clear absorption peaks at 0.53, 1.05, 1.11, 1.33, and 1.56 THz. The complete dehydration of α-lactose monohydrate takes only 15 mins at 145°C (418 K). Moreover, the THz refractive index of α-lactose monohydrate decreases during dehydration. The dehydration of α-lactose monohydrate was also studied using Beer–Lambert law to compare THz absorption spectra as functions of the heating time. The absorption coefficient spectra recorded at 0.53 and 1.35 THz for α-lactose monohydrate after different dehydration times vary linearly with the remaining water content.

Development of Microscopy Apparatus Switchable between Fluorescence and Ultralow-Frequency Raman Modes

In this study, a microscopy apparatus that can switch between the fluorescence microscopy and ultralow-frequency Raman microscopy modes was developed. The apparatus can be easily constructed by equipping a standard epi-illumination microscope with an additional port featuring a removable half mirror. Owing to the switchability, fluorescence imaging, and spectroscopy, Raman spectroscopy in the frequency range down to ∼10 cm^-1 can be performed using the apparatus. To demonstrate the advantageous features of this apparatus, micron-sized crystals of perylene, which have two polymorphic forms, were analyzed. The two polymorphs were clearly identified based on their shapes, fluorescence spectra, and ultralow-frequency Raman spectra, all of which can be observed with our apparatus alone. These results indicate that the apparatus is a powerful tool for the analysis and characterization of various nano-/micron-sized crystals.

Low-Frequency Vibrational Spectroscopy and Quantum Mechanical Simulations of the Crystalline Polymorphs of the Antiviral Drug Ribavirin

Crystal polymorphism is a common phenomenon in pharmaceutical solids and a critical issue when considering the formulation of therapeutics since multiple polymorphs may form during drug manufacturing. Low-frequency vibrational spectroscopy is sensitive to polymorphic content, and in this work, terahertz time-domain spectroscopy and low-frequency Raman spectroscopy were utilized in the study of crystalline ribavirin, a widely applicable antiviral. Characteristic spectra with numerous peaks in the sub-200 cm^–1 region were obtained of the more common polymorph of ribavirin (Form II). Solid-state density functional theory (ss-DFT) simulations were then used to optimize the crystal structure of this polymorph and calculate the frequencies and spectral intensities of the lattice vibrations in the low-frequency region. The near-harmonic thermal behavior of the sample with cooling enabled excellent agreement between experiment and theory to be achieved, emphasizing the quality of the applied model, and the observed spectral peaks could be assigned to specific atomic motions in the solid. Form I and Form II polymorphs of ribavirin were both investigated with ss-DFT to understand the different aspects governing the relative stabilities of these solids. The ss-DFT simulations of the polymorph energies revealed that Form II is more stable at all temperatures due to a stronger cohesive energy than Form I; however, ribavirin in Form I has a significantly lower conformational energy. The finding of monotropism appears to conflict with the reported enantiotropism of the ribavirin polymorphs but ultimately confirms that crystal defects in the real samples greatly affect the thermodynamic relationship of the crystals.

Stimulated Raman Scattering of an Organic Liquid in a Spherical-Shell Optical Cavity around an Aqueous Pendant Drop

Stimulated Raman scattering (SRS) in a liquid droplet enhances the Raman scattering intensity through optical resonances. We have previously used a pendant drop at the tip of a capillary as the Raman-enhancing medium. In this study, we develop a new optical cavity for SRS measurement that consists of a spherical shell of organic liquid. This enables us to extend the applicability of the pendant-drop SRS method to liquids with low viscosity or low interfacial tension. This method is used to observe low-frequency modes of liquid benzene. The results indicate that the SRS emerges locally with respect to the drop size. The developed method extends the study of liquid structures based on vibrational spectroscopy.

Diclofenac Ion Hydration: Experimental and Theoretical Search for Anion Pairs

Self-assembly of organic ions in aqueous solutions is a hot topic at the present time, and substances that are well-soluble in water are usually studied. In this work, aqueous solutions of sodium diclofenac are investigated, which, like most medicinal compounds, is poorly soluble in water. Classical MD modeling of an aqueous solution of diclofenac sodium showed equilibrium between the hydrated anion and the hydrated dimer of the diclofenac anion. The assignment and interpretation of the bands in the UV, NIR, and IR spectra are based on DFT calculations in the discrete-continuum approximation. It has been shown that the combined use of spectroscopic methods in various frequency ranges with classical MD simulations and DFT calculations provides valuable information on the association processes of medical compounds in aqueous solutions. Additionally, such a combined application of experimental and calculation methods allowed us to put forward a hypothesis about the mechanism of the effect of diclofenac sodium in high dilutions on a solution of diclofenac sodium.

On the use of a volume constraint to account for thermal expansion effects on the low-frequency vibrations of molecular crystals

A volume-constraint method is presented as a means to capture the influence of thermal expansion on the low-frequency vibrations in molecular crystals. In particular, the room-temperature terahertz absorption spectra of L-tartaric acid, α-lactose monohydrate, and α-para-aminobenzoic acid (PABA) have been simulated using dispersion-corrected, solid-state density functional theory (DFT-D). By comparing the normal modes obtained with a unit cell optimised without constraints to those obtained with a unit cell optimised while constrained to keep its experimental volume, wholesale improvements to the resultant spectrum is achieved when using the constrained geometry by inhibiting cell contraction. These improvements are demonstrated over a range of popular density functionals and basis sets up to triple-zeta complexity. A correlation method is then presented as a means to quantitatively compare the vibrational pattern of normal modes obtained from both unit cells. This analysis reveals that thermal expansion can effect the character and relative frequency of normal modes, with the choice of geometry ultimately affecting the assignment of the experimental absorptions. The sensibility of using the experimental volume as an approximation is then discussed, where it is speculated that large basis sets or hybrid functionals are necessary to ensure that the thermal expansion effect is not overestimated. The low-frequency absorption spectrum of PABA is then fully characterised using the PBE-D3BJ/6-311G(2d,2p) method.

Low- versus Mid-frequency Raman Spectroscopy for in Situ Analysis of Crystallization in Slurries

Slurry studies are useful for exhaustive polymorph and solid-state stability screening of drug compounds. Raman spectroscopy is convenient for monitoring crystallization in such slurries, as the measurements can be performed in situ even in aqueous environments. While the mid-frequency region (400–4000 cm^–1) is dominated by intramolecular vibrations and has traditionally been used for such studies, the low-frequency spectral region (<200 cm^–1) probes solid-state related lattice vibrations and is potentially more valuable for understanding subtle and/or complex crystallization behavior. The aim of the study was to investigate low-frequency Raman spectroscopy for in situ monitoring of crystallization of an amorphous pharmaceutical in slurries for the first time and directly compare the results with those simultaneously obtained with mid-frequency Raman spectroscopy. Amorphous indomethacin (IND) slurries were prepared at pH 1.2 and continuously monitored in situ at 5 and 25 °C with both low- and mid-frequency Raman spectroscopy. At 25 °C, both spectral regions profiled amorphous IND in slurries as converting directly from the amorphous form toward the α crystalline form. In contrast, at 5 °C, principal component analysis revealed a divergence in the detected conversion profiles: the mid-frequency Raman suggested a direct conversion to the α crystalline form, but the low-frequency region showed additional transition points. These were attributed to the appearance of minor amounts of the ε-form. The additional solid-state sensitivity of the low-frequency region was attributed to the better signal-to-noise ratio and more consistent spectra in this region. Finally, the low-frequency Raman spectrum of the ε-form of IND is reported for the first time.

Three-dimensional modeling of intense unipolar THz pulses formation during their amplification in nonequilibrium extended Xe plasma channel

We develop a three-dimensional (3D) fully self-consistent model for analysis of an ultrashort THz pulse propagation and amplification in a nonequilibrium plasma channel formed in xenon by a femtosecond UV laser pulse. The model is based on the self-consistent solution of a second order wave equation in the cylindrical geometry and the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) at different points of the spatially inhomogeneous nonequilibrium plasma channel. We analyze the wide range of plasma and seed pulse parameters and reveal the optimal regimes for producing high intensity outgoing THz fields as well as highly unipolar THz pulses within the proposed mechanism. It is demonstrated that the process of EVDF relaxation in plasma limits the amplification of THz pulses at the level of ∼10^{7}W/cm^{2}. Both focusing features of nonequilibrium plasma and the possibility of producing THz pulses with a high degree of unipolarity are confirmed for the case of 3D geometry.

Dual spectroscopic detection of THz energy modes of critical chemical compounds

Precise identification and sensing of organic and inorganic molecular systems are key factors in several applications in present industrial and scientific domains. While high energy modes, due to electronic interactions, are mostly impervious to the initial thermodynamic or chemical conditions, the low energy modes are sensitive to such alterations which makes them suitable for quality control purpose with sensitive spectral identification methods. Here we report for the first time, several low frequency peaks of specific nitrogen-based compounds and their derivatives, using the dual spectroscopic approach of Terahertz Time-Domain Spectroscopy (THz-TDS) and THz Raman Spectroscopy (THz-RS). Two different isomeric molecular systems have also been investigated to assess both the selectivity and specificity of low energy modes in their identification and spectral correlation in terms of molecular interactions. This information of low frequency modes can be utilized readily by pharmaceutical and agri-food industries, chemical engineering and crystal growth communities in identification, detection, quality control and industrial waste management.

Investigation on the Correlation between Dispersion Characteristics at Terahertz Range and Dielectric Permittivity at Low Frequency of Epoxy Resin Nanocomposites

Despite the extensive research on nanocomposites, a fundamental theory on the interface region is still difficult to achieve. In the present paper, we chose epoxy resin and nano-SiO2, nano-SiC, nano-ZnO to prepare three kinds of nanocomposites. The dispersion characteristics at the terahertz range and dielectric permittivity at 1 Hz of epoxy resin-based nanocomposites were investigated. The reduction of the permittivity of nanocomposites at a slight filler concentration was absent at the terahertz range. The measurement results at 1 Hz show that the interaction between nano-SiO2, nano-SiC particles and epoxy resin was strong with the modification of the silane coupling agent. However, the modification of nano-ZnO particles was invalid. The Lorentz harmonic oscillator model was employed to fit the dispersion characteristics. The relevance between the damping constant and the dielectric permittivity at low frequency was established, indicating that the increase in the damping coefficient results from the restriction of the molecular chain motion by the interfacial region. The present results in this paper reveal a bright prospect of terahertz time-domain spectroscopy in establishing the theory of nanocomposite dielectric.

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.

Terahertz Spectroscopic Analysis in Protein Dynamics: Current Status

Proteins play a key role in living organisms. The study of proteins and their dynamics provides information about their functionality, catalysis and potential alterations towards pathological diseases. Several techniques are used for studying protein dynamics, e.g., magnetic resonance, fluorescence imaging techniques, mid-infrared spectroscopy and biochemical assays. Spectroscopic analysis, based on the use of terahertz (THz) radiation with frequencies between 0.1 and 15 THz (3–500 cm−1), was underestimated by the biochemical community. In recent years, however, the potential of THz spectroscopy in the analysis of both simple structures, such as polypeptide molecules, and complex structures, such as protein complexes, has been demonstrated. The THz absorption spectrum provides some information on proteins: for small molecules the THz spectrum is dominated by individual modes related to the presence of hydrogen bonds. For peptides, the spectral information concerns their secondary structure, while for complex proteins such as globular proteins and viral glycoproteins, spectra also provide information on collective modes. In this short review, we discuss the results obtained by THz spectroscopy in the protein dynamics investigations. In particular, we will illustrate advantages and applications of THz spectroscopy, pointing out the complementary information it may provide.

Comparison of Proton Acceptor and Proton Donor Properties of H2O and H2O2 in Organic Crystals of Drug-like Compounds: Peroxosolvates vs. Crystallohydrates

Two new peroxosolvates of drug-like compounds were synthesized and studied by a combination of X-ray crystallographic, Raman spectroscopic methods, and periodic DFT computations. The enthalpies of H-bonds formed by hydrogen peroxide (H₂O₂) as a donor and an acceptor of protons were compared with the enthalpies of analogous H-bonds formed by water (H₂O) in isomorphic (isostructural) hydrates. The enthalpies of H-bonds formed by H₂O₂ as a proton donor turned out to be higher than the values of the corresponding H-bonds formed by H₂O. In the case of H₂O₂ as a proton acceptor in H-bonds, the ratio appeared reversed. The neutral O∙∙∙H-O/O∙∙∙H-N bonds formed by the lone electron pair of the oxygen atom of water were the strongest H-bonds in the considered crystals. In the paper, it was found out that the low-frequency Raman spectra of isomorphous crystalline hydrate and peroxosolvate of N-(5-Nitro-2-furfurylidene)-1-aminohydantoin are similar. As for the isostructural hydrate and peroxosolvate of the salt of protonated 2-amino-nicotinic acid and maleic acid monoanion, the Raman spectra are different.

Low frequency coherent Raman spectroscopy

We revisit low frequency coherent Raman spectroscopy (LF-CRS) and present a unified theoretical background that provides consistent physical pictures of LF-CRS signal generation. Our general framework allows to compute the signal to noise ratio in the multitude of possible LF-CRS, and more generally CRS, experimental implementations both in the spectral and time domain.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Short hydrogen bonds enhance nonaromatic protein-related fluorescence

Intrinsic fluorescence of nonaromatic amino acids is a puzzling phenomenon with an enormous potential in biophotonic applications. The physical origins of this effect, however, remain elusive. Herein, we demonstrate how specific hydrogen bond networks can modulate fluorescence. We highlight the key role played by short hydrogen bonds, present in the protein structure, on the ensuing fluorescence. We provide detailed experimental and molecular evidence to explain these unusual nonaromatic optical properties. Our findings should benefit the design of novel optically active biomaterials for applications in biosensing and imaging.

Fluorescence in biological systems is usually associated with the presence of aromatic groups. Here, by employing a combined experimental and computational approach, we show that specific hydrogen bond networks can significantly affect fluorescence. In particular, we reveal that the single amino acid L-glutamine, by undergoing a chemical transformation leading to the formation of a short hydrogen bond, displays optical properties that are significantly enhanced compared with L-glutamine itself. Ab initio molecular dynamics simulations highlight that these short hydrogen bonds prevent the appearance of a conical intersection between the excited and the ground states and thereby significantly decrease nonradiative transition probabilities. Our findings open the door to the design of new photoactive materials with biophotonic applications.

Population difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses

We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the [Formula: see text] molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.

Solid-State Analysis of Alpha-Cyclodextrin Inclusion Complexes Using Low-Frequency Raman Spectroscopy

A rapid and nondestructive analytical technique is critical for the analysis of cyclodextrin inclusion complexes in solid dosage forms. This study proposed a newly developed low-frequency Raman spectroscopy as a candidate technique for the analysis of cyclodextrin inclusion complexes. In this study, we selected a typical series of five crystalline cyclodextrin inclusion complexes and reported the usefulness of Raman spectroscopy for analyzing these inclusion complexes. Some inclusion complexes clearly differed from the raw materials in conventional Raman spectra. In another case, though specific differences were not observed between inclusion complexes and raw materials in conventional Raman spectra, clear differences were observed in low-frequency Raman spectra. Moreover, no characteristic differences between inclusion complexes consisting of different guest molecules were observed in conventional Raman spectra. The characteristic differences were observed only in low-frequency Raman spectra. Therefore, low-frequency Raman spectroscopy is a useful technique for solid-state analysis of crystalline inclusion complexes.

Refractive Indices of Ge and Si at Temperatures between 4–296 K in the 4–8 THz Region

Refractive indices of high resistivity Si and Ge were measured at temperatures between 4–296 K and at frequencies between 4.2–7.7 THz using a Fourier-transform spectrometer (FTS) in transmission mode. A phenomenological model of the temperature dependence of the refractive index is proposed.

Simulation and Assignment of the Terahertz Vibrational Spectra of Enalapril Maleate Cocrystal Polymorphs

The identification of crystalline drug polymorphs using terahertz vibrational spectroscopy is a powerful approach for the nondestructive and noninvasive characterization of solid-state pharmaceuticals. However, a complete understanding of the terahertz spectra of molecular solids is challenging to obtain because of the complex nature of the low-frequency vibrational motions found in the sub-3 THz (sub-100 cm^-1) range. Unambiguous assignments of the observed spectral features can be achieved through quantum mechanical solid-state simulations of crystal structures and lattice vibrations utilizing the periodic boundary condition approach. The terahertz spectra of two polymorphs of enalapril maleate are presented here to demonstrate that even large pharmaceuticals can be successfully modeled using solid-state density functional theory, including cocrystalline solids comprised of multiple distinct species. These simulations enable spectral assignments to be made, but also provide insights into the conformational and cohesion energies that contribute to the polymorph stabilities. The results reveal that the Form II polymorph of enalapril maleate is the more stable of the two under ambient conditions, and that this stability is driven by a greater intermolecular cohesion energy as compared to Form I.

Probing Self-Assembly in Arginine-Oleic Acid Solutions with Terahertz Spectroscopy and X-ray Scattering

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. A mechanistic pathway to the formation of the spongelike aggregates is described.

Resolving Anharmonic Lattice Dynamics in Molecular Crystals with X-Ray Diffraction and Terahertz Spectroscopy

Molecular crystals are increasingly being used for advanced applications, ranging from pharmaceutics to organic electronics, with their utility dictated by a combination of their three-dimensional structures and molecular dynamics-with anharmonicity in the low-frequency vibrations crucial to numerous bulk phenomena. Through the use of temperature-dependent x-ray diffraction and terahertz time-domain spectroscopy, the structures and dynamics of a pair of isomeric molecular crystals exhibiting nearly free rotation of a CF_{3} functional group at ambient conditions are fully characterized. Using a recently developed solid-state anharmonic vibrational correction, and applying it to a molecular crystal for the first time, the temperature-dependent spatial displacements of atoms along particular terahertz modes are obtained, and are found to be in excellent agreement with the experimental observations, including the assignment of a previously unexplained absorption feature in the low-frequency spectrum of one of the solids.

Transformation of terahertz vibrational modes of cytosine under hydration

Cytosine and cytosine monohydrate are representative biomolecules for investigating the effect of hydrogen bonds in deoxyribonucleic acid. To better understand intermolecular interactions, such as hydrogen bonds, between nucleobases it is necessary to identify the low-frequency vibrational modes associated with intermolecular interactions and crystalline structures. In this study, we investigated the characteristic low-frequency vibrational modes of cytosine and cytosine monohydrate using terahertz time-domain spectroscopy (THz-TDS). The crystal geometry was obtained by the powder X-ray diffraction technique. The optimized atomic positions and the normal modes in the terahertz region were calculated using density functional theory (DFT), which agreed well with the experimental results. We found that overall terahertz absorption peaks of cytosine and cytosine monohydrate consist of collective vibrations mixed with intermolecular and intramolecular vibrations in mode character analysis, and that the most intense peaks of both samples involve remarkable intermolecular translational vibration. These results indicate that THz-TDS combined with DFT calculations including mode character analysis can be an effective method for understanding how water molecules contribute to the characteristics of the low-frequency vibrational modes by intermolecular vibrations with hydrogen bonding in biological and biomedical applications.

Raman and Terahertz Spectroscopic Characterization of Solid-state Cocrystal Formation within Specific Active Pharmaceutical Ingredients

Cocrystallization of specific active pharmaceutical ingredients (APIs) in the solid-state phase is becoming a feasible way to improve their corresponding physicochemical properties and ultimate bioavailability without making and breaking any covalent bonds within them. Many recent reports deal with the characterization and analysis topics of pharmaceutical APIs-based cocrystals. In this mini-review, we will focus on the recent steady-state and time-dependent spectroscopic investigation into the cocrystallization of specific APIs based on both Raman and emerging terahertz spectroscopy in pharmaceutical fields. Distinctive spectral, structural and also kinetic information of pharmaceutical APIs-based cocrystals are obtained and discussed, which would highlight the potential of vibrational spectroscopy as an attractive technique for various drug research and development during cocrystallization of specific APIs.

Time-domain terahertz compressive imaging

We present an implementation of the single-pixel imaging approach into a terahertz (THz) time-domain spectroscopy (TDS) system. We demonstrate the indirect coherent reconstruction of THz temporal waveforms at each spatial position of an object, without the need of mechanical raster-scanning. First, we exploit such temporal information to realize (far-field) time-of-flight images. In addition, as a proof of concept, we apply a typical compressive sensing algorithm to demonstrate image reconstruction with less than 50% of the total required measurements. Finally, the access to frequency domain is also demonstrated by reconstructing spectral images of an object featuring an absorption line in the THz range. The combination of single-pixel imaging with compressive sensing algorithms allows to reduce both complexity and acquisition time of current THz-TDS imaging systems.

Resolution and contrast in terahertz pulse time-domain holographic reconstruction

Here, we present a comprehensive study of the reconstruction quality in terahertz (THz) pulse time-domain holography. We look into single wavelength reconstructions, as well as broadband recovery enabled by the ultrabroadband nature of radiation and coherent detection enabled by electro-optic or photoconductive sensing. We demonstrate the transverse resolution dependence for amplitude and phase objects on the solid angle of the inline recorded time-domain THz hologram, and then turn to the contrast of reconstructed binary amplitude objects, and further to longitudinal resolution of phase objects. We show that transverse resolution can reach values comparable to the wavelength of the radiation used, and longitudinally, phase objects can be resolved with even greater precision. We compare the obtained resolution with theoretical estimates and show that THz pulse time-domain holography is a powerful non-contact imaging tool.

Probing self-associated intermolecular H-bonding using low-frequency SERS coupled with mid-IR SERS and DFT study: a case study of 2-MBA adsorbed on ZnO nanoparticles

In the present study, low-frequency SERS is reported for the first time to investigate intermolecular interactions. Low-frequency SERS in the THz region (>50 cm-1) and the mid-IR region is used to probe the H-bonding interaction in 2-mercaptobenzoic acid (2-MBA) molecules adsorbed on the surface of ZnO nanoparticles. The self-association due to H-bonding leads to dimer formation of 2-MBA through carboxylic acid groups. The characteristic Raman bands of the 2-MBA dimer, H-bonded O-H stretching and out-of-plane O-H bending modes, are observed. Subsequently, this dimer formation causes the evolution of two new low-frequency modes at 90 cm-1 (shear dimer in-plane bending) and 110 cm-1 (shear dimer stretching) of intermolecular H-bonding and a blue-shift of the torsional mode of (-COOH) + (-SH). In the THz region (50-200 cm-1) the vibrational modes are blue-shifted, while in the mid-IR region the symmetric out-of-plane O-H bending is red-shifted. The present work shows that SERS can be used to study intermolecular H-bonding of molecules at very low concentrations.

Increase in terahertz radiation power of plasmonic photoconductive antennas by embedding buried three-stepped rods in electrodes

In this work, a periodic structure with three-stepped metallic rods is designed for increasing terahertz (THz) radiation power of plasmonic photoconductive antennas (PCAs). Embedding this structure in electrodes of PCAs increases power of incident laser beam at 0.8 µm wavelength transmitted into the LT-GaAs substrate from 35% in a plasmonic PCA with rectangular rods to approximately 100%. Obtained analytical results in this study along with measurement results, reported in the literature, confirm that generated THz current of plasmonic PCA is increased up to 82% by using the buried three-stepped rods instead of rectangular rods. Consequently, THz radiation power is enhanced by 226%. Therefore THz radiation power of conventional plasmonic PCAs can be effectively enhanced by the designed buried three-stepped metallic rods.

Low frequency Raman Spectroscopy for micron-scale and in vivo characterization of elemental sulfur in microbial samples

Elemental sulfur (S(0)) is an important intermediate of the sulfur cycle and is generated by chemical and biological sulfide oxidation. Raman spectromicroscopy can be applied to environmental samples for the detection of S(0), as a practical non-destructive micron-scale method for use on wet material and living cells. Technical advances in filter materials enable the acquisition of ultra-low frequency (ULF) Raman measurements in the 10–100 cm⁻¹ range using a single-stage spectrometer. Here we demonstrate the potency of ULF Raman spectromicroscopy to harness the external vibrational modes of previously unrecognized S(0) structures present in environmental samples. We investigate the chemical and structural nature of intracellular S(0) granules stored within environmental mats of sulfur-oxidizing γ-Proteobacteria (Thiothrix). In vivo intracellular ULF scans indicate the presence of amorphous cyclooctasulfur (S₈), clarifying enduring uncertainties regarding the content of microbial sulfur storage globules. Raman scattering of extracellular sulfur clusters in Thiothrix mats furthermore reveals an unexpected abundance of metastable β-S₈ and γ-S₈, in addition to the stable α-S₈ allotrope. We propose ULF Raman spectroscopy as a powerful method for the micron-scale determination of S(0) structure in natural and laboratory systems, with a promising potential to shine new light on environmental microbial and chemical sulfur cycling mechanisms.

Unusual terahertz waveforms from a resonant medium controlled by diffractive optical elements

Up to now, full tunability of waveforms was possible only in electronics, up to radio-frequencies. Here we propose a new concept of producing few-cycle terahertz (THz) pulses with widely tunable waveforms. It is based on control of the phase delay between different parts of the THz wavefront using linear diffractive optical elements. Suitable subcycle THz wavefronts can be generated via coherent excitation of nonlinear low-frequency oscillators by few-cycle optical pulses. Using this approach it is possible to shape the electric field rather than the slow pulse envelope, obtaining, for instance, rectangular or triangular waveforms in the THz range. The method is upscalable to the optical range if the attosecond pump pulses are used.

Characterization of the Solid Physical State of API and Its Distribution in Pharmaceutical Hot Melt Extrudates Using Terahertz Raman Imaging

In this study, a method employing THz Raman imaging to characterize the solid state of acetaminophen within a solid dispersion prepared by hot melt extrusion was reported. The peak at 89 cm^-1 originating from the crystalline lattice vibrations provides unambiguous discrimination between crystalline and amorphous N-acetyl-para-aminophenol (APAP; acetaminophen) contents in the hot melt extrusion (HME) extrudates. Extrudates from four different HME processes (two different temperatures and two levels of screw speeds) were analyzed and compared. The results show that both high process temperature and high screw speed favor the formation of amorphous APAP solid dispersions. Finally, the high spatial resolution (~ 1 μm) Raman images of the extrudates prepared at 170°C and 200 rpm show a near-complete amorphous APAP dispersion in an HPMC matrix, confirming an efficient HME process. The study demonstrates that THz Raman imaging is ideally suited for the identification of different solid physical states of the APIs in a polymer matrix, and provides direct visualization of their distribution in HME extrudates.

Chiral Purity of Crystals Using Low-Frequency Raman Spectroscopy

The pharmaceutical industry is in need of new techniques to identify the chirality of solids due to regulatory and safety concerns regarding the biological activity of enantiomers. In this study, we present for the first time the application of low-frequency Raman spectroscopy as a new and sensitive method for analyzing the chiral purity of crystals. Using this method, we were able to identify small amounts, as low as 1 % w/w, of an enantiomer in racemic crystals. To demonstrate the capabilities of the method, we used a model system based on chiral crystals of enantiopure, racemic crystals and their mixtures in various ratios. We found that the low-frequency Raman spectra of racemic and enantiopure crystals are significantly different, reflecting the different hydrogen bond networks. Moreover, a comparison of the sensitivity of enantiomeric excess in chiral crystals to that of circular dichroism and X-ray diffraction measurements showed that low-frequency Raman attains high sensitivity comparable to chiral optical methods used for solutions. Overall, our proposed approach of using Raman spectroscopy for determining enantiomeric excess in crystals is simple, fast, and offers a high degree of chiral sensitivity.

Relationship between electron-phonon interaction and low-frequency Raman anisotropy in high-mobility organic semiconductors

Recent theoretical studies have shown that charge transport in high-mobility organic semiconductors is limited by low-frequency vibrations because of strong non-local electron-phonon interaction. Here we investigate two high-electron-mobility organic semiconductors with similar molecular structures but considerably different crystal packings, TCNQ and F2-TCNQ, and reveal the relationship between the experimental low-frequency Raman spectra and the calculated contributions of various vibrational modes to the electron-phonon interaction. We suggest that the combination of Raman spectroscopy with solid-state DFT is a powerful tool for probing electron-phonon interaction and focused search for high-mobility organic semiconductors.