Applications of terahertz spectroscopy to pharmaceutical sciences

Flexible Terahertz Metasurface Absorbers Empowered by Bound States in the Continuum

Terahertz absorbers are crucial to the cutting-edge techniques in the next-generation wireless communications, imaging, sensing, and radar stealth, as they fundamentally determine the performance of detectors and cloaking capabilities. It has long been a pressing task to find absorbers with customizable performance that can adapt to various environments with low cost and great flexibility. Here, perfect absorption empowered by bound states in the continuum (BICs) is demonstrated, allowing for the tailoring of absorption coefficient, bandwidth, and field of view. The one-port absorbers are interpreted using temporal coupled-mode theory highlighting the dominant role of BICs in the far-field radiation properties. Through a thorough investigation of BICs from the perspective of lattice symmetry, the radiation features of three BIC modes are unraveled using both multipolar and topological analysis. The versatile radiation capabilities of BICs provide ample freedom to meet specific requirements of absorbers, including tunable bandwidth, stable performance in a large field of view, and multiband absorption using a thin and flexible film without extreme geometric demands. These findings offer a systematic approach to developing optoelectronic devices and demonstrate the significant potential of BICs for optical and photonic applications, which will stimulate further studies on terahertz photonics and metasurfaces.

THz-TDS as a PAT tool for monitoring blend homogeneity in pharmaceutical manufacturing of solid oral dosage forms: A proof-of-concept study

The process analytical technology (PAT) framework is well established and integral to facilitate process understanding, enable a transition from batch to continuous manufacturing, and improve product quality. Near-infrared (NIR) spectroscopy has been established as a standard PAT tool for many process analytical challenges, including monitoring powder blend homogeneity. However, alternative technologies for monitoring powder blending are of interest due to the importance of the blending step in manufacturing solid oral dosage forms. Terahertz time-domain spectroscopy (THz-TDS) is therefore explored in this study as an alternative tool for monitoring blend homogeneity with the potential for endpoint control in a batch blending process. Powder blends of microcrystalline cellulose (MCC) and dibasic calcium phosphate dihydrate and blends of MCC and granulated α-lactose monohydrate were investigated non-invasively at various compositions using THz-TDS in transmission mode for acquiring spectra from samples enclosed in the blending container. It was found that attenuation- and phase-related parameters acquired with THz-TDS could reliably resolve physical changes related to the homogeneity of the blend. Further evaluations revealed that changes in the bulk density of the blend, in addition to the intrinsic optical properties of the materials, played a critical role in the observed trends for both systems. In contrast, the scattering contribution of the powder was mainly crucial for the attenuation-related parameter in blends with materials of high refractive indices. Finally, THz-TDS measurements were acquired throughout a blending process mimicking a continuous acquisition. The method could follow blending dynamics and resulted in reasonable predictive errors of the content of 0.5 - 2.5 %. Relative standard deviations for high content blends (20 %) were acceptable (3 - 7 %) whereas at low contents (5 %) significantly higher values (9 - 35 %) were found. Based on these findings, THz-TDS is a feasible PAT tool for monitoring blend homogeneity and controlling high content blend processes, although precision and accuracy is considered to improve with a more suitable interface.

Terahertz Constant Velocity Flying Spot for 3D Tomographic Imaging

This work reports on a terahertz tomography technique using constant velocity flying spot scanning as illumination. This technique is essentially based on the combination of a hyperspectral thermoconverter and an infrared camera used as a sensor, a source of terahertz radiation held on a translation scanner, and a vial of hydroalcoholic gel used as a sample and mounted on a rotating stage for the measurement of its absorbance at several angular positions. From the projections made in 2.5 h and expressed in terms of sinograms, the 3D volume of the absorption coefficient of the vial is reconstructed by a back-projection method based on the inverse Radon transform. This result confirms that this technique is usable on samples of complex and nonaxisymmetric shapes; moreover, it allows 3D qualitative chemical information with a possible phase separation in the terahertz spectral range to be obtained in heterogeneous and complex semitransparent media.

Terahertz frequency-domain sensing combined with quantitative multivariate analysis for pharmaceutical tablet inspection

Near infrared (NIR) and Raman spectroscopy combined with multivariate analysis are established techniques for the identification and quantification of chemical properties of pharmaceutical tablets like the concentration of active pharmaceutical ingredients (API). However, these techniques suffer from a high sensitivity to particle size variations and are not ideal for the characterization of physical properties of tablets such as tablet density. In this work, we have explored the feasibility of terahertz frequency-domain spectroscopy, with the advantage of low scattering effects, combined with multivariate analysis to quantify API concentration and tablet density. We studied 33 tablets, consisting of Ibuprofen, Mannitol, and a lubricant with API concentration and filler particle size as the design factors. The terahertz signal was measured in transmission mode across the frequency range 750 GHz to 1.5 THz using a vector network analyzer, frequency extenders, horn antennas, and four off-axis parabolic mirrors. The attenuation spectral data were pre-processed and orthogonal partial least square (OPLS) regression was applied to the spectral data to obtain quantitative prediction models for API concentration and tablet density. The performance of the models was assessed using test sets. While a fair model was obtained for API concentration, a high-quality model was demonstrated for tablet density. The coefficient of determination (R²) for the calibration set was 0.97 for tablet density and 0.98 for API concentration, while the relative prediction errors for the test set were 0.7% and 6% for tablet density and API concentration models, respectively. In conclusion, terahertz spectroscopy demonstrated to be a complementary technique to Raman and NIR spectroscopy, which enables the characterization of physical properties of tablets like tablet density, and the characterization of API concentration with the advantage of low scattering effects.

Nanoscale biomaterials for terahertz imaging: A non-invasive approach for early cancer detection

Terahertz (THz) technology is developing a non-invasive imaging system for biosensing and clinical diagnosis. THz medical imaging mainly benefits from great sensitivity in detecting changes in water content and structural variations in diseased cells versus normal tissues. Compared to healthy tissues, cancerous tumors contain a higher level of water molecules and show structural changes, resulting in different THz absorption. Here we described the principle of THz imaging and advancement in the field of translational biomedicine and early detection of pathologic tissue, with a particular focus on oncology. In addition, although the main forte of THz imaging relies on detecting differences in water content to distinguish the exact margin of tumor, THz displays limited contrast in living tissue for in-vivo clinical imaging. In the last few years, nanotechnology has attracted attention to aid THz medical imaging and various nanoparticles have been investigated as contrast enhancements to improve the accuracy, sensitivity, and specificity of THz images. Most of these multimodal contrast agents take advantage of the temperature-dependent of THz spectrum to the conformational variation of the water molecule. We discuss advances in developing THz contrast agents to accelerate the advancement of non-invasive THz imaging with improved sensitivity and specificity for translational clinical oncology.

Non-Covalent Reactions Supporting Antiviral Development

Viruses are the current big enemy of the world's healthcare systems. As the small infector causes various deadly diseases, from influenza and HIV to COVID-19, the virus continues to evolve from one type to its mutants. Therefore, the development of antivirals demands tremendous attention and resources for drug researchers around the world. Active pharmaceutical ingredients (API) development includes discovering new drug compounds and developing existing ones. However, to innovate a new antiviral takes a very long time to test its safety and effectiveness, from structure modeling to synthesis, and then requires various stages of clinical trials. Meanwhile, developing the existing API can be more efficient because it reduces many development stages. One approach in this effort is to modify the solid structures to improve their physicochemical properties and enhance their activity. This review discusses antiviral multicomponent systems under the research phase and has been marketed. The discussion includes the types of antivirals, their counterpart compound, screening, manufacturing methods, multicomponent systems yielded, characterization methods, physicochemical properties, and their effects on their pharmacological activities. It is hoped that the opportunities and challenges of solid antiviral drug modifications can be drawn in this review as important information for further antiviral development.

Development of Inline Near-Infrared Spectroscopy Method for Real-Time Monitoring of Blend Uniformity of Direct Compression and Granulation-Based Products at Commercial Scales

Blending is a critical intermediate unit operation for all solid oral formulations. For blend uniformity testing, API content in the blend must be quantified precisely. A detailed study was conducted to demonstrate the suitability of inline NIR (near-infrared) spectroscopy for blend uniformity testing of two solid oral formulations: existing direct compression (DC) product with a multistep blending process and granulation-based product with API granules. Both qualitative and quantitative methods were developed at a laboratory scale using statistical moving block standard deviation (MBSD) and multivariate data analysis such as principal component analysis (PCA) and partial least squares (PLS) regression. The qualitative MBSD method demonstrated that there was no need for multiple steps for the existing DC product. Hence, a simplified single-step process was developed for blending. Quantitative PLS models for blending processes of both the products were developed, validated, and successfully implemented at a commercial scale for the real-time release of blends. Results obtained from the validated model were in good agreement with the current method of sampling and chromatography.

Terahertz Spectroscopic Analysis of Lactose in Infant Formula: Implications for Detection and Quantification

Lactose plays a significant role in daily lives as a constituent of various food and pharmaceutical products. Yet, lactose intolerance conditions demand low-lactose and lactose-free products in the market. These increasing nutritional claims and labels on food products entail simple and reliable methods of analysis that can be used for meeting quality standards, nutritional claims and legal requirements. In this study, terahertz time-domain spectroscopy (THz-TDS) was employed to analyse α-lactose monohydrate qualitatively and quantitatively in food products. Both absorption spectra and absorption coefficient spectra were investigated for their prediction performance. Regression models for lactose quantification using peak area and height of the absorption peaks 0.53 and 1.37 THz were developed and assessed in infant formula samples. Satisfactory prediction results were achieved in ideal conditions with pure standards, but not in all predictions of infant formula samples. Reasons and further implications are discussed.

Substance Detection and Identification Using Frequency Doubling of the THz Broadband Pulse

We propose and discuss an effective tool for substance detection and identification using a broadband THz pulse that is based on frequency conversion near the substance absorption frequencies. With this aim, we analyze the evolution of spectral intensities at the doubled absorption frequencies in order to prove their similarity to those at which the absorption of THz pulse energy occurs. This analysis is provided for both artificial THz signals and the real signals reflected from the substances under consideration. We demonstrate the feasibility of the proposed approach in the detection and identification of substances with an inhomogeneous surface, which is the most difficult case for practice, by using the method of spectral dynamic analysis and integral correlation criteria.

Thermal and Optical Properties of Porous Nanomesh Structures for Sensitive Terahertz Bolometric Detection

Terahertz (THz) electromagnetic waves are attractive for use in nondestructive and biocompatible sensing applications. Thermal sensors are widely used for THz detection owing to the small photon energies of THz radiation, where this requires materials with low thermal conductivity and a small heat capacity to ensure the sensitive and fast operation of the sensors. In this study, we investigated the thermal and optical properties of porous nanomesh structures for sensitive THz bolometric detection. Nanometer (nm)-scale hole array structures were formed on gallium arsenide (GaAs) microelectromechanical system (MEMS) beams to improve their thermal properties. The thermal conductance of the porous MEMS beams was obtained by measuring their thermal bandwidths; it was found to decrease by as much as ~90% when the porosity (P) of the porous nanostructure was increased to ~0.69. We also measured the THz absorptance of the porous hole array structure. The results show that although the porous nanostructure has a much smaller area than the bulk material, it maintained a high coefficient of THz absorptance because the featured size was much smaller than the THz wavelength. The measured absorptance agreed well with that calculated by using the Drude model. These results demonstrate that the porous nanomesh structure is promising for developing highly sensitive THz thermal sensors.

Critical spectroscopic considerations towards reliable detection of material using terahertz time-domain spectroscopy

Terahertz (THz) time-domain spectroscopic (TDS) and imaging techniques have been recognized as important tools in recent times for non-contact and non-destructive evaluation of materials, such as, food, pharmaceuticals, and other composite materials of interest. The application of the THz-TDS technique in both material identification and quantification, however, involves the analyses of extremely complex response of the constituents of these composite materials. For a spectroscopist, therefore, it is essential to consider certain critical spectroscopic parameters while acquiring the spectroscopic data using THz-TDS. In this work, using sorbic acid, a widely used preservative in processed food as the typical sample for the spectroscopic measurements, we have systematically investigated the impact of all these critical factors on the spectroscopic identification, quantification, and repeatability of the same. We observed that any sample inhomogeneity or clusters formed inside the composite pellet of the sorbic acid mixed with Teflon during pellet preparation can lead to false spectral responses, depending on the choice of spectroscopic probing point on the sample and number of spectroscopic averages. Furthermore, we analyzed the THz-TDS acquisition in frequency-domain and noted the effect of pellet thickness and sample concentration on the resultant frequency bandwidth and absorption features. Besides THz spectroscopists, a clear understanding of these aspects addressed in this present work, will also assist material engineers in selecting optimum concentration and weight towards formulating advanced composites.

Quality-by-design in pharmaceutical development: From current perspectives to practical applications

Current pharmaceutical research directions tend to follow a systematic approach in the field of applied research and development. The concept of quality-by-design (QbD) has been the focus of the current progress of pharmaceutical sciences. It is based on, but not limited, to risk assessment, design of experiments and other computational methods and process analytical technology. These tools offer a well-organized methodology, both to identify and analyse the hazards that should be handled as critical, and are therefore applicable in the control strategy. Once implemented, the QbD approach will augment the comprehension of experts concerning the developed analytical technique or manufacturing process. The main activities are oriented towards the identification of the quality target product profiles, along with the critical quality attributes, the risk management of these and their analysis through in silico aided methods. This review aims to offer an overview of the current standpoints and general applications of QbD methods in pharmaceutical development.

Discrimination of Pharmaceutical Tablets Based on the Analysis of Solid-State Structures of Ingredients Using Terahertz Transmission Spectroscopy with the Injection-Seeded Parametric Generation Technique

A frequency-domain terahertz (THz) spectrometer that uses a tunable source, called an injection-seeded THz parametric generator, was applied to the analysis of solid-state structures of ingredients in pharmaceutical tablets, and its performance on discriminating pharmaceutical products was evaluated. The spectrometer has a dynamic range of 70 dB at 2 THz and is suitable for analyzing materials such as pharmaceutical ingredients that often have characteristic absorption peaks between 0.5 and 2.5 THz. Nine ofloxacin (racemate) and four levofloxacin (levorotatory enantiomer) tablet products commercially available in Japan were used as samples. They contain 8-12 additives in addition to the API. The sample tablets were filed down to a thickness of 1.2 mm (ofloxacin tablets) and 1.6 mm (levofloxacin tablets) to obtain transmission spectra over the wide spectral range of 0.8-2.1 THz. The absorption spectra obtained from the spectrometer were preprocessed by the second derivative; then, principal component analysis (PCA) was conducted on the results. Next, quadratic discriminant analysis (DA) was conducted on the scores of the three PCA components. The accuracy of the DA for all 13 products was 96.1%. In addition to the difference in crystal forms of the active ingredient, the small differences in the formulation were clearly discriminated using the THz absorption spectra. The spectrometer combined with data analysis shows potential for applications such as identifying pharmaceutical tablets, monitoring the stability of production processes, evaluating the stability of formulations during storage, and detecting counterfeit drugs on the market.

Bend losses in flexible polyurethane antiresonant terahertz waveguides

One important shortcoming of terahertz technology is the relative absence of convenient, flexible, and reconfigurable waveguides with low attenuation and small bend losses. While recent years have been marked by remarkable progress in lowering the impact of material losses using hollow-core guidance, such waveguides often have centimeter-scale diameter and are therefore not flexible. Here we experimentally and numerically investigate antiresonant dielectric waveguides made of thermoplastic polyurethane, a commonly used dielectric with a low Young's modulus. The hollow-core nature of antiresonant fibers leads to low transmission losses using simple structures, whereas the low Young's modulus of polyurethane makes them extremely flexible. The structures presented enable millimeter-wave manipulation in the same spirit as conventional (visible- and near-IR-) optical fibers, i.e. conveniently and reconfigurably, despite their centimeter-thick diameter. We investigate two canonical antiresonant geometries formed by one- and six-tubes, experimentally comparing their transmission, bend losses and mode profiles. The waveguides under investigation have loss below 1 dB/cm in their sub-THz transmission bands, increasing by 1 dB/cm for a bend radius of about 10 cm. We find that the six-tube waveguide outperforms its one-tube counterpart for smaller bend radii (here: 10cm); for larger bend radii, coupling to cladding tube modes can lead to a drop in transmission at specific frequencies in the six-tube waveguide that does not occur in the one-tube waveguide.

Method of Monitoring the Number of Amide Bonds in Peptides Using Near-Infrared Spectroscopy

Using near-infrared (NIR) spectroscopy, we aimed to develop a method of monitoring the increasing number of amide bonds with the elongation of the chain length of peptides. Because peptide synthesis can be monitored by evaluating the increasing number of amide bonds with dehydration occurring between amino acids, polyglycine, which has the simplest structure among polyamino acids, was studied, and the key bands whose absorption intensities increased with the elongation of the chain length, such as the bands attributed to glycine, diglycine, triglycine, and tetraglycine, were searched. The bands due to the combinations of the amide A and amide II/III modes in the region of 5000-4500 cm^-1 were revealed to be good candidates for key bands, their second derivative intensities increased as the number of amide bonds increased, regardless of pH, solvent species, and the presence of protecting groups. The number of amide bonds was evaluated by a partial least square regression using the abovementioned combination bands, and a calibration model with a high determination coefficient (≥0.99) was constructed. These results not only have demonstrated the usefulness of NIR spectroscopy as a process analytical technology tool for the process of synthesizing the peptide in a microflow reactor but also have provided basic knowledge for analyzing amide bonds in the NIR spectra of proteins, polyamino acids, polypeptides, and polyamides.

An Efficient Photomixer Based Slot Fed Terahertz Dielectric Resonator Antenna

A slot fed terahertz dielectric resonator antenna driven by an optimized photomixer is proposed, and the interaction of the laser and photomixer is studied. It is demonstrated that in a continuous wave terahertz photomixing scheme, the generated THz power is proportional to the 4th power of the surface electric field of photocondutive layer. Consequently, the optical to THz conversion efficiency of the proposed photomixer has an enhancement factor of 487. This is due to the fact that the surface electric field of the proposed photomixer with a 2D-Photonic Crystal (PhC) superstrate has been improved from 2.1 to 9.9 V/m, which represents a substantial improvement. Moreover, the electrically thick Gallium-Arsenide (GaAs) supporting substrate of the device has been truncated to create a dielectric resonator antenna (DRA) that offers a typical radiation efficiency of more than 90%. By employing a traditional coplanar strip (CPS) biasing network, the matching efficiency has been improved to 24.4%. Therefore, the total efficiency has been considerably improved due to the enhancements in the laser-to-THz conversion, as well as radiation and matching efficiencies. Further, the antenna gain has been improved to 9dBi at the presence of GaAs superstrate. Numerical comparisons show that the proposed antenna can achieve a high gain with relatively smaller dimensions compared with traditional THz antenna with Si lens.

Terahertz Spectroscopy: Encoding the Discovery, Instrumentation, and Applications toward Pharmaceutical Prospectives

Terahertz (THz) spectroscopy is an emerging field for quality control of pharmaceuticals, which uses T-waves for detection. T-waves fall in between infrared and microwave radiations while possessing some of the characteristics of both. THz spectroscopy reveals its existence in between 0.1 and 10 THz. These radiations have the ability to penetrate a broad range of non-conductive materials and it is nonionizing. The first article stating the use of THz radiations was found in late 1960 for the generation of the astronomical images. This review essentially creates attention toward different forms and instrumentation of THz spectroscopy along with the updates for timely and upbeat pharmaceutical applications. The most frequently used technique is THz-TDS which has profoundly privileged applicability for the pharmaceuticals. The existing literature of THz spectroscopy further created albeit interest to explore the applications for future implementation in concern with the pharmaceuticals. The review critically outlines here all the pharmaceutical applications of THz spectroscopy including protein analyses, crystallinity studies, evaluating tablet films and coats, medicinal aging variations, and detection of illicit drugs, along with the advantages over traditional techniques. The other side of THz spectroscopy stating limitations is also studied and taken into the note to present here. This review is a genuine attempt to quote and crucially assess the possible as well as anticipated prospectives for the pharmaceuticals. The present article will further promote the awareness, opportunities, and scientific exploration of this exciting technology as THz spectroscopy.

Application of Process Analytical Technology for Pharmaceutical Coating: Challenges, Pitfalls, and Trends

Coating process is a critical unit operation for manufacturing solid oral dosage forms. For a long time, the coating weight gain has been discerned as the most important, if not only, characteristic describing the coating quality. As the introduction of quality by design (QbD) and advancement of process analytical technology (PAT), nowadays more techniques are available to analyze other quality attributes which have been overlooked but have substantial impacts on the performance of coated products. The techniques that permit rapid and non-destructive measurements are of particular importance to improve process operation and product quality. This article reviews the analytical techniques that have been and potentially could be used as PAT tools for characterizing the quality of pharmaceutical coating product. By identifying the challenges and pitfalls encountered during PAT application, the review aims at fostering the adoption of PAT for paving the way to enhanced quality and efficiency of the coating processes.

Molecular vibration and Boson peak analysis of glucose polymers and ester via terahertz spectroscopy

Complex permittivity spectra were obtained herein by performing broadband terahertz (THz) spectroscopy on cellulose, paramylon, and paramylon ester. Absorption peaks observed for cellulose and paramylon at approximately 3 THz are attributed to hydrogen bonds. In addition, a broad absorption peak around 2 THz was observed for all the polymers, demonstrating a general feature of polymer glasses derived from weak interatomic van der Waals forces. The boson peak was observed for cellulose and paramylon ester. The boson peak frequency for cellulose nearly equaled that for glassy glucose-a unit structure of the cellulose polymer. Additionally, the insensitivity of cellulose to the polymerization degree was consistent with recent results obtained via molecular dynamics simulations. In contrast, the boson peak frequency of paramylon ester was markedly smaller than that of cellulose. These results demonstrate the importance of hydrogen bonds as determinants of the boson peak frequency.

Recognition of Pharmacological Bi-Heterocyclic Compounds by Using Terahertz Time Domain Spectroscopy and Chemometrics

In this study, we presented the concept and implementation of a fully functional system for the recognition of bi-heterocyclic compounds. We have conducted research into the application of machine learning methods to correctly recognize compounds based on THz spectra, and we have described the process of selecting optimal parameters for the kernel support vector machine (KSVM) with an additional `unknown' class. The chemical compounds used in the study contain a target molecule, used in pharmacy to combat inflammatory states formed in living organisms. Ready-made medical products with similar properties are commonly referred to as non-steroidal anti-inflammatory drugs (NSAIDs) once authorised on the pharmaceutical market. It was crucial to clearly determine whether the tested sample is a chemical compound known to researchers or is a completely new structure which should be additionally tested using other spectrometric methods. Our approach allows us to achieve 100% accuracy of the classification of the tested chemical compounds in the time of several milliseconds counted for 30 samples of the test set. It fits perfectly into the concept of rapid recognition of bi-heterocyclic compounds without the need to analyse the percentage composition of compound components, assuming that the sample is classified in a known group. The method allows us to minimize testing costs and significant reduction of the time of analysis.

Superficial and Fundamental Correspondences in the Terahertz/IR (6-15 THz) Absorption Spectra of Aspirin and Benzoic Acid

The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200-500 cm^-1 (6-15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental.

Pilot study of freshly excised breast tissue response in the 300-600 GHz range

The failure to accurately define tumor margins during breast conserving surgery (BCS) results in a 20% re-excision rate. The present paper reports the investigation to evaluate the potential of terahertz imaging for breast tissue recognition within the under-explored 300 - 600 GHz range. Such a frequency window matches new BiCMOS technology capabilities and thus opens up the opportunity for near-field terahertz imaging using these devices. To assess the efficacy of this frequency band, data from 16 freshly excised breast tissue samples were collected and analyzed directly after excision. Complex refractive indices have been extracted over the as-mentioned frequency band, and amplitude frequency images show some contrast between tissue types. Principal component analysis (PCA) has also been applied to the data in an attempt to automate tissue classification. Our observations suggest that the dielectric response could potentially provide contrast for breast tissue recognition within the 300 - 600 GHz range. These results open the way for silicon-based terahertz subwavelength near field imager design, efficient up to 600 GHz to address ex vivo life-science applications.

Terahertz spectroscopy of 2,4,6-trinitrotoluene molecular solids from first principles

We present a computational analysis of the terahertz spectra of the monoclinic and the orthorhombic polymorphs of 2,4,6-trinitrotoluene. Very good agreement with experimental data is found when using density functional theory that includes Tkatchenko–Scheffler pair-wise dispersion interactions. Furthermore, we show that for these polymorphs the theoretical results are only weakly affected by many-body dispersion contributions. The absence of dispersion interactions, however, causes sizable shifts in vibrational frequencies and directly affects the spatial character of the vibrational modes. Mode assignment allows for a distinction between the contributions of the monoclinic and orthorhombic polymorphs and shows that modes in the range from 0 to ca. 3.3 THz comprise both inter- and intramolecular vibrations, with the former dominating below ca. 1.5 THz. We also find that intramolecular contributions primarily involve the nitro and methyl groups. Finally, we present a prediction for the terahertz spectrum of 1,3,5-trinitrobenzene, showing that a modest chemical change leads to a markedly different terahertz spectrum.

Generation of high-field terahertz pulses in an HMQ-TMS organic crystal pumped by an ytterbium laser at 1030 nm

We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

A Study of the Effect of a Protein on the Structure of Water in Solution Using Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) was used to determine the spectra (range = 1.2-120 cm^-1) of aqueous solutions of bovine serum albumin (BSA) at pH range 2.5-10. Under each of the selected pH, BSA molecules exist in a different conformation, compared to other pH values. The spectra were used to calculate the functions of the dielectric permittivity of BSA solutions. Dielectric functions of the aqueous phase of BSA solutions were calculated based on the Bruggeman model, without the contribution of BSA itself. Fitting of the dielectric functions was performed using a model which includes three water spectral bands: two relaxation bands with relaxation times of about 8.28 and 0.3 ps and a vibrational band with a maximum of about 180 cm^-1. The parameters of these bands were determined through fitting and physical interpretation at the molecular level can be provided for each of them. A comparison between the values of model parameters of solutions with BSA and without BSA allowed to conclude that the main effect of BSA is the formation of strongly bound hydration shells in the immediate proximity to the protein molecule. At the same time, the structure of more distant layers of the hydration shells is destroyed, with an increased formation of free water molecules. Some differences are observed in the effect of different BSA conformations on the aqueous phase of solution. The proposed approach can be generalized and applied for studying of a wide class of biological macromolecules in aqueous solutions.

Pharmaceutical cocrystals, salts and polymorphs: Advanced characterization techniques

The main goal of a novel drug development is to obtain it with optimal physiochemical, pharmaceutical and biological properties. Pharmaceutical companies and scientists modify active pharmaceutical ingredients (APIs), which often are cocrystals, salts or carefully selected polymorphs, to improve the properties of a parent drug. To find the best form of a drug, various advanced characterization methods should be used. In this review, we have described such analytical methods, dedicated to solid drug forms. Thus, diffraction, spectroscopic, thermal and also pharmaceutical characterization methods are discussed. They all are necessary to study a solid API in its intrinsic complexity from bulk down to the molecular level, gain information on its structure, properties, purity and possible transformations, and make the characterization efficient, comprehensive and complete. Furthermore, these methods can be used to monitor and investigate physical processes, involved in the drug development, in situ and in real time. The main aim of this paper is to gather information on the current advancements in the analytical methods and highlight their pharmaceutical relevance.

Analysis of granular packing structure by scattering of THz radiation

Scattering methods are widely used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using terahertz radiation are highlighted as they probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize a spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier transform spectroscopy at the THz beamline of the Bessy II storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

Investigation of Dielectric Properties of Polymers and their Discrimination Using Terahertz Time-Domain Spectroscopy with Principal Component Analysis

Polymers are among the most commonly used materials in our everyday life. They are generally transparent to terahertz (THz) radiation, but are quite difficult to differentiate using optical techniques as few or no characteristic features exist in the spectral range of <2.0 THz for small and portable radiation systems. In this work, we report experimental measurement of refractive indices and absorption coefficients of styrene acrylonitrile (SAN) and Bakelite in the spectral range of 0.2-2.0 THz for the first time. Additionally, we demonstrate that by combining principle component analysis (PCA) with THz time-domain spectroscopy one can differentiate such polymers. In this analysis, the first three principle components PC1, PC2, and PC3 depict >94% variance with a distribution of 72.45%, 11.52%, and 9.38%, respectively.

Terahertz gas-phase spectroscopy: chemometrics for security and medical applications

We describe a spectrometer consisting of a vector network analyzer, a gas absorption cell, and a quasi-optical bench that acquires terahertz spectra of gaseous substances and mixtures. We tested volatile organic compounds that are medical biomarkers or chemicals which can be found on the US Environment Protection Agency list of harmful substances. Absorption spectra at gas pressures between 10 Pa and 5000 Pa were recorded. A subsequent multivariate data analysis demonstrated excellent qualitative and quantitative identification of pure substances and complex mixtures. The applied multivariate algorithms are principal components analysis, partial least square regression and soft independent modelling of class analogy.

A rapid Fourier-transform infrared (FTIR) spectroscopic method for direct quantification of paracetamol content in solid pharmaceutical formulations

A transmission FTIR spectroscopic method was developed for direct, inexpensive and fast quantification of paracetamol content in solid pharmaceutical formulations. In this method paracetamol content is directly analyzed without solvent extraction. KBr pellets were formulated for the acquisition of FTIR spectra in transmission mode. Two chemometric models: simple Beer's law and partial least squares employed over the spectral region of 1800-1000 cm(-1) for quantification of paracetamol content had a regression coefficient of (R(2)) of 0.999. The limits of detection and quantification using FTIR spectroscopy were 0.005 mg g(-(1) and 0.018 mg g(-1), respectively. Study for interference was also done to check effect of the excipients. There was no significant interference from the sample matrix. The results obviously showed the sensitivity of transmission FTIR spectroscopic method for pharmaceutical analysis. This method is green in the sense that it does not require large volumes of hazardous solvents or long run times and avoids prior sample preparation.

Crystallization and phase changes in paracetamol from the amorphous solid to the liquid phase

For the case of paracetamol, we show how terahertz time-domain spectroscopy can be used to characterize the solid and liquid phase dynamics. Heating of supercooled amorphous paracetamol from 295 K in a covered sample under vacuum leads to its crystallization at 330 K. First, form III is formed followed by the transformation of form III to form II at 375 K, to form I at 405 K, and finally melting is observed around 455 K. We discuss the difference between the featureless spectra of the supercooled liquid and its liquid melt. Lastly, we studied the onset of crystallization from the supercooled liquid in detail and quantified its kinetics based on the Avrami-Erofeev model. We determined an effective rate constant of k = 0.056 min(-1) with a corresponding onset of crystallization at T = 329.5 K for a heating rate of 0.4 K min(-1).

Terahertz pulsed imaging as an advanced characterisation tool for film coatings--a review

Solid dosage forms are the pharmaceutical drug delivery systems of choice for oral drug delivery. These solid dosage forms are often coated to modify the physico-chemical properties of the active pharmaceutical ingredients (APIs), in particular to alter release kinetics. Since the product performance of coated dosage forms is a function of their critical coating attributes, including coating thickness, uniformity, and density, more advanced quality control techniques than weight gain are required. A recently introduced non-destructive method to quantitatively characterise coating quality is terahertz pulsed imaging (TPI). The ability of terahertz radiation to penetrate many pharmaceutical materials enables structural features of coated solid dosage forms to be probed at depth, which is not readily achievable with other established imaging techniques, e.g. near-infrared (NIR) and Raman spectroscopy. In this review TPI is introduced and various applications of the technique in pharmaceutical coating analysis are discussed. These include evaluation of coating thickness, uniformity, surface morphology, density, defects and buried structures as well as correlation between TPI measurements and drug release performance, coating process monitoring and scale up. Furthermore, challenges and limitations of the technique are discussed.

Terahertz wavefronts measured using the Hartmann sensor principle

We demonstrate for the first time that the Hartmann wavefront sensor (HWS) principle can be applied for characterizing the wavefronts of terahertz (THz) electromagnetic radiation. The THz Hartmann wavefront sensor consists of a metallic plate with an array of holes and a two-dimensional scanable pyro-electric detector. The THz radiation with different wavefronts was generated by a far-infrared gas laser operated at 2.5 THz in combination with a number of objects that result in known wavefronts. To measure the wavefront, a beam passing through an array of holes generates intensity spots, for which the positions of the individual spot centroids are measured and compared with reference positions. The reconstructed wavefronts are in good agreement with the model expectations.