Identification of microplastics using Raman spectroscopy: Latest developments and future prospects

Abstract

Widespread microplastic pollution is raising growing concerns as to its detrimental effects upon living organisms. A realistic risk assessment must stand on representative data on the abundance, size distribution and chemical composition of microplastics. Raman microscopy is an indispensable tool for the analysis of very small microplastics (<20 μm). Still, its use is far from widespread, in part due to drawbacks such as long measurement time and proneness to spectral distortion induced by fluorescence. This review discusses each drawback followed by a showcase of interesting and easily available solutions that contribute to faster and better identification of microplastics using Raman spectroscopy. Among discussed topics are: enhanced signal quality with better detectors and spectrum processing; automated particle selection for faster Raman mapping; comprehensive reference libraries for successful spectral matching. A last section introduces non-conventional Raman techniques (non-linear Raman, hyperspectral imaging, standoff Raman) which permit more advanced applications such as real-time Raman detection and imaging of microplastics.

Araujo, C.F.; Nolasco, M.M.; Ribeiro, A.M.P.; Ribeiro-Claro, P.J.A.


Full Article

Crystal Structure of Poly(trimethylene 2,5-furandicarboxylate) Redux – a new model supported by computational spectroscopy.

Abstract

Poly(trimethylene 2,5-furandicarboxylate) (PTF) is an emergent biobased polymer potentially able to outperform the fossil-based poly(ethylene terephthalate) counterpart. In this work, computational chemistry and vibrational spectroscopy tools are combined to elucidate the conformational preferences of PTF in both crystalline and amorphous regions. This approach departs from previous studies and leads to a new proposal for the crystal structure of this significant biobased polymer. In crystalline domains, PTF chains take on a helical conformation due to the gauche–gauche kinks present in 1,3-propanediol (PDO) segments, while 2,5-furandicarboxylate (FDCA) moieties adopt the syn–syn motif. Similarly to its counterparts, poly(ethylene 2,5-furandicarboxylate) (PEF) and poly(butylene 2,5-furandicarboxylate) (PBF), syn–syn FDCA units allow the formation of a vast array of C–H⋯O contacts between furanic hydrogens and adjacent carbonyl moieties. The proposed crystal structure of PTF consists of two-dimensional sheets of chains connected by C–H⋯O bonds, which are stacked upon one another forming π–π interactions among furanic rings. A thorough vibrational analysis of PTF’s infrared and inelastic neutron scattering intensity profiles, with identification of vibrational modes sensitive to conformation and degree of crystallinity, sets a blueprint for future studies employing vibrational spectroscopy techniques.

Catarina F. Araujo, Simão V. Pandeirada, Inês M. Oliveira, Guilherme B. Rosa, Beatriz Agostinho, Armando J. D. Silvestre, Andreia F. Sousa, Svemir Rudic, Pedro D. Vaz, Mariela M. Nolasco and Paulo Ribeiro-Claro.


Full Article

Good vibrations: understanding deep eutectic solvents through the lens of vibrational spectroscopy.

Abstract

This review highlights the variety of ways in which vibrational spectroscopy can be of use in understanding why deep eutectic solvents (DESs) behave the way they do. Throughout six themed sub-sections, selected studies are showcased to illustrate the kind of information that can be garnered using different vibrational spectroscopy techniques, warn against common pitfalls, and advise on best practices. More than 150 studies on the subject have been surveyed, encompassing a variety of techniques from those widely available, such as infrared and Raman spectroscopy, to niche tools, such as inelastic neutron scattering and two-dimensional infrared spectroscopy. The merging of computational chemistry and vibrational spectroscopy tools, arguably the most exciting partnership in the field, is discussed in a section of its own. With this polyvalent toolkit of techniques, one may assess hydrogen bond formation among DES constituents, probe their conformational preferences, determine what happens when DESs solidify (phase segregation into pure compounds or novel framework formation?) and investigate their molecular distribution at the nanoscale level (homogenous solution or nanodomain formation?). Hopefully, researchers who are starting to use these techniques will find this review a useful guide while new arrivals will be inspired by their potentialities.

Araújo, CF; Abranches, DO; Coutinho, JAP; Vaz, PD; Ribeiro-Claro, P; Nolasco, MM.


Full Article

Using Molecular Conformers in COSMO-RS to Predict Drug Solubility in Mixed Solvents.

Abstract

This work explores the impact of solute conformers on the conductor-like screening model for real solvents (COSMO-RS) solubility predictions of vanillin and ethyl vanillin in water, short alcohols, and their mixed solvents. Two major conformers of these solutes and changes with solvent polarity were experimentally established by Raman spectroscopy and further confirmed by density functional theory calculations. The COSMO-RS predictions using the individual conformers show a poor description of the solubilities. Estimation with the COSMO-RS default conformer distribution gave better predictions and an intermediate behavior between the predictions obtained using each individual conformer. To further improve the description of the solubilities, the weight of each conformer was fitted to the experimental solid–liquid equilibrium data of the solute in a pure solvent at different temperatures. Better solubility predictions in ternary systems describing solubility maxima were found, suggesting a semipredictive approach to COSMO-RS. This method can predict the liquid–liquid oiling-out effect in the studied binary and ternary systems.

Cordova, I.W.; Teixeira, G.; Ribeiro-Claro, P. J. A. ; Abranches, D. O.; Pinho, S. P.; Ferreira, O.; Coutinho, J. A. P.


Full Article

Exploring asymmetry induced entropy in tetraalkylammonium–urea DES systems: what can be learned from inelastic neutron scattering.

Abstract

In this work, inelastic neutron scattering (INS) spectroscopy is used to investigate the impact of entropic factors on the behaviour of deep eutectic solvents (DES). Periodic density functional theory calculations (DFT) provide a reliable assignment of the vibrational modes of pure compounds. This assignment guides the analysis of INS spectra of binary mixtures – with particular attention to methyl torsional modes. Deviations from ideality in the mixtures of tetraalkylammonium salts with urea are readily determined through a simplified thermodynamic approach. This study reports and discusses the relationship between the cation’s asymmetry, the INS spectra of the eutectic mixture and its deviation from ideality. Contrary to the majority of systems studied so far, the deep eutectic system comprised of [N2,2,2,1]Cl and urea appears to owe its deviation from ideality to entropic rather than enthalpic factors.

C. F. Araujo, P. Ribeiro-Claro, P. D. Vaz, S. Rudić, R. A. F. Serrano, L. P. Silva, J. A. P. Coutinho, M. M. Nolasco.


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©Copyright 2024 - Computational Spectroscopy Lab - Department of Chemistry - University of Aveiro - 3810-193 Aveiro