Circularly polarized component in surface-enhanced Raman spectra

Circular Polarization-Resolved Raman Optical Activity: A Perspective on Chiral Spectroscopies of Vibrational States

Circular polarization-resolved Raman scattering methods include Raman optical activity (ROA) and its derivative─surface-enhanced Raman optical activity (SEROA). These spectroscopic modalities are rapidly developing due to their high information content, stand-off capabilities, and rapid development of Raman-active chiral nanostructures. These methods enable a direct readout of the vibrational energy levels of chiral molecules, crystals, and nanostructured materials, making it possible to study complex interactions and the dynamic interfaces between them. They were shown to be particularly valuable for nano- and biotechnological fields encompassing complex particles with nanoscale chirality that combine strong scattering and intense polarization rotation. This perspective dives into recent advancements in ROA and SEROA, their distinction from surface-enhanced Raman scattering, and the potential of these information-rich label-free spectroscopies for the detection of chiral biomolecules.

Effective fully polarizable QM/MM approaches to compute Raman and Raman Optical Activity spectra in aqueous solution

Raman and Raman Optical Activity (ROA) signals are amply affected by solvent effects, especially in the presence of strongly solute-solvent interactions such as Hydrogen Bonding (HB). In this work, we extend the fully atomistic polarizable Quantum Mechanics/Molecular Mechanics approach, based on the Fluctuating Charges and Fluctuating Dipoles force field to the calculation of Raman and ROA spectra. Such an approach is able to accurately describe specific HB interactions, by also accounting for anisotropic contributions due to the inclusion of fluctuating dipoles. To highlight the potentiality of the novel approach, Raman and ROA spectra of L-Serine and L-Cysteine dissolved in aqueous solution are computed and compared both with alternative theoretical approaches and experimental measurements.

Plasmonic Chiral Metamaterials with Sub-10 nm Nanogaps

Sub-10 nm nanogaps are enantioselectively fabricated between two nanocrescents based on nanoskiving and show tailored circular dichroism (CD) activity. The mirror symmetry of the nanostructure is broken by subsequent deposition with different azimuthal angles. Strong plasmonic coupling is excited in the gaps and at the tips, leading to the CD activity. The dissymmetry g-factor of the chiral nanogaps with 5 nm gap-width is -0.055, which is 2.5 times stronger than that of the 10 nm gap-width. Moreover, the surface-enhanced Raman scattering (SERS) performance of l/d-cysteine absorbed on chiral nanogaps manifests as the emergence of enantiospecific Raman peaks and the appearance of distinct changes in SERS intensities, which affirms that chiral nanogaps can recognize specific cysteine enantiomers via standard Raman spectroscopy in the absence of circularly polarized light source and a chiral label molecule. The sub-10 nm chiral nanogaps with tailored chiroptical responses show great potential in a class of chiral applications, such as chiral sensing, polarization converters, label-free chiral recognition, and asymmetric catalysis.

Chiral detection by induced surface-enhanced Raman optical activity

Combination of optical activity with surface-enhanced Raman scattering has been a dream of physical chemists for a long time. We report a measurement protocol based on silver colloids and aromatic linkers where chiral acids could be detected in concentrations of about 10-5 M. We explain the mechanism by binding and self-assembly of the linkers into chiral aggregates on the silver surface. Following the "sergeants-and-soldiers" principle, the chirality is determined by the relatively minor acidic component. Such detection of biologically relevant molecules may be useful when other methods, such as electronic circular dichroism, are not sensitive enough. In the future, variations of the chemical structure of the linker or other conditions are needed to provide a more specific signal allowing one to better discriminate among the optically active molecules.

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

In 2010, Tian et al. reported the development of a new, relatively sensitive method of the chemical analysis of various surfaces, including buried interfaces (for example the surfaces of solid samples in a high-pressure gas or a liquid), which makes it possible to analyze various biological samples in situ. They called their method shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). SHINERS spectroscopy is a type of surface-enhanced Raman spectroscopy (SERS) in which an increase in the efficiency of the Raman scattering is induced by plasmonic nanoparticles acting as electromagnetic resonators that locally significantly enhance the electric field of the incident electromagnetic radiation. In the case of SHINERS measurements, the plasmonic nanoparticles are covered by a very thin transparent protective layer (formed, for example, from various oxides such as SiO2, MnO2, TiO2, or organic polymers) that does not significantly damp surface electromagnetic enhancement, but does separate the nanoparticles from direct contact with the probed material and keeps them from agglomerating. Preventing direct contact between the metal plasmonic structures and the analyzed samples is especially important when biological samples are investigated, because direct interaction between the metal nanoparticles and various biological molecules (e.g., peptides) may lead to a change in the structure of those biomolecules. In this mini-review, the state of the art of SHINERS spectroscopy is briefly described.

Helicene-SPP-Based Chiral Plasmonic Hybrid Structure: Toward Direct Enantiomers SERS Discrimination

Achieving chiral plasmon response based on the combination of achiral plasmonic nanostructures with highly chiral surrounding medium represents an attractive way for creation of hybrid optically active plasmonic materials. In this work, we present an attractive design and fabrication of chiral plasmon substrates based on a surface plasmon-polariton-supported structure coupled with extremely optically active helicene enantiomers. Such approach allows us to excite chiral plasmon waves and to design optically active surface-enhanced Raman spectroscopy substrates. Its further combination with standard Raman spectroscopy makes possible enantioselective detection/recognition of optical enantiomers with detection limits below those of standard spectral techniques. The chiral optical response of new plasmonic system was observed and controlled by the optical rotation of helicenes. Without necessity of previous chiral separation or implementation of sophisticated experimental equipment, we were able to estimate the concentration of enantiomers in their mixture by using left- or right-handed chiral plasmon substrates.

Improved synthesis of concave cubic gold nanoparticles and their applications for Raman analysis of surfaces

In this contribution, we present a modification of the procedure for producing concave cubic gold (cc-Au) nanoparticles; this modification significantly increases the homogeneity of the product obtained. The synthesis of cc-Au is carried out by the slow growth of seed nanostructures in a solution containing chloroauric acid, silver nitrate, ascorbic acid and hexadecyltrimethylammonium chloride. We show that, when nanoparticles synthesized in a solution containing both chloroauric acid and copper chloride (with the molar ratio equal to ca. 10 : 1) are used as seeds (instead of seeds formed without the addition of copper), one can observe a significant increase in the homogeneity of the cc-Au nanostructures formed. The resulting cc-Au, and cc-Au@Ag nanoparticles (cc-Au covered by a nanometric layer of silver) as well, have been used as plasmonic cores in nanoresonators dedicated for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). To our knowledge, the SHINERS nanoresonators produced in this work display a homogeneity that is significantly better than that of any anisotropic SHINERS nanostructures previously synthesized without the subsequent complex process of purifying the nanoparticles. Concave cubic nanoparticles were about 5 times more efficient as electromagnetic nanoresonators than spherical nanostructures of a similar size formed from the same material.

In this contribution, we present a modification of the procedure for producing concave cubic gold (cc-Au) nanoparticles; this modification significantly increases the homogeneity of the product obtained.

Recognition of chiral zwitterionic interactions at nanoscale interfaces by chiroplasmonic nanosensors

The ability to detect chiral molecules renders plasmonic nanosensors as promising tools for the study of chirality phenomena in living systems. Using gold nanorod based plasmonic nanosensors, we investigated here typically chiral zwitterionic electrostatic (Zw-Es) and hydrogen-bonding (Hb) interactions occurring via amine and carboxylic groups at nanoscale interfaces in aqueous solutions. Our results reveal that the plasmonic circular dichroism responses of the nanosensors can have both conformational sensitivity and chiral selectivity to the interfacial molecular interactions. Such a dual function of the plasmonic nanosensors enables a new chiroptical way to differentiate between chiral Zw-Es and Hb interactions, to monitor the transformation between these two interaction forces, and particularly to recognize homochiral Zw-Es interactions in solution. Together with the surface enhanced Raman scattering (SERS) technique, this plasmonic CD based biosensing could have important values for the insightful understanding of chirality-dependent molecular recognition in biological and pharmaceutical systems.

Plasmonic nanoparticles in chemical analysis

In this review various analytical techniques utilising the plasmonic properties of silver and gold nanoparticles have been presented.

Intense chirality induction in nitrile solvents by a helquat dye monitored by near resonance Raman scattering

Chirality induction phenomena attract attention because of their relevance to intermolecular interactions encountered in living matter. Usually, such effects are weak. However, enantiomers of a [6]helquat dye were found to induce exceptionally strong chirality in several achiral solvents containing nitrile groups. This effect was observable as an intense Raman optical activity (ROA) induced in acetonitrile, acetonitrile-d3, and liquid hydrogen cyanide solvents. The observation was verified by measurement of both helquat enantiomers which provided mirror image ROA spectra. Theoretical analysis indicated that the 532 nm laser excitation light was in a near resonance with electronic transitions of the dye, which made the effect observable in very dilute solutions (1 : 200 000 helquat to nitrile ratio) and thus the phenomenon can be generally useful in analytical chemistry.

Unfolding Pathway of a Globular Protein by Surfactants Monitored with Raman Optical Activity

Protein denaturation by surfactants has received increased attention in the last years due to its implications in topics such as pharmaceutics, cosmetics, paints, or biotechnology. This phenomenon is highly dependent on the physicochemical (structural) properties of the denaturing agents. In this work, we have measured for the first time the Raman optical activity (ROA) of bovine serum albumin (BSA) in the presence of three surfactants (anionic, cationic, and neutral), which has allowed us to detect new spectroscopic insights of the protein-surfactant interaction that conventional Raman spectroscopy cannot. Our work proposes two new groups of ROA marker bands to explore the unfolding of BSA induced by surfactants, which are related to "polar" (amide I and III modes) and "apolar" (methylene bending and phenyl breathing modes) protein sections. The appearance of the former groups is related to the initial attack of the surfactant, while the second groups relate to the hydrophobic unfolding.

Theoretical Modeling of the Surface-Enhanced Raman Optical Activity

Surface-enhanced Raman optical activity (SEROA) is a new technique combining the sensitivity of the surface-enhanced Raman scattering (SERS) with the detailed information about molecular structure provided by the chiral spectroscopies. So far, experimental SEROA spectra have been reported in several studies, but the interpretation and theoretical background are rather limited. In this work, general expressions for the electromagnetic contribution to SEROA are derived using the matrix polarization theory and used to investigate the enhancement in model systems. The results not only reveal a strong dependence of the enhancement on the distance between the molecule and a metal part but also the dependence of the ratio of ROA and Raman intensities (circular intensity difference, CID) on the distance and rotational averaging. For a ribose model, an optimal molecule-colloid distance was predicted which provided the highest CIDs. However, the CID maximum disappeared after a rotational averaging. For cysteine zwitterion, the simulated SEROA and SERS spectra provided a qualitative agreement with previous experiments.