Molecular Properties of 3d and 4f Coordination Compounds Deciphered by Raman Optical Activity Spectroscopy.

Molecular properties of coordination compounds can be efficiently studied by vibrational spectroscopy. The scope of Raman spectroscopy has been greatly enhanced by the introduction of Raman optical activity (ROA) sensitive to chirality. The present review describes some of its recent applications to study the coordination compounds. 3d and 4f metal complexes often absorb the excitation light, or exhibit luminescence. Therefore, effects caused in ROA spectra by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) must be taken into consideration.In 3d metal complexes ECD and circularly-polarized Raman scattering compete with the resonance ROA (RROA) signal. Pure RROA spectrum can thus be obtained by subtracting the so-called ECD-Raman component. CPL is frequently encountered in 4f systems. While it can mask the ROA spectra, it is useful to study molecular structure. These electronic effects can be reduced by using near-infrared excitation although vibrational ROA signal is much weaker compared to the usual green laser excitation scenario. The ROA methodology is thus complex, but capable of providing unique information about the molecules of interests and their interaction with light.

Aggregation-aided SERS: Selective detection of arsenic by surface-enhanced Raman spectroscopy facilitated by colloid cross-linking.

Arsenic of natural or industrial origin often occurs in water and makes it impotable. Due to its high toxicity, very sensitive detection is required. In the present study an ultra-sensitive arsenite (As3+) sensing is reported, based on aggregation-aided surface-enhanced Raman scattering (AA-SERS) of modified silver colloids. SERS intensity of mercapto-compounds attached to the colloidal silver nanoparticles surface is greatly increased in the presence of arsenic. Colloid aggregation is facilitated by cross-linking; a meshwork consisting of arsenic atoms and glutathione bridges is formed, as indicated by UV-Vis absorption spectroscopy, TEM and Raman imaging. The best 2-mercaptopyridine reporter molecule makes it possible to directly detect As3+ at concentrations as low as 0.5 ppb, which is better than achieved by the SERS technique so far.

Vibrational Circular Dichroism Unravels Supramolecular Chirality and Hydration Polymorphism of Nucleoside Crystals.

Invited for the cover of this issue are Valery Andrushchenko, Monika Krupová, and co-workers at the Institute of Organic Chemistry and Biochemistry (IOCB Prague) of the Czech Academy of Sciences. The image depicts a "crystal city" illuminated by "chiral suns" shining left- and right-circularly polarized light (L-CPL and R-CPL), which reveals differences in the structure of the chiral crystalline "skyscrapers". Designed by Tomás Bellon @ IOCB Prague. Read the full text of the article at 10.1002/chem.202201922.

Vibrational Circular Dichroism Unravels Supramolecular Chirality and Hydration Polymorphism of Nucleoside Crystals.

Vibrational circular dichroism (VCD) spectroscopy has been widely used to study (bio)molecules in solution. However, its solid-state applications have been restricted due to experimental limitations and artifacts. Having overcome some of them, the first VCD study of nucleoside crystals is now presented. A two-orders-of-magnitude enhancement of VCD signal was observed due to high molecular order in the crystals and resulting supramolecular chirality. This allowed to obtain high-quality VCD spectra within minutes using minute amounts of samples. The VCD technique is extremely sensitive in detecting changes in a crystal order and is able to distinguish different hydration states of crystals. This elevates it to a new level, as a fast and efficient tool to study chiral crystalline samples. This study demonstrates that VCD is capable of near-instantaneous detection of hydration polymorphs and crystal degradation, which is of substantial interest in pharmaceutical industry (quality and stability control).

Spectral counterstaining in luminescence-enhanced biological Raman microscopy.

Cell imaging heavily depends on fluorescent labels typically incompatible with Raman microscopy. The europium(iii) complex based on dipicolinic acid (DPA) presented here is an exception from this rule. Although its luminescence bands are very narrow, their intensity is comparable to the background Raman bands. This makes it complementary to less luminous compounds referred to as Raman tags. Through several examples we show that the complex provides a morphological context in otherwise unstained cells, thus acting as a spectral-counterstaining agent.

Testate amoebae examined by confocal and two-photon microscopy: implications for taxonomy and ecophysiology.

Testate amoebae (TA) are a group of free-living protozoa, important in ecology and paleoecology. Testate amoebae taxonomy is mainly based on the morphological features of the shell, as examined by means of light microscopy or (environmental) scanning electron microscopy (SEM/ESEM). We explored the potential applications of confocal laser scanning microscopy (CLSM), two photon excitation microscopy (TPEM), phase contrast, differential interference contrast (DIC Nomarski), and polarization microscopy to visualize TA shells and inner structures of living cells, which is not possible by SEM or environmental SEM. Images captured by CLSM and TPEM were utilized to create three-dimensional (3D) visualizations and to evaluate biovolume inside the shell by stereological methods, to assess the function of TA in ecosystems. This approach broadens the understanding of TA cell and shell morphology, and inner structures including organelles and endosymbionts, with potential implications in taxonomy and ecophysiology.

Correlation between off-axis illumination and apodized phase-contrast: two complementary microscopic phase-imaging modes.

Microscopic images of biological phase specimens of various optical thickness, acquired under off-axis illumination and apodized/conventional phase-contrast are compared. The luminance profiles in appropriately filtered apodized phase-contrast images compare well with those in the original off-axis illumination images. The two unfiltered image types also yield similar results in terms of quasi-three-dimensional surface (pseudo-relief) rendering, and thus are comparable in terms of the information contents (optical thickness map). However, the overall visual impression is very different as the visual cues to depth structure are present in the off-axis illumination images only. The comparison demonstrated in the present paper was made possible owing to apodization, which substantially reduces the "halo"/shade-off artifacts in the phase-contrast images. The results imply the possibility of combining the off-axis illumination and apodized phase-contrast imaging to examine specimens of medium optical thickness, in which the phase visualization capability of the two imaging modes substantially overlaps (e.g., larger cells or cell clusters).

Phase-contrast versus off-axis illumination: is a more complex microscope always more powerful?

In this article, a practical demonstration suitable for any biology college classroom is presented. With the examples of a complex biological specimen (slug's radula) and a simple reference specimen (electron microscopical grid imprint in gelatin), both of which can be easily prepared, the capabilities of two imaging modes commonly used in optical microscopy are demonstrated. The results obtained under phase contrast (a rather sophisticated method, 1953 Nobel Prize to Zernike) and off-axis illumination (a very simple method) are compared. The off-axis illumination setup is capable of delivering noticeably better microscopic images of these two particular specimens, yet it can be easily assembled in a laboratory classroom. The outcome of such a demonstration is expected to be the realization on the part of the students that one needs to carefully choose the apparatus to address a given biological problem, with the "bottom line" being that a more complex one may not necessarily yield better results. An attempt to explain this "paradox" is presented, in the particular case presented here, partly from the physiology of vision perspective (the shape-from-shading problem). The overall aim of the present article is to induce in students critical thinking about the capabilities of a laboratory equipment in general and about data interpretation.

An efficient way of high-contrast, quasi-3D cellular imaging: off-axis illumination.

An imaging system enabling a convenient visualisation of cells and other small objects is presented. It represents an adaptation of the optical microscope condenser, accommodating a built-in edge (relief) diaphragm brought close to the condenser iris diaphragm and enabling high-contrast pseudo-relief (quasi-3D) imaging. The device broadens the family of available apparatus based on the off-axis (or anaxial, asymmetric, inclined, oblique, schlieren-type, sideband) illumination. The simplicity of the design makes the condenser a user-friendly, dedicated device delivering high-contrast quasi-3D images of phase objects. Those are nearly invisible under the ordinary (axial) illumination. The phase contrast microscopy commonly used in visualisation of phase objects does not deliver the quasi-3D effect and introduces a disturbing 'halo' effect around the edges. The performance of the device presented here is demonstrated on living cells and tissue replicas. High-contrast quasi-3D images of cell-free preparations of biological origin (paper fibres and microcrystals) are shown as well.