Cholesterol Effect on the Specific Capacitance of Submicrometric DOPC Bilayer Patches Measured by in-Liquid Scanning Dielectric Microscopy

Raman scattering in protonated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC): Indicators of conformational and lateral orders

The use of deuterocarbons is an effective method in the Raman spectroscopy of multicomponent lipid materials and biological samples. Here, Raman spectra of hydrated multilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), its deuterated analog 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC_d62), and DPPC-DPPC_d62 mixtures were studied in a wide temperature range to specify the Raman indicators of conformational and lateral orders. The temperature dependence of the 985 cm^-1 line in the deuterated phospholipid unequivocally indicates that this line corresponds to the CC stretching vibrations of deuterated hydrocarbon chains in the all-trans conformation. It was also concluded that the ratio of Raman intensities at the maximum of the peak of the symmetric CD₂ stretching and at a maximum near 2168 cm^-1 reflects the conformational order of the hydrocarbon chain and can be used for an evaluation of the fraction of the all-trans sequences. The frequency of the symmetric CD₂ stretching peak is sensitive to the phase state (gel or fluid) but has a low sensitivity to the partial conformational disordering within the gel phase. The Raman study of DPPC-DPPC_d62 mixtures reveals that the lateral order contributes to the ratio of intensities of the antisymmetric and symmetric CH₂ stretching peaks as a prefactor enhancing the effect of conformational ordering.

Action of Mechanical Forces on Polymerization and Polymers

In this review, we summarize recent developments in the field of the mechanochemistry of polymers. The aim of the review is to consider the consequences of mechanical forces and actions on polymers and polymer synthesis. First, we review classical works on chemical reactions and polymerization processes under strong shear deformations. Then, we analyze two emerging directions of research in mechanochemistry—the role of mechanophores and, for the first time, new physical phenomena, accompanying external impulse mechanical actions on polymers. Mechanophores have been recently proposed as sensors of fatigue and cracks in polymers and composites. The effects of the high-pressure pulsed loading of polymers and composites include the Dzyaloshinskii–Moriya effect, emission of superradiation and the formation of metal nanoparticles. These effects provide deeper insight into the mechanism of chemical reactions under shear deformations and pave the way for further research in the interests of modern technologies.

Electrical properties of outer membrane extensions from Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 is a metal-reducing bacterium that is able to exchange electrons with solid-phase minerals outside the cell. These bacterial cells can produce outer membrane extensions (OMEs) that are tens of nanometers wide and several microns long. The capability of these OMEs to transport electrons is currently under investigation. Tubular chemically fixed OMEs from S. oneidensis have shown good dc conducting properties when measured in an air environment. However, no direct demonstration of the conductivity of the more common bubble-like OMEs has been provided yet, due to the inherent difficulties in measuring it. In the present work, we measured the electrical properties of bubble-like OMEs in a dry air environment by Scanning Dielectric Microscopy (SDM) in force detection mode. We found that at the frequency of the measurements (∼2 kHz), OMEs show an insulating behavior, with an equivalent homogeneous dielectric constant ε_OME = 3.7 ± 0.7 and no dephasing between the applied ac voltage and the measured ac electric force. The dielectric constant measured for the OMEs is comparable to that obtained for insulating supramolecular protein structures (ε_protein = 3-4), pointing towards a rich protein composition of the OMEs, probably coming from the periplasm. Based on the detection sensitivity of the measuring instrument, the upper limit for the ac longitudinal conductivity of bubble-like OMEs in a dry air environment has been set to σ_OME,ac < 10^-5 S m^-1, a value several orders of magnitude smaller than the dc conductivity measured in tubular chemically fixed OMEs. The lack of conductivity of bubble-like OMEs can be attributed to the relatively large separation between cytochromes in these larger OMEs and to the suppression of cytochrome mobility due to the dry environmental conditions.

Fast Label-Free Nanoscale Composition Mapping of Eukaryotic Cells Via Scanning Dielectric Force Volume Microscopy and Machine Learning

Mapping the biochemical composition of eukaryotic cells without the use of exogenous labels is a long-sought objective in cell biology. Recently, it has been shown that composition maps on dry single bacterial cells with nanoscale spatial resolution can be inferred from quantitative nanoscale dielectric constant maps obtained with the scanning dielectric microscope. Here, it is shown that this approach can also be applied to the much more challenging case of fixed and dry eukaryotic cells, which are highly heterogeneous and show micrometric topographic variations. More importantly, it is demonstrated that the main bottleneck of the technique (the long computation times required to extract the nanoscale dielectric constant maps) can be shortcut by using supervised neural networks, decreasing them from weeks to seconds in a wokstation computer. This easy-to-use data-driven approach opens the door for in situ and on-the-fly label free nanoscale composition mapping of eukaryotic cells with scanning dielectric microscopy.

Dielectric properties and lamellarity of single liposomes measured by in-liquid scanning dielectric microscopy

Liposomes are widely used as drug delivery carriers and as cell model systems. Here, we measure the dielectric properties of individual liposomes adsorbed on a metal electrode by in-liquid scanning dielectric microscopy in force detection mode. From the measurements the lamellarity of the liposomes, the separation between the lamellae and the specific capacitance of the lipid bilayer can be obtained. As application we considered the case of non-extruded DOPC liposomes with radii in the range ~ 100-800 nm. Uni-, bi- and tri-lamellar liposomes have been identified, with the largest population corresponding to bi-lamellar liposomes. The interlamellar separation in the bi-lamellar liposomes is found to be below ~ 10 nm in most instances. The specific capacitance of the DOPC lipid bilayer is found to be ~ 0.75 µF/cm² in excellent agreement with the value determined on solid supported planar lipid bilayers. The lamellarity of the DOPC liposomes shows the usual correlation with the liposome's size. No correlation is found, instead, with the shape of the adsorbed liposomes. The proposed approach offers a powerful label-free and non-invasive method to determine the lamellarity and dielectric properties of single liposomes.

Dielectric Imaging of Fixed HeLa Cells by In-Liquid Scanning Dielectric Force Volume Microscopy

Mapping the dielectric properties of cells with nanoscale spatial resolution can be an important tool in nanomedicine and nanotoxicity analysis, which can complement structural and mechanical nanoscale measurements. Recently we have shown that dielectric constant maps can be obtained on dried fixed cells in air environment by means of scanning dielectric force volume microscopy. Here, we demonstrate that such measurements can also be performed in the much more challenging case of fixed cells in liquid environment. Performing the measurements in liquid media contributes to preserve better the structure of the fixed cells, while also enabling accessing the local dielectric properties under fully hydrated conditions. The results shown in this work pave the way to address the nanoscale dielectric imaging of living cells, for which still further developments are required, as discussed here.