No abstract

We report on the preparation and structural characterization of powder samples of the perovskite‐slab layered polycrystalline Sr2Nb2O7 (hereafter named SNO) ferroelectric compound. Comparison is performed on samples grown without (SNO) and with an applied electric field of 3.34 kV/cm (SNOE). The Raman effect produced by these samples as a function of temperature, from 27°C (room temperature) to 400°C is presented and discussed. Significant differences are observed and discussed among SNO and SNOE. The electric field promotes the growth of platelets, preferentially oriented along the (0b0) planes. The temperature coefficients of 35 phonon wavenumbers were determined for both samples. A majority of the phonons exhibit monotonic wavenumber softening's with increasing temperatures, but a significant number of them show discontinuities on the wavenumber temperature slopes at two temperatures 215°C and 307 ± 5°C. The first temperature corresponds to the well‐known incommensurate phase to the ferroelectric phase transition (Phases III to II). The second temperature is indicative of an unreported phase transition for both the SNO and SNOE samples. For the sample SNOE, a phonon f7 (at 121 cm−1 at room temperature) displays a behavior that may be indicative of the existence of another ordering parameter that vanishes at 488 ± 5°C in the SNOE sample, induced by the electric field applied during the growth. The phonon broadenings with temperature are explained in terms of the Klemens model, which considers that the broadenings are due to the thermal expansion of the lattice, with a substantial contribution in magnitude from anharmonic phonon–phonon interactions.

No abstract

In the present work the temperature response of the constitutive S1 segment of the SARS-CoV-2 Spike Glycoprotein (GPS) has been studied. The intensity of the Raman bands remained almost constant before reaching a temperature of 133 °C. At this temperature a significant reduction of peak intensities was observed. Above 144 °C the spectra ceased to show any recognizable feature as that of the GPS S1, indicating that it had transformed after the denaturation process that it was subjected. The GPS S1 change is irreversible. Hence, Raman Spectroscopy (RS) provides a precision method to determine the denaturation temperature (T D ) of dry powder GPS S1. The ability of RS was calibrated through the reproduction of T D of other well studied proteins as well as those of the decomposition temperature of some amino acids (AA). Through this study we established a T D of 139 ± 3 °C for powder GPS S1 of SARS-CoV-2.

We present a combined experimental and theoretical study dedicated to analyzing the surface-enhanced Raman spectra of solutions of citrate-covered silver nanoparticles (NPs) in the presence of acetil-neuraminic acid (Neu5Ac). The Raman signals from Neu5Ac (particularly the bands located at 1002 and 1237 cm −1 ) can easily be detected for concentrations as low as 1 mg/dl, providing outstanding molecular sensing properties for our synthesized Ag-NPs. When compared to its solid phase, Neu5Ac adsorption on citrate-covered Ag particles leads to enhanced Raman intensities; many vibrational frequencies are shifted; and relative intensities undergo significant changes. These variations in the spectra complicate molecular identification, especially in mixed overlayers as the ones considered in this work. Consequently, experimental results are discussed on the basis of extensive density functional theory (DFT) calculations on model citrate-covered silver clusters with coadsorbed Neu5Ac species. Several citrate−Neu5Ac-cluster complexes are optimized, and the Raman spectra of every stable structure are calculated. The theoretical data reveal a complex interplay between the chemical nature and distribution of the adsorbates. Citrate molecules prefer to aggregate on the silver surface, leaving thus sizable regions of the Ag cluster exposed for Neu5Ac adsorption. The simulated Raman spectra of citrate−Neu5AcAg complexes show acceptable similarities with experimental SERS measurements, allowing for a clear identification of the different vibrational bands. By changing the functional groups attached to the Ag cluster it is possible to modulate the adsorption properties and Raman response of Neu5Ac. DFT calculations reveal that propanethiol adsorption on model Ag clusters allows the formation of a more uniform molecular overlayer, effectively protecting the silver surface from the surrounding media. We predict that Ag-NPs functionalized with propanethiol species are not efficient substrates for Raman detection of Neu5Ac. The previous predictions are corroborated by performing SERS measurements on propanethiolcovered Ag-NPs exposed to different concentrations of Neu5Ac from which the absence of Neu5Ac spectral features is observed. The here-used DFT scheme provides an adequate theoretical frame to predict and understand the physical origin of the Raman response of adsorbed molecules in Ag-NPs, and we expect that this predictability may extend to other metallic NPs.

In this paper, we report a fast and easy method to detect histamine dihydrochloride using gold nanostars in colloidal aqueous solution as a highly active SERS platform with potential applications in biomedicine and food science. This colloid was characterized with SEM and UV–Vis spectroscopy. Also, numerical calculations were performed to estimate the plasmonic resonance and electric field amplification of the gold nanoparticles to compare the difference between nanospheres and nanostars. Finally, aqueous solutions of histamine dihydrochloride were prepared in a wide range of concentrations and the colloid was added to carry out SERS. We found SERS amplified the Raman signal of histamine by an enhancement factor of 1 . 0 × 10 7 , demonstrating the capability of the method to detect low concentrations of this amine molecule.

We report the synthesis of MWNTs/ZnO hybrid nanostructures. A simple, affordable, chemical procedure to functionalize MWNT with ZnO nanoparticles was performed. A significant portion of the surface of MWNTs is covered with ZnO nanoparticles, such particles form highly porous spherical nodules of 50-150 nm in diameter, sizes that are in values one order of magnitude larger than similar ZnO nanonodules reported in the literature. Hence, in the self-assembled nanocomposite the ZnO exhibits a large surface to volume ratio, which is a very advantageous property for potential catalytic applications. The resultant MWNTs/ZnO nanocomposites were characterized by X-ray diffraction, scanning and high-resolution transmission electron microscopy, and UV-Vis and Raman spectroscopies. The temperature coefficient of resistance (TCR) of the nanocomposites was measured and reported. The average TCR value goes from -5.6%/K, and up to -18%/K, on temperature change intervals from 10 K to 1 K, respectively. Based on TCR results, the nanocomposite MWNTs/ZnO prepared in this work is a promising material with potential application as a bolometric sensor.

We report the efficient wet-chemical production of self-assembled gold-copper bimetallic nanoparticles (diameter of ∼2 nm) into two-dimensional flexible ribbonlike nanostructures. The direct observation of a layered arrangement of particles into nanoribbons was provided through high-resolution transmission electron microscopy and electron tomography. These nanoribbons showed photoluminesce and efficient photocatalytic activity for the conversion of 4-nitrophenol. The thermal stability of the nanoribbons was also measured by in situ heat treatment in the electron microscope, confirming that the self-assembled gold-copper nanoribbons efficiently supported up to 350 °C. The final morphology of the nanoparticles and their ability to self-assemble into flexible nanoribbons were dependent on concentration and the ratio of precursors. Therefore, these experimental factors were discussed. Remarkably, the presence of copper was found to be critical to triggering the self-assembly of nanoparticles into ordered layered structures. These results for the synthesis and stability of self-assemblies of metallic nanoparticles present a potential extension of the method to producing materials with catalytic applications.