Synthesis, Thermal Analysis, Spectroscopic Properties, and Degradation Process of Tutton Salts Doped with AgNO3 or H3BO3.

In this work, we synthesized and studied the spectroscopic properties of (NH4)2(SO4)2Y(H2O)6 (Y = Ni, Mg) crystals doped with AgNO3 or H3BO3. These crystals constitute a series of hexahydrated salts known as Tutton salts. We investigated the influence of dopants on the vibrational modes of the tetrahedral ligands NH4 and SO4, octahedral complexes Mg(H2O)6 and Ni(H2O)6, and H2O molecules present in these crystals through Raman and infrared spectroscopies. We were able to identify bands that are attributed to the presence of Ag and B dopants, as well as band shifts caused by the presence of these dopants in the crystal lattice. A detailed study of the crystal degradation processes was performed by thermogravimetric measurements, where there was an increase in the initial temperature of crystal degradation due to the presence of dopants in the crystal lattice. Raman spectroscopy of the crystal residues after the thermogravimetric measurements helped us to elucidate the degradation processes occurring after the crystal pyrolysis process.

Surface-enhanced Raman spectroscopy of one and a few molecules of acid 4-mercaptobenzoic in AgNP enabled by hot spots generated by hydrogen bonding.

The present study reports a direct approach to generate efficient hot spots using a nonresonant molecule bound to the inner part or hot spots that can increase the SERS sensitivity to obtain spectra of one and a few molecules. The 4-Mercaptobenzoic acid (4-MBA) adsorbate, connected to the Ag surface by a thiolate-Ag bonding, was able to trigger a self-assembly process of AgNP, which occurred by cooperative hydrogen bonds between the carboxylic groups of 4-MBA located in different nanoparticles when the pH was adjusted to 4. The self-assembly structure was characterized by UV-Vis spectroscopy and SERS (Surface Enhancement Raman Scattering), and DFT-based calculation of the model complex [AgNP-(4-MBA)2-AgNP] was employed to improve the understanding of the self-assembled complex formation through the comparison of calculated and experimental SERS spectra. The SERS signal of 4-MBA on AgNP above the pKa of the carboxyl group was not observed below 1 × 10-6 mol L-1 in any condition. Additionally, the SERS spectra of 4-bromobezenothiol (4-BrBT) at 5.0 × 10-7 mol L-1 had no bands assignable to 4-BrBT, reinforcing the hypothesis that the SERS intensification for 4-MBA in low concentrations (below 1 × 10-6 mol L-1) is due to the hydrogen bonding triggered self-assembly of AgNP below 4-MBA pKa. The average SERS of the 4-MBA in low surface coverage shows a mixture of structures, such as protonated and deprotonated 4-MBA, as well as a small amount of benzenethiol coming from decarboxylation of part of 4-MBA molecules. The few molecules SERS detection of 4-MBA was demonstrated experimentally and the experimental results were associated with a greater number of hot spots formed, being befitting with the generalized Mie theory simulations.

Structural characterization and plasmonic properties of manganese oxide-coated gold nanorods.

The preparation of metal@(dielectric or semiconductor) core@shell hybrid materials have been shown promising for both SERS and SEF applications due to improved stability in the presence of ions and the adsorbate compared to non-covered metallic nanoparticles. However, fine control over the thickness of the covering layer is essential to maximize the intrinsic trade-off between the plasmonic enhancement and the chemical stability improvement. Here, the preparation of manganese dioxide ultrathin layers covered gold nanorods (AuNR@MnO2) with varying thicknesses of the MnO2 layer is reported, and the characterization and evaluation of the resulting materials as SERS and SEF substrate. The MnO2 layer over the AuNR was prepared by reducing potassium permanganate by sodium oxalate in a basic medium. The AuNR@MnO2 hybrid material was characterized by UV-Vis spectroscopy, transmission electron microscopy, X-ray powder diffraction, and cyclic voltammetry. It was studied the SEF effect of the cyanine dye IR-820 excited at 785 nm with high performance for several thicknesses of the MnO2 ultrathin film. The enhancement factor increased for thicker oxide layers. The SERS effect of the IR-820 dye excited at 633 nm showed the most significant enhancement factor for thinner layers. The seemly opposite behavior of the two plasmonic effects may be assigned to the distance dependence of the electromagnetic field generated in the AuNR, which results in decreasing SERS performance. For SEF, the thinner layers resulted in the Au nanoparticles' emission quenching, so a more significant distance was necessary to observe enhancement.

Biosynthesis of silver nanoparticles using Caesalpinia ferrea (Tul.) Martius extract: physicochemical characterization, antifungal activity and cytotoxicity.

BACKGROUND: Green synthesis is an ecological technique for the production of well characterized metallic nanoparticles using plants. This study investigated the synthesis of silver nanoparticles (AgNPs) using a Caesalpinia ferrea seed extract as a reducing agent. METHODS: The formation of AgNPs was identified by instrumental analysis, including ultraviolet-visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) of the AgNPs, and surface-enhanced Raman scattering (SERS) spectra of rhodamine-6G (R6G). We studied the physicochemical characterization of AgNPs, evaluated them as an antifungal agent against Candida albicans, Candida kruzei, Candida glabrata and Candida guilliermondii, and estimated their minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) values. Lastly, this study evaluated the cytotoxicity of the AgNPs in murine L929 fibroblasts cells using an MTT assay. RESULTS: The UV-Vis spectroscopy, SERS, SEM and XRD results confirmed the rapid formation of spheroidal 30-50 nm AgNPs. The MIC and MFC values indicated the antifungal potential of AgNPs against most of the fungi studied and high cell viability in murine L929 fibroblasts. In addition, this study demonstrated that C. ferrea seed extracts may be used for the green synthesis of AgNPs at room temperature for the treatment of candidiasis.