Unusual Luminescence of Quartz from La Sassa, Tuscany: Insights on the Crystal and Defect Nanostructure of Quartz

Quartz from La Sassa (Tuscany, Italy) presents a unique luminescence related to intrinsic and extrinsic defects in the crystal lattice due to the growth mechanisms in hydrothermal conditions. The bright fluorescence under the UV lamp was apparent to collectors since the early 1970s, and it entered the literature as a reference case of yellow-luminescent quartz. Early reports present the history of the discovery, the geological context, and preliminary luminescence measurements of the quartz nodules, suggesting various activators as potentially responsible of the peculiar luminescence effects: uranyl groups (UO22+), rare earths (Tb3+, Eu3+, Dy3+, Sm3+, Ce3+) and polycyclic aromatic compounds (PAH). Here, we report a full investigation of the La Sassa material, by a multi-analytical approach encompassing cathodoluminescence optical microscopy (OM-CL), laser-induced fluorescence (LIF), wavelength resolved thermally stimulated luminescence (WR-TSL), trace elements analysis by mass spectrometry (ICP-MS) and Raman spectroscopy (RS). The results provide a significant step forward in the interpretation of the luminescence mechanisms: the main luminescent centres are identified as alkali-compensated (mainly Li+ and Na+, K+ and H+) aluminum [AlO4/M+]0 centres substituting for Si, where the recombination of a self-trapped exciton (STE) or an electron at a nonbridging oxygen hole centre (NBOHC) are active.

THE STUDY IN DIFFUSION MECHANISM BY VORONOI POLYHEDRON IN SODIUM TETRA-SILICATE MELT

Molecular dynamic simulation is carried out for Sodium tetra-silicate (NS4) melt at 1873 K and pressure of 0.1 MPa. The diffusion mechanism of Na atoms is investigated in terms of Voronoi polyhedron around network former and displacement of Na atoms between them. The simulation shows that Na atoms are not uniformly distributed through polyhedrons, but they mainly gather in nonbridging oxygen (NBO) and free oxygen (FO) polyhedrons. More than 75.22% of total Na atoms are place in NBO polyhedrons, although the number of NBO polyhedrons is only 22.27%. The two motion types give mainly contribution to Na diffusion: hopping of isolated Na atom or collective displacement. During 150 ps, the system comprises two separate regions: Na-poor regions formed by Si-O subnets and Na-rich regions formed by O2 clusters. The two regions have strongly different chemical composition, the density of Na atoms as well as motion type of Na atoms.

Investigation and Study of CaO-Borate Glass

Recent days the special interest towards Glasses is specifically due to their vast applications that they range. Glass series Calcium borate metallic glass (CaO–B2O3) has been prepared by melt quenching technique. CaO influences like density/molar volume ratio on the properties have analyzed and glass sample different modulus has found between them. The optical properties such as direct and indirect bandgap, Urbach energy, band gap, which also includes heat‐treated glasses, has studied by Fourier transform infrared spectroscopy and UV‐visible, respectively. According to this model elastic moduli increase, this is because of the CaO content increase. The Debye temperature, elastic moduli, other acoustic parameters and Poisson's ratio has acquired from experimental data. The obtained results show that the CaO enters into the glass network as a modifier by occupying increasing the nonbridging oxygen atoms (NBOs) and the interstitial spaces in the network.

Extracellular Nucleotides Selectively Induce Migration of Chondrocytes and Expression of Type II Collagen

The migration of chondrocytes from healthy to injured tissues is one of the most important challenges during cartilage repair. Additionally, maintenance of the chondrogenic phenotype remains another limitation, especially during monolayer culture in vitro. Using both the differentiated and undifferentiated chondrogenic ATDC5 cell line, we showed that extracellular nucleotides are able to increase the migration rate of chondrocytes without affecting their chondrogenic phenotype. We checked the potency of natural nucleotides (ATP, ADP, UTP, and UDP) as well as their stable phosphorothioate analogs, containing a sulfur atom in the place of one nonbridging oxygen atom in a phosphate group. We also detected P2y1, P2y2, P2y4, P2y6, P2y12, P2y13, and P2y14 mRNA transcripts for nucleotide receptors, demonstrating that P2y1 and P2y13 are highly upregulated in differentiated ATDC5 cells. We showed that ADPβS, UDPβS, and ADP are the best stimulators of migration of differentiated chondrocytes. Additionally, ADP and ADPβS positively affected the expression of type II collagen, a structural component of the cartilage matrix.

Structural Analysis of an L-Cysteine Desulfurase from an Ssp DNA Phosphorothioation System

ABSTRACT DNA phosphorothioate (PT) modification, in which the nonbridging oxygen in the sugar-phosphate backbone is substituted by sulfur, is catalyzed by DndABCDE or SspABCD in a double-stranded or single-stranded manner, respectively. In Dnd and Ssp systems, mobilization of sulfur in PT formation starts with the activation of the sulfur atom of cysteine catalyzed by the DndA and SspA cysteine desulfurases, respectively. Despite playing the same biochemical role, SspA cannot be functionally replaced by DndA, indicating its unique physiological properties. In this study, we solved the crystal structure of Vibrio cyclitrophicus SspA in complex with its natural substrate, cysteine, and cofactor, pyridoxal phosphate (PLP), at a resolution of 1.80 Å. Our solved structure revealed the molecular mechanism that SspA employs to recognize its cysteine substrate and PLP cofactor, suggesting a common binding mode shared by cysteine desulfurases. In addition, although the distance between the catalytic Cys314 and the substrate cysteine is 8.9 Å, which is too far for direct interaction, our structural modeling and biochemical analysis revealed a conformational change in the active site region toward the cysteine substrate to move them close to each other to facilitate the nucleophilic attack. Finally, the pulldown analysis showed that SspA could form a complex with SspD, an ATP pyrophosphatase, suggesting that SspD might potentially accept the activated sulfur atom directly from SspA, providing further insights into the biochemical pathway of Ssp-mediated PT modification. IMPORTANCE Apart from its roles in Fe-S cluster assembly, tRNA thiolation, and sulfur-containing cofactor biosynthesis, cysteine desulfurase serves as a sulfur donor in the DNA PT modification, in which a sulfur atom substitutes a nonbridging oxygen in the DNA phosphodiester backbone. The initial sulfur mobilization from l-cysteine is catalyzed by the SspA cysteine desulfurase in the SspABCD-mediated DNA PT modification system. By determining the crystal structure of SspA, the study presents the molecular mechanism that SspA employs to recognize its cysteine substrate and PLP cofactor. To overcome the long distance (8.9 Å) between the catalytic Cys314 and the cysteine substrate, a conformational change occurs to bring Cys314 to the vicinity of the substrate, allowing for nucleophilic attack.

Defects in nanosilica catalytically convert CO2to methane without any metal and ligand

Active and stable metal-free heterogeneous catalysts for CO2fixation are required to reduce the current high level of carbon dioxide in the atmosphere, which is driving climate change. In this work, we show that defects in nanosilica (E′ centers, oxygen vacancies, and nonbridging oxygen hole centers) convert CO2to methane with excellent productivity and selectivity. Neither metal nor complex organic ligands were required, and the defect alone acted as catalytic sites for carbon dioxide activation and hydrogen dissociation and their cooperative action converted CO2to methane. Unlike metal catalysts, which become deactivated with time, the defect-containing nanosilica showed significantly better stability. Notably, the catalyst can be regenerated by simple heating in the air without the need for hydrogen gas. Surprisingly, the catalytic activity for methane production increased significantly after every regeneration cycle, reaching more than double the methane production rate after eight regeneration cycles. This activated catalyst remained stable for more than 200 h. Detailed understanding of the role of the various defect sites in terms of their concentrations and proximities as well as their cooperativity in activating CO2and dissociating hydrogen to produce methane was achieved.

Optical Properties of La3+ NPs/Ag+ Co-Doped Zinc Borotellurite Glass

Lanthanum and silver co-doped zinc borotellurite glasses having chemical composition of [{[(TeO2)0.7(B2O3)0.3]0.7(ZnO)0.3}0.96 (La NPs)0.04]1-x(Ag2O)x with 0.02 ≤ x ≤ 0.10 molar fraction were fabricated via melt-quenching technique. X-ray diffraction (XRD) analysis confirmed the amorphous nature of the samples through the presence of broad hump instead of sharp peaks. Optical properties of the samples were determined through ultraviolet-visible spectroscopy (UV-Vis) in the range of 220 to 800 nm. Fundamental absorption edge observed in the optical absorption spectra shifts to longer wavelength as the concentration of silver in the glass system increases. Decreasing trend for indirect energy band gap and increasing trend of Urbach energy values with the increment of Ag content suggest the contribution of Ag to the formation of nonbridging oxygen. Enhanced optical properties of the prepared glass hints the possible application of the glass material as optical fiber in photonic field.

Distribution of sodium and dynamical heterogeneity in sodium silicate liquid

The structural and dynamical properties in sodium silicate liquid were investigated by molecular dynamics method. To clarify the distribution of sodium atoms in model, characteristics of simplex have been investigated. The simulation results reveal that Na2O⋅4SiO2 (NS4) liquid has a lot of simplexes with four sodium atoms inside but about half of simplexes do not have sodium. The spatial distribution of sodium is nonuniform, sodium tends to be in the nonbridging oxygen-simplexes and in larger-radius simplex. Moreover, the sodium density for nonbridging oxygen region is significantly higher than the one for Si-region. Namely, link-cluster function F[Formula: see text](r, t) has been used to clarify dynamical heterogeneity in NS4 liquid. The F[Formula: see text](r, t) for sets of random, immobile and mobile network atoms is quite different, which indicates that the dynamics of network atoms is heterogeneous. The Si–O network has the structure with two separated domains (immobile and mobile domains). These types of domain are significantly different in local microstructure, mobility of atoms and chemical composition.

Effect of basicity on cementitious activity of modified electric arc furnace steel slag

Electric arc furnace steel slag was modified by wastes in hot-stage process with the aim to improve the cementitious activity. Effect of basicity on cementitious activity of modified steel slag was investigated in this research. The modified slag was characterized by XRD, DTA and Raman spectra techniques to investigate the correlations between structural features and the cementitious activity. The results showed that modified steel slag with a basicity index of 1.67 possessed the highest activity index, 107%, due to the precipitation of Belite, high content of amorphous phase and low polymerization degree. The polymerization degree of modified slag was demonstrated by nonbridging oxygen per tetrahedrally coordinated cations value which was calculated through curve-fitted Raman spectra. The value of modified slag increased as the basicity promoted from 1.05 to 1.67, indicating a lower degree of polymerization. Modified slag with a basicity index of 1.86 possessed the poor cementitious activity mainly because of the significant decrease of glass phase content.

Simulation of structural characteristics of Mullite melt at high pressure

Molecular dynamics (MD) simulation is applied to investigate the structural characteristics of Al2O3–SiO2 system with the Al2O3 content at 60 mol.% (Mullite — 3Al2O[Formula: see text]2SiO2). MD models containing 5250 atoms in cubic box with periodic boundary conditions are constructed at 3500 K in 0–100 GPa pressure range. The Born–Mayer–Huggins potential is applied in this study. Based on the cluster analyzes and MD data visualization technique, the structural characteristics of Mullite melt, such as the short-range and intermediate-range order, the local environments of oxygen and network structure will be clarified. Under compression, the Al–O bonds are broken, leading to incorporation of Al atoms into Si–O subnet through both nonbridging oxygen (NBO) and bridging oxygen (BO), forming –Al–O–Si-network (Al–O–Si, Si–O–Al2, Si2–O–Al, Si–O–Al3, Si2–O–Al2 and Si3–O–Al). The degree of polymerization (DOP) of the silica network and alumina network increase with pressure and the DOP of SiOx-network is lower than that of AlO[Formula: see text]-network under compression. The statistics of corner, edge and face-sharing bonds between two adjacent TO[Formula: see text] units (T is Si, Al; x = 3–7) as well as characteristics of all type OT[Formula: see text] linkages (y = 2–6) are investigated in detail. Structural and compositional heterogeneities are clarified via analysis of structure and size distribution of TO[Formula: see text]-clusters.