Temperature-induced valence transition and associated lattice collapse in samarium fulleride.

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-T(c)) copper oxide superconductors. Metal fulleride salts-metal intercalation compounds of C60--and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system--they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + epsilon) + to nearly 2 +. The negative thermal expansion--heating from 4.2 to 32 K leads to contraction rather than expansion--occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm(2+) towards the smaller Sm((2+epsilon)+), analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

Elucidation of one step synthesis of PEGylated CuFe bimetallic nanoparticles. Antimicrobial activity of CuFe@PEG vs Cu@PEG.

There is a growing field of research on the physicochemical properties of bimetallic nanoparticles (BMNPs) and their potential use in different applications. Meanwhile, their antimicrobial activity is scarcely reported, although BMNPs can potentially achieve unique chemical transformations and synergetic effects can be presented. Towards this direction a reproducible simple hybrid polyol process under moderate temperature solvothermal conditions has been applied for the isolation of non-oxide contaminated bimetallic CuFe nanoparticles (NPs). 1,2-propylene glycol (PG), tetraethylene glycol (TEG) and polyethylene glycol (PEG 8000), that exhibit different physicochemical properties, have been utilized to regulate the size, structure, composition and the surface chemistry of NPs. The BMNPs were found to be of small crystalline size, 30-45nm, and high hydrophilicity, different wt% percentage of organic coating and variable hydrodynamic size and surface charge. The antimicrobial activity of the BMNPs was evaluated against the bacterial strains B. subtilis, E. coli and fungus S. cerevisiae. The IC50 values for CuFe NPs were found significantly lower compared with Cu NPs of the same size, revealing an enhancement in the antimicrobial activity when iron and copper coexist in the crystal structure. The reactive oxygen species (ROS) production was measured intracellularly and extracellularly by the nitroblue tetrazolium assay in the fungal cultures. No extracellular ROS were measured suggesting that both CuFe and Cu NPs enter the fungal cells during the incubation, also verified by optical imaging of the fungal cells in the presence of NPs. Higher ROS concentrations were generated intracellularly for CuFe NPs supporting different red/ox reaction mechanisms.

Hetero-nanocomposites of magnetic and antifungal nanoparticles as a platform for magnetomechanical stress induction in Saccharomyces cerevisiae.

Copper(i) oxide (Cu2O) nanoparticles (NPs) of 30 nm with antifungal properties have been functionalized with 9 nm nickel ferrite (NiFe2O4) magnetic nanoparticles (MNPs) to construct hetero-nanocomposites (NCs) of a submicron hydrodynamic size for magnetomechanical stress induction in the yeast, Saccharomyces cerevisiae. A post-synthetic approach involving the assembly through hydrophobic interactions of the preformed NPs of non-uniform sizes, albeit coated with the same surfactant (oleylamine), is reported. Solvents of different polarity were implemented during the synthetic procedure resulting in NCs of similar composition consisting mainly of MNPs randomly decorated onto the bigger Cu2O NPs. The antifungal properties of the building NPs and the NCs were studied in terms of fungistatic and fungicidal activity, whereas the ionic leaching was found to be negligible, highlighting the nanosize effect. Although S. cerevisiae cells were found to be resistant to individual NiFe2O4 MNPs because of their small size, their sensitivity to NCs significantly increased upon short-time exposure to a rotating low-frequency magnetic field (10 min, 30 Hz, 35 G) and this arises from the collective properties. The magnetomechanical cell stress induction was accompanied by alteration of cellular membrane integrity and programmed cell death signaling.

Hydrothermal synthesis of copper based nanoparticles: antimicrobial screening and interaction with DNA.

Copper based nanoparticles (Cu-based NPs) of different compositions and sizes have been hydrothermally synthesized by varying the reaction time in the presence of the biocompatible surfactants polyoxyethylene (20) sorbitan laurate (Tween 20) and polyethylene glycol 8000 (PEG 8000). Effective control of the above synthetic parameters gave rise to Cu, Cu2O and Cu/Cu2O NPs of 10-44 nm. The antibacterial activity of the NPs was screened against Gram-positive (Bacillus subtilis, Bacillus cereus, Staphylococcus aureus) and Gram-negative (Xanthomonas campestris, Escherichia coli) bacteria. The Cu-based NPs induce pDNA degradation in a dose-dependent manner as well as extensive ds CT-DNA degradation. Cu2O NPs of 16 nm and 12 nm exhibit the lowest IC50 values (2.13 µg/mL and 3.7 µg/mL) against B. cereus and B. subtilis, respectively. The agarose gel electrophoresis of ds CT-DNA treated with Cu2O NPs demonstrated degradation at high concentration. In lower concentrations, viscosity measurements indicated groove binding. In regard to the enhanced antibacterial effect and specificity of Cu2O NPs against the Gram-positive strains, the activity pathway was further explored and ROS production and lipid peroxidation verified. The released copper ions 5.15 mg/L in distilled water and 16.32 mg/L in nutrient medium, found below the critical value to inhibit bacterial growth and thus nanosized composition effect is predominant.

Raman spectroscopic study of the rare-earth fullerides Eu6-xSrxC60.

We present Raman spectroscopic studies of the isostructural and isoelectronic Eu(6-x)Sr(x)C(60) (x = 0, 3, 5, 6) and Ba(6)C(60) compounds. The Raman spectra of the Eu-based fullerides show dramatic changes compared to the pure alkaline-earth systems, including significant broadening, splitting and frequency shifts of the fivefold degenerate H(g) intramolecular modes of C(60). Moreover, the A(g)(2) mode exhibits an even larger downshift and a remarkable broadening. These findings are consistent with distortions of the C(60) molecular cages and a considerable electron-phonon coupling strength-strongly enhanced in the Eu containing systems-originating from the strong orbital hybridization between the metal atom and the C(60) molecule.

Lattice collapse in mixed-valence samarium fulleride Sm(2.75)C(60) at high pressure.

Sm(2.75)C(60) displays large negative thermal expansion behaviour in the temperature range 4.2-32 K at ambient pressure as a result of a quasi-continuous valence change from the larger Sm(2+) towards the smaller Sm(2.3+) ion. Here we use the powder synchrotron X-ray diffraction technique at ambient temperature and elevated pressures to study the compression behaviour in the pressure range 0-6 GPa. An abrupt hysteretic phase transition, accompanied by a dramatic volume decrease (approximately =6.0%) and a change in colour from black to golden was found at approximately =4 GPa induced by a sudden Sm valence transition from +2.3 towards +3. Such behaviour is typical of highly correlated Kondo insulators like SmS and makes Sm(2.75)C(60) the first known molecular-based member of this fascinating class of materials.