Silver nanoparticles linked by a Pt-containing organometallic dithiol bridge: study of local structure and interface by XAFS and SR-XPS.

Silver nanoparticles (AgNPs) functionalized with an organometallic bifunctional thiol containing Pt(ii) centers, generated in situ from trans-trans-[thioacetyl-bistributylphosphine-diethynylbiphenyl-diplatinum(ii)], were synthesized with different sulphur/metal molar ratios (i.e. AgNPs-1 and AgNPs-2) with the aim to obtain nanosystems of different mean size and self-organization behaviour. AgNPs spontaneously self-assemble, giving rise to 2D networks, as previously assessed. In this work a deeper insight into the chemico-physical properties of these AgNPs is proposed by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and X-ray absorption fine structure spectroscopy (XAFS) techniques. The results are discussed in order to probe the interaction at the interface between a noble metal and a thiol ligand at the atomic level and the aim of this study is to shed light on the chemical structure and self-organization details of nanosystems. The nature of the chemical interaction between the dithiol ligand and the Ag atoms on the nanoparticle surface was investigated by combining SR-XPS (S2p, Ag3d core levels) and XAS (S and Ag K-edges) analysis. UV-visible absorption and emission measurements were also carried out on all samples and compared with TD-DFT calculations so as to get a better understanding of their optical behavior and establish the nature of the excitation and emission processes.

Nature of "disorder" in the ordered double perovskite Sr2FeMoO6.

The degree of B/B;{'} alternate cation order is known to heavily influence the magnetic properties of A_{2}BB;{'}O_{6} double perovskites although the nature of such disorder has never been critically studied. Our detailed x-ray absorption fine structure studies in conjunction with synchrotron radiation x-ray diffraction experiments on polycrystalline Sr_{2}FeMoO_{6} samples with various degrees of disorder reveal that a very high degree of short range order is preserved even in samples with highly reduced long range chemical order. Based on these experimental results and with the help of detailed structural simulations, we are able to model the nature of the disorder in this important class of materials and discuss the consequent implications on its physical properties.

Local structure of Sr(2)FeMo(x)W(1-x)O(6) double perovskites across the composition-driven metal to insulator transition.

Sr(2)FeMoO(6) oxides exhibit a half-metallic ferromagnetic (HM-FM) ground state and peculiar magnetic and magnetotransport properties, which are interesting for applications in the emerging field of spintronics and attractive for fundamental research in the field of heavily correlated electron systems. Sr(2)FeWO(6) is an insulator with an antiferromagnetic (I-AFM) ground state. The solid solutions Sr(2)FeMo(x)W(1-x)O(6) also have peculiar properties-W doping enhances chemical order which allows stabilization of the HM-FM state; as the W content exceeds a certain value a metal to insulator transition (MIT) occurs. The role of W in determining the physical properties of Sr(2)FeMo(x)W(1-x)O(6) systems has been a matter of intense investigation. This work deals with the problem of the structural and electronic changes related to the MIT from a local perspective by means of x-ray absorption spectroscopy (XAS). This technique allows one to probe in detail the local structure and electronic modifications around selected absorber ions (W, Mo, Fe and Sr in our case). The results of XAS analysis in the whole composition range (0≤x≤1), in the near edge (XANES) and extended (EXAFS) regions, demonstrate an abrupt change of the local structure around the Fe and Mo sites at the critical composition, x(c). This change represents the microstructural counterpart associated with the MIT. Conversely, the local structure and electronic configuration of W ions remain unaltered in the whole composition range, suggesting indirect participation of W in the MIT.

Antiferromagnetic-paramagnetic insulating transition in Cr-doped V(2)O(3) investigated by EXAFS analysis.

We have performed extended x-ray absorption fine-structure (EXAFS) spectroscopy on a 2.8% Cr-doped V(2)O(3) sample, with the aim of studying its structural evolution in a wide temperature range across the paramagnetic-antiferromagnetic insulating phase transition at T(c). The data were registered with two different set-ups in fluorescence and transmission geometries, for polarized and unpolarized spectra, respectively. Our idea, based on previous experiments reported in the literature, is that extended structural modifications of the nominal trigonal symmetry are present in the paramagnetic insulating phase for several tens of degrees above T(c), involving further-nearest-neighbor vanadium ions. Our data confirm that the paramagnetic insulating phase is not structurally homogeneous in a temperature range of about 30 K around T(c), where local distortions of monoclinic symmetry involving further-nearest neighbors are present. Moreover, the analysis of the absorption profile at Cr K-edge suggests that Cr ions enter the lattice randomly. We finally analyze our findings in light of current theoretical models.

Thermal dependent anharmonicity effects on gold bulk studied by extended x-ray-absorption fine structure.

The structural parameters of the first five coordination shells of an Au bulk obtained from high accuracy L(3)-edge extended x-ray absorption fine structure (EXAFS) spectra in the temperature range 20-300 K are reported. Good agreement with previously reported studies is found. The effective second and third order force constants evaluated using EXAFS data are compatible with those calculated from phonon dispersion curves. A careful comparison of the variations of the EXAFS first shell distance with x-ray diffraction data provided the mean squared relative displacement of the atomic vibrations perpendicular to the first interatomic bond. An alternative new approach that is useful in achieving this parameter when x-ray diffraction data are not available is proposed.

Microarchitectural and physical changes during fetal growth in human vertebral bone.

The ossification process in human vertebra during the early stage of its formation was studied by X-ray diffraction (XRD) and X-ray microtomography (microCT) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Twenty-two samples taken from vertebral ossification centers of human fetal bone (gestational age ranging between 16 and 26 weeks) were investigated. The analysis of three-dimensional images at high spatial resolution (approximately 10 and approximately 2 microm) allows a detailed quantitative description of bone microarchitecture. A denser trabecular network was found in fetal bone compared with that of adult bone. The images evidenced a global isotropic structure clearly composed of two regions: a central region (trabecular bone) and a peripheral region (immature bone). XRD experiments evidenced hydroxyapatite-like crystalline structure in the mineral phase at any fetal age after 16 weeks. Interestingly, the analysis of XRD patterns highlighted the evolution of crystalline structure of mineralized bone as a function of age involving the growth of the hydroxyapatite crystallites.

Time-resolved X-ray powder diffraction on a three-way catalyst at the GILDA beamline.

Time-resolved X-ray diffraction experiments carried out at the beamline BM08-GILDA of ESRF allowed a study of the structural modifications taking place in a Pt/ceria-zirconia catalyst while the CO oxidation reaction was in progress. The capillary tube in which the sample is stored acts effectively as a chemical microreactor that ensures homogeneity of the sample treatments and minimization of diffusion effects. During the flowing of the reactant CO/He mixture, the investigated catalyst undergoes a fast Ce(IV)-Ce(III) partial reduction that involves the release of one O atom for every two reduced Ce cations. Because Ce(III) has a larger ionic radius than Ce(IV), the structural modification produces an increase of the lattice constant of the ceria-zirconia mixed oxide, and this increase is monitored by the translating imaging-plate device implemented at GILDA. The CO(2) resulting from the oxidation of the fluxed CO is monitored by a quadrupole mass spectrometer during the recording of the time-resolved X-ray diffraction pattern. The chemical and structural information was combined to show that the CO(2) yield is nearly constant until the catalytic system can provide oxygen for the reaction, while the structural rearrangement of the catalyst is delayed with respect to the switching on of the CO/He flux. After this induction time, during which CO(2) is produced with no structural modification of the catalyst, a fast increase of the lattice constant takes place.

Observation of the segregation and the dissolution of the Co and the Cu in CoCu metastable alloys.

Metastable Co(x)Cu100-x(x=5, 10, 15, 20) alloys have been annealed at increasing temperatures in order to study the evolution of the Co cluster and its relation with the magnetotransport properties. The structure was investigated by X-ray Absorption Spectroscopy on the Co K-edge as a function of composition and annealing temperature. An anomalous trend in the structural evolution has been evidenced and related to the preculiar features observed in the magnetotransport properties.

Curved optics for x-ray phase contrast imaging by synchrotron radiation.

Conventional radiographic techniques have strong limitations when low-absorption contrast samples are imaged. Phase contrast radiography has been shown to produce high-quality images of soft tissues. In this technique the recorded intensity patterns are related to gradients in the refractive index of the sample. A critical point of this new technique is the need to employ crystal analysers, which results in an appreciable reduction in the beam intensity and consequently in rather long exposure times. In this paper the use of focused beams is suggested to overcome this aspect. Biological samples with small structures and low absorption variations were imaged using both flat and curved monochromator crystals, demonstrating that the use of curved optics leads to a decrease in the exposure time with only a limited degradation of the spatial resolution. This opens up the possibility of using the phase contrast technique with laboratory sources.

On the coordination and oxidation states of the active-site copper ion in prokaryotic Cu,Zn superoxide dismutases.

The active-site copper ion of the prokaryotic Cu,Zn superoxide dismutase from P. leiognathi is found to undergo reversible reduction upon irradiation of the protein solution with a high-intensity X-ray beam from a third-generation synchrotron source. The same phenomenon is observed for the enzyme crystals, whose diffraction pattern has been obtained from synchrotron sources. In this case the active-site copper-ligand coordination bond lengths and in particular the Cu-NE2(His61) distance are consistent with a copper ion in the reduced state. These results are in line with previous studies on the eukaryotic Cu,Zn superoxide dismutases and suggest the conservation of an identical catalytic mechanism in both the prokaryotic and eukaryotic enzymes.

Observation of horizontal focusing of X-rays using a toothed-crystal monochromator.

The behaviour of a toothed-crystal monochromator for synchrotron radiation was studied for the first time on the GILDA CRG beamline at the ESRF. A new kind of horizontal focusing was observed which may be attributed to the properties of inclined diffraction. We have also observed a vertical focusing of the beam, the nature of which remains unexplained.

Anomalous wide-angle X-ray scattering apparatus on the GILDA beamline at the ESRF.

The experimental apparatus for anomalous wide-angle X-ray scattering (AWAXS) on the GILDA beamline at the ESRF is described. The main features are the high beam stability and reproducibility which allow anomalous scattering effects to be resolved also on dilute elements, the large spectral range which allows AWAXS experiments at the K edges of heavy elements, and the use of a high-efficiency detection system. The apparatus has been tested in extreme conditions by performing AWAXS experiments at the Eu K edge in Eu-doped Sr metaphosphate glasses.

X-ray Optics of a Dynamical Sagittal-Focusing Monochromator on the GILDA Beamline at the ESRF.

The performance of a dynamical sagittal-focusing monochromator for hard X-rays is described. It consists of a flat first crystal and a diamond-shaped ribbed second crystal which is clamped by its central rib and dynamically bent by applying a force on its two apices. The system has proved to perform very well on the GILDA beamline at the ESRF. The horizontal acceptance varies with energy and with focusing geometry as predicted theoretically; the total available horizontal fan of radiation (3.6 mrad) is in fact collected in the 1:3 geometry. The system is routinely run in a dynamical focusing mode for XAFS spectroscopy in the energy range 5-30 keV with Si(311) crystals, with a constant spot size FWHM ~1 mm on the sample and without degradation of energy resolution or reproducibility. Using simple geometrical considerations we calculate the variations of the horizontal profile of the reflected beam during rocking-curve scans in different focusing geometries and find them in agreement with observed ones. Not only is this is a practical aid in alignment but it illustrates the X-ray optics of sagittal focusing in an elegant way.

Sequence specificity of tetrahydrobenzopsoralen photobinding to DNA.

The sequence specificity of photobinding to DNA of two tetrahydrobenzopsoralen derivatives has been investigated by testing the photoreactivity toward a number of self-complementary oligonucleotides. The thermodynamic constant for noncovalent binding to each DNA sequence was evaluated. The extent of photoreactivity was greatly dependent upon base composition. The two tetracyclic compounds show similar behavior in comparison to other bifunctional derivatives. Their overall rate constants were greatly enhanced in comparison to classical psoralens. However, their high efficiency of covalent binding is counterbalanced by low affinity for noncovalent interaction with DNA.

Antiproliferative activity of some unsubstituted angular analogoues of ellipticine.

With the aim of obtaining further knowledge on the antiproliferative activity of pyrroloquinolines and isoquinolines, we prepared four unsubstituted angular pyridotetrahydrocarbazoles having a fourth non-aromatic ring, via modified Fischer synthesis. These compounds may be considered as simpler analogues of ellipticine. They induced evident antiproliferative effects in Ehrlich ascites and in CHO cells in vitro, but were ineffective on T2 bacteriophage. These compounds formed molecular complexes with DNA in vitro, while in CHO cells in vivo, they induced double-strand breaks in DNA and DNA-protein cross-links. These data suggest that these ellipticine analogoues are capable of inhibiting topoisomerase II, as the parent compound does. The most active derivative was 2N-5H-6,7,8,9-tetrahydropyrido[2,3-a]carbazole, which represents an interesting model for the study of new antitumor drugs.

Synthesis and characterization of new methylpsoralens as potential photochemotherapeutic agents.

Three new psoralens with methyl groups on carbons involved in their reactive double bonds (compounds 9-11 in Scheme 1) were synthesized from the corresponding 7-hydroxycoumarins by cyclization of acetonyl derivatives of the latter in an alkaline medium. In preliminary tests, the new methyl-substituted psoralens exhibited considerable interaction in the dark with DNA, good photoreactivity against the macromolecule, and also interesting antiproliferative activity.