Copper clusters as novel fluorescent probes for the detection and photocatalytic elimination of lead ions.

A new homogeneous assay for a fast, selective and sensitive detection and elimination of lead ions has been developed using copper clusters as novel fluorescent probes in aqueous solutions. At the same time, their elimination is achieved using the efficient photocatalytic activity of such clusters. Both effects are explained in terms of an efficient electron transfer due to the LUMO energy of the cluster overlapping with the ion redox potential. The mechanism, which allows the explanation of previously reported results for different types of metal clusters, is further confirmed using smaller and larger Cu clusters displaying non-selective and non-quenching luminescence in the presence of different cations.

Cage-like effect in Au-Pt nanoparticle synthesis in microemulsions: a simulation study.

The different distributions of metals in bimetallic nanoparticles synthesized in microemulsions were studied by computer simulation. The simulations demonstrated that if the difference between the reduction potentials of both metals is about 0.15-0.3 V, the compartmentalization of the reaction media causes the accumulation of slower reduction reactants in the microemulsions droplets, which favours the chemical reaction like a cage effect: increasing the local concentration of the slower reduction metal salt gives rise to a faster reduction, so the differences in reduction rates of both metals are attenuated. A more coincidental reduction of both metals deeply affects the nanoparticle structure, causing a better mixed alloy. This effect will be more pronounced when the concentration is higher and the intermicellar exchange rate is faster. This means that for any fixed microemulsion the nanoparticle structure can be modified by changing the reactant concentration: the core can be enriched in the faster reduction metal by lower concentrations, and the shell can be enriched in the slower metal by higher concentrations. Based on these observations, this study suggests a route to help experimentalists better create nanoparticles with a pre-defined structure.

Insight into the surface composition of bimetallic nanocatalysts obtained from microemulsions.

The enhancement of catalysts efficiency of bimetallic nanoparticles depends on the ability to exert control over surface composition. However, results relating surface composition and feeding solution of bimetallic nanoparticles synthesized in microemulsions are controversial and apparently contradictory. In order to comprehend how the resulting surface can be modified under different synthesis conditions and for different pairs of metals, a computer simulation study was carried out. The resulting surface compositions are explained based on the relative rates of deposition of the two metals, which depend on the particular metal pair, the concentration of reactants and the microemulsion composition. This study provides a satisfactory understanding of experimental results and allows us to identify the main factors affecting the nanoparticle's surface composition. Consequently, concrete and practical guidelines can be established to facilitate the experimental synthesis of bimetallic nanoparticles with tailored surfaces.

Investigation of the effect of S/In molar ratio on physical properties of sprayed In2S3 thin films.

Indium sulfide (In2S3) thin films have been synthesized on glass substrates using the spray technique (CSP). The S : In molar ratio was varied from 1 to 4 in the starting solution. The Raman analysis confirms the formation of the ß-In2S3 material and the absence of a secondary phase. The EDS analysis reveals that our layers are pure. The thin film surface is free of cracks, as observed in AFM images. Optical transmission achieved 80% in the visible and near infrared region. The refractive index (n) is affected by the changes in the S/In molar ratio. The optical parameters, single oscillator energy (E 0), dispersion energy (E d) and high frequency dielectric constant (ε ∞), are calculated via the Wemple-DiDomenico model. In addition, the photoconductivity kinetics in In2S3 films for S/In = 2 were investigated and analyzed. The I-V characteristics and the photoresponse were also studied.

Percolation threshold and scattering power law of gelatin gels.

Gelation of gelatin was broadly studied by experimental and theoretical methods. Power laws observed on the gel point-mainly obtained by dynamic light scattering (DLS)-are considered to be the signature of some special dynamic phenomena ascribed to the appearance of a percolation cluster. We present here experimental (DLS and rheometric measurements) and Monte Carlo simulation studies showing that the percolation threshold and DLS power-law decay occur on different times. We ascribe the percolation point to the time where the scattering medium mode diverges. This mode is sensitive to the clusters' growth and diverges when the system attains the percolation threshold. The power-law behavior is obtained only in the postpercolation regime.

Magnetic relaxation of gamma-Fe2O3 nanoparticles arrangements and electronic phase-segregated systems.

Gamma-Fe2O3 nanoparticles have been synthesized and dispersed in a polymeric matrix, forming a series of composites with different concentrations of magnetic particles. The effect of volume polydispersity and dipolar interactions on the relaxation behavior is discussed. We have paid special attention to the dynamic approach to discuss a possible true superspin-glass transition in highly concentrated composites. To avoid the practical limitations that appear in highly concentrated systems of particles, like the formation of aggregates, etc., we have studied the glassy phase that appears spontaneously in certain strongly electronic correlated materials close to a metal-insulator transition. It must be emphasized that from a theoretical point of view these inhomogenous magnetic states could present important advantages over classical dispersions of particles, like field-control of the effective particle size. The results are compared with other recently obtained for classical systems of particles.

Novel synthetic routes of large-pore magnetic mesoporous nanocomposites (SBA-15/Fe3O4) as potential multifunctional theranostic nanodevices.

In this paper, novel magnetic silica nanocomposites were prepared by anchoring magnetite nanoparticles onto the outer surface of mesoporous SBA-15 silica; the magnetic nanoparticles were prepared by microemulsion and solvothermal methods, varying the synthesis conditions in order to control the final physicochemical, textural and magnetic properties. The morphology and mesostructure of the materials were characterized by X-ray diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR), N2 adsorption-desorption, and Transmission and Scanning Electron Microscopy (TEM and SEM). Magnetic silica nanocomposites feature a two-dimensional hexagonal arrangement constituted by a homogeneous pore channel system with diameters between 13 and 18 nm and a Brunauer-Emmett-Teller (BET) surface area higher than 260 m2 g-1. The different morphologies of the samples are given by the presence of diverse magnetic nanoparticle arrangements covalently linked onto the outer surface of the mesoporous silica rods. This confers on them a superparamagnetic behaviour with a magnetic response between 50-80 emu g-1, even though the weight percent of magnetite present in the samples does not exceed 21.7%. In addition, the magnetic nanocomposites exhibit magnetic hyperthermia with moderate Specific Absorption Rate (SAR) values.

Interaction of silver atomic quantum clusters with living organisms: bactericidal effect of Ag3 clusters mediated by disruption of topoisomerase-DNA complexes.

Essential processes for living cells such as transcription and replication depend on the formation of specific protein-DNA recognition complexes. Proper formation of such complexes requires suitable fitting between the protein surface and the DNA surface. By adopting doxorubicin (DOX) as a model probe, we report here that Ag3 atomic quantum clusters (Ag-AQCs) inhibit the intercalation of DOX into DNA and have considerable influence on the interaction of DNA-binding proteins such as topoisomerase IV, Escherichia coli DNA gyrase and the restriction enzyme HindIII. Ag-AQCs at nanomolar concentrations inhibit enzyme activity. The inhibitory effect of Ag-AQCs is dose-dependent and occurs by intercalation into DNA. All these effects, not observed in the presence of Ag+ ions, can explain the powerful bactericidal activity of Ag-AQCs, extending the knowledge of silver bactericidal properties. Lastly, we highlight the interest of the interaction of Ag clusters with living organisms, an area that should be further explored due to the potential consequences that it might have, both beneficial and harmful.

Synthesis of water-soluble gold clusters in nanosomes displaying robust photoluminescence with very large Stokes shift.

This paper reports a novel procedure using nanosomes, made of bola-hydroxyl and mercapto-palmitic acids, for the production of gold clusters with robust luminescent emissions and very large Stokes shifts. It shows that these results cannot be explained by the currently accepted mechanism based on ligand-to-metal charge transfer absorptions involving electron-rich ligands attached to the cluster core. Exhaustive characterization of the cluster samples using Mass Spectrometry, HR-TEM/STEM, XPS, EXAFS, and steady-state and time-resolved luminescence allows to deduce that a mixture of two cluster sizes, having non-closed shell electronic configurations, are firstly generated inside the nanosome compartments due to the difference in bonding strength of the two types of terminal groups in the fatty acids. This initial bimodal cluster size distribution slowly evolves into very stable, closed-shell Au cluster complexes (Au6-Au16 and Au5-Au14) responsible for the observed luminescent properties. The very small (≈1.2 nm) synthesized cluster complexes are water soluble and suitable to be used for the conjugation of biomolecules (through the terminal COO(-) groups) making these systems very attractive as biomarkers and offering, at the same time, a novel general strategy of fabricating stable atom-level quantum dots with large Stokes shifts of great importance in many sensor applications.

Microemulsions for topical delivery of 8-methoxsalen.

8-Methoxsalen (8-MOP) and related furocumarins have been extensively used for the treatment of hyperproliferative skin diseases in association with long-wavelength UVA light. In order to develop alternative formulations for the topical administration of 8-MOP, microemulsions were evaluated as delivery vehicles. Six microemulsion formulations were prepared using water, isopropyl myristate (IPM) and Tween((R)) 80: Span((R)) 80: 1,2-Octanediol (3:1:1.2 w/w). The microemulsions were characterized using conductimetric and dynamic light scattering analyses. The ability of the systems to deliver 8-MOP into and through the skin was evaluated in vitro using newborn pig-skin. The in vitro permeation data showed that the novel microemulsions increased the 8-MOP total penetration through the skin by order of 1.9-4.5, as compared with IPM. In general, the accumulation of 8-MOP into the skin was increased by a factor of 1.5-4.5 by the microemulsion systems with respect to their total amount of drug delivered across the skin. These results suggest that the studied microemulsion systems may be appropriate vehicles for the topical delivery of 8-MOP.

Simulation of the kinetics of nanoparticle formation in microemulsions.

Monte Carlo simulations were carried out to explain experimental results concerning the different sizes obtained for Ag and Au nanoparticles synthesized in microemulsions. Computer simulations allowed to study the interplay between the chemical reaction rate and the material interdroplet exchange, and their consequences on the mechanism and size distribution of nanoparticles synthesized in microemulsions. It has been shown that, although the material interdroplet exchange depends primarily on the flexibility of the surfactant film, a slow reaction rate leads to a more effective material interdroplet exchange for a given microemulsion. Two factors contribute to this result. Firstly, a slow reaction implies that autocatalytic growth takes place for a longer period of time, because there are available reactants. If the reaction is faster, the reactants are almost exhausted at early stages of the process. As a consequence, autocatalytic growth is only possible at the beginning. Secondly, a slow reaction rate implies the continuous production of seed nuclei, which can be exchanged between micelles due to their small size, allowing the coagulation of two nanoparticles (growth by ripening). Once again, this exchange can only take place at early stages of the synthesis when the reaction is faster. Both factors, autocatalysis and ripening, favour the slow growth of the biggest nanoparticles leading to the production of larger particles when the reaction is slower.

Enhanced dimerization of TiOCl under pressure: spin-Peierls to Peierls transition.

We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced Ti3+-Ti3+ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti3+ ions along the b axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.

Enhanced pressure dependence of magnetic exchange in A2+[V2]O4 spinels approaching the itinerant electron limit.

We report a systematic enhancement of the pressure dependence of T(N) in A(2+)[V(2)]O(4) spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant-electron behavior is identified in Zn[V(2)]O(4) and Mg[V(2)]O(4) exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in Zn[V(2)]O(4) below T(s) approximately T(N) and dT(N)/dP<0, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.

Origin of the glassy magnetic behavior of the phase segregated state of the perovskites.

In this Letter we demonstrate that the phase segregated state observed in many rare-earth perovskites constitutes a sort of self-generated assembly of magnetic clusters in which magnetic interaction introduces collectivity among them. We show that the observed glassy behavior (memory, aging, etc.) can be perfectly understood taking into account only the intercluster interactions. We address the fundamental question about whether this state constitutes classical spin glass or if, on the other hand, a new universality class must be defined.

Enhancement of the recollision rate in diffusion-influenced reactions in an inhomogeneous medium.

Brownian dynamics simulations were performed to determine the first collision and recollision rates of spherical reagent particles in a reaction medium made heterogeneous by the presence of randomly located inert spherical obstacles in a continuum solvent. The recollision rate vp (and hence the overall reactive collision rate when activation energy is high) was always enhanced by the presence of obstacles, the degree of enhancement increasing with the volume fraction occupied by obstacles (phi) and with decreasing reagent concentration phiR. Enhancement increased with obstacle size at high phiR, and fell with increasing obstacle size at low phiR. The vp-phiR data follow a power law, where the scaling factor betap decreased with decreasing obstacle size and increasing phi, and the prefactor kp initially increased with phi and then fell (except for large obstacles). The behavior of betap appears to be largely due to the obstacles reducing the probability that reagent particles escape from each other after collision, while the dominant factors responsible for the behavior of kp appear to be initially the effect of obstacles in enhancing effective local reagent concentration, and then (for small obstacles), their reduction of the reagent-particle coordination number. As the energy of activation falls, the reactive collision rate becomes less influenced by the reagent recollision rate and more influenced by the rate of first collision. For low-activation-energy reactions, the presence of obstacles depresses the reactive collision rate if reagent concentration is low or if the obstacles are small and their concentration high. The fall in the reactive collision rate with decreasing activation energy is steeper, the lower the reagent concentration and the smaller the obstacles.

Kinetic studies on the formation of nitrosamines I. Formation of dimethylnitrosamine in aqeous solution of perchloric acid.

The kinetics of nitrosation of dimethylamine (DMA) in aqueous perchloric acid solution have been studied using a differential spectrophotometric technique. The rate law is Initial rate = e[DMA]0 [nitrite]2 0 [H+]/(f + [H+])2 where [DMA]0 and [nitrite]0 represent initial stoichiometric concentrations. At 310.0 K and mu = 2.0 M, e = (2.2 +/- 0.2) X 10(-5) M-1 s-1 and f = (1.28 +/- 0.02) X 10(-3) M. The associated activation energy is 56 +/- 3 kJ mol-1. A clear inhibition of the nitrosation rate by ionic strength has been observed in which only the kinetic parameter (f) has an effective change. It is concluded that under the experimental conditions of this work only the dinitrogen trioxid is the effective carrier for the nitrosation.