Yuri Antonio Diaz-Fernandez

Synthesis, Characterization and Antibacterial Activity against Gram Positive and Gram Negative Bacteria of Biomimetically Coated Silver Nanoparticles

In the present work, we describe a simple procedure to produce biomimetically coated silver nanoparticles (Ag NPs), based on the postfunctionalization and purification of colloidal silver stabilized by citrate. Two biological capping agents have been used (cysteine Cys and glutathione GSH). The composition of the capped colloids has been ascertained by different techniques and antibacterial tests on GSH-capped Ag NPs have been conducted under physiological conditions, obtaining values of Minimum Inhibitory Concentration (MIC) of 180 and 15 μg/mL for Staphylococcus aureus and Escherichia coli, respectively. The antibacterial activity of these GSH capped NPs can be ascribed to the direct action of metallic silver NPs, rather than to the bulk release of Ag(+).

Self-assembled monolayers of silver nanoparticles firmly grafted on glass surfaces: Low Ag + release for an efficient antibacterial activity

A two-step, easy synthetic strategy in solution has been optimized to prepare authentic monolayers of silver nanoparticles (NP) on MPTS-modified glass surfaces, that were investigated by AFM imaging and by quantitative silver determination techniques. NP in the monolayers remain firmly grafted (i.e. not released) when the surfaces are exposed to air, water or in the physiological conditions mimicked by phosphate saline buffer, as UV-Vis spectroscopy and AFM studies demonstrate. About 15% silver release as Ag(+) ions has been found after 15days when the surfaces are exposed to water. The released silver cations are responsible of an efficient local microbicidal activity against Escherichia coli and Staphylococcus aureus bacterial strains.

Synthesis of branched Au nanoparticles with tunable near-infrared LSPR using a zwitterionic surfactant

Asymmetric branched gold nanoparticles are obtained using for the first time in the seed-growth approach a zwitterionic surfactant, laurylsulfobetaine, whose concentration in the growth solution allows to control both the length to base-width ratio of the branches and the LSPR position, that can be tuned in the 700-1100 nm near infrared range.

Enhancement of room temperature ferromagnetism in N-doped TiO2−x rutile: Correlation with the local electronic properties

The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.

Influence of organic additives on the cloud point of PONPE-7.5

The effect of the addition of different organic compounds on the critical temperature (cloud point) of micellar solutions of a non-ionic surfactant was studied. In particular, the addition of aliphatic alcohols and some hydrocarbons to polyoxyethylene(7.5) nonylphenyl ether (PONPE-7.5) solutions was investigated. It was found that the short-chain alcohols (methanol, ethanol, propanol and butanol) increased the cloud point temperature, while for the longer-chain alcohols, the temperature decreased. The effect of the first group can be explained by considering that these species tend to remain in solution and promote the formation of expanded water structures, favoring micelle hydration and, in consequence, increasing the cloud point. This influence is more significant as the concentration of the additive increases, and can be negligible at very low concentrations. The effect of longer-chain alcohols suggests a preferential solubilization of these compounds in the inner core of the aggregate, inducing changes to the shape and size of the micelles. Since the volume-to-area ratio in a micelle increases with these additions, there is a general increase of the micelle size and the cloud point decreases. The effects of branching and cyclization were studied using two series: n-butanol, isobutanol and tert-butanol, and n-hexane, cyclohexane and benzene, respectively. For both series, the addition of the compound provoked a marked decrease in the cloud point temperature. These effects can be explained in view of the site of solubilization inside the micelle. Benzene molecules seem to interact preferentially with the ethylene groups, leading to strong dehydration, while the aliphatic compounds are solubilized in the inner core.

Micelles as Containers for Self‐Assembled Nanodevices: A Fluorescent Sensor for Lipophilicity

Potentiometric titrations, fluorescence versus pH titrations, dynamic light scattering and fluorescence polarization anisotropy studies demonstrate that inside the nanodimensioned Triton X-100 micelles, 1-pyrenecarboxylic acid, PCOO(-), forms an apical complex with the Zn(2+) cation encircled by a lipophilic cyclen ligand and hugely increasing its fluorescence. The ability of the Zn(2+)-cyclen-PCOO(-) complex plus its micellar container to act as a fluorescent sensor to evaluate the lipophilicity of molecular species is demonstrated on the fatty acid series CH(3)(CH(2))(x)COOH (x=0-16). At pH 7.4 a decrease in fluorescence is observed on the addition of fatty acids that is directly related to their chain length, that is, to their tendency to enter the micellar containers, where they dislocate PCOO(-) from the Zn(2+) centre. The independent determination of fatty acid pK(a) values in the presence of Triton X-100 micelles confirms that our fluorescent micellar device is capable of sensing their lipophilicity.

Optical Method for Predicting the Composition of Self-Assembled Monolayers of Mixed Thiols on Surfaces Coated with Silver Nanoparticles

With a simple optical method, based on UV-vis absorption spectra on glass slides, it is possible to predict the composition of self-assembled monolayers of mixed thiols, grafted on monolayers of silver nanoparticles. Glass slides are modified with the layer-by-layer technique, first forming a monolayer of mercaptopropyltrimethoxysilane, then grafting a monolayer of silver nanoparticles on it. These surfaces are further coated by single or mixed thiol monolayers, by dipping the slides in toluene solutions of the chosen thiols. Exchange constants are calculated for the competitive deposition between the colorless 1-dodecanethiol or PEG5000 thiol and BDP-SH, with the latter being a thiol-bearing molecule containing the strongly absorbing BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) moiety, synthesized on purpose. The constants are calculated by determining the fraction of BDP-SH deposited on the surface from a solution with a given molar fraction, directly measuring the absorption spectra of BDP-SH on the slides. Then, the exchange constant for the competitive deposition between 1-dodecanethiol and PEG5000 thiol is calculated by combining their exchange constants with BDP-SH. This allows to predict the fraction of the two colorless thiols coating the silver nanoparticles slides obtained from a toluene solution with a given molar fraction, for example, of PEG5000 thiol. The correctness of the calculated surface fraction is verified by studying the coating competition between 1-dodecanethiol and a PEG5000 thiol remotely modified with a strongly absorbing fluorescein fragment.

The Cu(II) complex of a C-lipophilized 13aneN4 macrocycle with an additional protonable amino group as micellar anion receptor

Three 13aneN4 macrocyclic ligands have been prepared bearing a -CH(2)NHR side arm (R = H, n-C(5)H(11), n-C(10)H(21)) on a carbon atom. When Cu(2+) is complexed in the macrocyclic ring, the amino group of the side arm undergoes an acid-base protonation equilibrium but it is not able to coordinate apically the metal cation even when it is deprotonated. The Cu(2+) complex with the ligand bearing the longest appended aliphatic chain is fully confined inside Triton X-100 micelles, and its ability to bind and sequestrate a series of anions inside micelles has been studied at two different pH values, i.e. both with protonated and neutral side-arm amino group. The favourable role played by the protonated amino group in the side arm has been demonstrated.

Labeling interacting configurations through an analysis of excitation dynamics in a resonant photoemission experiment: the case of rutile TiO2

A detailed study of resonant photoemission at Ti L(2,3) edges of insulating rutile TiO(2-x) thin film is presented. Pure TiO(2) resonating structures, defect-related resonances, resonant Raman-Auger and normal LVV Auger emissions are tracked, including an unpredicted two-hole correlated satellite below the non-bonding part of the valence band. The analysis of excitation dynamics unambiguously addresses the origin of these features and, in particular, the extent of charge transfer effects on the Ti-O bonding in the valence band of rutile, disclosing further applications to the more general case of, formally, d(0) oxides.

Structure and magnetism of HoBaCo2O5+ \delta layered cobaltites with 0.02≤δ≤0.22

In this paper we have studied, by means of high-resolution neutron powder diffraction and magnetic susceptibility, the structural and magnetic features of selected samples of the HoBaCo2O5+δ layered cobaltite in the low oxygen content range 0.02≤δ≤0.22. The results show a strong antiferromagnetic contribution at room temperature coupled to an intermediate spin state of the Co3+ ions.