Yuri Diaz Fernandez

Antibacterial Activity of Glutathione-Coated Silver Nanoparticles against Gram Positive and Gram Negative Bacteria

In the present paper, we study the mechanism of antibacterial activity of glutathione (GSH) coated silver nanoparticles (Ag NPs) on model Gram negative and Gram positive bacterial strains. Interference in bacterial cell replication is observed for both cellular strains when exposed to GSH stabilized colloidal silver in solution, and microbicidal activity was studied when GSH coated Ag NPs are (i) dispersed in colloidal suspensions or (ii) grafted on thiol-functionalized glass surfaces. The obtained results confirm that the effect of dispersed GSH capped Ag NPs (GSH Ag NPs) on Escherichia coli is more intense because it can be associated with the penetration of the colloid into the cytoplasm, with the subsequent local interaction of silver with cell components causing damages to the cells. Conversely, for Staphylococcus aureus, since the thick peptidoglycan layer of the cell wall prevents the penetration of the NPs inside the cytoplasm, the antimicrobial effect is limited and seems related to the interaction with the bacterial surfaces. Experiments on GSH Ag NPs grafted on glass allowed us to elucidate more precisely the antibacterial mechanism, showing that the action is reduced because of GSH coating and the limitation of the translational freedom of NPs.

Hydride formation thermodynamics and hysteresis in individual Pd nanocrystals with different size and shape

Physicochemical properties of nanoparticles may depend on their size and shape and are traditionally assessed in ensemble-level experiments, which accordingly may be plagued by averaging effects. These effects can be eliminated in single-nanoparticle experiments. Using plasmonic nanospectroscopy, we present a comprehensive study of hydride formation thermodynamics in individual Pd nanocrystals of different size and shape, and find corresponding enthalpies and entropies to be nearly size- and shape-independent. The hysteresis observed is significantly wider than in bulk, with details depending on the specifics of individual nanoparticles. Generally, the absorption branch of the hysteresis loop is size-dependent in the sub-30u2009nm regime, whereas desorption is size- and shape-independent. The former is consistent with a coherent phase transition during hydride formation, influenced kinetically by the specifics of nucleation, whereas the latter implies that hydride decomposition either occurs incoherently or via different kinetic pathways.

Using micelles for a new approach to fluorescent sensors for metal cations

A new approach based on self-assembly inside micelles has been individuated to prepare a system behaving as a water-operating selective fluorescent sensor for Cu2+ and Ni2+.

A Versatile Self-Assembly Strategy for the Synthesis of Shape-Selected Colloidal Noble Metal Nanoparticle Heterodimers

The self-assembly of individual nanoparticles into dimers—so-called heterodimers—is relevant for a broad range of applications, in particular in the vibrant field of nanoplasmonics and nanooptics. In this paper we report the synthesis and characterization of material- and shape-selected nanoparticle heterodimers assembled from individual particles via electrostatic interaction. The versatility of the synthetic strategy is shown by assembling combinations of metal particles of different shapes, sizes, and metal compositions like a gold sphere (90 nm) with either a gold cube (35 nm), gold rhombic dodecahedron (50 nm), palladium truncated cube (120 nm), palladium rhombic dodecahedron (110 nm), palladium octahedron (130 nm), or palladium cubes (25 and 70 nm) as well as a silver sphere (90 nm) with palladium cubes (25 and 70 nm). The obtained heterodimer combinations are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDX), dynamic light scattering (DLS), and zeta-potential measurements. We describe the optimal experimental conditions to achieve the highest yield of heterodimers compared to other aggregates. The experimental results have been rationalized using theoretical modeling. A proof-of-principle experiment where individual Au–Pd heterodimers are exploited for indirect plasmonic sensing of hydrogen finally illustrates the potential of these structures to probe catalytic processes at the single particle level.

Research Update: Progress in synthesis of nanoparticle dimers by self-assembly

This article highlights recent advances in the controlled self-assembly of nanoparticles to produce dimeric nanoparticle structures. The relevance of this emergent field is discussed in terms of recent applications in plasmonics and chemical catalysis. The concept of bond-valence applied to nanoparticles will be discussed, emphasizing some general approaches that have been successfully used to build these structures. Further, the asymmetric functionalization of nanoparticles surfaces as a path to drive selective aggregation, the use of biomolecules to self-assemble nanoparticles into dimers in solution, and the confinement of aggregates in small cavities are discussed.

Mixing thiols on the surface of silver nanoparticles: preserving antibacterial properties while introducing SERS activity

Controlling the surface composition of self-assembled monolayers is one of the major experimental challenges in nanotechnology. Despite the significant interest of the scientific community and the considerable number of publications related to this topic, the potential in this field is still far from being fully exploited.We present in this study a versatile method to coat silver nanoparticles (AgNPs) having average diameter of 7xa0nm with mixed monolayers of two thiols, achieving a precise control of surface composition. Different combinations of thiols have been investigated, and the nanomaterials obtained have been characterized by complementary experimental techniques, addressing the composition of the mixed monolayer. The surface-enhanced Raman spectroscopy (SERS) effect on a Raman reporter (7-mercapto-4-methylcoumarine) introduced into the mixed monolayers has also been investigated. The antibacterial activity of the coated AgNPs was investigated, showing that the colloids were active against Escherichia coli and Staphilococcus aureus irrespective of the nature of the mixed monolayer. These materials are good candidates as SERS-tags for biological applications.

Double helical and monomeric Ag(I) and Zn(II) complexes of 1,2-cyclohexanediyl-bis(iminophenanthridine) ligands

The tetradentate ligands tcp and ccp, made of two bidentate iminophenanthridine halves, separated, respectively, by trans- and cis-cyclohexanediyl spacers, have been studied in acetonitrile as regards their ability of coordinating the d10 cations Ag+ and Zn2+. Spectrophotometric and spectrofluorimetric titrations revealed the tendency to form 1 ∶ 2 and 2 ∶ 2 metal–ligand complexes for Ag+, when metal–ligand molar ratios lower than 1 ∶ 1 are chosen. Determination of the formation constants of the 1 ∶ 2 and 2 ∶ 2 complex species demontrate that at metal–ligand molar ratio = 1 ∶ 1 the preferred species are the helicates [AgI2(tcp)2]2+ and [AgI(ccp)2]2+, whose nature was been further supported by 1H NMR and ESI-MS spectra. The crystal and molecular structure of [Ag2(ccp)2](CF3SO3)(Et2O) has also been determined by X-ray diffraction, confirming the double helical nature of the molecular cation. The same ligands tend to form 1 ∶ 2 and 1 ∶ 1 metal–ligand complexes with Zn2+ in acetonitrile, at molar ratios lower than 1 ∶ 1. However, only the monomeric [Zn(tcp)]2+ and [Zn(ccp)]2+ complexes are formed at 1 ∶ 1 stoichiometry, as indicated by calculation of formation constants, and by 1H NMR and ESI-MS spectra. The luminescent properties of the ligands have been also studied, which are connected to the presence of the luminescent phenanthridine heterocycle. In the free ligands, intramolecular excimer formation between the two phenanthridine halves of the molecule occurs. Complexation of the ligands with Ag+ affords non-luminescent helicates, while the monomeric complexes obtained with Zn2+ are strongly luminescent.

Exploiting micelle-driven coordination to evaluate the lipophilicity of molecules.

We present a systematic study based on the calculation of complexation constants between a Zn-complex solubilized in Triton X-100 micellar solutions and a series of linear mono- and dicarboxylic acids, under physiological pH conditions, that allowed the evaluation of the lipophilicity of these molecules. This empirical lipophilicity parameter describes conveniently the partition of organic molecules between hydrophobic microdomains and water. The results can be used to predict the lipophilicity of molecules with similar structure and allows the distinction of intrinsic contributions of the carboxylates and of the methylene groups to the lipophilicity of the molecule.

Bis-bidentate vs. bis-tridentate imino-heterocycle ligands in the formation of dinuclear helical complexes of Fe(II)

By reaction of the bis-bidentate imino-quinoline ligand 1 with the octahedral cation Fe2+ in a 3 ∶ 2 metal/ligand molar ratio, a partial hydrolysis of the ligands is observed instead of the formation of the expected triple helicate, due to the excessive crowding of the trans-1,2-cyclohexyl spacers. The monomeric [FeII(2)2]2+ complex is obtained, whose crystal and molecular structure have been determined, in which the amino-imino-quinoline tridentate ligands 2, originating from 1 by loss of one quinoline aldehyde, coordinate Fe2+ with a mer disposition. Double helical complexes can instead be obtained by using the bis-tridentate ligand 3, featuring the same spacer of ligand 1, i.e.trans-1,2-cyclohexyl, and two imino-phenanthroline donor sets. The spectral and electrochemical properties of the low-spin double helical [FeII2(3)2]4+ complex have been studied and compared with those of the low spin [FeII(4)]2+ complex, which can be considered the monomeric half of the helicate. Significantly enhanced kinetic inertness and resistance to oxidation were found for the double helical complex.