Physicochemical characteristics of protein-NP bioconjugates: the role of particle curvature and solution conditions on human serum albumin conformation and fibrillogenesis inhibition.

Gold nanoparticles (Au NPs) from 5 to 100 nm in size synthesized with HAuCl(4) and sodium citrate were complexed with the plasma protein human serum albumin (HSA). Size, surface charge, and surface plasmon bands of the Au NPs are largely modified by the formation of a protein corona via electrostatic interactions and hydrogen bonding as revealed by thermodynamic data. Negative values of the entropy of binding suggested a restriction in the biomolecule mobility upon adsorption. The structure of the adsorbed protein molecules is slightly affected by the interaction with the metal surface, but this effect is enhanced as the NP curvature decreases. Also, it is observed that the protein molecules adsorbed onto the NP surface are more resistant to complete thermal denaturation than free protein ones as deduced from the increases in the melting temperature of the adsorbed protein. Differences in the conformations of the adsorbed protein molecules onto small (<40 nm) and large NPs were observed on the basis of ζ-potential data and FTIR spectroscopy, also suggesting a better resistance of adsorbed protein molecules to thermal denaturing conditions. We think this enhanced protein stability is responsible for a reduced formation of HSA amyloid-like fibrils in the presence of small Au NPs under HSA fibrillation conditions.

In situ formation of one-dimensional assemblies of gold nanoparticles in confined media.

The spontaneous in situ formation of one-dimensional (1D) assemblies of gold nanoparticles (NP) in oleylamine/bis(2-ethylhexyl) sulfosuccinate sodium salt/water/octane (OAm/AOT/w/o) microemulsions by exploiting both the aurophilic bonding between OAm and gold salt, and the interactions between OAm and AOT surfactant is presented. Control on the structure of the resulting assemblies is achieved by changing in the solvent quality, the [Au]/[AOT] molar ratio and the presence of different cosolutes. A possible mechanism of the formation of the 1D parallel Au NP arrays is proposed.

Modulation of size and shape of Au nanoparticles using amino-X-shaped poly(ethylene oxide)-poly(propylene oxide) block copolymers.

In the present work, the formation and stabilization of gold nanoparticles in a one-pot water-based synthesis has been achieved in the presence of a four-arm, star-shaped polyoxyethyelene-polyoxypropylene (PEO-PPO) block copolymer, Tetronic T904, which acts as both reductant and stabilizer. The influence of several parameters such as copolymer and gold salt concentration, reaction temperature, and solution pH on both the size and shape of the resulting nanocrystals has been established. Low copolymer/gold salt molar ratios favor the formation of either triangular or hexagonal planar nanostructures due to a low reduction rate which turns the reaction into kinetic control. As the molar ratio increases, reduction becomes faster with the subsequent increase in the number of crystal seeds and, thus, the decrease in particle size. In addition, there is an increase in the reduction rate which causes the reduction reaction to be governed by thermodynamics, and consequently, spherical geometries are favored. A particle spherical shape can also be promoted as a consequence of the accumulation of block copolymer molecules on different crystallographic planes, homogenizing the metal surface structure and disabling the growth in different crystallographic directions. The same behavior was observed when the reaction temperature was raised. The size and shape of gold nanoparticles could also be controlled by varying the pH of the medium. As the pH becomes more acidic, protons prevent the oxyethylene part of the copolymer from the reduction of metal ions, and consequently, the number of nuclei decreases. This explains the overall increase in the particle size and the change in shape when the synthesis is carried out in acid medium. Finally, comparison with nanoparticles obtained in the presence of a structurally related linear block copolymer Pluronic P105, with a similar number of EO and PO units as T904, denoted an important incidence of the arrangement of PEO and PPO blocks on the reduction reaction rate and the size and shape of the resulting nanoparticles.

Additional supra-self-assembly of human serum albumin under amyloid-like-forming solution conditions.

Protein aggregation has a multitude of consequences ranging from affecting protein expression to its implication in different diseases. Of recent interest is the specific form of aggregation leading to the formation of amyloid fibrils, structures associated with diseases such as Alzheimer's disease. These fibrils can further associate in other more complex structures such as fibrillar gels, plaques, or spherulitic structures. In the present work, we describe the physical and structural properties of additional supraself-assembled structures of human serum albumin under solution conditions in which amyloid-like fibrils are formed. We have detected the formation of ordered aggregates of amyloid fibrils, i.e., spherulites which possess a radial arrangement of the fibrils around a disorganized protein core and sizes of several micrometers by means of polarized optical microscopy, laser confocal microscopy, and transmission electron microscopy. These spherulites are detected both in solution and embedded in an isotropic matrix of fibrillar gels. In this regard, we have also noted the formation of protein gels when the protein concentration and/or ionic strength exceds a threshold value (the gelation point) as observed by rheometry. Fibrillar gels are formed through intermolecular nonspecific association of amyloid fibrils at a pH far away from the isolectric point of the protein where protein molecules seem to display a "solid-like" behavior due to the existence of non-DLVO (Derjaguin-Landau-Verwey-Overbeck) intermolecular repulsive forces. As the solution ionic strength increases, a coarsening of this type of gel is observed by environmental scanning microscopy. In contrast, at pH close to the protein isoelectric point, particulate gels are formed due to a faster aggregation process, which does not allow substantial structural reorganization to enable the formation of ordered structures. This behavior also additionally corroborates that the existence of particulates might also be a generic property of all polypeptide chains as amyloid fibril formation under suitable conditions.

Block copolymer-mediated synthesis of size-tunable gold nanospheres and nanoplates.

We have successfully controlled the size and shape of isotropic and anisotropic gold nanocrystals through a one-step reaction by using amphiphilic polyethylene oxide-polystyrene oxide block copolymers as both reductant and stabilizing agents in water solution. Spherical or quasispherical nanoparticles were obtained at room temperature with tunable mean sizes and polydispersities depending on reaction conditions, that is, on copolymer block length, and copolymer and gold salt concentrations. By moderate increases of reaction temperature up to 65 degrees C, progressive formation of single-crystalline gold nanoplates in good yields takes place (up to 70%) without the necessity of additional reactants or growing solutions. These nanoplates are characterized by lateral mean sizes between 0.1-1.2 microm depending on copolymer concentration and reaction temperature, with mainly truncated or rounded triangular shapes with {111} planes as two basal surfaces. This allows us to tune the surface plasmon band of the nanoplates from ca. 850 nm to more than 1100 nm, well inside the near-infrared region (NIR), which enables the use of these type of nanostructures as a very promsing materials in applications such as optical coatings, SERS, and cancer cell hyperthermia. We proposed that the growth of these nanostructures can stem from a decrease in the reaction rate as temperature increases due to an enhanced copolymer hydrophobicity, which gives rise to a structure of interacting micelles formed from the fluid via a percolation transition (known as "soft gel") at elevated temperatures. In this way, reduction becomes slow enough to allow kinetic control of the reaction, and preferential adsorption of the copolymer molecules/micelles on certain crystallographic planes can favor the growth of certain nanocrystal facets to give the final structure. This alternative water-based system provides a more convenient and environmentally benign route to the synthesis of shape-controlled noble-metal nanocrystals in high yield because it does not involve toxic organic solvents or reagents and serves as a bridge between two frontline discipline: the block copolymeric science and anisotropic nanoparticles.