Importance of the Structural and Physicochemical Properties of Silica Nanoshells in the Photothermal Effect of Silica-Coated Au Nanoparticles Suspensions.

In this work, monodisperse silica-coated gold nanoparticles (NPs) were synthesized and used for obtaining aqueous colloidal dispersions with an optimum relationship between colloidal stability and photothermal activity. The idea behind this design was to produce systems with the advantages of the presence of a silica shell (biocompatibility, potential for surface modification, and protecting effect) with a minimal loss of optical and thermal properties. With this aim, the photothermal properties of NPs with silica shells of different thicknesses were analyzed under conditions of high radiation extinction. By using amorphous, gel-like silica coatings, thicknesses higher than 40 nm could be obtained without an important loss of the light absorption capacity of the colloids and with a significant photothermal response even at low NP concentrations. The effects produced by changes in the solvent and in the NP concentration were also analyzed. The results show that the characteristics of the shell control both, the photothermal effect and the optical properties of the colloidal dispersions. As the presence of a silica shell strongly enhances the possibilities of adding cargo molecules or probes, these colloids can be considered of high interest for biomedical therapies, sensing applications, remote actuation, and other technological applications.

Effect of Light Intensity on the Aggregation Behavior of Primary Particles during In Situ Photochemical Synthesis of Gold/Polymer Nanocomposites.

Metal/polymer nanocomposites have attracted much attention in recent years due to their exceptional properties and wide range of potential applications. A key challenge to obtain these materials is to stabilize the metal nanoparticles in the matrix, avoiding uncontrolled aggregation processes driven by the high surface free energy of nanosized particles. Here, we investigate the aggregation mechanism of primary particles in gold-epoxy nanocomposites prepared via light-assisted in situ synthesis, under different irradiation conditions. The growth and aggregation of gold nanoparticles were monitored in situ by time-resolved small-angle X-ray scattering experiments, whereas spectroscopic measurements were performed to interpret how matrix polymerization influences the aggregation process. It was found that light intensity has a greater influence on the reduction rate than on the polymerization rate. Under irradiation, gold nanostructures evolve through five time-defined stages: nuclei-mass fractals-surface fractals-spherical nanoparticles-aggregates. If the maximum in the polymerization rate is reached before the aggregation step, individual primary nanoparticles will be preserved in the polymer matrix due to diffusional constraints imposed by the reaction medium. Because the light intensity has a different influence on the reduction rate than on the polymerization rate, this parameter can be used as a versatile tool to avoid aggregation of gold nanoparticles into the polymer matrix.

Tuning the Photothermal Effect of Carboxylated-Coated Silver Nanoparticles through pH-Induced Reversible Aggregation.

The photothermal response of mercaptoundecanoic acid (MUA)-coated Ag nanoparticles (Ag@MUA NPs) in both aqueous dispersions and paper substrates was determined as a function of pH when irradiated with a green laser or a blue LED source. Aqueous dispersions of Ag@MUA NPs showed an aggregation behavior by acidification that was used for the formation of NPs clusters of variable sizes. Aggregation was induced by changing the pH across the apparent pKa of the acid, higher than the pKa of the free acid. Formation of these aggregates was completely reversible allowing the return to the well-dispersed initial state by simply increasing the pH by the addition of a base. Aggregation produced a shift of the plasmon band that changed the spectra of the dispersions and their ability to be remotely heated when irradiated with visible light. These aggregates could be transferred to paper by simple impregnation of the substrates with the dispersion. On the solid substrate, a higher photothermal response than in the liquid medium was observed. A high local increase of up to 75 °C could be recorded on paper after only 30 s of irradiation with a green laser, whereas a blue LED array was enough for inducing the melting of a solid paraffin (Tm = 36-38 °C) deposited on it. This work demonstrates that photothermal heating can be controlled by the reversible aggregation of NPs to induce different thermal responses in liquid and solid media.

Mechanism of Particle Formation in Silver/Epoxy Nanocomposites Obtained through a Visible-Light-Assisted in Situ Synthesis.

A detailed understanding of the processes taking place during the in situ synthesis of metal/polymer nanocomposites is crucial to manipulate the shape and size of nanoparticles (NPs) with a high level of control. In this paper, we report an in-depth time-resolved analysis of the particle formation process in silver/epoxy nanocomposites obtained through a visible-light-assisted in situ synthesis. The selected epoxy monomer was based on diglycidyl ether of bisphenol A, which undergoes relatively slow cationic ring-opening polymerization. This feature allowed us to access a full description of the formation process of silver NPs before this was arrested by the curing of the epoxy matrix. In situ time-resolved small-angle X-ray scattering investigation was carried out to follow the evolution of the number and size of the silver NPs as a function of irradiation time, whereas rheological experiments combined with near-infrared and ultraviolet-visible spectroscopies were performed to interpret how changes in the rheological properties of the matrix affect the nucleation and growth of particles. The analysis of the obtained results allowed us to propose consistent mechanisms for the formation of metal/polymer nanocomposites obtained by light-assisted one-pot synthesis. Finally, the effect of a thermal postcuring treatment of the epoxy matrix on the particle size in the nanocomposite was investigated.

Synthesis of silver nanoparticles coated with OH-functionalized organic groups: dispersion and covalent bonding in epoxy networks.

The introduction of reactive functionalities in the organic groups used to stabilize inorganic nanoparticles (NPs) enables multiple applications based on their covalent fixation to a variety of materials, substrates and interfaces. In this paper we report the synthesis of silver nanoparticles (NPs) with an average diameter of about 4 nm, coated with particular organic groups that allow their solubility in a variety of organic solvents and the covalent bonding through secondary hydroxyl groups present in their structure. Water-dispersible NPs stabilized with 11-mercaptoundecanoate anions were first synthesized. The esterification of carboxylate groups with phenyl glycidyl ether generated 2-hydroxyester functionalities and made the NPs dispersible in a variety of organic solvents. To illustrate the multiple possible applications of the synthesized NPs, their incorporation to an epoxy network is discussed. A solution of the silver NPs in diglycidyl ether of bisphenol A was polymerized in the presence of benzyldimethylamine as initiator. This led to an epoxy network containing a homogeneous dispersion of silver NPs as revealed by the constancy of the plasmon band location. Covalent bonding of the NPs to the epoxy matrix was produced by chain transfer reactions involving the hydroxyl groups. Nanocomposites were strongly colored and exhibited a dependence of the glass transition temperature on the concentration of NPs. Several applications are envisaged for these materials.

Tailoring the self-assembly of linear alkyl chains for the design of advanced materials with technological applications.

The self-assembly of n-alkyl chains at the bulk or at the interface of different types of materials and substrates has been extensively studied in the past. The packing of alkyl chains is driven by Van der Waals interactions and can generate crystalline or disordered domains, at the bulk of the material, or self-assembled monolayers at an interface. This natural property of alkyl chains has been employed in recent years to develop a new generation of materials for technological applications. These studies are dispersed in a variety of journals. The purpose of this article was to discuss some selected examples where these advanced properties arise from a process involving the self-assembly of alkyl chains. We included a description of electronic devices and new-generation catalysts with properties derived from a controlled two-dimensional (2D) or three-dimensional (3D) self-assembly of alkyl chains at an interface. Then, we showed that controlling the crystallization of alkyl chains at the bulk can be used to generate a variety of advanced materials such as superhydrophobic coatings, shape memory hydrogels, hot-melt adhesives, thermally reversible light scattering (TRLS) films for intelligent windows and form-stable phase change materials (FS-PCMs) for the storage of thermal energy. Finally, we discussed two examples where advanced properties derive from the formation of disordered domains by physical association of alkyl chains. This was the case of photoluminescent nanocomposites and materials used for reversible optical storage.

Formation of fractal-like structures driven by carbon nanotubes-based collapsed hollow capsules.

Carbon nanotubes (CNTs) based hollow capsules were obtained by degradation under acidic conditions of core-shell nanocomposites build up upon adsorption of multilayers of CNTs (shell) onto melamine-formaldehyde (MF) spheres (core). By evaporation of the dispersions obtained, polymeric fractal patterns from the degradation products of the MF core were formed onto silicon wafers. The proposed mechanism for the formation of these structures is based on the role of the capsules as arrangements of heterogeneities that facilitate the dewetting of the liquid polymeric films.

Epoxy-Based Organogels for Thermally Reversible Light Scattering Films and Form-Stable Phase Change Materials.

Alkyl chains of ß-hydroxyesters synthesized by the capping of terminal epoxy groups of diglycidylether of bisphenol A (DGEBA) with palmitic (C16), stearic (C18), or behenic (C22) fatty acids self-assemble forming a crystalline phase. Above a particular concentration solutions of these esters in a variety of solvents led to supramolecular (physical) gels below the crystallization temperature of alkyl chains. A form-stable phase change material (FS-PCM) was obtained by blending the ester derived from behenic acid with eicosane. A blend containing 20 wt % ester was stable as a gel up to 53 °C and exhibited a heat storage capacity of 161 J/g, absorbed during the melting of eicosane at 37 °C. Thermally reversible light scattering (TRLS) films were obtained by visible-light photopolymerization of poly(ethylene glycol) dimethacrylate-ester blends (50 wt %) in the gel state at room temperature. The reaction was very fast and not inhibited by oxygen. TRLS films consisted of a cross-linked methacrylic network interpenetrated by the supramolecular network formed by the esters. Above the melting temperature of crystallites formed by alkyl chains, the film was transparent due to the matching between refractive indices of the methacrylic network and the amorphous ester. Below the crystallization temperature, the film was opaque because of light dispersion produced by the organic crystallites uniformly dispersed in the material. Of high significance for application was the fact that the contrast ratio did not depend on heating and cooling rates.

Reprint of: self-assembly of nanoparticles employing polymerization-induced phase separation.

Nanoparticles (NPs) may be homogeneously dispersed in the precursors of a polymer (reactive solvent) by an adequate selection of their stabilizing ligands. However, the dispersion can become metastable or unstable in the course of polymerization. If this happens, NP-rich domains can be segregated by a process called polymerization-induced phase separation (PIPS). This occurs mainly due to the decrease in the entropic contribution of the reactive solvent to the free energy of mixing (increase in its average size) and, for a reactive solvent generating a cross-linked polymer, the additional contribution of the elastic energy in the post-gel stage. The extent of PIPS will depend on the competition between phase separation and polymerization rates. It can be completely avoided, limited to a local scale or conveyed to generate different types of NPs' aggregates such as crystalline platelets, self-assembled structures with a hierarchical order and partitioning at the interface, and bidimensional patterns of NPs at the film surface. The use of a third component in the initial formulation such as a linear polymer or a block copolymer, provides the possibility of generating an internal template for the preferential location and self-assembly of phase-separated NPs. Some illustrative examples of morphologies generated by PIPS in solutions of NPs in reactive solvents, are analyzed in this feature article.

Self-assembly of nanoparticles employing polymerization-induced phase separation.

Nanoparticles (NPs) may be homogeneously dispersed in the precursors of a polymer (reactive solvent) by an adequate selection of their stabilizing ligands. However, the dispersion can become metastable or unstable in the course of polymerization. If this happens, NP-rich domains can be segregated by a process called polymerization-induced phase separation (PIPS). This occurs mainly due to the decrease in the entropic contribution of the reactive solvent to the free energy of mixing (increase in its average size) and, for a reactive solvent generating a cross-linked polymer, the additional contribution of the elastic energy in the post-gel stage. The extent of PIPS will depend on the competition between phase separation and polymerization rates. It can be completely avoided, limited to a local scale or conveyed to generate different types of NPs' aggregates such as crystalline platelets, self-assembled structures with a hierarchical order and partitioning at the interface, and bidimensional patterns of NPs at the film surface. The use of a third component in the initial formulation such as a linear polymer or a block copolymer, provides the possibility of generating an internal template for the preferential location and self-assembly of phase-separated NPs. Some illustrative examples of morphologies generated by PIPS in solutions of NPs in reactive solvents, are analyzed in this feature article.

Poly(dodecyl methacrylate) as solvent of paraffins for phase change materials and thermally reversible light scattering films.

Paraffins are typical organic phase change materials (PCM) used for latent heat storage. For practical applications they must be encapsulated to prevent leakage or agglomeration during fusion. In this study it is shown that eicosane (C20H42 = C20) in the melted state could be dissolved in the hydrophobic domains of poly(dodecyl methacrylate) (PDMA) up to concentrations of 30 wt %, avoiding the need of encapsulation. For a 30 wt % solution, the heat of phase change was close to 69 J/g, a reasonable value for its use as a PCM. The fully converted solution remained transparent at 80 °C with no evidence of phase separation but became opaque by cooling as a consequence of paraffin crystallization. Heating above the melting temperature regenerated a transparent material. A high contrast ratio and abrupt transition between opaque and transparent states was observed for the 30 wt % blends, with a transparent state at 35 °C and an opaque state at 23 °C. This behavior was completely reproducible during consecutive heating/cooling cycles, indicating the possible use of this material as a thermally reversible light scattering (TRLS) film.

Hierarchical assemblies of gold nanoparticles at the surface of a film formed by a bridged silsesquioxane containing pendant dodecyl chains.

Hierarchical aggregates of gold nanoparticles (NPs) on different length scales were in situ generated at the surface of a bridged silsesquioxane during the process of film formation by polycondensation and solvent evaporation. A precursor of a bridged silsesquioxane based on the reaction product of (glycidoxypropyl)trimethoxysilane (2 mol) with dodecylamine (1 mol) was hydrolytically condensed in a THF solution at room temperature in the presence of formic acid, water, and variable amounts of dodecanethiol-stabilized gold NPs (average diameter of 2 nm). The initial compatibility of the precursor with gold NPs was achieved by the presence of dodecyl chains in both components. Phase separation of gold NPs accompanied by partitioning to the air-polymer interface took place driven by the polycondensation reaction and solvent evaporation. A hierarchical organization of gold NPs in the structures generated at the air-polymer interface was observed. Small body-centered cubic (bcc) crystals of about 20 nm diameter were formed in the first step, in which the 2 nm gold NPs kept their individuality (high-resolution transmission electron microscopy, field emission scanning electron microscopy, and small-angle X-ray diffraction). In the second step, bcc crystals aggregated, forming compact micrometer-sized spherical particles. Under particular evaporation rates a third step of the self-assembly process was observed where micrometer-sized particles formed fractal structures. Increasing the initial concentration of gold NPs in the formulation led to more compact fractal structures in agreement with theoretical simulations. The surface percolation of NPs in fractal structures can be the basis of useful applications.

Formation of gold branched plates in diluted solutions of poly(vinylpyrrolidone) and their use for the fabrication of near-infrared-absorbing films and coatings.

Ribbon-like and branched gold nano- and microstructures were produced by simple heating of diluted aqueous solutions of poly(vinylpyrrolidone) (PVP) and HAuCl4. The reaction was carried out in a one-pot, one-step process at mild temperatures. Modification of the synthesis variables allowed the obtaining of structures with different sizes and branching degrees which formed stable hydrosols with characteristic colors. A mechanism for the growth of the crystals was proposed, based on the aggregation of metal units followed by reorientation and attachment processes facilitated by the presence of low concentrations of the polymer. These anisotropic structures were used to obtain large-area porous coatings on metallic, plastic, and glass substrates and to synthesize homogeneous polymer composites. The resulting gold-modified materials showed an important increase of absorption in the near-infrared (NIR) region of the electromagnetic spectrum, which could find interesting applications in the development of NIR-absorbing filters and coatings.

One-step synthesis of gold and silver hydrosols using poly(N-vinyl-2-pyrrolidone) as a reducing agent.

Synthesis of gold and silver hydrosols was carried out in a one-step process by reduction of aqueous solutions of metal salts using poly(N-vinyl-2-pyrrolidone) (PVP). Both kinds of metal nanoparticles were obtained without the addition of any other reducing agent, at low temperatures and using water as the synthesis solvent. Shape, size, and optical properties of the particles could be tuned by changing the employed PVP/metal salt ratio. It is proposed that PVP acts as the reducing agent suffering a partial degradation during the nanoparticles synthesis. Two possible mechanisms are proposed to explain the reduction step: direct hydrogen abstraction induced by the metal ion and/or reducing action of macroradicals formed during degradation of the polymer. Initial formation of the macroradicals might be associated with the metal-accelerated decomposition of low amounts of peroxides present in the commercial polymer.