Governing the morphology of Pt-Au heteronanocrystals with improved electrocatalytic performance

Platinum-gold heteronanostructures comprising either dimer (Pt-Au) or core-satellite (Pt@Au) configurations were synthesized by means of a seeded growth procedure using platinum nanodendrites as seeds. Careful control of the reduction kinetics of the gold precursor can be used to direct the nucleation and growth of gold nanoparticles on either one or multiple surface sites simultaneously, leading to the formation of either dimers or core-satellite nanoparticles, respectively, in high yields. Characterization by electron tomography and high resolution electron microscopy provided a better understanding of the actual three-dimensional particle morphology, as well as the Au-Pt interface, revealing quasi-epitaxial growth of Au on Pt. The prepared Pt-Au bimetallic nanostructures are highly efficient catalysts for ethanol oxidation in alkaline solution, showing accurate selectivity, high sensitivity, and improved efficiency by generating higher current densities than their monometallic counterparts.

Effect of the cross-linking density on the thermoresponsive behavior of hollow PNIPAM microgels

We report on the fabrication of thermally responsive hollow pNIPAM particles through the oxidation of the metal core in an Au@pNIPAM system. The selective oxidation of the Au core is achieved by addition of AuCl4(-) to an aqueous dispersion of Au@pNIPAM particles in the presence of cetyltrimethylammonium bromide (CTAB). We fabricate hollow pNIPAM particles with three cross-linking densities (N,N'-methylenebis(acrylamide), BA, at 5%, 10%, and 17.5%). The study of the effect of the amount of BA within the microgel network was performed by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM), showing its key role in determining the final hollow structure and thermal response. While the thermal responsiveness is largely achieved at low cross-linking densities, the hollow structure only remains at larger cross-linking densities. This was further confirmed by cryo-TEM analysis of hollow pNIPAM particles below and above the volume phase transition temperature (VPTT). Thus, it clearly shows (i) the shrinking of particle size with the temperature at low cross-linking density and (ii) the dependence of particle size on the amount of cross-linker for the final hollow pNIPAM structure. Observed differences in the hollow pNIPAM structure are attributed to different elastic contributions (Π(elas)), showing higher elasticity for microgels synthesized at lower amount of BA.

Palladium Nanoparticle-Loaded Cellulose Paper: A Highly Efficient, Robust, and Recyclable Self-Assembled Composite Catalytic System

We present a novel strategy based on the immobilization of palladium nanoparticles (Pd NPs) on filter paper for development of a catalytic system with high efficiency and recyclability. Oleylamine-capped Pd nanoparticles, dispersed in an organic solvent, strongly adsorb on cellulose filter paper, which shows a great ability to wick fluids due to its microfiber structure. Strong van der Waals forces and hydrophobic interactions between the particles and the substrate lead to nanoparticle immobilization, with no desorption upon further immersion in any solvent. The prepared Pd NP-loaded paper substrates were tested for several model reactions such as the oxidative homocoupling of arylboronic acids, the Suzuki cross-coupling reaction, and nitro-to-amine reduction, and they display efficient catalytic activity and excellent recyclability and reusability. This approach of using NP-loaded paper substrates as reusable catalysts is expected to open doors for new types of catalytic support for practical applications.

Enhanced electrochemical sensing of polyphenols by an oxygen-mediated surface

We report a straightforward heat treatment in air of commercial screen-printed carbon electrodes (SPCE) at different temperatures and times (ht-SPCE) that produces considerable electrocatalytic effects.

Metal nanoparticles and supramolecular macrocycles: a tale of synergy

In this minireview, we summarize current research dealing with the combination of noble-metal nanoparticles and different families of supramolecular macrocycles (cyclodextrins, cucurbit[n]urils, calixarenes, and pillar[n]arenes). We intended to select relevant publications on the synthesis of noble-metal nanoparticles with macrocycles acting as capping agents or/and reducing agents, as well as on the post-synthetic metal-nanoparticle modification with macrocycles. We also discuss strategies in which supramolecular chemistry is applied to direct the self-assembly of nanoparticles and formation of polymer composites. We finally describe the main applications of these materials in various fields.

Plasmon Mapping in Au@Ag Nanocube Assemblies

Surface plasmon modes in metallic nanostructures largely determine their optoelectronic properties. Such plasmon modes can be manipulated by changing the morphology of the nanoparticles or by bringing plasmonic nanoparticle building blocks close to each other within organized assemblies. We report the EELS mapping of such plasmon modes in pure Ag nanocubes, Au@Ag core-shell nanocubes, and arrays of Au@Ag nanocubes. We show that these arrays enable the creation of interesting plasmonic structures starting from elementary building blocks. Special attention will be dedicated to the plasmon modes in a triangular array formed by three nanocubes. Because of hybridization, a combination of such nanotriangles is shown to provide an antenna effect, resulting in strong electrical field enhancement at the narrow gap between the nanotriangles.

Nickel nanoparticle-doped paper as a bioactive scaffold for targeted and robust immobilization of functional proteins

Cellulose-based materials are widely used in analytical chemistry as platforms for chromatographic and immunodiagnostic techniques. Due to its countless advantages (e.g., mechanical properties, three-dimensional structure, large surface to volume area, biocompatibility and biodegradability, and high industrial availability), paper has been rediscovered as a valuable substrate for sensors. Polymeric materials such as cellulosic paper present high protein capture ability, resulting in a large increase of detection signal and improved assay sensitivity. However, cellulose is a rather nonreactive material for direct chemical coupling. Aiming at developing an efficient method for controlled conjugation of cellulose-based materials with proteins, we devised and fabricated a hybrid scaffold based on the adsorption and in situ self-assembly of surface-oxidized Ni nanoparticles on filter paper, which serve as "docking sites" for the selective immobilization of proteins containing polyhistidine tags (His-tag). We demonstrate that the interaction between the nickel substrate and the His-tagged protein G is remarkably resilient toward chemicals at concentrations that quickly disrupt standard Ni-NTA and Ni-IDA complexes, so that this system can be used for applications in which a robust attachment is desired. The bioconjugation with His-tagged protein G allowed the binding of anti-Salmonella antibodies that mediated the immuno-capture of live and motile Salmonella bacteria. The versatility and biocompatibility of the nickel substrate were further demonstrated by enzymatic reactions.

Laser heating tunability by off-resonant irradiation of gold nanoparticles

Temperature changes in the vicinity of a single absorptive nanostructure caused by local heating have strong implications in technologies such as integrated electronics or biomedicine. Herein, the temperature changes in the vicinity of a single optically trapped spherical Au nanoparticle encapsulated in a thermo-responsive poly(N-isopropylacrylamide) shell (Au@pNIPAM) are studied in detail. Individual beads are trapped in a counter-propagating optical tweezers setup at various laser powers, which allows the overall particle size to be tuned through the phase transition of the thermo-responsive shell. The experimentally obtained sizes measured at different irradiation powers are compared with average size values obtained by dynamic light scattering (DLS) from an ensemble of beads at different temperatures. The size range and the tendency to shrink upon increasing the laser power in the optical trap or by increasing the temperature for DLS agree with reasonable accuracy for both approaches. Discrepancies are evaluated by means of simple models accounting for variations in the thermal conductivity of the polymer, the viscosity of the aqueous solution and the absorption cross section of the coated Au nanoparticle. These results show that these parameters must be taken into account when considering local laser heating experiments in aqueous solution at the nanoscale. Analysis of the stability of the Au@pNIPAM particles in the trap is also theoretically carried out for different particle sizes.

Size tunable Au@Ag core-shell nanoparticles: synthesis and surface-enhanced Raman scattering properties

We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core-shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core-shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core-shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core-shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core-shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core-shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

Controllable nitric oxide release in the presence of gold nanoparticles

A major problem associated with nitric oxide (NO) donors is the release of the desired amount of NO at a specific site. A number of platforms have been developed for the regulation of NO dosage. We present the use of citrate-stabilized gold nanoparticles as a platform to regulate NO release. Because of the affinity between gold and thiols, the characteristic -S-NO bond of S-nitrosothiols (RSNOs) breaks in the presence of gold nanoparticles, thereby releasing NO and modifying the gold nanoparticle surface with the corresponding thiol. This system allows for surface-controlled NO release, where the amount of NO released is proportional to the number of thiols bound to the gold nanoparticle surface. Moreover, by employing an amperometric technique to detect the maximum NO release, we were able to estimate the stoichiometry of the reaction, that is, the number of adsorbed RSNO molecules per gold nanoparticle. A kinetic model for NO release and its subsequent decomposition is proposed and used to fit the experimental results. The reaction was found to be zeroth- and first-order with respect to RSNO and gold nanoparticles, respectively.

Dimethylformamide-mediated synthesis of water-soluble platinum nanodendrites for ethanol oxidation electrocatalysis

Herein we describe the synthesis of water-soluble platinum nanodendrites in dimethylformamide (DMF), in the presence of polyethyleneimine (PEI) as a stabilizing agent. The average size of the dendrites is in the range of 20-25 nm while their porosity can be tuned by modifying the concentration of the metal precursor. Electron tomography revealed different crystalline orientations of nanocrystallites in the nanodendrites and allowed a better understanding of their peculiar branching and porosity. The high surface area of the dendrites (up to 22 m(2) g(-1)) was confirmed by BET measurements, while X-ray diffraction confirmed the abundance of high-index facets in the face-centered-cubic crystal structure of Pt. The prepared nanodendrites exhibit excellent performance in the electrocatalytic oxidation of ethanol in alkaline solution. Sensing, selectivity, cycleability and great tolerance toward poisoning were demonstrated by cyclic voltammetry measurements.

Size-dependent surface plasmon resonance broadening in nonspherical nanoparticles: single gold nanorods

The dependence of the spectral width of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanorods protected by a silica shell is investigated as a function of their size. Experiments were performed using the spatial modulation spectroscopy technique that permits determination of both the spectral characteristics of the LSPR of an individual nanoparticle and its morphology. The measured LSPR is shown to broaden with reduction of both the nanorod length and its diameter, which is in contrast with the predictions of existing classical and quantum theoretical models. This behavior can be reproduced assuming the LSPR width linearly depends on the inverse of an effective length proportional to the square root of the particle surface with the same slope as that recently determined for silica-coated silver nanospheres.

Hydrophobic interactions modulate self-assembly of nanoparticles

Hydrophobic interactions constitute one of the most important types of nonspecific interactions in biological systems, which emerge when water molecules rearrange as two hydrophobic species come close to each other. The prediction of hydrophobic interactions at the level of nanoparticles (Brownian objects) remains challenging because of uncontrolled diffusive motion of the particles. We describe here a general methodology for solvent-induced, reversible self-assembly of gold nanoparticles into 3D clusters with well-controlled sizes. A theoretical description of the process confirmed that hydrophobic interactions are the main driving force behind nanoparticle aggregation.

Highly transparent and conductive films of densely aligned ultrathin Au nanowire monolayers

The combination of low electrical resistance and high optical transparency in a single material is very uncommon. Developing these systems is a scientific challenge and a technological need, to replace ITO in flexible electronic components and other highly demanding applications. Here we report a facile method to prepare single layers of densely aligned ultrathin Au-nanowires, homogeneous over cm(2) areas. The as-deposited films show an electrical/optical performance competitive with ITO and graphene-based electrodes. Moreover, the Au-films show a good stability under ambient conditions, and the large aspect ratio of the ultrathin nanowires makes them perfect for deposition in flexible substrates.

Effects of gold nanoparticles on the stability of microbubbles

Surfactant-coated microbubbles are utilized in a wide variety of applications, from wastewater purification to contrast agents in medical ultrasound imaging. In many of these applications, the stability of the microbubbles is crucial to their effectiveness. Controlling this, however, represents a considerable challenge. In this study, the potential for stabilizing microbubbles using solid nanoparticles adsorbed onto their surfaces was explored. A new theoretical model has been developed to describe the influence of interfacially adsorbed solid particles upon the dissolution of a gas bubble in a liquid. The aim of this work was to test experimentally the prediction of the model that the presence of the nanoparticles would inhibit gas diffusion and coalescence/disproportionation, thus increasing the life span of the bubbles. Near-monodisperse microbubbles (~100 μm diameter) were prepared using a microfluidic device and coated with a surfactant, with and without the addition of a suspension of spherical gold nanoparticles (~15 nm diameter). The experimental results confirmed the theoretical predictions that as the surface concentration of gold nanoparticles increased the bubbles underwent negligible changes in their size and size distribution over a period of 30 days at the ambient temperature and pressure. Under the same conditions, bubbles coated with the same surfactant but no nanoparticles survived only a matter of hours.

Steric hindrance induces crosslike self-assembly of gold nanodumbbells

In the formation of colloidal molecules, directional interactions are crucial for controlling the spatial distribution of the building blocks. Anisotropic nanoparticles facilitate directional clustering via steric constraints imposed by each specific shape, thereby restricting assembly along certain directions. We show in this Letter that the combination of patchiness (attraction) and shape (steric hindrance) allows assembling gold nanodumbbell building blocks into crosslike dimers with well-controlled interparticle distance and relative orientation. Steric hindrance between interacting dumbbell-like particles opens up a new synthetic approach toward low-symmetry plasmonic clusters, which may significantly contribute to understand complex plasmonic phenomena.

Antibonding plasmon modes in colloidal gold nanorod clusters

The optical response of nanoplasmonic colloids in disperse phase is strictly related to their shape. However, upon self-assembly, new optical features, for example, bonding or antibonding modes, emerge as a result of the mutual orientations of nanoparticles. The geometry of the final assemblies often determines which mode is dominating in the overall optical response. These new plasmon modes, however, are mostly observed in silico, as self-assembly in the liquid phase leads to cluster formation with a broad range of particle units. Here we show that low-symmetry clustering of gold nanorods (AuNRs) in solution can also reveal antibonding modes. We found that UV-light irradiation of colloidal dispersions of AuNRs in N-methyl-2-pyrrolidone (NMP), stabilized by poly(vinylpyrrolidone) (PVP) results in the creation of AuNRs clusters with ladderlike morphology, where antibonding modes can be identified. We propose that UV irradiation induces formation of radicals in solvent molecules, which then promote cross-linking of PVP chains on the surface of adjacent particles. This picture opens up a number of relevant questions in nanoscience and is expected to find application in light induced self-assembly of particles with various compositions and morphologies.

Ordered arrays of gold nanostructures from interfacially assembled Au@PNIPAM hybrid nanoparticles

In this Article, we report on the assembly of hybrid Au@PNIPAM core-shell particles at the air/water interface, their transfer onto solid substrates, and the controlled combustion of the organic material to produce arrays of gold nanoparticles. A detailed investigation on the assembly behavior of such soft hybrid colloids at the air/water interface was performed by correlating the surface pressure-area isotherms with SEM and AFM images from samples transferred at different surface pressures. The hybrid particles display a complex behavior at the interface, and we could distinguish three distinct phases with varying interparticle spacings at different compression. The transfer process presented enables the decoration of topologically structured substrates with gold nanoparticle arrays, and the order of the initial monolayers is retained in the arrays of inorganic gold nanoparticles. The change in monolayer morphology upon compression can therefore be used to tailor the interparticle distance between approximately 650 and 300 nm without exchanging the colloids. More sophisticated gold nanostructures can be patterned into symmetric arrays using a similar protocol, which we demonstrate for nanostars and nanorods.

Colloidal synthesis of gold semishells

This work describes a novel and scalable colloid chemistry strategy to fabricate gold semishells based on the selective growth of gold on Janus silica particles (500 nm in diameter) partly functionalized with amino groups. The modulation of the geometry of the Janus silica particles allows us to tune the final morphology of the gold semishells. This method also provides a route to fabricating hollow gold semishells through etching of the silica cores with hydrofluoric acid. The optical properties were characterized by visible near-infrared (vis-NIR) spectroscopy and compared with simulations performed using the boundary element method (BEM). These revealed that the main optical features are located beyond the NIR region because of the large core size.

Acoustic Vibrations in Bimetallic Au@Pd Core-Shell Nanorods

The acoustic vibrations of gold nanorods coated with palladium were investigated as a function of Pd amount using ultrafast pump-probe spectroscopy. Both the extensional and breathing vibrational modes of the nanorods were coherently excited and detected. This permits precise determination of their periods, which were found to decrease and increase with Pd deposition, for the extensional and vibrational modes, respectively. These opposite behaviors reflect changes of the nanoparticle size and mechanical properties, in agreement with numerical simulations. Comparison of experimental and computed periods yields information on the amount of deposited Pd, providing a novel tool to characterize bicomponent nano-objects for small fractions of one of the components (Pd/Au atomic fraction down to 5%).

Growth and branching of gold nanoparticles through mesoporous silica thin films

Composite materials made of mesoporous oxide thin films containing metallic nanoparticles are of high interest in various fields, including catalysis, biosensing and non-linear optics. We demonstrate in this work the fabrication of such composite materials containing a sub-monolayer of gold nanoparticles (GNPs) of various shapes covered with mesoporous silica thin films. Additionally, the shape of the GNPs (and thus their optical properties) can be modified in situ through seeded growth and branching. Such growth proceeds upon wetting with HAuCl(4) solution, a surfactant (cetyltrimethylammonium bromide, CTAB) and a mild reducing agent (ascorbic acid, AA). The effect of varying several reaction parameters (time and CTAB and AA concentrations) was evaluated, showing that more anisotropic particles are obtained at longer reaction times, lower CTAB concentration and higher AA concentration. The final shape of the GNPs was also found to depend on their initial shape and size, as well as the pore size of the mesoporous film covering them. Because the growth proceeds through the pores of the film, it may lead to shapes that are not easily obtained in solution, such as particles with branches on one side only. Finally, we have confirmed that no damage was induced to the mesoporous silica structure during the growth process and thus the final particles remain well covered by the thin film, which can eventually be used as a filter between the GNPs and the outer medium.