Benito Rodríguez-González

High-yield synthesis and optical response of gold nanostars

Multipod Au nanoparticles (nanostars) with single crystalline tips were synthesized in extremely high yield through the reduction of HAuCl(4) in a concentrated solution of poly(vinylpyrrolidone) (PVP) in N,N-dimethylformamide (DMF), in the presence of preformed Au nanoparticle seeds, but with no need for external energy sources. Nanostar dispersions display a well-defined optical response, which was found (through theoretical modeling) to comprise a main mode confined within the tips and a secondary mode confined in the central body. Calculations of the surface enhanced Raman scattering (SERS) response additionally show that this morphology will be relevant for sensing applications.

Multishell bimetallic AuAg nanoparticles: synthesis, structure and optical properties

Core–shell and multishell bimetallic AuAg nanoparticles have been synthesized by successive reduction of metal salts with ascorbic acid on pre-made seeds in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The coverage of the seeds is extremely uniform, although in some cases deviations from a spherical shape are observed with the formation of nanorods or nanoprisms. The evolution of the optical properties as further metal layers are deposited is very dramatic and can be modelled using Mie theory for multilayer spheres. However, preliminary results using high-resolution STEM-XEDS elemental mapping suggest that the actual distribution of the two metals within the multilayer spheres may involve (partial) alloying of the metals.

Design of SERS-Encoded, Submicron, Hollow Particles Through Confined Growth of Encapsulated Metal Nanoparticles

The synthetic architectures of complex inorganic nanostructures, including multifunctional hollow capsules, are expected to play key roles in many different applications, such as drug delivery, photonic crystals, nanoreactors, and sensing. Implementation of novel strategies for the fabrication of such materials is needed because of the infancy of this knowledge, which still limits progress in certain areas. Herein we report a straightforward synthetic approach for the development of multifunctional submicron reactors comprising catalytic gold nanoparticles (2-3 nm) confined inside hollow silica capsules. Additionally, the confined growth of encapsulated metal nanoparticles was carried out to evidence the usefulness and functionality of these reactors in catalytic applications and as an approach for the development of novel complex nanostructures. Their potential and multifunctionality have been pointed out by fabrication of SERS-encoded submicrometer particles with shape and size uniformity for use in antigen biosensing; this was accomplished via codification of gold nanoparticle islands grown onto their inner surfaces.

Temperature, pH, and ionic strength induced changes of the swelling behavior of PNIPAM-poly(allylacetic acid) copolymer microgels

The volume phase transition of colloidal microgels made of N-isopropylacrylamide (NIPAM) is well-studied and it is known that the transition temperature can be influenced by copolymerization. A series of poly( N-isopropylacrylamide- co-allylacetic acid) copolymers with different contents of allylacetic acid (AAA) was synthesized by means of a simple radical polymerization approach. The thermoresponsive behavior of these particles was studied using dynamic light scattering (DLS). Further characterization was done by employing transmission electron microscopy (TEM) and zeta potential measurements. TEM observations reveal the approximately spherical shape and low polydispersity of the copolymer particles. In addition, the measured zeta potentials provide information about the relative surface charge. Since these copolymers are much more sensitive to external stimuli such as pH and ionic strength than their pure PNIPAM counterparts, the volume phase transition was investigated at two different pH values and various salt concentrations. At pH 10 for the copolymer microgels with the highest AAA content, a significant shift of the volume phase transition temperature toward higher values is found. For higher AAA content, a change in pH from 8 to 10 can induce a change in radius of up to 100 nm making the particles interesting as pH controlled actuators.

Loading of Exponentially Grown LBL Films with Silver Nanoparticles and Their Application to Generalized SERS Detection

Feature film: Thin films made by exponential layer-by-layer growth display high diffusivity and can be readily infiltrated with inorganic nanoparticles. They can sequestrate molecular systems from solution as a function of the composition of their layers, while providing intense surface-enhanced Raman scattering (SERS) signals (see picture).

Highly Active Nanoreactors: Nanomaterial Encapsulation Based on Confined Catalysis**

It happens inside: highly active nanoreactors are prepared by encapsulating dendritic Pt nanoparticles (NPs) grown on a polystyrene template inside hollow porous silica capsules. The catalytic activity of these Pt NPs is preserved after encapsulation and template removal. Different metals, such as Ni, can thus be reduced inside the capsules, thereby leading to the formation of composites with tunable magnetic properties.

Influence of silver ions on the growth mode of platinum on gold nanorods

Gold nanorods were used as seeds for platinum growth, using a mild reducing agent, ascorbic acid, in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Highly preferential growth on the tips or complete overcoating can be achieved by manipulation of the reduction conditions, among which the presence of silver ions was found to be highly dominant. In either case, the growth was found to be epitaxial, as demonstrated by high resolution electron microscopy and Fourier transform analysis. Additionally, the deposition mode leads to very different effects on the optical properties of the nanoparticles, with tip growth inducing huge surface plasmon red shifts.

Multiwalled Carbon Nanotubes Drive the Activity of Metal@oxide Core–Shell Catalysts in Modular Nanocomposites

Rational nanostructure manipulation has been used to prepare nanocomposites in which multiwalled carbon nanotubes (MWCNTs) were embedded inside mesoporous layers of oxides (TiO(2), ZrO(2), or CeO(2)), which in turn contained dispersed metal nanoparticles (Pd or Pt). We show that the MWCNTs induce the crystallization of the oxide layer at room temperature and that the mesoporous oxide shell allows the particles to be accessible for catalytic reactions. In contrast to samples prepared in the absence of MWCNTs, both the activity and the stability of core-shell catalysts is largely enhanced, resulting in nanocomposites with remarkable performance for the water-gas-shift reaction, photocatalytic reforming of methanol, and Suzuki coupling. The modular approach shown here demonstrates that high-performance catalytic materials can be obtained through the precise organization of nanoscale building blocks.

Aerobic synthesis of cu nanoplates with intense plasmon resonances.

Nanoparticles of coinage metals (Au, Ag, Cu) are known to display attractive optical properties, arising from localized surface plasmon resonances in the visible and the nearinfrared (NIR) frequencies. Such properties have stimulated the development of numerous synthetic (colloid chemistry) strategies for tuning the optical response through control of gold and silver nanoparticle size and shape. However, copper is less popular, mainly because the fabrication of chemically stable Cu colloids with intense plasmon resonance bands is far more complicated, first because they are prone to fast oxidation, but also because of the lower ‘‘free-electron character’’ of copper. The free-electron behavior of Au and Ag colloids in the visible range is reflected in the fairly constant value of the imaginary part of their dielectric functions, which is responsible for the sharp and prominent extinction bands displayed by colloids of these metals. For Cu metal, the real and imaginary parts of the dielectric function vary markedly in the UV–Vis range, so that electronic interband transitions from the valence band to the Fermi level overlap the plasmon resonances up to 600 nm. Since interband transitions can efficiently damp surface plasmon resonances through dephasing of the optical polarization associated with the electron oscillation, well-defined plasmon bands can only be achieved if the resonance wavelength is shifted away from the interband transitions. This can in principle be achieved for non-spherical nanoparticles, but it is still difficult for copper because of the extended range of the interband transitions. Although a number of methods have been reported for the fabrication of Cu nanoparticles, such as UV-light irradiation, pulsed sonoelectrochemical reduction, g-irradiation, chemical or polyol reduction of copper salts, and growth in reverse micelles, few of them

Rapid Epitaxial Growth of Ag on Au Nanoparticles: From Au Nanorods to Core–Shell Au@Ag Octahedrons

The strongly shape-dependent optical properties of metal nanoparticles have motivated the rapid development of new and efficient strategies toward morphology control. However, a highly efficient control over shape and size has been mainly achieved for gold. Therefore, an interesting route toward the production of other metal nanoparticles with tailored morphology would be the use of pre-formed gold nanocrystals as templates, on which other metals could be grown. This would allow not only a tight control over the growth, and morphology of the nanocrystals, but also an interesting enhancement of the functionality of such nanomaterials, the properties of which would differ from those found in similar nanostructures made of the individual constituent metals. In particular, various approaches have been developed to fabricate Au@Ag core–shell nanoparticles by the epitaxial growth of Ag on preformed Au nanoparticles, which were in general based on either chemical or photoinduced reduction processes. 12] The former often make use of a weak reducing agent, such as ascorbic acid or hydroxylamine, so that the reduction takes place exclusively on the surface of the metallic seed particles, which act as catalysts. 34] However, this can only be achieved within a narrow pH range so that homogeneous nucleation of Ag nanoparticles in solution is avoided. Herein, we describe a simple and rapid method to grow silver on single-crystal Au nanorods, resulting in single-crystal core–shell Au@Ag nanoparticles with tailored morphology, ranging from nanorods all the way to spheres, through octahedrons, and thereby giving rise to a remarkable control over the optical response spanning the whole visible range and into the near IR. The growth method is based on the use of hydroquinone as reducing agent. Although the preparation of silver nanoparticles using hydroquinone has been previously reported, this typically resulted in a rather poor control over shape and size. 18] Additionally, hydroquinone has also been used to grow thin silver shells on gold nanoparticles as a means to amplify their scattering properties, but this was restricted to very thin shells on small spherical particles. 20] However, we demonstrate here that these processes can be utilized in a much more controlled manner, thus allowing exquisite morphology control. Based on our previous experience on the reshaping of single-crystal gold nanorods into octahedrons, we decided to explore the coating of the same type of nanorods with silver, so as to tune the morphology of the resulting core–shell particles. Interestingly, we found that silver grows preferentially on the lateral facets of the Au nanorods, so that, indeed complete reshaping of the initial rods into Au@Ag octahedrons and even spheres was achieved, which might be related to the prior capping agent exchange from cetyltrimethylammonium bromide (CTAB) to methoxy-poly(ethylene glycol)-thiol (mPEG-SH). Detailed analysis of the optical response of a number of Au@Ag nanoparticles with varying Ag shell and thickness, as well as theoretical modeling by means of the boundary element method (BEM), revealed that this system provides an excellent opportunity to gradually change the localized surface plasmon resonance (LSPR) frequency from the NIR, all the way through the complete visible range. The synthetic method is thus based on the use of hydroquinone (HQ) as a mild reducing agent to reduce Ag ions selectively onto the gold nanorods surface. Single-crystal Au nanorods with an average aspect ratio of 4.6 0.6 (61.7 5.2 13.5 1.2 nm) were prepared by standard seeded growth in CTAB (see the Experimental Section in the [a] A. S nchez-Iglesias, E. Carb -Argibay, Dr. A. Glaria, Dr. B. Rodr guez-Gonz lez, Dr. J. P rez-Juste, Dr. I. Pastoriza-Santos, Prof. Dr. L. M. Liz-Marz n Departamento de Qu mica F sica and Unidad Asociada CSIC-Universidade de Vigo 36310 Vigo (Spain) Fax: (+34) 986812556 E-mail : pastoriza@uvigo.es Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000144.