Showing papers by "Jorge Pérez-Juste published in 2010"

Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials

Abstract: This chapter provides a brief overview of recent progress in the synthesis of silica-coated nanomaterials and their significant impact in different areas such as spectroscopy, magnetism, catalysis, and biology. The need to increase the topological complexity of colloid-based structures for the creation of designer materials with specific functionalities demands a better understanding of the relationships between material topology and material function. A number of reports have been devoted to silica coating of colloidal nanoparticles by aqueous classical methods such as Stober synthesis, use of silane coupling agents, and the sodium silicate water-glass methodology. Microemulsions are macroscopically homogeneous mixtures of water, organic solvent (oil), and surfactant, which on the microscopic level consist of heterogeneous domains of water and organic solvent separated by a surfactant monolayer. Various general approaches toward silica coating of inorganic nanoparticles within W/O microemulsions have been considered so far.

Catalysis by Au@pNIPAM Nanocomposites: Effect of the Cross-Linking Density

Abstract: Gold nanoparticles encapsulated in a thermoresponsive microgel (pNIPAM) were used as catalysts in the electron-transfer reaction between hexacyanoferrate(III) and borohydride ions. The thermosensitive pNIPAM network can act as a “nanogate” that can be opened or closed to a certain extent, thereby controlling the diffusion of reactants toward the catalytic core. Interestingly, the crosslinker density plays an important role, because it defines the thermal response of the Au@pNIPAM system and, in turn, the extent of the volume change and therefore the polymeric density. The catalytic activity of the encapsulated gold nanoparticles is thus affected both by temperature and by the composition of the shell. A mathematical model reproducing the key features of the temperature-controlled catalysis by our thermosensitive nanoparticles confirms the effect of diffusion rate through the shell on the actual reaction rate.

Modulation of Localized Surface Plasmons and SERS Response in Gold Dumbbells through Silver Coating

Abstract: We describe the modulation of localized surface plasmons in gold nanodumbbells through stepwise silver coating, along with a detailed discussion regarding the experimental parameters affecting the final core−shell morphology. Interestingly, whereas conformal growth was observed for thin coatings, for intermediate and high silver salt concentrations, the final nanoparticles end up with either rod-like or irregular faceted morphologies as a consequence of anisotropic silver growth. Upon silver reduction, pronounced changes in the optical properties were observed, which could be modeled using the boundary element method (BEM), which also allowed the assignment of different plasmon modes. Such core−shell Au@Ag nanoparticles are expected to serve as excellent SERS substrates, as significantly higher enhancement factors are expected for silver as compared to gold. Optical enhancing properties for SERS were tested with two laser lines, evidencing significantly larger enhancement factors for the bimetallic nanopa...

Colloidal Gold-Catalyzed Reduction of Ferrocyanate (III) by Borohydride Ions: A Model System for Redox Catalysis

Abstract: We report results on the large catalytic effect of spherical gold nanoparticles on the rate of reduction of hexacyanoferrate (III) by sodium borohydride in aqueous solution. Because the gold nanoparticles remain stable and no aggregation takes place during the reaction, it can be monitored until completion. The presence of colloidal gold leads to a considerable increase in the observed reaction rate and to a change in the order of reaction. The reaction is first-order with respect to the hexacyanoferrate (III) concentration and gold particle concentration, but the reaction order with respect to borohydride ion is more complex. The activation energy is found to be 15 kJ/mol for 15 nm gold particles. The redox reaction is activation-controlled under most conditions, but the rate of reaction approaches the diffusion limit for higher borohydride concentrations and is over 104 times faster than in the absence of the gold catalyst.

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

Abstract: 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.

Growing Au/Ag Nanoparticles within Microgel Colloids for Improved Surface‐Enhanced Raman Scattering Detection

Abstract: The synthesis, characterization, and assembly of different types of nanoparticles, which was established as a necessary prerequisite for the application of nanotechnology, have dramatically advanced over the last 20 years. However, it has recently been realized that the incorporation of multiple functionalities within nanoscale systems would become much more useful for most of the foreseen applications. Thus, the fabrication of multifunctional nanoparticles has become a major challenge. Among these systems, the incorporation of active (optically, catalytically, magnetically...) nanoparticles within so-called “smart” thermosensitive microgels has received significant attention over the last few years. The incorporation of nanoparticles can be accomplished either by in situ formation, by post-synthesis assembly or by direct polymerization on the nanoparticles surface. We have recently reported the growth of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels on the surface of gold nanoparticles, involving several steps, including the formation of a first polystyrene thin layer, followed by pNIPAM polymerization after the required purification process. Although gold nanoparticle growth could be achieved within the microgel shell, this synthesis was restricted to spherical nanoparticle seeds, whereas, for example, nanorods (which display a much more interesting optical response) were not properly incorporated. Thus, there was a need to both simplify the coating process and make it more widely applicable. We report here a novel procedure where butenoic acid is used for the synthesis of gold nanoparticles in aqueous surfactant solutions, in the presence of preformed Au seeds. Apart from the interesting observation that butenoic acid can be used as a reducing agent, this is particularly interesting because it provides the particles with a vinyl functionality, which should be useful for the direct pNIPAM polymerization on the nanoparticles surface and their subsequent encapsulation, while avoiding complicated surface functionalization steps. Although we have only optimized the reduction process for nanosphere growth, we also demonstrate that butenoic acid can replace cetyltrimethylammonium bromide (CTAB) molecules from Au nanoparticle surfaces, including Au nanorods, and adsorb on the metal surface, thereby facilitating the polymerization of pNIPAM on the metal core. The improved stability of the nanocomposites and the porosity of the pNIPAM shell allows subsequent reduction of metal atoms on the metal core, which was exploited for the overgrowth of pNIPAM encapsulated Au spheres and rods with both Au and Ag under mild conditions. In a previous publication we demonstrated the ability of these composite colloids to mechanically trap non-common surface-enhanced Raman scattering (SERS) analytes. However, the use of 60 nm gold spheres as colloidal cores and the impossibility of forming hot spots due to the physical barrier imposed by the pNIPAM shell severely limited the enhancing capability and thus the detection limit. In the present work, the SERS intensity was significantly increased through manipulation of the composition and the morphology of the nanoparticle cores. Thus, the first modification involved the controlled growth of uniform silver shells, a much more efficient plasmonic material, on the gold cores. Second, we exploited the near field concentration at the ends of nanorods to further increase the signal. Finally, we demonstrate that the molecular affinity of the pNIPAM shells toward analytes can be extended by tuning the surface charge, which would allow the electrostatic attraction of charged molecules. [a] R. Contreras-C ceres, Dr. I. Pastoriza-Santos, Dr. R. A. Alvarez-Puebla, Dr. J. P rez-Juste, Prof. 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 : juste@uvigo.es lmarzan@uvigo.es [b] R. Contreras-C ceres, Prof. A. Fern ndez-Barbero Departamento de F sica Aplicada Universidad de Almer a Almer a (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001261.

Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods in Aqueous Solution

Abstract: We report experimental and theoretical demonstration regarding the hydrogen-bonding mechanism behind the end-to-end assembly of gold nanorods modified with bifunctional linking molecules. Time-dependent assembly studies were carried out for different linking molecules at both higher and lower pH values with respect to their respective pKa values and were all found to be in agreement with a hydrogen-bonding theory. The results indicate that hydrogen bonding between protonated and unprotonated linking molecules is responsible for nanorod assembly in aqueous solution. Complementary information regarding the stability of the hydrogen-bonded configurations was obtained by density functional calculations for different protonation states.

Growth of Sharp Tips on Gold Nanowires Leads to Increased Surface-Enhanced Raman Scattering Activity.

Abstract: We report the formation of gold nanoparticles with a novel and useful morphology, comprising nanowires fully covered with sharp tips (thorned nanowires). The synthesis is based on a seeded-growth approach based the rapid overgrowth of ultrathin gold wires in N,N-dimethylformamide, in the presence of poly(vinylpyrrolidone). The process allows a fine control over the thickness of the final wires, as well as the tunability of the number and sharpness of the thorns. These new plasmonic nanostructures display extremely strong optical enhancing properties and can be readily used as platforms for SERS and for integration in ultrasensitive optical devices.

Growth of pentatwinned gold nanorods into truncated decahedra

Abstract: The growth mechanism from pentatwinned (PTW) gold nanorods into truncated quasi-decahedral particles when a gold salt (HAuCl₄) is reduced by N, N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone), was elucidated through a combination of different techniques, including transmission and scanning electron microscopy, high resolution TEM and selected area electron diffraction. Particles with intermediate shapes between the original pentatwinned Au nanorods, used as seeds, and the final quasi-decahedral particles were obtained by simply tuning the [HAuCl₄] to [seeds] ratio. From the thorough structural analysis of all the intermediate morphologies obtained, it was concluded that gradual morphology changes are related to the preferential growth of higher energy crystallographic facets. As a result of the particle growth and concomitant decreased anisotropy, a progressive blue-shift of the surface plasmon resonance bands of the nanoparticles was registered by vis-NIR extinction spectroscopy.

Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials

Abstract: This chapter provides a brief overview of recent progress in the synthesis of silica-coated nanomaterials and their significant impact in different areas such as spectroscopy, magnetism, catalysis, and biology. The need to increase the topological complexity of colloid-based structures for the creation of designer materials with specific functionalities demands a better understanding of the relationships between material topology and material function. A number of reports have been devoted to silica coating of colloidal nanoparticles by aqueous classical methods such as Stober synthesis, use of silane coupling agents, and the sodium silicate water-glass methodology. Microemulsions are macroscopically homogeneous mixtures of water, organic solvent (oil), and surfactant, which on the microscopic level consist of heterogeneous domains of water and organic solvent separated by a surfactant monolayer. Various general approaches toward silica coating of inorganic nanoparticles within W/O microemulsions have been considered so far.

Interaction of light and fast electrons with metallic nanoparticles

Abstract: The shape and size of nanoparticles can be tuned through chemical synthesis parameters, which results in a variety of morphologies. The electromagnetic response of highly anisotropic metallic nanoparticles on external stimuli (light and fast electrons) has been modelled using a full 3D boundary element method. A good agreement between numerical calculations and experimental spectra has been obtained.