Ana Sánchez-Iglesias

Quantitative Determination of the Size Dependence of Surface Plasmon Resonance Damping in Single Ag@SiO2 Nanoparticles

The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.

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.

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.

Synthesis of Multifunctional Composite Microgels via In Situ Ni Growth on pNIPAM-Coated Au Nanoparticles

Novel colloidal composites have been designed to incorporate multiple functionalities that allow optical detection, magnetic manipulation, molecular trapping, and thermal response. Such particles are made of gold nanoparticle cores covered by a thin layer of metallic nickel and a poly(N-isopropylacrylamide) (pNIPAM) shell. While the gold cores provide efficient optical response through localized surface plasmon resonances, nickel allows external magnetic manipulation and the pNIPAM shell can be swollen or collapsed as a function of temperature, thus allowing capture and release of various types of molecules.

Glycans as Biofunctional Ligands for Gold Nanorods: Stability and Targeting in Protein-Rich Media

Poly(ethylene glycol) (PEG) has become the gold standard for stabilization of plasmonic nanoparticles (NPs) in biofluids, because it prevents aggregation while minimizing unspecific interactions with proteins. Application of Au NPs in biological environments requires the use of ligands that can target selected receptors, even in the presence of protein-rich media. We demonstrate here the stabilizing effect of low-molecular-weight glycans on both spherical and rod-like plasmonic NPs under physiological conditions, as bench-marked against the well-established PEG ligands. Glycan-coated NPs are resistant to adsorption of proteins from serum-containing media and avoid phagocytosis by macrophage-like cells, but retain selectivity toward carbohydrate-binding proteins in protein-rich biological media. These results open the way toward the design of efficient therapeutic/diagnostic glycan-decorated plasmonic nanotools for specific biological applications.

AuAg bimetallic nanoparticles: formation, silica-coating and selective etching

A time-resolved study of the formation of AuAg alloy nanoparticles during boiling of AgNO3 and HAuCl4 in the presence of sodium citrate has been performed by monitoring the UV-visible spectra of the solutions. This study reveals clear differences with respect to the formation of pure Au particles, and suggests that gold and silver nanoparticles nucleate separately, but lattice rearrangement eventually leads to formation of alloy nanoparticles with the expected composition, which was confirmed by high resolution TEM. Additionally, we studied the chemical reaction of the alloy nanoparticles with NH4OH, observing that part of the Ag atoms get oxidized, but reorganization of the crystal lattice during the reaction prevents full transformation into pure gold nanoparticles. This allows the synthesis of silica-coated bimetallic particles with tailored plasmon resonance between ca. 430 and 520 nm, as well as homogeneous incorporation of the coated alloy nanoparticles within silica gels.

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.

Rabi Splitting in Photoluminescence Spectra of Hybrid Systems of Gold Nanorods and J-Aggregates

We experimentally and theoretically investigate the interactions between localized plasmons in gold nanorods and excitons in J-aggregates under ambient conditions. Thanks to our sample preparation procedure we are able to track a clear anticrossing behavior of the hybridized modes not only in the extinction but also in the photoluminescence (PL) spectra of this hybrid system. Notably, while previous studies often found the PL signal to be dominated by a single mode (emission from so-called lower polariton branch), here we follow the evolution of the two PL peaks as the plasmon energy is detuned from the excitonic resonance. Both the extinction and PL results are in good agreement with the theoretical predictions obtained for a model that assumes two interacting modes with a ratio between the coupling strength and the plasmonic losses close to 0.4, indicative of the strong coupling regime with a significant Rabi splitting estimated to be ∼200 meV. The evolution of the PL line shape as the plasmon is detuned depends on the illumination wavelength, which we attribute to an incoherent excitation given by decay processes in either the metallic rods or the J-aggregates.

Measuring Lattice Strain in Three Dimensions through Electron Microscopy

The three-dimensional (3D) atomic structure of nanomaterials, including strain, is crucial to understand their properties. Here, we investigate lattice strain in Au nanodecahedra using electron tomography. Although different electron tomography techniques enabled 3D characterizations of nanostructures at the atomic level, a reliable determination of lattice strain is not straightforward. We therefore propose a novel model-based approach from which atomic coordinates are measured. Our findings demonstrate the importance of investigating lattice strain in 3D.