For gold nanorods the intrinsic shape-anisotropy offers the prospect of anisotropic assembly, provided their regionselective surface modification can be realized. Here we developed a nanorod with patchy surface chemistry, featuring positively charged molecules in the tip region and polymer molecules at the sides by careful control of molecule concentrations during ligand exchange. When these patchy nanorods are assembled with small negatively spherical particles, electric double layer interaction can direct the assembly of two nanospheres at the opposite ends of the nanorods. The PEG chains promote the selectivity of the procedure. As the size of the nanospheres increases, they start to shift towards the side of the nanorod due to increased van der Waals interaction. When the relative size of the nanospheres is even larger, only a single nanoshpere is assembled, but instead of the tip region, they are attached to side of the nanorods. The apparent cross-over of the region-selectivity can be interpreted in terms of colloidal interactions, i.e. the second spherical particle is excluded due to nanosphere-nanosphere electric double layer repulsion, while the large vdW attraction result in a side positioning of the single adsorbed spherical particle. The results underline the importance of absolute values of the different interaction strengths and length scales in the programmed assembly of pathy nanoscale building blocks.

In this study controlled clustering kinetics is demonstrated for PEG grafted gold nanoparticles, in response to applied environmental stimuli; the temperature and ionic strength of the medium. It is also found that the rate of assembly determines the structure of the prepared clusters. After the system is brought out of equilibrium, time-dependent extinction and dynamic light scattering data are used to follow the evolution of nanoparticle cluster formation in real time. The results show that the rate of assembly increases with increasing ionic strength or temperature of the medium. As a result the nanoparticle cluster size scales with ionic strength and temperature, over a cluster size range from a few particle sizes up to the micron-scale. It is found that, even at the lowest ionic strength, the electric double layer repulsion is eliminated; hence the observed differences in kinetics and in cluster structure arise from modulation of the repulsive steric interactions between nanoparticles. The approach should be extendable to suspensions of other nanoparticle types, where the nanoparticle stability is determined by surface-grafted responsive macromolecules.

Substrate properties might significantly influence the scattering spectra of supported plasmonic nanoparticles because of different damping mechanisms. In this work, indium tin oxide substrates are modified by the combination of nanosphere lithography and ion-bombardment to create a nanopattern with sharp boundaries between the irradiated and masked regions. The single-particle scattering spectra of gold nanorods distributed over the nanopattern are investigated in detail. For nanorods located purely on either the masked or implanted areas, the spectra can be adequately interpreted in terms of a classical damped harmonic oscillator model, taking the chemical interface damping into account. When the particles overlap the masked and irradiated areas, however, markedly a different behavior is found depending on the actual arrangement. For the rods experiencing a symmetric inhomogeneity (i.e., by bridging between two masked regions), damping varies smoothly with the extent of substrate inhomogeneity. For the asymmetric case (rods overlapping the boundary between the implanted and masked zones), a sudden increase of the damping is found, which is rather independent on the specific extent of substrate inhomogeneity. Comparing the damping variations with the related intensity changes indicates that substrate inhomogeneity at such length scales results in a different behavior than predicted by the classical damped harmonic oscillator model applied for nanoparticles encapsulated or homogeneously surrounded by molecular coatings.

It is shown that scanning-probe and optical measurements performed on individual gold particles provide direct experimental evidence on the inhomogeneous ligand distribution of tip-selectively cysteamine-modified gold nanorods. At higher (10 −2 mM) cysteamine concentration, a well-defined patch is formed at the tips of 115 × 55 nm gold nanorods. While at lower cysteamine concentration binding of the cysteamine still takes place at the rod tips, it only provides a partial coverage, allowing other thiol molecules to bind at the rod tip. The findings allow for a more rational design of functional patchiness at the nanoparticle level.

Complex shaped nanoparticles featuring structural or surface chemical patchiness are of special interest in both fundamental and applied research areas. Here we report the preparation and optical properties of gold/silica "mushroom" nanoparticles, where a gold particle is only partially covered by the silica cap. The synthetic approach allows precise control over the particle structure. The interfacial preparation method relies on partially embedding the gold particles in a polystyrene layer that masks the immersed part of the gold particle during silica shell growth from an aqueous solution. By adjusting sacrificial polystyrene film thickness and silica growth time, precise control over the coverage and cap thickness can be achieved. Correlative electron microscopy and single particle scattering spectroscopy measurements underline the high precision and reproducibility of the method.The good agreement between the measured and simulated single particle spectra supported by near-field calculations indicates that the observed changes in the dipolar plasmon resonance are influenced by the extent of coverage of the gold core by the silica cap. The straightforward methods readily available for gold and silica surface modification using range of different (bio)molecules make these well-defined nanoscale objects excellent candidates to study fundamental processes of programmed self-assembly or application as theranostic agents.2

Signal enhancement related to indentations in a gold surface layer during micro-Raman scattering experiments was investigated. The indentations were prepared based on colloidal templating and the voids filled with 4mercaptobenzoic acid (MBA)-loaded gold nanospheres. The periodic void structure has been designed to allow selective excitation of a single void in such a way that at the laser wavelength of the micro-Raman setup the cavity-type plasmon modes localized at the metallic void interface can be effectively excited. The surface modification of the gold particles by MBA was studied in detail, and the number of MBA molecules present on a single gold nanoparticle inferred from optical and electrophoretic-mobility measurements was found to be ca. 210. Correlative scanning electron microscopy and micro-Raman measurements allowed the investigations at the single void level. The Raman signal from a single MBA-loaded gold nanoparticle in the cavity was already detectable. The number of particles present at a single void site provided a straightforward way to limit the number of molecules excited during the experiments. By measuring the signal strength as a function of particle number trapped inside a single nanovoid and comparing it with a reference sample (clusters of given number of particles on a flat gold surface), a 25-fold experimental signal enhancement attributed to the nanostructured nature of the interface could be inferred.

The self-assembly of nanoscopic building blocks into higher order macroscopic patterns is one possible approach for the bottom-up fabrication of complex functional systems. Macroscopic pattern formation, in general, is determined by the reaction and diffusion of ions and molecules. In some cases macroscopic patterns emerge from diffusion and interactions existing between nanoscopic or microscopic building blocks. In systems where the distribution of the interaction-determining species is influenced by the presence of a diffusion barrier, the evolving macroscopic patterns will be determined by the spatiotemporal evolution of the building blocks. Here we show that a macroscopic pattern can be generated by the spatiotemporally controlled aggregation of like-charged carboxyl-terminated gold nanoparticles in a hydrogel, where clustering is induced by the screening effect of the sodium ions that diffuse in a hydrogel. Diffusion fronts of the sodium ions and the induced nanoparticle aggregation generate Voronoi diagrams, where the Voronoi cells consist of aggregated nanoparticles and their edges are aggregation-free and nanoparticle-free zones. We also developed a simple aggregation-diffusion model to adequately describe the evolution of the experimentally observed Voronoi patterns.

No abstract