Templated growth of gold satellites on dimpled silica cores

Bottom-up synthesis of meta-atoms as building blocks in self-assembled metamaterials : Recent advances and perspectives

Meta-atoms interact with light in interesting ways and offer a large range of exciting properties. They exhibit optical properties inaccessible by natural atoms but their fabrication is notoriously difficult because of the precision required. In this perspective, we present the current research landscape in making meta-atoms, with a focus on the most promising self-assembly approaches and main challenges to overcome, for the development of materials with novel properties at optical frequencies.

Linear Assembly of Two-Patch Silica Nanoparticles and Control of Chain Length by Coassembly with Colloidal Chain Stoppers

The self-assembly of patchy nanosized building blocks is an efficient strategy for producing highly organized materials. Herein we report the chaining of divalent silica nanoparticles with polystyrene patches dispersed in tetrahydrofuran triggered by lowering the solvent quality. We study the influence of the patch-to-particle size ratio and show that the nature of the added nonsolvent, for example, ethanol, water, or salty water, and its volume fraction should be carefully adjusted. We demonstrate that colloidal assembly initially obeys the kinetic model of step-growth polymerization and that beyond a certain length, the chains have the possibility to cyclize. We also show that the length of the chains can be controlled by the addition of one-patch silica nanoparticles, which act as colloidal analogues of chain stoppers.

Toward Huygens' Sources with Dodecahedral Plasmonic Clusters

The design and chemical synthesis of plasmonic nanoresonators exhibiting a strong magnetic response in the visible is a key requirement to the realization of efficient functional and self-assembled metamaterials. However, novel applications like Huygens' metasurfaces or mu-near-zero materials require stronger magnetic responses than those currently reported. Our numerical simulations demonstrate that the specific dodecahedral morphology, whereby 12 silver satellites are located on the faces of a nanosized dielectric dodecahedron, provides sufficiently large electric and magnetic dipolar and quadrupolar responses that interfere to produce so-called generalized Huygens' sources, fulfilling the generalized Kerker condition. Using a multistep colloidal engineering approach, we synthesize highly symmetric plasmonic nanoclusters with a controlled silver satellite size and show that they exhibit a strong forward scattering that may be used in various applications such as metasurfaces or perfect absorbers.

Dimpled SiO2@γ-Fe2O3 nanocomposites - fabrication and use for arsenic adsorption in aqueous medium

We report the synthesis of nanocomposites made of silica nanoparticles whose six surface dimples are decorated with magnetic maghemite nanoparticles and their use for detection and recovery of arsenic in aqueous media. Precursor silica nanoparticles have aminated polystyrene chains at the bottom of their dimples and the maghemite nanoparticles are surface functionalized with carboxylic acid groups in two steps: amination with 3-aminopropyltrimethoxysilane, then derivatization with succinic anhydride in the presence of triethylamine. In the end, the colloidal assembly consists of the regioselective grafting of the carboxylic acid-modified iron oxide nanoparticles onto the 6-dimple silica nanoparticles. Several characterization techniques such as transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) are employed to assess the grafting process and study the influence of the maghemite functional groups on the quality of the composites formed. The resulting magnetic nanocomposites are used for the environmentally benign detection and removal of arsenic from aqueous medium, being readily extracted through means of magnetic separation.

The process of grafting maghemite NFs onto silica dimples.

Bioinspired One-Step Synthesis of Pomegranate-like Silica@Gold Nanoparticles with Surface-Enhanced Raman Scattering Activity

Gold-silica (Au-SiO₂) nanohybrids are of great technological importance, and it is crucial to develop facile synthetic protocols to prepare Au-SiO₂ nanohybrids with novel structures. Here we report the bioinspired synthesis of pomegranate-like SiO₂@Au nanoparticles (P-SiO₂@Au NPs) via one-step aqueous synthesis from chloroauric acid and tetraethyl orthosilicate mediated by a basic amino acid, arginine. Effects of chloroauric acid, tetraethyl orthosilicate, and arginine on the morphology and optical property of the products are investigated in detail. The P-SiO₂@Au NPs achieve tunable plasmon resonance depending on the amount of chloroauric acid, which affects the size and shape of the P-SiO₂@Au NPs. Finite-difference time-domain simulations are performed, revealing that the plasmon peak red-shifts with increasing particle size. Arginine serves as the reducing and capping agents for Au as well as the catalyst for SiO₂ formation and also promotes the combination of Au and SiO₂. Formation process of the P-SiO₂@Au NPs is clarified through time-course analysis. The P-SiO₂@Au NPs show good sensitivity for both colloidal and paper-based surface-enhanced Raman scattering measurements. They achieve enhancement factors of 4.3 × 10⁷-8.5 × 10⁷ and a mass detection limit of ca. 1 ng using thiophenol as the model analyte.

Colloidal chemistry with patchy silica nanoparticles

We report a new route to synthesize clusters, or so-called colloidal molecules (CMs), which mimic the symmetry of molecular structures made of one central atom. We couple site-specifically functionalized patchy nanoparticles, i.e., valence-endowed colloidal atoms (CAs), with complementary nanospheres through amide bonds. By analogy with the Gillespie formalism, we show that AX4, AX3E1 and AX2E2 CMs can be obtained from tetravalent sp3-like CAs when the relative amount of both building units is varied in a controlled manner. We obtain AX2 CMs from divalent sp-like CAs. We also show that it is possible to covalently attach two different types of satellites to the same central patchy nanoparticle to create more complex CMs, opening the way to the fabrication of new multifunctional nanostructures with well-controlled shape and composition.

Regiospecific Nucleation and Growth of Silane Coupling Agent Droplets onto Colloidal Particles

Nucleation-and-growth processes are used extensively in the synthesis of spherical colloids, and more recently regiospecific nucleation-and-growth processes have been exploited to prepare more complex colloids such as patchy particles. We demonstrate that surface geometry alone can be made to play the dominant role in determining the final particle geometry in such syntheses, meaning that intricate chemical surface patternings are not required. We present a synthesis method for "lollipop"-shaped colloidal heterodimers (patchy particles), combining a recently published nucleation-and-growth technique with our recent findings that particle geometry influences the locus of droplet adsorption onto anisotropic template particles. Specifically, 3-methacryloxypropyl trimethoxysilane (MPTMS) is nucleated and grown onto bullet-shaped and nail-shaped colloids. The shape of the template particle can be chosen such that the MPTMS adsorbs regiospecifically onto the flat ends. In particular, we find that particles with a wider base increase the range of droplet volumes for which the minimum in the free energy of adsorption is located at the flat end of the particle compared with bullet-shaped particles of the same aspect ratio. We put forward an extensive analysis of the synthesis mechanism and experimentally determine the physical properties of the heterodimers, supported by theoretical simulations. Here we numerically optimize, for the first time, the shape of finite-sized droplets as a function of their position on the rod-like silica particle surface. We expect that our findings will give an impulse to complex particle creation by regiospecific nucleation and growth.