Gold nanoparticle shape dependence of colloidal stability domains.

Controlling the spatial arrangement of plasmonic nanoparticles is of particular interest to utilize inter-particle plasmonic coupling, which allows changing their optical properties. For bottom-up approaches, colloidal nanoparticles are interesting building blocks to generate more complex structures via controlled self-assembly using the destabilization of colloidal particles. For plasmonic noble metal nanoparticles, cationic surfactants, such as CTAB, are widely used in synthesis, both as shaping and stabilizing agents. In such a context, understanding and predicting the colloidal stability of a system solely composed of AuNPs and CTAB is fundamentally crucial. Here, we tried to rationalize the particle behavior by reporting the stability diagrams of colloidal gold nanostructures taking into account parameters such as the size, shape, and CTAB/AuNP concentration. We found that the overall stability was dependent on the shape of the nanoparticles, with the presence of sharp tips being the source of instability. For all morphologies evaluated here, a metastable area was systematically observed, in which the system aggregated in a controlled way while maintaining the colloidal stability. Combining different strategies with the help of transmission electron microscopy, the behavior of the system in the different zones of the diagrams was addressed. Finally, by controlling the experimental conditions with the previously obtained diagrams, we were able to obtain linear structures with a rather good control over the number of particles participating in the assembly while maintaining good colloidal stability.

Insight on Chirality Encoding from Small Thiolated Molecule to Plasmonic Au@Ag and Au@Au Nanoparticles.

Chiral plasmonic nanomaterials exhibiting intense optical activity are promising for numerous applications. In order to prepare those nanostructures, one strategy is to grow metallic nanoparticles in the presence of chiral molecules. However, in such approach the origin of the observed chirality remains uncertain. In this work, we expand the range of available chiral plasmonic nanostructures and we propose another vision of the origin of chirality in such colloidal systems. For that purpose, we investigated the synthesis of two core-shell Au@Ag and Au@Au systems built from gold nanobipyramid cores, in the presence of cysteine. The obtained nanoparticles possess uniform shape and size and show plasmonic circular dichroism in the visible range, and were characterized by electron microscopy, circular dichroism, and UV-vis-NIR spectroscopy. Opto-chiral responses were found to be highly dependent on the morphology and the plasmon resonance. It revealed (i) the importance of the anisotropy for Au@Au nanoparticles and (ii) the role of the multipolar modes for Au@Ag nanoparticles on the way to achieve intense plasmonic circular dichroism. The role of cysteine as shaping agent and as chiral encoder was particularly evaluated. Our experimental results, supported by theoretical simulations, contrast the hypothesis that chiral molecules entrapped in the nanoparticles determine the chiral properties, highlighting the key role of the outmost part of the nanoparticles shell on the plasmonic circular dichroism. Along with these results, the impact of enantiomeric ratio of cysteine on the final shape suggested that the presence of a chiral shape or chiral patterns should be considered.

Versatile template-directed synthesis of gold nanocages with a predefined number of windows.

Highly symmetrical gold nanocages can be produced with a controllable number of circular windows of either 2, 3, 4, 6 or 12 via an original fabrication route. The synthetic pathway includes three main stages: the synthesis of silica/polystyrene multipod templates, the regioselective seeded growth of a gold shell on the unmasked part of the silica surface and the development of gold nanocages by dissolving/etching the templates. Electron microscopy and tomography provide evidence of the symmetrical features of the as-obtained nanostructures. The optical properties of nanocages with 4 and 12 windows were measured at the single particle level by spatial modulation spectroscopy and correlated with numerical simulations based on finite-element modeling. The new multi-step synthesis approach reported here also allows the synthesis of rattle-like nanostructures through filling of the nanocages with a guest nano-object. With the potential to adjust the chemical composition, size and geometry of both the guest particle and the host cage, it opens new routes towards the fabrication of hollow nanostructures of high interest for a variety of applications including sensing devices, catalytic reactors and biomedicine.

Heterogeneous singlet oxygen generation: in-operando visible light EPR spectroscopy.

The use of photosensitizers immobilized on mesoporous materials to produce singlet oxygen (1O2) has opened a new way to synthetic and environmental applications due to the fast development of flow photochemistry and continuous-flow microreactors. 1O2-based photosensitized processes can be employed for the degradation of organic pollutants in an aqueous medium and the photosensitizer can be covalently attached to the support and separated from the effluent reducing the environmental impact. The aim of the present paper is to evaluate the 1O2 generation of Rose Bengal (RB) in homogeneous and heterogeneous systems using in-operando evaluation. Mesoporous SiO2 nanoparticles (MSNs) were successfully conjugated with RB (MSN-RB) and electron paramagnetic resonance (EPR) spectroscopy in combination with the spin trap TEMP was employed to obtain paramagnetic TEMPO via generated 1O2 when RB or MSN-RB are exposed to visible light. Additionally, EPR/DMPO was used to exclude the possible generation of other reactive oxygen species (ROS) by the functionalized nanoparticles. We found that in situ 1O2 generation was enhanced when the same amount of RB is immobilized inside of mesoporous SiO2.

Au nanobipyramids@mSiO2 core-shell nanoparticles for plasmon-enhanced singlet oxygen photooxygenations in segmented flow microreactors.

The plasmonic features of gold nanomaterials provide intriguing optical effects which can find potential applications in various fields. These effects depend strongly on the size and shape of the metal nanostructures. For instance, Au bipyramids (AuBPs) exhibit intense and well-defined plasmon resonance, easily tunable by controlling their aspect ratio, which can act synergistically with chromophores for enhancing their photophysical properties. In Rose Bengal-nanoparticle systems it is now well established that the control of the dye-to-nanoparticle distance ranging from 10 to 20 nm as well as spectral overlaps is crucial to achieve appropriate coupling between the plasmon resonance and the dye, thus affecting its ability to generate singlet oxygen (1O2). We have developed AuBPs@mSiO2 core-shell nanostructures that provide control over the distance between the metal surface and the photosensitizers for improving the production of 1O2 (metal-enhanced 1O2 production - ME1O2). A drastic enhancement of 1O2 generation is evidenced for the resulting AuBPs and AuBPs@mSiO2 in the presence of Rose Bengal, using a combination of three indirect methods of 1O2 detection, namely in operando Electron Paramagnetic Resonance (EPR) with 2,2,6,6-tetramethylpiperidine (TEMP) as a chemical trap, photooxygenation of the fluorescence probe anthracene-9,10-dipropionic acid (ADPA), and photooxygenation of methionine to methionine sulfoxide in a segmented flow microreactor.

Beyond the Concentration Limitation in the Synthesis of Nanobipyramids and Other Pentatwinned Gold Nanostructures.

Gold nanoparticles offer unique optoelectronic properties relevant for a wide range of processes and products, in biology and medicine (therapeutic agents, diagnostic, drug delivery), as well as in electronics, photovoltaics, and catalysis. So far, various synthesis methods proposed have led to rather limited concentration and purity of the colloidal suspensions, severely hindering their use. Here, we present a simple and versatile procedure for the synthesis of gold pentatwinned nanostructures, including nanobipyramids based on a seed-mediated growth process that overcomes the concentration limitations of current methods by 2 orders of magnitude. Moreover, our novel process offers quantitative yields while easily allowing a fine control of the particles' shape, size (with a high monodispersity), and plasmonic properties. Finally, we demonstrate that our method can be easily upscaled to produce large amounts of nanostructures, up to the gram scale, with minimal waste and postprocessing, thus facilitating their use for further applications and industrial developments.

Self-ordering promoted by the nanoconfinement of poly(3-hexylthiophene) and its nanocomposite with single-walled carbon nanotubes.

Nanostructuration and self-ordering of semiconducting organic materials are required to fabricate highly efficient photovoltaic and photoemissive devices. In this work, we investigated the combined effect of melt-assisted template processing and self-ordering of high purity regio-regular poly (3-hexylthiophene) (P3HT) to obtain nanofibers of P3HT and of P3HT-single-walled carbon nanotubes (SWNT) nanocomposites. An original ordering of the polymer and the carbon nanotubes within the nanofibers, as well as their surprising anisotropic photoluminescent properties were determined by vibrational and optical spectroscopy. It was attributed to the combined effect of the melt-assisted wetting confined within alumina nanopores, altogether with the self-organization of both P3HT chains on the one hand, and of the P3HT charged with SWNT on the other hand. It is proposed that the well-ordered regio-regular P3HT matrix orientation is promoted by the interaction with the alumina pore surface and the 1D confinement. For the composite case, the P3HT matrix imposes additionally a preferential orientation of the SWNT transversal to the nanofiber axis. This original organization is responsible for the unexpected polarization of the composite nanofibers photoluminescence. This work opens the way to alternative methods for tackling challenges of nanofabrication to obtain more efficient optoelectronic nanodevices.

Plasmonic Nanoparticles Driven Enhanced Light Amplification in a Local 2D and 3D Self-Assembly.

We present fluorescence and a random lasing enhancement effect due to the interaction between gold nanoparticles (AuNPs) and Rhodamine 6G (Rh6G) dye. Non-covalently bounded dyes in the proximity of nanoparticles are studied in three systems of varying dimensionality: from (i) three-dimensional freely distributed suspensions, through (ii) quasi-two-dimensional multilamellar liposomes, to (iii) solid two-dimensional thin layers. Liposomes facilitate the formation of stable AuNPs/Rh6G composition showing enhanced fluorescence, while solid thin films exhibit plasmon-assisted random lasing.

Charging and aggregation of latex particles in aqueous solutions of ionic liquids: towards an extended Hofmeister series.

The effect of ionic liquid (IL) constituents and other monovalent salts on the stability of polystyrene latex particles was studied by electrophoresis and light scattering in dilute aqueous suspensions. The surface charge and the aggregation rate were both sensitive to the type of ion leading to different critical coagulation concentration (CCC) values. Systematic variation of the type of IL cation and anion allows us to place these ions within the Hofmeister series. We find that the dicyanoamide anion should be placed between iodide and thiocyanate, while all 1-alkyl-3-methylimidazolium cations can be positioned to the left of the tetramethylammonium and ammonium ions. The hydrophobicity of the 1-butyl-1-methylpyrrolidinium (BMPL(+)) ion is intermediate between 1-ethyl-3-methylimidazolium (EMIM(+)) and 1-butyl-3-methylimidazolium (BMIM(+)). With increasing alkyl chain length, the 1-alkyl-3-methylimidazolium cations adsorb on the latex particles very strongly, and 1-hexyl-3-methylimidazolium (HMIM(+)) and 1-octyl-3-methylimidazolium (OMIM(+)) lead to pronounced charge reversal and to an intermediate restabilization region.

Multipod-like silica/polystyrene clusters.

Multipod-like clusters composed of a silica core and PS satellites are prepared according to a seeded-growth emulsion polymerization of styrene in the presence of size-monodisperse silica particles previously surface-modified with methacryloxymethyltriethoxysilane. Tuning the diameter and concentration of the silica seeds affords homogeneous batches of tetrapods, hexapods, octopods, nonapods and dodecapods with morphology yields as high as 80%. Three-dimensional reconstructions by cryo-electron tomography are presented on large fields for the first time to show the high symmetry and regularity of the clusters demonstrating the good control of the synthesis process. These synthesis experiments are visited again digitally, in order to successfully refine an original simulation model and better understand the correlation between the history of the cluster growth and the final composition of the cluster mixture. Finally, using the model as a predictive tool and varying the extra experimental conditions, e.g. the composition of the surfactant mixture and the styrene concentration, result in trapping other cluster morphologies, such as tripods.

Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Q(i)8L(a)6](n-) octahedral nano-building blocks.

New luminescent poly(methylmethacrylate) (PMMA) nanocomposites with high content of different hexanuclear octahedral cluster building blocks, namely [Mo6I8(C2F5COO6)](2-), [Re6Se8(CN)6](4-) and [W6Cl14](2-) have been prepared by free-radical polymerisation. To do so, cluster complexes bearing a polymerisable ammonium counter-cation have been synthesised. In this way, we demonstrate that ionic assembling is a powerful tool to functionalise easily any type of anionic cluster units to be introduced in a PMMA organic matrix. All samples remain homogeneous, stable during several months, and retain the luminescence properties of the cluster precursor.

Synthesis of multivalent silica nanoparticles combining both enthalpic and entropic patchiness.

Silica particles with a controlled number of entropic patches, i.e. dimples, are synthesized through the growth of the silica core of binary multipods that have been produced by a seeded-growth emulsion polymerization reaction. Transmission electron microscopy studies indicate that the silica surface conforms to the shape of the polystyrene (PS) nodules of the multipods while growing, allowing good control of the final shape of the dimpled silica particles. The PS nodules are also used as protecting masks to regioselectively graft amino groups, as revealed by the adsorption of gold markers. After dissolution of the PS nodules, some polymer chains remain grafted onto the silica surface, forming organic bumps. These residues are also selectively functionalized, leading to silica particles with both entropic and enthalpic patches.

Particle aggregation mechanisms in ionic liquids.

Aggregation of sub-micron and nano-sized polystyrene latex particles was studied in room temperature ionic liquids (ILs) and in their water mixtures by time-resolved light scattering. The aggregation rates were found to vary with the IL-to-water molar ratio in a systematic way. At the water side, the aggregation rate is initially small, but increases rapidly with increasing IL content, and reaches a plateau value. This behaviour resembles simple salts, and can be rationalized by the competition of double-layer and van der Waals forces as surmised by the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). At the IL side, aggregation slows down again. Two generic mechanisms could be identified to be responsible for the stabilization in ILs, namely viscous stabilization and solvation stabilization. Viscous stabilization is important in highly viscous ILs, as it originates from the slowdown of the diffusion controlled aggregation due to the hindrance of the diffusion in a viscous liquid. The solvation stabilization mechanism is system specific, but can lead to a dramatic slowdown of the aggregation rate in ILs. This mechanism is related to repulsive solvation forces that are operational in ILs due to the layering of the ILs close to the surfaces. These two stabilization mechanisms are suspected to be generic, as they both occur in different ILs, and for particles differing in surface functionalities and size.

Synthesis and site-specific functionalization of tetravalent, hexavalent, and dodecavalent silica particles.

Different shapes: Tetravalent, hexavalent, and dodecavalent silica particles were obtained by the growth of the silica core of binary tetrapods, hexapods, and dodecapods, respectively. The surface of the multivalent particles can be regioselectively functionalized, thereby leading to particles with anisotropic geometry and chemistry.

Microfluidic-induced growth and shape-up of three-dimensional extended arrays of densely packed nanoparticles.

We use evaporation within a microfluidic device to extract the solvent of a (possibly very dilute) dispersion of nanoparticles and concentrate the dispersion until a solid made of densely packed nanoparticles grows and totally invades the microfluidic geometry. The growth process can be rationalized as an interplay between evaporation-induced flow and kinetic and thermodynamic coefficients which are system-dependent; this yields limitations to the growth process illustrated here on two main cases: evaporation- and transport-limited growth. Importantly, we also quantify how colloidal stability may hinder the growth and show that care must be taken as to the composition of the initial dispersion, especially regarding traces of ionic species that can destabilize the suspension upon concentration. We define a stability chart, which, when fulfilled, permits us to grow and shape-up solids, including superlattices and extended and thick arrays of nanoparticles made of unary and binary dispersions, composites, and heterojunctions between distinct types of nanoparticles. In all cases, the geometry of the final solid is imparted by that of the microfluidic device.

Spheres growing on a sphere: a model to predict the morphology yields of colloidal molecules obtained through a heterogeneous nucleation route.

Through the heterogeneous nucleation of polymer nodules on a surface-modified silica particle, the high-yield achievement of hybrid colloidal molecules with a well-controlled multipod-like morphology was recently demonstrated. However, as the formation mechanism of these colloidal molecules has not been completely understood yet, some opportunities remain to reduce the tedious empirical process needed to optimize the chemical recipes. In this work, we propose a model to help understand the formation mechanism of almost pure suspensions of well-defined colloidal molecules. The outcomes of the model allow proposing probable nucleation growth scenario able to explain the experimental results. Such a model should make easier the determination of the optimal recipe parameters for a targeted morphology. The reasonably good agreements between the model and the experimental results show that the most important processes have been captured. It is thus a first step toward the rational design of large quantities of chemically prepared colloidal molecules.

Design and elaboration of colloidal molecules: an overview.

The concept of colloidal molecules was first evoked by van Blaaderen in 2003 for describing small non-spherical colloids made of the aggregation of a small number of particles. He predicted original properties to the complex assemblies of such colloids, in particular in optics. This critical review deals with the different strategies reported for creating robust clusters of spherical particles which could mimic the space-filling models of simple conventional molecules. These routes concern either the controlled clustering of preformed colloids directed by coalescence, physical routes, chemical routes, or 2-D/3-D geometrical confinement, or strategies starting from a single colloid which is decorated by satellite colloids by taking advantage of controlled phase separation or nucleation and growth phenomena. These routes are compared from the viewpoint of the accessible shapes, their tunability and scalability (146 references).

SAXS exploration of the synthesis of ultra monodisperse silica nanoparticles and quantitative nucleation growth modeling.

The production of highly monodisperse nanoparticles of precisely controlled size is a very important research field. It has important applications notably for the optical properties of nanoparticles (e.g. quantum dot) or nanoparticle assemblies (e.g. photonic band gap crystals) and for electromagnetic properties (e.g. information storage). Understanding monodisperse nanoparticle synthesis mechanism is based mostly on the Classical Nucleation Theory (CNT). It has been shown in the literature and in this work that CNT is able to predict the nanoparticle concentration and average size correctly. However, until recently only a few models based on CNT were able to predict the size distribution of the synthesized objects. In this work, we show that a CNT based model is not able to predict the size distribution of silica nanoparticles formed in a pure La Mer like nucleation growth process. Reasons for this discrepancy are discussed and should be taken into account to develop more complete models able to predict the size distribution especially if it is desired to use them as tools to optimize monodispersity.