Prismatic Confinement Induces Tunable Orientation in Plasmonic Supercrystals.

Throughout history scientists have looked to Nature for inspiration and attempted to replicate intricate complex structures formed by self-assembly. In the context of synthetic supercrystals, achieving such complexity remains a challenge due to the highly symmetric nature of most nanoparticles (NPs). Previous works have shown intricate coupling between the self-assembly of NPs and confinement in templates, such as emulsion droplets (spherical confinement) or tubes (cylindrical confinement). This study focuses on the interplay between anisotropic NP shape and tunable "prismatic confinement" leading to the self-assembly of supercrystals in cavities featuring polygonal cross sections. A multiscale characterization strategy is employed to investigate the orientation and structure of the supercrystals locally and at the ensemble level. Our findings highlight the role of the mold interface in guiding the growth of distinct crystal domains: each side of the mold directs the formation of a monodomain that extends until it encounters another, leading to the creation of grain boundaries. Computer simulations in smaller prismatic cavities were conducted to predict the effect of an increased confinement. Comparison between prismatic and cylindrical confinements shows that flat interfaces are key to orienting the growth of supercrystals. This work shows a method of inducing orientation in plasmonic supercrystals and controlling their textural defects, thus offering insight into the design of functional metasurfaces and hierarchically structured devices.

Out-of-Equilibrium Mechanical Disruption of ß-Amyloid-Like Fibers using Light-Driven Molecular Motors.

Artificial molecular motors have the potential to generate mechanical work on their environment by producing autonomous unidirectional motions when supplied with a source of energy. However, the harnessing of this mechanical work to subsequently activate various endoenergetic processes that can be useful in materials science remains elusive. Here, it is shown that by integrating a light-driven rotary motor through hydrogen bonds in a ß-amyloid-like structure forming supramolecular hydrogels, the mechanical work generated during the constant rotation of the molecular machine under UV irradiation is sufficient to disrupt the ß-amyloid fibers and to trigger a gel-to-sol transition at macroscopic scale. This melting of the gel under UV irradiation occurs 25 °C below the temperature needed to melt it by solely using thermal activation. In the dark, a reversible sol-gel transition is observed as the system fully recovers its original microstructure, thus illustrating the possible access to new kinds of motorized materials that can be controlled by advanced out-of-equilibrium thermodynamics.

Confinement Effects on the Structure of Entropy-Induced Supercrystals.

Depletion-induced self-assembly is routinely used to separate plasmonic nanoparticles (NPs) of different shapes, but less often for its ability to create supercrystals (SCs) in suspension. Therefore, these plasmonic assemblies have not yet reached a high level of maturity and their in-depth characterization by a combination of in situ techniques is still very much needed. In this work, gold triangles (AuNTs) and silver nanorods (AgNRs) are assembled by depletion-induced self-assembly. Small Angle X-ray Scattering (SAXS) and scanning electron microscopy (SEM) analysis shows that the AuNTs and AgNRs form 3D and 2D hexagonal lattices in bulk, respectively. The colloidal crystals are also imaged by in situ Liquid-Cell Transmission Electron Microscopy. Under confinement, the affinity of the NPs for the liquid cell windows reduces their ability to stack perpendicularly to the membrane and lead to SCs with a lower dimensionality than their bulk counterparts. Moreover, extended beam irradiation leads to disassembly of the lattices, which is well described by a model accounting for the desorption kinetics highlighting the key role of the NP-membrane interaction in the structural properties of SCs in the liquid-cell. The results shed light on the reconfigurability of NP superlattices obtained by depletion-induced self-assembly, which can rearrange under confinement.

Extracting the morphology of gold bipyramids from small-angle X-ray scattering experiments via form factor modelling.

Accurate shape description is a challenge in materials science. Small-angle X-ray scattering (SAXS) can provide the shape, size and polydispersity of nanoparticles by form factor modelling. However, simple geometric models such as the ellipsoid may not be enough to describe objects with complex shapes. This work shows that the form factor of gold nanobipyramids is accurately described by a truncated bicone model, which is validated by comparison with transmission electron microscopy (TEM) data for nine different synthesis batches; the average shape parameters (width, height and truncation) and the sample polydispersity are obtained. In contrast, the ellipsoid model yields worse fits of the SAXS data and exhibits systematic discrepancies with the TEM results.

Polymorphous Packing of Pentagonal Nanoprisms.

Packing solid shapes into regular lattices can yield very complex assemblies, not all of which achieve the highest packing fraction. In two dimensions, the regular pentagon is paradigmatic, being the simplest shape that does not pave the plane completely. In this work, we demonstrate the packing of plasmonic nanoprisms with pentagonal cross section, which form extended supercrystals. We do encounter the long-predicted ice-ray and Dürer packings (with packing fractions of 0.921 and 0.854, respectively) but also a variety of novel polymorphs that can be obtained from these two configurations by a continuous sliding transformation and exhibit an intermediate packing fraction. Beyond the fundamental interest of this result, fine control over the density and symmetry of such plasmonic assemblies opens the perspective of tuning their optical properties, with potential applications in metamaterial fabrication, catalysis, or molecular detection.

Formation of kinetically trapped small clusters of PEGylated gold nanoparticles revealed by the combination of small-angle X-ray scattering and visible light spectroscopy.

Gold nanoparticles coated with polyethylene glycol (PEG) are able to form clusters due to the collapse of the surface-grafted polymer chains when the temperature and ion concentration of the aqueous medium are increased. The chain collapse reduces the steric repulsion, leading to particle aggregation. In this work, we combine small angle X-ray scattering (SAXS) and visible light spectroscopy to elucidate the structure of the developing clusters. The structure derived from the SAXS measurements reveals a decrease in interparticle distance and drastic narrowing of its distribution in the cluster, indicating restricted particle mobility and displacement within the cluster. Surprisingly, instead of forming a large crystalline phase, the evolving clusters are composed of about a dozen particles. The experimental optical extinction spectra measured during cluster formation can be very well reproduced by optical simulations based on the SAXS-derived structural data.

Unique orientation of 1D and 2D nanoparticle assemblies confined in smectic topological defects.

New collective optical properties have emerged recently from organized and oriented arrays of closely packed semiconducting and metallic nanoparticles (NPs). However, it is still challenging to obtain NP assemblies which are similar everywhere on a given sample and, most importantly, share a unique common orientation that would guarantee a unique behavior everywhere on the sample. In this context, by combining optical microscopy, fluorescence microscopy and synchrotron-based grazing incidence X-ray scattering (GISAXS) of assemblies of gold nanospheres and of fluorescent nanorods, we study the interactions between NPs and liquid crystal smectic topological defects that can ultimately lead to unique NP orientations. We demonstrate that arrays of one-dimensional - 1D (dislocations) and two-dimensional - 2D (grain boundaries) topological defects oriented along one single direction confine and organize NPs in closely packed networks but also orient both single nanorods and NP networks along the same direction. Through the comparison between smectic films associated with different kinds of topological defects, we highlight that the coupling between the NP ligands and the smectic layers below the grain boundaries may be necessary to allow for fixed NP orientation. This is in contrast with 1D defects, where the induced orientation of the NPs is intrinsically induced by the confinement independently of the ligand nature. We thus succeeded in achieving the fixed polarization of assemblies of single photon emitters in defects. For gold nanospheres confined in grain boundaries, a strict orientation of hexagonal networks has been obtained with the 〈10〉 direction strictly parallel to the defects. With such closely packed and oriented NPs, new collective properties are now foreseen.

Double-Lattice Packing of Pentagonal Gold Bipyramids in Supercrystals with Triclinic Symmetry.

Pentagonal packing is a long-standing issue and a rich mathematical topic, brought to the fore by recent progress in nanoparticle design. Gold pentagonal bipyramids combine fivefold symmetry and anisotropy and their section varies along the length. In this work, colloidal supercrystals of pentagonal gold bipyramids are obtained in a compact arrangement that generalizes the optimal packing of regular pentagons in the plane. Multimodal investigations reveal a two-particle unit cell with triclinic symmetry, a lower symmetry than that of the building blocks. Monte Carlo computer simulations show that this lattice achieves the densest possible packing. Going beyond pentagons, further simulations show an odd-even effect of the number of sides on the packing: odd-sided bipyramids are non-centrosymmetric and require the double-lattice arrangement to recover inversion symmetry. The supercrystals display a facet-dependent optical response that is promising for sensing, metamaterials applications, and for fundamental studies of self-assembly processes.

Softness-driven complexity in supercrystals of gold nanoparticles.

Many soft matter systems are composed of roughly spherical objects that can self-assemble in ordered structures. Unlike hard spheres, at high volume fraction these soft spheres adapt their shape to the local geometrical constraints and the question of space filling needs to be entirely revisited. Hydrophobically coated gold nanocrystals self-assemble in supercrystals and are good candidates to explore this question. When the soft coating is thin compared to the rigid core, a FCC structure is obtained, with a behaviour similar to that of hard spheres. In the opposite case, for a thick soft coating, a BCC structure is found instead. This paper focus on the intermediate region between these two classical structures. By varying the gold core radius R and the ligand fully extended length L, we establish a structure diagram based on a large experimental data set. The hexagonal Frank-Kasper C14 structure is observed for various values of R and L and can coexist with a FCC phase. Depending on the structure, values of the minimum thickness e of the ligand shell compared to L are different. These experimental results confirm that the C14 Frank-Kasper phase is a solution to the problem of filling the space with soft particles even with a rigid core and should help to establish pertinent models in order to predict the structures of the superlattices built by gold nanoparticles.

Doxorubicin-Loaded Metal-Organic Frameworks Nanoparticles with Engineered Cyclodextrin Coatings: Insights on Drug Location by Solid State NMR Spectroscopy.

Recently developed, nanoscale metal-organic frameworks (nanoMOFs) functionalized with versatile coatings are drawing special attention in the nanomedicine field. Here we show the preparation of core-shell MIL-100(Al) nanoMOFs for the delivery of the anticancer drug doxorubicin (DOX). DOX was efficiently incorporated in the MOFs and was released in a progressive manner, depending on the initial loading. Besides, the coatings were made of biodegradable γ-cyclodextrin-citrate oligomers (CD-CO) with affinity for both DOX and the MOF cores. DOX was incorporated and released faster due to its affinity for the coating material. A set of complementary solid state nuclear magnetic resonance (ssNMR) experiments including 1H-1H and 13C-27Al two-dimensional NMR, was used to gain a deep understanding on the multiple interactions involved in the MIL-100(Al) core-shell system. To do so, 13C-labelled shells were synthesized. This study paves the way towards a methodology to assess the nanoMOF component localization at a molecular scale and to investigate the nanoMOF physicochemical properties, which play a main role on their biological applications.

Interplay of Structure and Dynamics in Lithium/Ionic Liquid Electrolytes: Experiment and Molecular Simulation.

Despite their promising use in electrochemical and electrokinetic devices, ionic-liquid-based electrolytes often exhibit complex behavior arising from a subtle interplay of their structure and dynamics. Here, we report a joint experimental and molecular simulation study of such electrolytes obtained by mixing 1-butyl 3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate. More in detail, experiments consisting of X-ray scattering, pulsed field gradient NMR, and complex impedance spectroscopy are analyzed in the light of molecular dynamics simulations to probe the structural, dynamical, and electrochemical properties of this ionic-liquid-based electrolyte. Lithium addition promotes the nanostructuration of the liquid as evidenced from the appearance of a scattering prepeak that becomes more pronounced. Microscopically, using the partial structure factors determined from molecular dynamics, this prepeak is shown to correspond to the formation of well-ordered positive/negative charge series and also large aggregates (Lin(BF4)4-m)(4-m+n)-, which develop upon lithium addition. Such nanoscale ordering entails a drastic decrease in both the molecular mobility and ionic conductivity. In particular, the marked association of Li+ cations with four BF4- anions and long ion pairing times, which are promoted upon lithium addition, are found to severely hinder the Li+ transport properties.

Correction: Solution self-assembly of plasmonic Janus nanoparticles.

Correction for 'Solution self-assembly of plasmonic Janus nanoparticles' by Nicolò Castro et al., Soft Matter, 2016, 12, 9666-9673, DOI: .

Grazing Incidence X-ray Diffraction Studies of Lipid-Peptide Mixed Monolayers during Shear Flow.

Grazing incidence X-ray diffraction (GIXD) studies of monolayers of biomolecules at an air-water interface give quantitative information of in-plane packing, coherence length of crystalline domains, etc. Rheo-GIXD measurements can reveal quantitative changes in the nanocrystalline domains of a monolayer under shear. Here, we report GIXD studies of monolayers of alamethicin peptide, DPPC lipid, and their mixtures at an air-water interface under steady shear stress. The alamethicin monolayer and the mixed monolayer show a flow jamming transition. On the other hand, the pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer under constant stress flows steadily with a notable enhancement of the area/molecule and coherence lengths, suggesting the fusion of nanocrystallites during flow. The DPPC-alamethicin mixed monolayer shows no significant change in the area/DPPC molecule, but the coherence lengths of the individual phases (DPPC and alamethicin) increase, suggesting that the crystallites of individual phases grow bigger by merging of domains. More phase separation occurs in the system during flow. Our results show that rheo-GIXD has the potential to explore in situ molecular structural changes under rheological conditions for a diverse range of confined biomolecules at interfaces.

Real-Time In Situ Observations Reveal a Double Role for Ascorbic Acid in the Anisotropic Growth of Silver on Gold.

Rational nanoparticle design is one of the main goals of materials science, but it can only be achieved via a thorough understanding of the growth process and of the respective roles of the molecular species involved. We demonstrate that a combination of complementary techniques can yield novel information with respect to their individual contributions. We monitored the growth of long aspect ratio silver rods from gold pentatwinned seeds by three in situ techniques (small-angle X-ray scattering, optical extinction spectroscopy and liquid-cell transmission electron microscopy). Exploiting the difference in reaction speed between the bulk synthesis and the nanoparticle formation in the TEM cell, we show that the anisotropic growth is thermodynamically controlled (rather than kinetically) and that ascorbic acid, widely used for its mild reductive properties, plays a shape-directing role, by stabilizing the {100} facets of the silver cubic lattice, in synergy with the halide ions. This approach can easily be applied to a wide variety of synthesis strategies.

From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects.

In this Letter, we show how advanced hierarchical structures of topological defects in the so-called smectic oily streaks can be used to sequentially transfer their geometrical features to gold nanospheres. We use two kinds of topological defects, 1D dislocations and 2D ribbon-like topological defects. The large trapping efficiency of the smectic dislocation cores not only surpasses that of the elastically distorted zones around the cores but also surpasses the one of the 2D ribbon-like topological defect. This enables the formation of a large number of aligned NP chains within the dislocation cores that can be quasi-fully filled without any significant aggregation outside of the cores. When the NP concentration is large enough to entirely fill the dislocation cores, the LC confinement varies from 1D to 2D. We demonstrate that the 2D topological defect cores induce a confinement that leads to planar hexagonal networks of NPs. We then draw the phase diagram driven by NP concentration, associated with the sequential confinements induced by these two kinds of topological defects. Owing to the excellent large-scale order of these defect cores, not only the NP chains but also the NP hexagonal networks can be oriented along the desired direction, suggesting a possible new route for the creation of either 1D or 2D highly anisotropic NP networks. In addition, these results open rich perspectives based on the possible creation of coexisting NP assemblies of different kinds, localized in different confining areas of a same smectic film that would thus interact thanks to their proximity but also would interact via the surrounding soft matter matrix.

Determining the morphology and concentration of core-shell Au/Ag nanoparticles.

Accurately measuring the shape, structure and concentration of nanoparticles (NPs) is a crucial step towards understanding their formation and a prerequisite for any applications. While determining these parameters for single-metal NPs is by now rather routine, reliably characterizing bimetallic NPs is still a challenge. Using four complementary techniques: transmission electron microscopy (TEM), light absorbance spectroscopy (AS), small-angle X-ray scattering (SAXS) and inductively coupled plasma mass spectrometry (ICP-MS) we study bimetallic nanoparticles obtained by growing a silver shell on top of a gold seed. The initial quasi-spherical objects become faceted and grow into a rounded cube as the molar silver-to-gold ratio K increases. The shape evolution is well described by SAXS and TEM. The shell thickness, overall size polydispersity and number particle concentration obtained by the various methods are in good agreement, validating the use of non-invasive in situ techniques such as AS and SAXS for the study of bimetallic NPs.

Plasmonic Oligomers with Tunable Conductive Nanojunctions.

Engineering plasmonic hot spots is essential for applications of plasmonic nanoparticles. A particularly appealing route is to weld plasmonic nanoparticles together to form more complex structures sustaining plasmons with symmetries targeted to given applications. However, control of the welding and subsequent hot spot characteristics is still challenging. Herein, we demonstrate an original method that connects gold particles to their neighbors by another metal of choice. We first assemble gold bipyramids in a tip-to-tip configuration, yielding short chains of variable length, and grow metallic junctions in a second step. We follow the chain formation and the deposition of the second metal (i.e., silver or palladium) via UV/vis spectroscopy, and we map the plasmonic properties using electron energy loss spectroscopy. The formation of silver bridges leads to a huge red shift of the longitudinal plasmon modes into the mid-infrared region, while the addition of palladium results in a red shift accompanied by significant plasmon damping.

Insights into the Formation Mechanism of CdSe Nanoplatelets Using in Situ X-ray Scattering.

Two-dimensional ultrathin CdSe nanoplatelets have attracted a large interest due to their optical properties but their formation mechanism is not well understood. Several different mechanisms have been proposed: confined growth in a surfactant mesophase acting as a template, anisotropic ripening of small seeds into 2D nanoplatelets, or continuous anisotropic growth of a limited number of nuclei. However, quantitative in situ data that could validate or disprove these formation scenarios are lacking. We use synchrotron-based small-angle and wide-angle X-ray scattering to probe the formation mechanism of CdSe nanoplatelets synthesized using a heating-up method. We prove the absence of a molecular mesophase in the reactive medium at the onset of nanoplatelet formation ruling out a templating effect. We also show that our data are inconsistent with the anisotropic ripening of small seeds whereas the evolution of the SAXS patterns during the reaction is consistent with the continuous lateral growth of nanoplatelets fed by reactive monomers. Finally, we show that when the final temperature of the synthesis is lowered, nanoplatelets with larger lateral dimensions form. We reveal that they bend in solution during their growth to yield nanoscrolls.

Design of Engineered Cyclodextrin Derivatives for Spontaneous Coating of Highly Porous Metal-Organic Framework Nanoparticles in Aqueous Media.

Nanosized metal-organic frameworks (nanoMOFs) MIL-100(Fe) are highly porous and biodegradable materials that have emerged as promising drug nanocarriers. A challenging issue concerns their surface functionalization in order to evade the immune system and to provide molecular recognition ability, so that they can be used for specific targeting. A convenient method for their coating with tetraethylene glycol, polyethylene glycol, and mannose residues is reported herein. The method consists of the organic solvent-free self-assembly on the nanoMOFs of building blocks based on ß-cyclodextrin facially derivatized with the referred functional moieties, and multiple phosphate groups to anchor to the nanoparticles' surface. The coating of nanoMOFs with cyclodextrin phosphate without further functional groups led to a significant decrease of macrophage uptake, slightly improved by polyethylene glycol or mannose-containing cyclodextrin phosphate coating. More notably, nanoMOFs modified with tetraethylene glycol-containing cyclodextrin phosphate displayed the most efficient "stealth" effect. Mannose-coated nanoMOFs displayed a remarkably enhanced binding affinity towards a specific mannose receptor, such as Concanavalin A, due to the multivalent display of the monosaccharide, as well as reduced macrophage internalization. Coating with tetraethylente glycol of nanoMOFs after loading with doxorubicin is also described. Therefore, phosphorylated cyclodextrins offer a versatile platform to coat nanoMOFs in an organic solvent-free, one step manner, providing them with new biorecognition and/or "stealth" properties.

Interaction and structuration of membrane-binding and membrane-excluding colloidal particles in lamellar phases.

Within the framework of a discrete Gaussian model, we present analytical results for the interaction induced by a lamellar phase between small embedded colloidal particles. We consider the two limits of particles strongly adherent to the adjacent membranes and of particles impenetrable to the membranes. Our approach takes into account the finite size of the colloidal particles, the discrete nature of the layers, and includes the Casimir-like effect of fluctuations, which is very important for dilute phases. Monte Carlo simulations of the statistical behavior of the membrane-interacting colloidal particles account semi-quantitatively, without any adjustable parameters, for the experimental data measured on silica nanospheres inserted within lyotropic smectics. We predict the existence of finite-size and densely packed particle aggregates originating from the competition between attractive interactions between colloidal particles in the same layer and repulsion between colloidal particles one layer apart.