Tunability of Interactions between the Core and Shell in Rattle-Type Particles Studied with Liquid-Cell Electron Microscopy.

Yolk-shell or rattle-type particles consist of a core particle that is free to move inside a thin shell. A stable core with a fully accessible surface is of interest in fields such as catalysis and sensing. However, the stability of a charged nanoparticle core within the cavity of a charged thin shell remains largely unexplored. Liquid-cell (scanning) transmission electron microscopy is an ideal technique to probe the core-shell interactions at nanometer spatial resolution. Here, we show by means of calculations and experiments that these interactions are highly tunable. We found that in dilute solutions adding a monovalent salt led to stronger confinement of the core to the middle of the geometry. In deionized water, the Debye length κ-1 becomes comparable to the shell radius Rshell, leading to a less steep electric potential gradient and a reduced core-shell interaction, which can be detrimental to the stability of nanorattles. For a salt concentration range of 0.5-250 mM, the repulsion was relatively long-ranged due to the concave geometry of the shell. At salt concentrations of 100 and 250 mM, the core was found to move almost exclusively near the shell wall, which can be due to hydrodynamics, a secondary minimum in the interaction potential, or a combination of both. The possibility of imaging nanoparticles inside shells at high spatial resolution with liquid-cell electron microscopy makes rattle particles a powerful experimental model system to learn about nanoparticle interactions. Additionally, our results highlight the possibilities for manipulating the interactions between core and shell that could be used in future applications.

Compartmentalization of gold nanoparticle clusters in hollow silica spheres and their assembly induced by an external electric field.

Assembly of plasmonic nanoparticle clusters having hotspots in a specific space is an effective way to efficiently utilize their plasmonic properties. In the assembly, however, bulk-like aggregates of the nanoparticles are readily formed by strong van der Waals forces, inducing a decrease of the properties. The present work proposes an advanced method to avoid aggregation of the clusters by encapsulating into a confined space of hollow silica interior. Hollow spheres incorporating gold nanoparticle clusters were synthesized by a surface-protected etching process. The observation of inner nanoparticles with liquid cell transmission electron microscopy experimentally proved that the nanoparticles moved as a cluster instead of as dispersed nanoparticles within the water-filled hollow compartment. The hollow spheres incorporating the nanoparticle clusters were assembled in the vicinity of electrodes by application of an external AC electric field, resulting in the enhancement of Raman intensities of probe molecules. The nanoparticle-cluster-containing hollow spheres were redispersed when the electric field was turned off, showing that the hollow silica spheres can act as a physical barrier to avoid the cluster aggregation. The Raman intensities were reversibly changed by switching the electric field on and off to control the assembled or dispersed states of the hollow spheres.

Photo-stability of lutein in surfactant-free lutein-zein composite colloidal particles.

The ability of nanoparticles from the plant protein zein to protect lutein from light degradation was studied under various conditions. Lutein-zein nanoparticles were synthesized, after zein purification, by anti-solvent precipitation. Particle sizes, ranging from 25 to 75 nm, measured by dynamic light scattering, were tuned by varying zein concentrations in the solvent phase (before anti-solvent precipitation), which was linked to the encapsulation efficiency. However, changes in particle sizes did not result in significant changes in photo-stability. Zein-lutein nanoparticles showed increased photo-stability of lutein when compared to lutein dispersions in water. To further promote the lutein stability, ascorbic acid was used as an antioxidant in the aqueous dispersion. The addition of ascorbic acid to lutein-zein particles resulted in dispersions with similar properties. However, the photo-stability of lutein in dispersions stabilized with ascorbic acid improved significantly compared to samples without ascorbic acid or to pure lutein dispersions (about 25% increased relative stability).

Seeded-Growth of Silica Rods from Silica-Coated Particles.

Seeded growth of silica rods from colloidal particles has emerged as a facile method to develop novel complex particle structures with hybrid compositions and asymmetrical shapes. However, this seeded-growth technique has been so far limited to colloidal particles of only a few materials. Here, we first develop a general synthesis for the seeded-growth of silica rods from silica particles. We then demonstrate the growth of silica rods from silica-coated particles with three different cores which highlight the generality of this synthesis: fluorescently labeled organo-silica (fluorescein), metallic (Ag), and organic (PS latex). We also demonstrate the assembly of these particles into supraparticles. This general synthesis method can be extended to the growth of silica rods from any colloidal particle which can be coated with silica.

Dynamic Electric Field Alignment of Metal-Organic Framework Microrods.

Alignment of metal-organic framework (MOF) crystals has previously been performed via careful control of oriented MOF growth on substrates, as well as by dynamic magnetic alignment. We show here that bromobenzene-suspended microrod crystals of the MOF NU-1000 can also be dynamically aligned via electric fields, giving rise to rapid electrooptical responses. This method of dynamic MOF alignment opens up new avenues of MOF control which are important for integration of MOFs into switchable electronic devices as well as in other applications such as reconfigurable sensors or optical systems.

Shaping Silica Rods by Tuning Hydrolysis and Condensation of Silica Precursors.

We present the synthesis of colloidal silica particles with new shapes by manipulating the growth conditions of rods that are growing from polyvinylpyrrolidone-loaded water-rich droplets containing ammonia and ethanol. The silica rods grow by ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS). The lengthwise growth of these silica rods gives us the opportunity to change the conditions at any time during the reaction. In this work, we vary the availability of hydrolyzed monomers as a function of time and study how the change in balance between the hydrolysis and condensation reactions affects a typical synthesis (as described in more detail by our group earlier1). First, we show that in a "standard" synthesis, there are two silica growth processes occurring; one in the oil phase and one in the droplet. The growth process in the water droplet causes the lengthwise growth of the rods. The growth process in the oil phase produces a thin silica layer around the rods, but also causes the nucleation of 70 nm silica spheres. During a typical rod growth, silica formation mainly takes place in the droplet. The addition of partially hydrolyzed TEOS or tetramethoxysilane (TMOS) to the growth mixture results in a change in balance between the hydrolysis and condensation reaction. As a result, the growth also starts to take place on the surface of the water droplet and thus from the oil phase, not only from inside the droplet onto a silica rod sticking out of the droplet. Carefully tuning the growth from the droplet and the growth from the oil phase allowed us to create nanospheres, hollow silica rods, hollow sphere rod systems (colloidal matchsticks), and bent silica rods.

Impact of the electron beam on the thermal stability of gold nanorods studied by environmental transmission electron microscopy.

In-situ transmission electron microscopy experiments are of great interest to nanoscience and nanotechnology. However, it is known that the electron beam can have a significant impact on the structure of the sample which makes it important to carefully interpret in-situ data. In this work, we studied the thermal stability of CTAB-stabilized gold nanorods under different gaseous environments in an environmental transmission electron microscope and compared the outcome to ex-situ heating experiments. We observed a remarkable influence of the electron beam: While the nanorods were stable under inert conditions when exposed to the electron beam even at 400°C, the same nanorods reshaped at temperatures as low as 100°C under ex-situ conditions. We ascribe the stabilizing effect to the transformation of the CTAB bi-layer into a thin carbon layer under electron beam irradiation, preventing the nanorods from deforming. When exposed to an oxidizing environment in the environmental transmission electron microscope, this carbon layer was gradually removed and the gold atoms became mobile allowing for the deformation of the rod. This work highlights the importance of understanding the phenomena taking place under electron beam irradiation, which can greatly affect in-situ experiments and conclusions drawn from these. It stresses that in-situ electron microscopy data, taken on measuring the temperature dependence of nanoparticle properties, should be carefully assessed and accompanied by ex-situ experiments if possible.

Fully-biobased UV-absorbing nanoparticles from ethyl cellulose and zein for environmentally friendly photoprotection.

Effective photoprotection is a vital consumer issue. However, there are many concerns regarding the adverse environmental and health impacts associated with current organic and inorganic UV filters. Here, we prepare fully-biobased UV-absorbing nanoparticles from ethyl cellulose (ECNPs) and zein (ZNPs) with encapsulated biobased photoprotectants obtainable from plants and foods (quercetin, retinol, and p-coumaric acid), which have the potential to satisfy both environmental and health issues in photoprotection. We show the ability of ECNPs and ZNPs to be easily tuned compositionally to obtain uniform, broadband UV spectrum absorbance profiles, and prepare transparent UV-absorbing coatings from the ECNPs. We find that the maximum loadings for retinol, quercetin, and p-coumaric acid into the ECNPs are 31 wt%, 14 wt%, and 13 wt% respectively. The ECNP size remains constant (except for the largest loading of retinol, 31 wt%) and the absolute zeta potential increases upon increasing the loading of quercetin and retinol, whereas increasing the loading of p-coumaric acid results in increasing the particle size and a lower absolute zeta potential. We find that quercetin and retinol are effectively retained inside the ECNPs at 64-70% after 72 hours. These results have significant implications for the development of novel photoprotection technologies and functional nanoparticles.

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.

Sculpting Silica Colloids by Etching Particles with Nonuniform Compositions.

We present the synthesis of new shapes of colloidal silica particles by manipulating their chemical composition and subsequent etching. Segments of silica rods, prepared by the ammonia catalyzed hydrolysis and condensation of tetraethylorthosilicate (TEOS) from polyvinylpyrrolidone loaded water droplets, were grown under different conditions. Upon decreasing temperature, delaying ethanol addition, or increasing monomer concentration, the rate of dissolution of the silica segment subsequently formed decreased. A watery solution of NaOH (∼mM) selectively etched these segments. Further tuning the conditions resulted in rod-cone or cone-cone shapes. Deliberately modulating the composition along the particle's length by delayed addition of (3-aminopropyl)-triethoxysilane (APTES) also allowed us to change the composition stepwise. The faster etching of this coupling agent in neutral conditions or HF afforded an even larger variety of particle morphologies while in addition changing the chemical functionality. A comparable step in composition was applied to silica spheres. Biamine functional groups used in a similar way as APTES caused a charge inversion during the growth, causing dumbbells and higher order aggregates to form. These particles etched more slowly at the neck, resulting in a biconcave silica ring sandwiched between two silica spheres, which could be separated by specifically etching the functionalized layer using HF.

Microelectrophoresis of Silica Rods Using Confocal Microscopy.

The electrophoretic mobility and the zeta potential (ζ) of fluorescently labeled colloidal silica rods, with an aspect ratio of 3.8 and 6.1, were determined with microelectrophoresis measurements using confocal microscopy. In the case where the colloidal particles all move at the same speed parallel to the direction of the electric field, we record a xyz-stack over the whole depth of the capillary. This method is faster and more robust compared to taking xyt-series at different depths inside the capillary to obtain the parabolic flow profile, as was done in previous work from our group. In some cases, rodlike particles do not move all at the same speed in the electric field, but exhibit a velocity that depends on the angle between the long axis of the rod and the electric field. We measured the orientation-dependent velocity of individual silica rods during electrophoresis as a function of κa, where κ-1 is the double layer thickness and a is the radius of the rod associated with the diameter. Thus, we determined the anisotropic electrophoretic mobility of the silica rods with different sized double layers. The size of the double layer was tuned by suspending silica rods in different solvents at different electrolyte concentrations. We compared these results with theoretical predictions. We show that even at already relatively high κa when the Smoluchowski limiting law is assumed to be valid (κa > 10), an orientation dependent velocity was measured. Furthermore, we observed that at decreasing values of κa the anisotropy in the electrophoretic mobility of the rods increases. However, in low polar solvents with κa < 1, this trend was reversed: the anisotropy in the electrophoretic mobility of the rods decreased. We argue that this decrease is due to end effects, which was already predicted theoretically. When end effects are not taken into account, this will lead to strong underestimation of the experimentally determined zeta potential.

Yolk/Shell Colloidal Crystals Incorporating Movable Cores with Their Motion Controlled by an External Electric Field.

Yolk/shell particles composed of a submicrometer-sized movable core and a silica shell are promising building blocks for novel optical colloidal crystals, because the locations of cores in the shell compartment can be reversibly changed by using external stimuli. Two dimensional arrays of yolk/shell particles incorporating movable cores were prepared by a self-assembly method. The movable cores of colloidal crystals in water could be observed with an optical microscope under application of external electric field. The motions of inner silica cores depended on the electric field strength and frequency and were categorized into three cases: (1) Random Brownian motion, (2) anisotropic motion of cores moving in a direction orthogonal to a field, and (3) suppressed motion fixed in the center of shell compartment. Random Brownian motion of cores was scarcely affected by field strength when a high frequency (in the MHz range) electric field was applied. On the other hand, an increase in field strength at low-frequency fields (kHz) transiently changed the core motion from (1) to (2) and a further increase in field strength changed it from (2) to (3). When the silica core was incorporated in a large void a stronger electric field was needed to suppress its motion than when it was in a small void. The high responsivity to electric fields in a low-frequency range indicated the importance of electric double layer (EDL) interaction between core and inner shell in controlling the core location in yolk/shell colloidal crystals. It was also shown that movable titania cores in yolk/shell particles required a low-frequency field with a high strength to change from the random to anisotropic motion. The result suggested that the electrostatic interaction between EDLs of the silica core and the inner silica wall could be stronger than that between EDLs of the titania core and the silica shell.

Biobased Nanoparticles for Broadband UV Protection with Photostabilized UV Filters.

Sunscreens rely on multiple compounds to provide effective and safe protection against UV radiation. UV filters in sunscreens, in particular, provide broadband UV protection but are heavily linked to adverse health effects due to the generation of carcinogenic skin-damaging reactive oxygen species (ROS) upon solar irradiation. Herein, we demonstrate significant reduction in the ROS concentration by encapsulating an antioxidant photostabilizer with multiple UV filters into biobased ethyl cellulose nanoparticles. The developed nanoparticles display complete broadband UV protection and can form transparent and flexible films. This system therefore shows significant potential toward effective and safe nanoparticle-based UV protective coatings.

Phase diagram of binary colloidal rod-sphere mixtures from a 3D real-space analysis of sedimentation-diffusion equilibria.

Self-assembly of binary particle systems offers many new opportunities for materials science. Here, we studied sedimentation equilibria of silica rods and spheres, using quantitative 3D confocal microscopy. We determined not only pressure, density and order parameter profiles, but also the experimental phase diagram exhibiting a stable binary smectic liquid-crystalline phase (Sm2). Using computer simulations we confirmed that the Sm2-phase can be stabilized by entropy alone, which opens up the possibility of combining new materials properties at a wide array of length scales.

Random three-dimensional jammed packings of elastic shells acting as force sensors.

In a jammed solid of granular particles, the applied stress is in-homogeneously distributed within the packing. A full experimental characterization requires measurement of all the interparticle forces, but so far such measurements are limited to a few systems in two and even fewer in three dimensions. Particles with the topology of (elastic) shells are good local force sensors as relatively large deformations of the shells result from relatively small forces. We recently introduced such fluorescent shells as a model granular system in which force distributions can be determined in three dimensions using confocal microscopy and quantitative image analysis. An interesting aspect about these shells that differentiates them from other soft deformable particles is their buckling behavior at higher compression. This leads to deformations that do not conserve the inner volume of the particle. Here we use this system to accurately measure the contact forces in a three-dimensional packing of shells subjected to a static anisotropic compression and to shear. At small deformations forces are linear, however, for a buckled contact, the restoring force is related to the amount of deformation by a square root law, as follows from the theory of elasticity of shells. Near the unjamming-jamming transition (point J), we found the probability distribution of the interparticle forces P(f) to decay nearly exponentially at large forces, with little evidence of long-range force chains in the packings. As the packing density is increased, the tail of the distribution was found to crossover to a Gaussian, in line with other experimental and simulation studies. Under a small shear strain, up to 0.216, applied at an extremely low shear rate, we observed a shear-induced anisotropy in both the pair correlation function and contact force network; however, no appreciable change was seen in the number of contacts per particle.

Synthesis of Cone-Shaped Colloids from Rod-Like Silica Colloids with a Gradient in the Etching Rate.

We present the synthesis of monodisperse cone-shaped silica colloids and their fluorescent labeling. Rod-like silica colloids prepared by ammonia-catalyzed hydrolysis and condensation of tetraethyl orthosilicate in water droplets containing polyvinylpyrrolidone cross-linked by citrate ions in pentanol were found to transform into cone-shaped particles upon mild etching by NaOH in water. The diameter and length of the resulting particles were determined by those of the initial rod-like silica colloids. The mechanism responsible for the cone-shape involves silica etching taking place with a varying rate along the length of the particle. Our experiments thus also lead to new insights into the variation of the local particle structure and composition. These are found to vary gradually along the length of the rod, as a result of the way the rod grows out of a water droplet that keeps itself attached to the flat end of the bullet-shaped particles. Subtle differences in composition and structure could also be resolved by high-resolution stimulated emission depletion confocal microscopy on fluorescently labeled particles. The incorporation of a fluorescent dye chemically attached to an amine-based silane coupling agent resulted in a distribution of fluorophores mainly on the outside of the rod-shaped particles. In contrast, incorporation of the silane coupling agent alone resulted in a homogeneous distribution. Additionally, we show that etching rods, where a silane coupling agent alone was incorporated and subsequently coupled to a fluorescent dye, resulted in fluorescent silica cones, the orientation of which can be discerned using super-resolution confocal microscopy.

Confinement Induced Plastic Crystal-to-Crystal Transitions in Rodlike Particles with Long-Ranged Repulsion.

Colloidal particles in geometrical confinement display a complex variety of packing structures different from their three-dimensional (3D) bulk counterpart. Here, we confined charged rodlike colloids with long-ranged repulsions to a thin wedge-shaped cell and show, by quantitative 3D confocal microscopy, that not only their positional but also their orientational order depends sensitively upon the slit width. Synchronized with transitions in lattice symmetry and number of layers confinement induces plastic crystal-to-crystal transitions. A model analysis suggests that this complex sequence of more or less rotationally ordered states originates from the subtle competition between the electrostatic repulsion of a rod with the wall and with its neighbors.

Directed Self-Assembly of Micron-Sized Gold Nanoplatelets into Oriented Flexible Stacks with Tunable Interplate Distance.

A growing demand for control over the interparticle spacing and the orientation of anisotropic metallic particles into self-assembled structures is fuelled by their use in potential applications such as in plasmonics, catalysis, sensing, and optoelectronics. Here, we present an improved high yield synthesis method to fabricate micron- and submicron-sized gold nanoplatelets with a thickness less than 20 nm using silver nanoplatelets as seeds. By tuning the depth of the secondary minimum in the DLVO interaction potential between these particles, we are able to assemble the platelets into dynamic and flexible stacks containing thousands of platelets arranged face-to-face with well-defined spacing. Moreover, we demonstrate that the length of the stacks, and the interplate distance can be controlled between tens and hundreds of nm with the ionic strength. We use a high frequency external electric field to control the orientation of the stacks and direct the stacks into highly organized 2D and 3D assemblies that strongly polarize light.

Nematic ordering of polarizable colloidal rods in an external electric field: theory and experiment.

We employ the coupled dipole method [B. W. Kwaadgras, et al., J. Chem. Phys., 2011, 135, 134105] to calculate the orientation-dependent interaction of polarizable colloidal rods with an external electric field. We project the angular distribution function of a system of such rods on a quasi-two-dimensional slab, corresponding to the focal plane of a microscope, and show that the 3D nematic order parameter and its measurable projected analogue are very similar. We compare our results to confocal microscopy measurements on the orientation distribution function of systems of polarizable colloidal silica rods in an external electric field, demonstrating reasonably good agreement between theory and experiment without any free fit parameter.

Pores with longitudinal irregularities distinguish objects by shape.

The resistive-pulse technique has been used to detect and size objects which pass through a single pore. The amplitude of the ion current change observed when a particle is in the pore is correlated with the particle volume. Up to date, however, the resistive-pulse approach has not been able to distinguish between objects of similar volume but different shapes. In this manuscript, we propose using pores with longitudinal irregularities as a sensitive tool capable of distinguishing spherical and rod-shaped particles with different lengths. The ion current modulations within resulting resistive pulses carry information on the length of passing objects. The performed experiments also indicate the rods rotate while translocating, and displace an effective volume that is larger than their geometrical volume, and which also depends on the pore diameter.