Bulk Electrosynthesis of Patchy Particles with Highly Controlled Asymmetric Features.

Asymmetric modification of particles with various patches of different composition and size at predefined positions is an important challenge in contemporary surface chemistry, as such particles have numerous potential applications, ranging from materials science and (photo)catalysis to self-assembly and drug delivery. However, approaches allowing the synthesis of this kind of complex objects in the bulk of a solution in a straightforward way are currently lacking. In this context, bipolar electrochemistry (BE) is a powerful technique for the asymmetric modification of conducting objects. Herein, this approach is used for the highly controlled modification of particles with different metal patches, generated at specific locations of isotropic objects. The synthesis is carried out in the bulk of the solution and leads to predefined patterns of increasing complexity, including even a specific chiral arrangement of the patches.

Engineering Azeotropy to Optimize the Self-Assembly of Colloidal Mixtures.

The goal of inverse self-assembly is to design interparticle interactions capable of assembling the units into a desired target structure. The effective assembly of complex structures often requires the use of multiple components, each new component increasing the thermodynamic degrees of freedom and, hence, the complexity of the self-assembly pathway. In this work we explore the possibility to use azeotropy, i.e., a special thermodynamic condition where the system behaves effectively as a one-component system, as a way to control the self-assembly of an arbitrary number of components. Exploiting the mass-balance equations, we show how to select patchy particle systems that exhibit azeotropic points along the desired self-assembly pathway. As an example we map the phase diagram of a binary mixture that, by design, fully assembles into cubic (and only cubic) diamond crystal via an azeotropic point. The ability to explicitly include azeotropic points in artificial designs reveals effective pathways for the self-assembly of complex structures.

Engineering the surface patchiness and topography of polystyrene colloids: From spheres to ellipsoids.

HYPOTHESIS: Colloidal surface morphology determines suspension properties and applications. While existing methods are effective at generating specific features on spherical particles, an approach extending this to non-spherical particles is currently missing. Synthesizing un-crosslinked polymer microspheres with controlled chemical patchiness would allow subsequent thermomechanical stretching to translate surface topographical features to ellipsoidal particles. EXPERIMENTS: A systematic study using seeded emulsion polymerization to create polystyrene (PS) microspheres with controlled surface patches of poly(tert-butyl acrylate) (PtBA) was performed with different polymerization parameters such as concentration of tBA monomer, co-swelling agent, and initiator. Thermomechanical stretching converted seed spheres to microellipsoids. Acid catalyzed hydrolysis (ACH) was performed to remove the patch domains. Roughness was characterized before and after ACH using atomic force microscopy. FINDINGS: PS spheres with controlled chemical patchiness were synthesized. A balance between two factors, domain coalescence from reduced viscosity and domain growth via monomer absorption, dictates the final PtBA) patch features. ACH mediated removal of patch domains produced either golf ball-like porous particles or multicavity particles, depending on the size of the precursor patches. Patchy microspheres were successfully stretched into microellipsoids while retaining their surface characteristics. Particle roughness is governed by the patch geometry and increases after ACH. Overall, this study provides a facile yet controllable platform for creating colloids with highly adjustable surface patterns.

Fabrication of Patchy Silica Microspheres with Tailor-Made Patch Functionality using Photo-Iniferter Reversible-Addition-Fragmentation Chain-Transfer (PI-RAFT) Polymerization.

Their inherent directional information renders patchy particles interesting building blocks for advanced applications in materials science. In this study, a feasible method to fabricate patchy silicon dioxide microspheres is demonstrated, which they are able to equip with tailor-made polymeric materials as patches. Their fabrication method relies on a solid-state supported microcontact printing (µCP) routine optimized for the transfer of functional groups to capillary-active substrates, which is used to introduce amino functionalities as patches to a monolayer of particles. Acting as anchor groups for polymerization, photo-iniferter reversible addition-fragmentation chain-transfer (RAFT) is used to graft polymer from the patch areas. Accordingly, particles with poly(N-acryloyl morpholine), poly(N-isopropyl acrylamide), and poly(n-butyl acrylate) are prepared as representative acrylic acid-derived functional patch materials. To facilitate their handling in water, a passivation strategy of the particles for aqueous systems is introduced. The protocol introduced here, therefore, promises a vast degree of freedom in engineering the surface properties of highly functional patchy particles. This feature is unmatched by other techniques to fabricate anisotropic colloids. The method, thus, can be considered a platform technology, culminating in the fabrication of particles that possess locally precisely formed patches on particles at a low µm scale with a high material functionality.

Advances in Colloidal Building Blocks: Toward Patchy Colloidal Clusters.

The scalable synthetic route to colloidal atoms has significantly advanced over the past two decades. Recently, colloidal clusters with DNA-coated cores called "patchy colloidal clusters" have been developed, providing a directional bonding with specific angle of rotation due to the shape complementarity between colloidal clusters. Through a DNA-mediated interlocking process, they are directly assembled into low-coordination colloidal structures, such as cubic diamond lattices. Herein, the significant progress in recent years in the synthesis of patchy colloidal clusters and their assembly in experiments and simulations is reviewed. Furthermore, an outlook is given on the emerging approaches to the patchy colloidal clusters and their potential applications in photonic crystals, metamaterials, topological photonic insulators, and separation membranes.

Sorting of heterogeneous colloids by AC-dielectrophoretic forces in a microfluidic chip with asymmetric orifices.

HYPOTHESIS: The synthesis of compositionally heterogeneous particles is central to the development of complex colloidal units for self-assembly and self-propulsion. Yet, as the complexity of particles grows, synthesis becomes more prone to "errors". We hypothesize that alternating-current dielectrophoretic forces can efficiently sort Janus particles, as a function of patch size and material, and colloidal dumbbells by size. EXPERIMENTS: We prepared Janus particles with different patch size and material by physical vapor deposition and colloidal dumbbells via capillarity-assisted particle assembly. We then performed sorting experiments in a microfluidic chip comprising electrodes with asymmetric orifices, specifically exploiting the dielectric contrast between different portions of the particles or their size difference to steer them towards different outlets. FINDINGS: We calculated that the DEP force for Janus particles may switch from positive to negative as a function of composition at a critical AC frequency, thus enabling sorting different particles crossing the electrodes' region. The predictions are confirmed by optical microscopy experiments. We also show that intact and "broken" dumbbells can be simply separated as they experience different DEP forces. The integration of multiple asymmetric orifices leads a larger zone with high field gradient to increase separation efficiency and makes it a promising tool to select precise particle populations, isolating fractions with narrowly distributed characteristics.

Closed Loop Control of Bubble-Propelled Microrobots.

Bubble-propelled microrobots have an advantage of relatively swift movement compared to most other types of microrobots, which makes them well suited for applications such as micromanipulation or movement in flows, but their high speed also poses challenges in precisely controlling their motion. This study proposes automated control of the microrobots using visual feedback and steering with uniform magnetic fields to constrain the microrobot's moving direction. The implementation of a closed-loop control mechanism ensures precise autonomous navigation along prescribed trajectories. Experimental results demonstrate that this approach achieves satisfactory tracking performance, with an average error of 6. 7 µm for a microrobot with a diameter of 24 µm.

Modulating Functionality of Starch-Based Patchy Particles by Manipulating Architecture and Environmental Factors.

Starch as a food-grade thickener has been commonly used in food products to modulate textural properties. Improving viscosity-enhancing ability, so as to be able to use less starch for the same texture, has been considered as an approach to reduce the dietary consumption of carbohydrates. We have positively charged amaranth starch (∼1 µm) and negatively charged corn starch (>10 µm) and physically fused the particles together to create a starch with a heterogeneous pattern. This starch has a negatively charged main body, due to the larger corn particles, and positively charged patches from the amaranth starch. These patchy particles self-assembled through electrostatic interactions into a shear-reversible thickener. The impact of patchiness and charge density on material functionality was investigated. The degree of patchiness was controlled by manipulating the ratio between the two starches, and results showed that viscosity was reduced when the patchiness was higher. With the same patchy area, a higher charge density did not contribute to higher water-holding capacity. The more charged particles were able to enhance the viscosity, however, due to the stronger interparticle electrostatic interaction. The effects of environmental factors including pH level and ionic strength were also investigated, and the results showed that at extreme pH levels, or in the presence of Na+ or Ca2+, the charges on the starch particles were screened, and this inhibited interaction and reduced viscosity. The present work demonstrates that the texture of starch slurries can be fine-tuned by manipulating the degree of patchiness and the charge density of patchy particles. It also evaluates the application potential in food products with different pH levels and ionic strengths.

The physics of empty liquids: from patchy particles to water.

Empty liquids represent a wide class of materials whose constituents arrange in a random network through reversible bonds. Many key insights on the physical properties of empty liquids have originated almost independently from the study of colloidal patchy particles on one side, and a large body of theoretical and experimental research on water on the other side. Patchy particles represent a family of coarse-grained potentials that allows for a precise control of both the geometric and the energetic aspects of bonding, while water has arguably the most complex phase diagram of any pure substance, and a puzzling amorphous phase behavior. It was only recently that the exchange of ideas from both fields has made it possible to solve long-standing problems and shed new light on the behavior of empty liquids. Here we highlight the connections between patchy particles and water, focusing on the modelling principles that make an empty liquid behave like water, including the factors that control the appearance of thermodynamic and dynamic anomalies, the possibility of liquid-liquid phase transitions, and the crystallization of open crystalline structures.

Low-Symmetry MOF-Based Patchy Colloids and Their Precise Linking via Site-Selective Liquid Bridging to Form Supra-Colloidal and Supra-Framework Architectures.

Colloids with surface patches (or patchy particles) can bind and assemble with directionality. However, the bonding between the usually high-symmetry, dome-shaped patches is not precise, as it cannot lock the exact position and orientation of the relevant particles. This issue prevents the assembly of well-defined colloidal superstructures by design. Herein, we introduce low-symmetry, metal-organic framework (MOF)-based patchy colloids, which feature a polyhedral matrix and flat hexagonal patches, along with anisotropic surfaces and compositions. Guided by the encoded shape/chemical information and mediated by a site-selective liquid-bridging interaction, the distinct patchy particles self-assemble into supra-colloidal (or supra-framework) structures with unprecedented precision. In this case, the valence, position, and orientation of the particles within assemblies are fully coordinated and precisely aligned. The dynamic nature of the liquid bridges also allows us to investigate the unique assembly kinetics. Our strategy not only defines new modes of colloidal bonding, but also provides a powerful means toward creating hierarchical and multi-component MOF materials.

Determination of the yield, mass and structure of silver patches on colloidal silica using multiwavelength analytical ultracentrifugation.

Anisotropic nanoparticles offer considerable promise for applications but also present significant challenges in terms of their characterization. Recent developments in the electroless deposition of silver patches directly onto colloidal silica particles have opened up a simple and scalable synthesis method for patchy particles with tunable optical properties. Due to the reliance on patch nucleation and growth, however, the resulting coatings are distributed in coverage and thickness and some core particles remain uncoated. To support process optimization, new methods are required to rapidly determine patch yield, thickness and coverage. Here we present a novel approach based on multiwavelength analytical ultracentrifugation (MWL-AUC) which permits simultaneous hydrodynamic and spectroscopic characterization. The patchy particle colloids are produced in a continuous flow mixing process that makes use of a KM-type micromixer. By varying the process flow rate or metal precursor concentration we show how the silver to silica mass ratio distribution derived from the AUC-measured sedimentation coefficient distribution can be influenced. Moreover, through reasoned assumptions we arrive at an estimation of the patch yield that is close to that determined by arduous analysis of scanning electron microscopy (SEM) images. Finally, combining MWL-AUC, electrodynamic simulations and SEM image analysis we establish a procedure to estimate the patch thickness and coverage.

Self-Assembly of All-DNA Rods with Controlled Patchiness.

Double-stranded DNA (dsDNA) fragments exhibit noncovalent attractive interactions between their tips. It is still unclear how DNA liquid crystal self-assembly is affected by such blunt-end attractions. It is demonstrated that stiff dsDNA fragments with moderate aspect ratio can specifically self-assemble in concentrated aqueous solutions into different types of smectic mesophases on the basis of selectively screening of blunt-end DNA stacking interactions. To this end, this type of attractions are engineered at the molecular level by constructing DNA duplexes where the attractions between one or both ends are screened by short hairpin caps. All-DNA bilayer and monolayer smectic-A type of phases, as well as a columnar phase, can be stabilized by controlling attractions strength. The results imply that the so far elusive smectic-A in DNA rod-like liquid crystals is a thermodynamically stable phase. The existence of the bilayer smectic phase is confirmed by Monte-Carlo simulations of hard cylinders decorated with one attractive terminal site. This work demonstrates that DNA blunt-ends behave as well-defined monovalent attractive patches whose strength and position can be potentially precisely tuned, highlighting unique opportunities concerning the stabilization of nonconventional DNA-based lyotropic liquid crystal phases assembled by all-DNA patchy particles with arbitrary geometry and composition.

Facile self-assembly of colloidal diamond from tetrahedral patchy particles via ring selection.

Diamond-structured crystals, particularly those with cubic symmetry, have long been attractive targets for the programmed self-assembly of colloidal particles, due to their applications as photonic crystals that can control the flow of visible light. While spherical particles decorated with four patches in a tetrahedral arrangement-tetrahedral patchy particles-should be an ideal building block for this endeavor, their self-assembly into colloidal diamond has proved elusive. The kinetics of self-assembly pose a major challenge, with competition from an amorphous glassy phase, as well as clathrate crystals, leaving a narrow widow of patch widths where tetrahedral patchy particles can self-assemble into diamond crystals. Here we demonstrate that a two-component system of tetrahedral patchy particles, where bonding is allowed only between particles of different types to select even-member rings, undergoes crystallization into diamond crystals over a significantly wider range of patch widths conducive for experimental fabrication. We show that the crystallization in the two-component system is both thermodynamically and kinetically enhanced, as compared to the one-component system. Although our bottom-up route does not lead to the selection of the cubic polytype exclusively, we find that the cubicity of the self-assembled crystals increases with increasing patch width. Our designer system not only promises a scalable bottom-up route for colloidal diamond but also offers fundamental insight into crystallization into open lattices.

Divalent Multilinking Bonds Control Growth and Morphology of Nanopolymers.

Assembly of nanoscale objects into linear architectures resembling molecular polymers is a basic organization resulting from divalent interactions. Such linear architectures occur for particles with two binding patches on opposite sides, known as Janus particles. However, unlike molecular systems where valence bonds can be envisioned as pointlike interactions nanoscale patches are often realized through multiple molecular linkages. The relationship between the characteristics of these linkages, the resulting interpatch connectivity, and assembly morphology is not well-explored. Here, we investigate assembly behavior of model divalent nanomonomers, DNA nanocuboid with tailorable multilinking bonds. Our study reveals that the characteristics of individual molecular linkages and their collective properties have a profound effect on nanomonomer reactivity and resulting morphologies. Beyond linear nanopolymers, a common signature of divalent nanomonomers, we observe an effective valence increase as linkages lengthened, leading to the nanopolymer bundling. The experimental findings are rationalized by molecular dynamics simulations.

Mikto-Arm Stars as Soft-Patchy Particles: From Building Blocks to Mesoscopic Structures.

We present an atomistic molecular dynamics study of self-assembled mikto-arm stars, which resemble patchy-like particles. By increasing the number of stars in the system, we propose a systematic way of examining the mutual orientation of these fully penetrable patchy-like objects. The individual stars maintain their patchy-like morphology when creating a mesoscopic (macromolecular) self-assembled object of more than three stars. The self-assembly of mikto-arm stars does not lead to a deformation of the stars, and their shape remains spherical. We identified characteristic sub-units in the self-assembled structure, differing by the mutual orientation of the nearest neighbor stars. The current work aims to elucidate the possible arrangements of the realistic, fully penetrable patchy particles in polymer matrix and to serve as a model system for further studies of nanostructured materials or all-polymer nanocomposites using the mikto-arm stars as building blocks.

Directional and Reconfigurable Assembly of Metallodielectric Patchy Particles.

Colloidal particles with surface patches can self-assemble with high directionality, but the resulting assemblies cannot reconfigure unless the patch arrangement (number, symmetry, etc.) is altered. While external fields with tunable inputs can guide the assembly of dynamic structures, they encourage particle alignment relative to its shape rather than the surface patterns. Here, we report on the synthesis of metallodielectric patchy particles and their assembly under the AC electric field, which gives rise to a series of structures including two-layer alternating chains, open-brick walls, staggering stacks, and vertical chains that are directed by the patches yet reconfigurable by the field. The configurations of the assemblies (e.g., the chains) can be further switched between a rigid and a flexible state emulating the conformations of polymers. Our work suggests that, for directed colloidal assembly, the particle complexities (patches and shapes) can be coupled with the external manipulations in a cooperative manner for creating materials with precise yet reconfigurable structures.

Towards Polymeric Nanoparticles with Multiple Magnetic Patches.

Fabricating future materials by self-assembly of nano-building blocks programmed to generate specific lattices is among the most challenging goals of nanotechnology and has led to the recent concept of patchy particles. We report here a simple strategy to fabricate polystyrene nanoparticles with several silica patches based on the solvent-induced self-assembly of silica/polystyrene monopods. The latter are obtained with morphological yields as high as 99% by seed-growth emulsion polymerization of styrene in the presence of 100 nm silica seeds previously modified with an optimal surface density of methacryloxymethyl groups. In addition, we fabricate "magnetic" silica seeds by silica encapsulation of preformed maghemite supraparticles. The polystyrene pod, i.e., surface nodule, serves as a sticky point when the monopods are incubated in a bad/good solvent mixture for polystyrene, e.g., ethanol/tetrahydrofuran mixtures. After self-assembly, mixtures of particles with two, three, four silica or magnetic silica patches are mainly obtained. The influence of experimental parameters such as the ethanol/tetrahydrofuran volume ratio, monopod concentration and incubation time is studied. Further developments would consist of obtaining pure batches by centrifugal sorting and optimizing the relative position of the patches in conventional repulsion figures.

Top-Down Heterogeneous Colloidal Engineering Using Capillary Assembly of Liquid Particles.

Capillary assembly of liquid particles (CALP) is a microfabrication strategy for engineering arbitrarily shaped polymer colloids. The method entails depositing emulsion particles into patterned microarrays within a fluidic cell: coalescence, polymerization, and extraction of the deposited material engender faceted colloids. Herein, the versatility of CALP is demonstrated by using both consecutive assembly and heterogeneous coassembly to engineer geometrically diverse Janus and patchy colloids. Liquid particles (LPs) can be patterned laterally across the plane of the template by manipulating the capillary immersion force, liquid particle hardness, and rate of coalescence. Bilayers of different polymeric LPs and patchy microarrays are fabricated, comprising solid colloids made from various materials including poly(styrene), p-styryltrimethoxysilane, and iron oxide. Eleven different structures including concentric Janus squares, triblock ellipsoids, and planar tetramer and pentagonal patchy particles are described. All particles are fluorescently labeled, resist flocculation, withstand extended heating, and endure dispersion in organic solvent. Further crystallization and processing into colloid-based microscale devices is therefore anticipated. Heterogeneous CALP combines top-down microfabrication with bottom-up synthesis to engineer nonequilibrium particle structures that cannot be made with wet chemistry. CALP enables the design and fabrication of colloids with complex internal construction to target hierarchical functional materials. Ultimately, the integration of colloidal building blocks comprising multiple components that are independently addressable is crucial for the development of nano/micromaterials such as filtration devices, sensors, diagnostics, solid-state catalysts, and optical electronics.

Integration of Multitargeted Polymer-Based Contrast Agents with Photoacoustic Computed Tomography: An Imaging Technique to Visualize Breast Cancer Intratumor Heterogeneity.

One of the primary challenges in breast cancer diagnosis and treatment is intratumor heterogeneity (ITH), i.e., the coexistence of different genetically and epigenetically distinct malignant cells within the same tumor. Thus, the identification of ITH is critical for designing better treatments and hence to increase patient survival rates. Herein, we report a noninvasive hybrid imaging technology that integrates multitargeted and multiplexed patchy polymeric photoacoustic contrast agents (MTMPPPCAs) with single-impulse panoramic photoacoustic computed tomography (SIP-PACT). The target specificity ability of MTMPPPCAs to distinguish estrogen and progesterone receptor-positive breast tumors was demonstrated through both fluorescence and photoacoustic measurements and validated by tissue pathology analysis. This work provides the proof-of-concept of the MTMPPPCAs/SIP-PACT system to identify ITH in nonmetastatic tumors, with both high molecular specificity and real-time detection capability.

Solvent-Induced Assembly of One-Patch Silica Nanoparticles into Robust Clusters, Wormlike Chains and Bilayers.

We report the synthesis and solvent-induced assembly of one-patch silica nanoparticles in the size range of 100-150 nm. They consisted, as a first approximation, of silica half-spheres of which the truncated face was itself concave and carried in its center a polymeric patch made of grafted polystyrene chains. The multistage synthesis led to 98% pure batches and allowed a fine control of the patch-to-particle size ratio from 0.69 to 1.54. The self-assembly was performed in equivolume mixtures of tetrahydrofuran and ethanol, making the polymeric patches sticky and ready to coalesce together. The assembly kinetics was monitored by collecting samples over time and analyzing statistically their TEM images. Small clusters, such as dimers, trimers, and tetramers, were formed initially and then evolved in part into micelles. Accordingly to previous simulation studies, more or less branched wormlike chains and planar bilayers were observed in the long term, when the patch-to-particle size ratio was high enough. We focused also on the experimental conditions that could allow preparing small clusters in a good morphology yield.