Spherical Building Blocks Research Articles

Universal Way to "Glue" Capsules and Gels into 3D Structures by Electroadhesion.

We demonstrate the use of electroadhesion (EA), i.e., adhesion induced by an electric field, to connect a variety of soft materials into 3D structures. EA requires a cationic and an anionic material, but these can be of diverse origin, including covalently cross-linked hydrogels made by polymerizing charged monomers or physical gels/capsules formed by the ionic cross-linking of biopolymers (e.g., alginate and chitosan). Between each cationic/anionic pair, EA is induced rapidly (in ∼10 s) by low voltages (∼10 V DC)─and the adhesion is permanent after the field is turned off. The adhesion is strong enough to allow millimeter-scale capsules/gels to be assembled in 3D into robust structures such as capsule-capsule chains, capsule arrays on a base gel, and a 3D cube of capsules. EA-based assembly of spherical building blocks can be done more precisely, rapidly, and easily than by any alternative techniques. Moreover, the adhesion can be reversed (by switching the polarity of the field)─hence any errors during assembly can be undone and fixed. EA can also be used for selective sorting of charged soft matter─for example, a 'finger robot' can selectively 'pick up' capsules of the opposite charge by EA and subsequently 'drop off' these structures by reversing the polarity. Overall, our work shows how electric fields can be used to connect soft matter without the need for an adhesive or glue.

Structural investigations of Au-Ni aerogels: morphology and element distribution.

The physical properties of nanomaterials are determined by their structural features, making accurate structural control indispensable. This carries over to future applications. In the case of metal aerogels, highly porous networks of aggregated metal nanoparticles, such precise tuning is still largely pending. Although recent improvements in controlling synthesis parameters like electrolytes, reductants, or mechanical stirring, the focus has always been on one particular morphology at a time. Meanwhile, complex factors, such as morphology and element distributions, are studied rather sparsely. We demonstrate the capabilities of precise morphology design by deploying Au-Ni, a novel element combination for metal aerogels in itself, as a model system to combine common aerogel morphologies under one system for the first time. Au-Ni aerogels were synthesized via modified one- and two-step gelation, partially combined with galvanic replacement, to obtain aerogels with alloyed, heterostructural (novel metal aerogel structure of interconnected nanoparticles and nanochains), and hollow spherical building blocks. These differences in morphology are directly reflected in the physisorption behavior, linking the isotherm shape and pore size distribution to the structural features of the aerogels, including a broad-ranging specific surface area (35-65 m2 g-1). The aerogels were optimized regarding metal concentration, destabilization, and composition, revealing some delicate structural trends regarding the ligament size and hollow sphere character. Hence, this work significantly improves the structural tailoring of metal aerogels and possible up-scaling. Lastly, preliminary ethanol oxidation tests demonstrated that morphology design extends to the catalytic performance. All in all, this work emphasizes the strengths of morphology design to obtain optimal structures, properties, and (performances) for any material application.

Elaborate Size-Tuning of Silica Aerogel Building Blocks Enables Laser-Driven Lighting.

Silica aerogels with accurate building-block control are realized by adjusting the surfactant concentration during the synthesis process. The resulting silica-aerogel monolith with spherical building blocks of ≈24-40nm, together with a deliberately created hole along the incident light direction, shows an incredibly promising application in monochromatic laser-driven lighting. The resulting coefficient of illuminance variation is as low as 8.1%, significantly outperforming commercially available ground-glass diffusers (139.0%) and polymer diffusers (249.1%); the speckle contrast is lower, as well as better, than that can be recognized by the human eye (4%), and the illuminance uniformity in the range of 0.770-0.862 is much better (higher) than that indoor workplace lighting required by the International Organization for Standardization. Lighting with any color in the visible spectrum, including white, can be obtained by using three primary color lasers (450, 532, and 638nm) with different powers simultaneously as the light source. The resulting silica aerogel, which has excellent thermal stability, high laser-damage threshold, outstanding mechanical performance, and superhydrophobicity, can be further applied to long-distance and noncontact laser-driven lighting in rain or underwater without any additional encapsulation components.

Electromagnetic field visualization in virtual reality

We developed two virtual reality demonstrations for electromagnetism. For statics, we use spherical building blocks and superposition to calculate the electric field of a user-generated configuration of charge. The electric field information is displayed on a three-dimensional grid using vector arrows. A second demonstration displays field information for electromagnetic fields generated by a particle moving along a user-generated path (at arbitrary speed). We used the Unity game creation engine to efficiently develop the demonstrations, and tested them on an HTC Vive VR headset with handheld controllers.

Biomimetic, Strong, Tough, and Self-Healing Composites Using Universal Sealant-Loaded, Porous Building Blocks.

Many natural materials, such as nacre and dentin, exhibit multifunctional mechanical properties via structural interplay between compliant and stiff constituents arranged in a particular architecture. Herein, we present, for the first time, the bottom-up synthesis and design of strong, tough, and self-healing composite using simple but universal spherical building blocks. Our composite system is composed of calcium silicate porous nanoparticles with unprecedented monodispersity over particle size, particle shape, and pore size, which facilitate effective loading and unloading with organic sealants, resulting in 258% and 307% increases in the indentation hardness and elastic modulus of the compacted composite. Furthermore, heating the damaged composite triggers the controlled release of the nanoconfined sealant into the surrounding area, enabling moderate recovery in strength and toughness. This work paves the path towards fabricating a novel class of biomimetic composites using low-cost spherical building blocks, potentially impacting bone-tissue engineering, insulation, refractory and constructions materials, and ceramic matrix composites.

Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS

Surface-enhanced Raman scattering (SERS) substrates play important roles for the enhancement of inelastic scattering signals. Traditional substrates such as roughened electrodes and colloidal aggregates suffer from well-known signal reproducibility issues, whereas for current dominant two-dimensional planar systems, the hot spot distributions are limited by the zero-, one- or two-dimensional plane. The introduction of a three-dimensional (3D) system such as a pyramid geometry breaks the limitation of a single Cartesian SERS-active area and extends it into the z-direction, with the tip potentially offering additional benefits of strong field enhancement and high sensitivity. However, current 3D pyramidal designs are restricted to film deposition on prepared pyramid templates or self-assembly in pyramidal molds with spherical building blocks, hence limiting their SERS effectiveness. Here, we report on the fabrication of a new class of low cost and well-defined plasmonic nanoparticle pyramid arrays from different anisotropic shaped nanoparticles using combined top-down lithography and bottom-up self-assembly approach. These pyramids exhibit novel optical scattering properties that can be exploited for the design of reproducible and sensitive SERS substrate. The SERS intensity was found to decrease drastically in accordance with a power law function as the focal planes move from the apex of the pyramid structure towards the base. In comparison to sphere-based building blocks, pyramids assembled from anisotropic rhombic dodecahedral gold nanocrystals with numerous sharp tips exhibited the strongest SERS performance. Macroscale pyramidal array films with plasmonic tunability as a new class of SERS substrate for sensitive detection of chemicals.

Switchable Supracolloidal Coassembly of Microgels Mediated by Host/Guest Interactions.

Supramolecular engineering of multibody colloidal systems provides flexible ways of manipulating superstructures and material properties. We investigate a coassembling microgel (MG) system, in which host- and guest-modified MG partners coassemble by molecular recognition, and show in detail how electrostatic repulsion needs to be balanced for the supramolecular recognition to take place. We observe a gradual change from repellent MGs to stable clusters and ordered flocculates upon decreasing electrostatic repulsion. The adaptive nature of the multivalent interactions embedded in the soft MG shell leads to kinetically trapped scenarios and fibril formation from spherical building blocks.

Preparation of highly uniform 1-dimensional α-Ag2WO4nanostructures with controllable aspect ratio and study of the growth mechanism

The synthesis of α-Ag2WO4 crystals using conventional methods typically leads to large micrometer-sized and/or non-uniform products. In this study, we demonstrate the preparation of monodisperse and uniform α-Ag2WO4 nanorods and nanowires using a facile precipitation approach. The key strategy of this approach is to allow (1) crystal growth to occur undisturbed and (2) sufficient time for equilibrium structures to be formed or to let nature take its course. The aspect ratio of the 1D products can be tuned by varying the duration of stirring during the initial reaction period. Analysis of the reaction intermediates revealed a three-stage growth mechanism involving the directed aggregation of spherical building blocks. The uniformity of the products can be attributed to self-focusing phenomena, in which sedimentation and the redistribution of mass yield 1D nanostructures with uniform lengths and diameters, respectively. α-Ag2WO4 nanowires and sub-micrometer rods were found to exhibit dissimilar absorption characteristics that led to the photodegradation of rhodamine B via different mechanisms.

Designing Robust Hierarchically Textured Oleophobic Fabrics

Commercially available woven fabrics (e.g., nylon- or PET-based fabrics) possess inherently re-entrant textures in the form of cylindrical yarns and fibers. We analyze the liquid repellency of woven and nanotextured oleophobic fabrics using a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks. At each level of hierarchy, the density of the topographical features is captured using a dimensionless textural parameter D(n)*. For a plain-woven mesh comprised of chemically treated fiber bundles (n = 2), the tight packing of individual fibers in each bundle (D2* ≈ 1) imposes a geometric constraint on the maximum oleophobicity that can be achieved solely by modifying the surface energy of the coating. For liquid droplets contacting such tightly bundled fabrics with modified surface energies, we show that this model predicts a lower bound on the equilibrium contact angle of θ(E) ≈ 57° below which the Cassie–Baxter to Wenzel wetting transition occurs spontaneously, and this is validated experimentally. We demonstrate how the introduction of an additional higher order micro-/nanotexture onto the fibers (n = 3) is necessary to overcome this limit and create more robustly nonwetting fabrics. Finally, we show a simple experimental realization of the enhanced oleophobicity of fabrics by depositing spherical microbeads of poly(methyl methacrylate)/fluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) onto the fibers of a commercial woven nylon fabric.

Bifacial DNA Origami-Directed Discrete, Three-Dimensional, Anisotropic Plasmonic Nanoarchitectures with Tailored Optical Chirality

Discrete three-dimensional (3D) plasmonic nanoarchitectures with well-defined spatial configuration and geometry have aroused increasing interest, as new optical properties may originate from plasmon resonance coupling within the nanoarchitectures. Although spherical building blocks have been successfully employed in constructing 3D plasmonic nanoarchitectures because their isotropic nature facilitates unoriented localization, it still remains challenging to assemble anisotropic building blocks into discrete and rationally tailored 3D plasmonic nanoarchitectures. Here we report the first example of discrete 3D anisotropic gold nanorod (AuNR) dimer nanoarchitectures formed using bifacial DNA origami as a template, in which the 3D spatial configuration is precisely tuned by rationally shifting the location of AuNRs on the origami template. A distinct plasmonic chiral response was experimentally observed from the discrete 3D AuNR dimer nanoarchitectures and appeared in a spatial-configuration-dependent manner. This study represents great progress in the fabrication of 3D plasmonic nanoarchitectures with tailored optical chirality.

Self-assembly of soft-matter quasicrystals and their approximants

The surprising recent discoveries of quasicrystals and their approximants in soft-matter systems poses the intriguing possibility that these structures can be realized in a broad range of nanoscale and microscale assemblies. It has been theorized that soft-matter quasicrystals and approximants are largely entropically stabilized, but the thermodynamic mechanism underlying their formation remains elusive. Here, we use computer simulation and free-energy calculations to demonstrate a simple design heuristic for assembling quasicrystals and approximants in soft-matter systems. Our study builds on previous simulation studies of the self-assembly of dodecagonal quasicrystals and approximants in minimal systems of spherical particles with complex, highly specific interaction potentials. We demonstrate an alternative entropy-based approach for assembling dodecagonal quasicrystals and approximants based solely on particle functionalization and shape, thereby recasting the interaction-potential-based assembly strategy in terms of simpler-to-achieve bonded and excluded-volume interactions. Here, spherical building blocks are functionalized with mobile surface entities to encourage the formation of structures with low surface contact area, including non-close-packed and polytetrahedral structures. The building blocks also possess shape polydispersity, where a subset of the building blocks deviate from the ideal spherical shape, discouraging the formation of close-packed crystals. We show that three different model systems with both of these features-mobile surface entities and shape polydispersity-consistently assemble quasicrystals and/or approximants. We argue that this design strategy can be widely exploited to assemble quasicrystals and approximants on the nanoscale and microscale. In addition, our results further elucidate the formation of soft-matter quasicrystals in experiment.

3D Brownian Diffusion of Submicron-Sized Particle Clusters

We report on the translation and rotation of particle clusters made through the combination of spherical building blocks. These clusters present ideal model systems to study the motion of objects with complex shape. Since they could be separated into fractions of well-defined configurations on a sufficient scale and because their overall dimensions were below 300 nm, the translational and rotational diffusion coefficients of particle doublets, triplets, and tetrahedrons could be determined by a combination of polarized dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS). The use of colloidal clusters for DDLS experiments overcomes the limitation of earlier experiments on the diffusion of complex objects near surfaces because the true 3D diffusion can be studied. When the exact geometry of the complex assemblies is known, different hydrodynamic models for calculating the diffusion coefficients for objects with complex shapes could be applied. Because hydrodynamic friction must be restricted to the cluster surface, the so-called shell model, in which the surface is represented as a shell of small friction elements, was most suitable to describe the dynamics. A quantitative comparison of the predictions from theoretical modeling with the results obtained by DDLS showed an excellent agreement between experiment and theory.

Cross-linking nanostructured spherical capsules as building units by crystal engineering: related chemistry

Abstract The compound [Mo72Fe30O252(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3 (H2O)91]·ca. 140 H2O 3≡3a·ca. 140 H2O, an important educt for an unusual solid state reaction, can now be obtained easily by reacting (NH4)42[{MoV2O4(CH3COO)}30{(Mo)Mo5O21(H2O)6}12]·10 CH3COONH4·ca. 300 H2O 1 with FeCl3·6 H2O in water. Interestingly, the freshly precipitated crystals of 3 contain discrete spherical clusters of the type {MoVI72FeIII30} with as yet unprecedented 30×5 unpaired electrons (S=150/2 at room temperature). Upon drying 3, its cluster units 3a get covalently linked to form layers in a step by step solid state reaction, according to the scheme described below, resulting finally in the crystalline reaction product [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3(H2O)87]·ca. 80 H2O 44a·ca. 80 H2O. The linking process at the Fe sites follows the well known inorganic condensation process leading to FeIII polycations in aqueous solution according to the scheme Fe(OH2)+(H2O)Fe → − H + Fe(OH)+(H2O)Fe → − H + − H 2 O Fe–O–Fe and thus is based on a type of crystal engineering with nanostructured spherical building blocks. This process does not allow chaotic characteristics in contrast to the mentioned polycation formation. Careful investigation leads to the identification of an intermediate 5 containing clusters 5a — with the same cluster composition as 3a and 4a — in the closest possible non-covalent contact. The related materials are of tremendous interest for magnetochemistry (nano-magneto-technology). [ 3 ] → − crystal water [ 5 ] → deprotonation and condensation [ 4 ]