Janus particles with tunable patch symmetry and their assembly into chiral colloidal clusters.

Janus particles, which have an attractive patch on the otherwise repulsive surface, have been commonly employed for anisotropic colloidal assembly. While current methods of particle synthesis allow for control over the patch size, they are generally limited to producing dome-shaped patches with a high symmetry (C∞). Here, we report on the synthesis of Janus particles with patches of various tunable shapes, having reduced symmetries ranging from C2v to C3v and C4v. The Janus particles are synthesized by partial encapsulation of an octahedral metal-organic framework particle (UiO-66) in a polymer matrix. The extent of encapsulation is precisely regulated by a stepwise, asymmetric dewetting process that exposes selected facets of the UiO-66 particle. With depletion interaction, the Janus particles spontaneously assemble into colloidal clusters reflecting the particles' shapes and patch symmetries. We observe the formation of chiral structures, whereby chirality emerges from achiral building blocks. With the ability to encode symmetry and directional bonding information, our strategy could give access to more complex colloidal superstructures through assembly.

Biomimetic thermoresponsive superstructures by colloidal soft-and-hard co-assembly.

Soft-and-hard hybrid structures are ubiquitous in biological systems and have inspired the design of man-made mechanical devices, actuators, and robots. The realization of these structures, however, has been challenging at microscale, where material integration and actuation become exceedingly less practical. Here, through simple colloidal assembly, we create microscale superstructures consisting of soft and hard materials, which, serving as microactuators, have thermoresponsive shape-transforming properties. In this case, anisotropic metal-organic framework (MOF) particles as the hard components are integrated with liquid droplets, forming spine-mimicking colloidal chains via valence-limited assembly. The chains, with alternating soft and hard segments, are referred to as MicroSpine and can reversibly change shape, switching between straight and curved states through a thermoresponsive swelling/deswelling mechanism. By solidification of the liquid parts within a chain with prescribed patterns, we design various chain morphologies, such as "colloidal arms," with controlled actuating behaviors. The chains are further used to build colloidal capsules, which encapsulate and release guests by the temperature-programmed actuation.

Low-dimensional assemblies of metal-organic framework particles and mutually coordinated anisotropy.

Assembling metal-organic framework (MOF)-based particles is an emerging approach for creating colloidal superstructures and hierarchical functional materials. However, realization of this goal requires strategies that not only regulate particle interactions but also harness the anisotropic morphologies and functions of various frameworks. Here, by exploiting depletion interaction induced by ionic amphiphiles, we show the assembly of a broad range of low-dimensional MOF colloidal superstructures, including 1D straight chains, alternating or bundled chains, 2D films of hexagonal, square, centered rectangular, and snowflake-like architectures, and quasi-3D supercrystals. With well-defined polyhedral shapes, the MOF particles are mutually oriented upon assembly, producing super-frameworks with hierarchically coordinated crystallinity and micropores. We demonstrate this advantage by creating functional MOF films with optical anisotropy, in our cases, birefringence and anisotropic fluorescence. Given the variety of MOFs available, our technique should allow access to advanced materials for sensing, optics, and photonics.

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.

Polyhedral Micromotors of Metal-Organic Frameworks: Symmetry Breaking and Propulsion.

Colloidal micromotors can autonomously propel due to their broken symmetry that leads to unbalanced local mechanical forces. Most commonly, micromotors are synthesized to possess a Janus structure or its variants, having two components distinct in shape, composition, or surface joined together on opposite sides. Here, we report on an alternative approach for creating micromotors, where microcrystals of metal-organic frameworks (MOFs) with various polyhedral shapes are propelled under an AC electric field. In these cases, symmetry breaking is realized by orienting the polyhedral particles in a unique direction to generate uneven electrohydrodynamic flow. The particle orientations are controlled by a delicate competition between the electric and gravitational forces exerted on the particle, which we rationalize using experiments and a theoretical model. Furthermore, by leveraging the MOF types and shapes, or surface properties, we show that the propulsion of MOF motors can be tuned or reversed. Because of the flexibility in designing MOFs and their one-step scalable synthesis, our strategy is simple yet versatile for making well-defined functional micromotors.