Harnessing metal-organic frameworks for precise, hierarchical, and reconfigurable colloidal self-assembly

Abstract

The self-assembly of colloidal particles offers a remarkable avenue for creating intricate superstructures with tailored functionalities. The utilization of metal-organic framework (MOF)-based particles presents an exciting opportunity to broaden the repertoire of colloids and explore colloidal superstructures for functional materials. However, the translation of the anisotropic morphology and molecular features of MOFs into effective pathways for colloidal self-assembly remains elusive. This thesis explores various strategies that utilize MOF’s characteristics, aiming to decipher the anisotropic bonding information associated with MOF particles and manipulate colloidal interaction for controlled self-assembly. As a result, new opportunities emerge for the creation of diverse superstructures with advanced properties.

First, depletion interaction is introduced as a versatile approach to assemble MOF particles with different polyhedral shapes. Through the recognition of anisotropic shapes via facet-to-facet bonding, a wide range of low-dimensional superstructures have been obtained. Due to the mutual alignment of MOF particles within these superstructures, this method allows for the coordination of MOF’s molecular frameworks and micropores, thereby producing MOF films with anisotropic optical properties (Chapter 2). This method has also facilitated the design of more complex structures that would otherwise be unattainable. A notable example is demonstrated, in which MOF particles with a truncated hexagonal bipyramidal shape are assembled into open channel supercrystals. The specific facets of those particles serve as virtual patches, enabling anisotropic bonding that directs the particles into a unique, 3D honeycomb-like lattice with hierarchical pore structures (Chapter 3).

To encode more bonding information, MOF-based patchy particles have been designed. The patches, located on selected facets of MOF particles, possess chemistry and shape information that enable precise directional bonding with prescribed translational and orientational freedom. Through site-specific liquid bridging between patches, the particles self-assemble into supra-colloidal structures with unprecedented precision, forming rigid, facet-aligned chains (Chapter 4). These chains, featuring alternating soft-hard components, have been further combined with a thermoresponsive swelling agent to mimic the flexibility of a spine. Termed as MicroSpine, they exhibit reconfigurable shape transformation capabilities, switching between straight and curved conformations as the swelling agent diffuses in and out of the soft segments. Such dynamic features enable the chains to build capsules capable of encapsulating and releasing colloidal guests as regulated by temperature (Chapter 5).

Overall, this work demonstrates the achievement of precise, hierarchical, and reconfigurable colloidal self-assembly by harnessing the anisotropic traits and functions of MOF colloids. By unlocking new possibilities for material design, this research contributes to the advancement of both the fields of colloids and MOFs.