Colloidal metal-organic frameworks (CMOFs), nanoporous colloidal-sized crystals that are uniform in both size and polyhedral shape, are crystals composed of metal ions and organic bridging ligands, which can be used as building blocks for self-assembly in organic and aqueous liquids. They stand in contrast to conventional metal-organic frameworks (MOFs), which scientists normally study in the form of bulk crystalline powders. However, powder MOFs generally have random crystal size and shape and therefore do not possess either a definite mutual arrangement with adjacent particles or uniformity. CMOFs do have this quality, which can be important in vital uptake and release kinetics. In this Account, we present the diverse methods of synthesis, pore chemistry control, surface modification, and assembly techniques of CMOFs. In addition, we survey recent achievements and future applications in this emerging field. There is potential for a paradigm shift, away from using just bulk crystalline powders, towards using particles whose size and shape are regulated. The concept of colloidal MOFs takes into account that nanoporous MOFs, conventionally prepared in the form of bulk crystalline powders with random crystal size, shape, and orientation, may also form colloidal-sized objects with uniform size and morphology. Furthermore, the traditional MOF functions that depend on porosity present additional control over those MOF functions that depend on pore interactions. They also can enable controlled spatial arrangements between neighboring particles. To begin, we discuss progress regarding synthesis of MOF nano- and microcrystals whose crystal size and shape are well regulated. Next, we review the methods to modify the surfaces with dye molecules and polymers. Dyes are useful when seeking to observe nonluminescent CMOFs in situ by optical microscopy, while polymers are useful to tune their interparticle interactions. Third, we discuss criteria to assess the stability of CMOFs for various applications. In another section of this Account, we give examples of supracrystal assembly in liquid, on substrates, at interfaces, and under external electric fields. We end this Account with discussion of possible future developments, both conceptual and technological.

SignificanceCatalysis and mobility of reactants in fluid are normally thought to be decoupled. Violating this classical paradigm, this paper presents the catalyst laws of motion. Comparing experimental data to the theory presented here, we conclude that part of the free energy released by chemical reaction is channeled into driving catalysts to execute wormlike trajectories by piconewton forces performing work of a few kBT against fluid viscosity, where the rotational diffusion rate dictates the trajectory persistence length. This active motion agitates the fluid medium and produces antichemotaxis, the migration of catalyst down the gradient of the reactant concentration. Alternative explanations of enhanced catalyst mobility are examined critically.

A colloidal suspension of exfoliated, layered cobalt oxide nanosheets has been synthesized through the intercalation of quaternary tetramethylammonium ions into protonated lithium cobalt oxide. According to atomic force microscopy, exfoliated nanosheets of layered cobalt oxide show a plateau-like height profile with nanometer-level height, underscoring the formation of unilamellar 2D nanosheets. The exfoliation of layered cobalt oxide was cross-confirmed by X-ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The maintenance of the hexagonal in-plane structure of the cobalt oxide lattice after the exfoliation process was evidenced by selected-area electron diffraction and Co K-edge X-ray absorption near-edge structure analysis. The zeta-potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets. Adopting the nanosheets of layered cobalt oxide as a precursor, we were able to prepare the monodisperse CoO nanocrystals with a particle size of approximately 10 nm as well as the heterolayered film composed of cobalt oxide monolayer and polycation.

Methods are described to synthesize shape-selectable, monodisperse, aqueous-stable metal-organic frameworks (MOFs) by the reaction of aluminium nitrate with benzene tricarboxylic acid in various aqueous solvent mixtures and acetic acid as the capping ligand. Environmental stability was confirmed by thermal analysis and immersion in aqueous acidic media.

The exfoliation of layered Li[Mn(1/3)Co(1/3)Ni(1/3)]O(2) into individual monolayers could be achieved through the intercalation of quaternary tetramethylammonium (TMA(+)) ions into protonated metal oxide. An effective exfoliation occurred when the TMA(+)/H(+) ratio was 0.5-50. Reactions outside this range produced no colloidal suspension, but all the manganese cobalt nickel oxides precipitated. Atomic force microscopy and transmission electron microscopy clearly demonstrated that exfoliated manganese cobalt nickel oxide nanosheets have a nanometer-level thickness, underscoring the formation of unilamellar nanosheets. The maintenance of the hexagonal atomic arrangement of the manganese cobalt nickel oxide layer upon the exfoliation was confirmed by selected area electron diffraction analysis. According to diffuse reflectance ultraviolet--visible spectroscopy, the exfoliated manganese cobalt nickel oxides displayed distinct absorption peaks at approximately 354 and approximately 480 nm corresponding to the d-d transitions of octahedral metal ions, which contrasted with the featureless spectrum of the pristine metal oxide. In the light of zeta potential data showing the negative surface charge of manganese cobalt nickel oxide nanosheets, a heterolayered film of manganese cobalt nickel oxide and conductive polymers could be prepared through the successive coating process with colloidal suspension and polycations. The UV--vis and X-ray diffraction studies verified the layer-by-layer ordered structure of the obtained heterolayered film, respectively.