Size control over metal–organic framework porous nanocrystals†
Porous nanocrystals of metal–organic frameworks (MOFs) offer greater bioavailability, higher surface-to-volume ratios, superior control over MOF membrane fabrication, and enhanced guest-sorption kinetics compared to analogous bulk phases, but reliable synthesis of uniformly sized particles remains an outstanding challenge. Here, we identify the smallest and most probable sizes of known MOF nanocrystals and present an exhaustive comparative summary of nano- versus bulk-MOF syntheses. Based on critical analysis of reported size data and experimental conditions, an alternate to the LaMer model is proposed that describes nanocrystal formation as a kinetic competition between acid-base and metal–ligand reactivity. Particle growth terminates when ligands outcompete metal-ion diffusion, thereby arresting polymerization to produce kinetically trapped particle sizes. This model reconciles disparate trends in the literature and postulates that minimum particle sizes can be achieved by minimizing the relative ratios of metal-to-linker local concentrations. By identifying conditions that disfavor small nanocrystal sizes, this model also provides routes towards macroscopic MOF single crystals. A universal “seesaw” relationship between nanocrystal sizes and the concentrations of acidic surface-capping ligands provides a roadmap for achieving precise synthetic control. Best practices in synthesis, characterization, and data presentation are recommended for future investigations so that MOF nanocrystals may achieve their full potential as advanced nanomaterials.