Chemical separation membranes, drug delivery agents, and other applications of metal- organic frameworks (MOFs) benefit from preparing MOFs as nanoparticles (nanoMOFs) and by controlling the particle surface. Despite the lack of deliberately added surface ligands, common examples of nanoMOFs exhibit multi-week colloidal stability in a range of polar solvents, in stark contrast with most conventional nanoparticles requiring surface functionalization with bulky ligands, and yet the origin of this stability remains unknown. Here, we demonstrate that the “solubility” of nanoMOFs depends on the solubility of the constituent linkers and on the amount of exposed external surface area. Although the nanoMOF zeta potentials exceed |±20 mV|, these results indicate electrostatics alone cannot explain colloidal stability. Indeed, the identity of electrolyte ions at fixed ionic strengths induces divergent particle stabilities akin to the Hofmeister effect observed with proteins. Furthermore, nanoMOFs self-assemble into coronas with proteins to become stable under physiological conditions, even when similar surface charges between MOF and protein would be expected to repel. These results highlight the uniqueness of defining a MOF “surface” and suggest that nanoMOFs form a bridge between conventional nanoparticles and macromolecules, while opening fundamental questions into interfacial chemistry.
