For the first time, ∼100 nm zeolitic imidazolate framework-90 (ZIF-90) particles were produced by a water–alcohol-based system, taking advantage of viscosity effects using an optimized H₂O/tert-butanol/glycerol/PVP system. Furthermore, an in vitro cytotoxicity assay revealed the half-maximal effective concentration (EC₅₀) of ZIF-90 nanoparticles to be ∼92 μg ml⁻¹.