To prepare highly crystalline ultra-small nanoparticles of UiO-66(Zr) with maximal defect content, we first considered carefully the main relevant state-of-the-art strategies. For instance, the acidity of the solution was shown to significantly influence the kinetics of crystallization due to the changes in protonation state of the carboxylic acids that lead to faster kinetics at higher pH. In addition, the presence of water in the reaction mixture appeared to be a critical factor in determining the defects resulting from the formation of Zr-OH or Zr-OH2 bonds rather than Zr-ligand connections. Using a low synthesis temperature was also shown to be beneficial towards both the defect engineering and downsizing due to the inhibited formation of coordination bonds and to limited Ostwald ripening (illustrated in Figure ). Thus, to achieve our ambitious goal to prepare ultra-small nanoMOFs with a high defect content, we developed a new simple green strategy that : (i) avoids the use of very acidic Zr salts (e.g., ZrCl4, ZrOCl2.xH2O) and slightly acidic modulators (e.g. formic acid) by using pre-synthesized Zr6 acetate oxoclusters (Figure ); (ii) discards dimethylformamide (DMF) and replaces it by water and ethanol to avoid the release of formates upon DMF degradation (and enables a sustainable approach); (iii) ensures dissolution of the organic linker by diluting the reaction media in ethanol, subsequently fastening the synthesis kinetics; and (iv) is operated at room temperature.