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[Display omitted] Although it is well-accepted that iron oxide nanoparticles are considered as artificial enzymes when their surface is hydrophilic, the enzyme-like properties of iron oxide nanoparticles with hydrophobic surface coating is unexplored. This work demonstrates that hydrophobic iron oxide nanocrystals coated with a layer of oleic acid could serve as artificial enzymes when their surface is covered by a layer of ionic surfactant. Furthermore, the co-assembly of iron oxide nanocrystals and porous organic cages could modulate their enzyme-like activities. Co-assembly of iron oxide (Fe 3 O 4) nanocrystals with different size and porous organic cages (POCs) was performed by an emulsion-confined strategy to achieve hybridized Fe 3 O 4 /POCs co-assemblies. The peroxidase-mimic activity of these co-assemblies were assessed in the presence of 3, 3′, 5, 5′-Tetramethylbenzidine (TMB) and hydrogen peroxide. Finally, these co-assemblies were applied as sensors to detect glucose and hydrogen peroxide. Co-assembly of Fe 3 O 4 nanocrystals and POCs resulted in the self-assembly of Fe 3 O 4 nanoparticles into two-dimensional nanoparticle superlattices on the eight (1 1 1) facets of the octahedral POCs colloidal crystals. The unique oil-in-water (O/W) emulsion confined assembly method switches the Fe 3 O 4 nanoparticles and POC crystals from hydrophobic to hydrophilic because of the strong hydrophobic interactions. Importantly, these co-assemblies dispersed in water showed strong peroxidase-mimic activity in water despite that their surface is covered by a bilayer of aliphatic chains. Furthermore, the intrinsic enzymatic activity of the co-assemblies is highly dependent on the size of the nanocrystals, and a higher catalytic activity is achieved from a larger sized Fe 3 O 4 nanocrystal. [ABSTRACT FROM AUTHOR]