The self-organization of colloids into defi ned structures offers the possibility to develop novel materials with exciting properties. However, this requires the understanding and application of the forces governing interparticle association. A novel approach for a size-dependent anisotropic assembly between nanoparticles is achieved. Janus-like iron oxide/polystyrene hybrid nanocolloids are prepared by heterophase polymerization and selectively coated with silica on the iron oxide face to gradually form a cavity. Hence, a shallow surface around the hydrophobic polymer face is created, enabling smaller particles of the same nature to be locked by shape complementarity and colloidal steric stabilization. Further coating with silica fi xes these assemblies and allows quantitative analysis of the interaction on a nanoscale. biospecifi c interactions, [ 10 ] among other strategies, [ 11,12 ] it is possible to arrange nanopieces into predesigned hierarchical superstructures. [ 13 ] This enables the development of novel materials with tunable properties and interesting applications like, for example, optical metamaterials [ 14 ] and plasmonic photoconductors. [ 15 ] Moreover, when the building blocks used to construct the self-assembled materials are anisotropic, a richer variety of supramolecular morphologies can be attained, giving rise to systems with more complex interactions and unique properties. [ 16,17 ] Anisotropy usually plays a decisive role in order to render directional association between the single components, defi ned by their shape. Knowing that spherical particles, limited by their high symmetry generally array in closepacked superlattices, [ 18,19 ] no effort has been spared in affording colloids with more complex topologies. [20][21][22][23] As a result, anisotropic building blocks could be self-organized by geometry recognition. [ 24 ] Resembling the highly specifi c interaction between a substrate and an enzyme, colloids can selectively interact and associate by means of fi gure complementarity. [ 25,26 ] In this way, colloidal particles with defi ned cavities are found to arrange themselves into pairs whose components perfectly match in size and morphology. Moreover, by fabricating colloids