The effects of hierarchical design and its tuned interfaces on the composites' mechanical properties were evaluated at multiple scales. Nanoindentation with a Berkovich tip was used to assess the elastic modulus and hardness of the final material systems at different indentation depths. To facilitate the comparison, the obtained values of the neat epoxy and a bulk supercrystal of OA crosslinked Fe 3 O 4 NCs are also presented. As shown in Fig. and B, the untreated SP-composites (SPs-epoxy, without crosslinking nor amine surface functionalization) already show slightly higher stiffness and hardness values compared to the used neat epoxy matrix. The mechanical properties significantly raised upon crosslinking OA ligands (i.e. xSP-composite), associated to the strengthened NC-NC interface. This effect was further increased after functionalizing the SPs' surface with amine moieties (i.e. xSP-NH 2 -composites). Several other effects can be inferred from the indentation analysis. While the hardness gradually decreases with increasing indentation depth due to the so called indentation size effect, the elastic modulus typically remains constant unless energy is consumed via fracture phenomena. For small indentation depths of 300 nm (i.e. the same order of magnitude as the supraparticles' diameter), xSPs and xSP-NH 2 show similar E-moduli. This is not surprising, since they only differ in the epoxy-binding affinity but not in their interior cohesion. Moreover, these values show a larger scatter associated to whether the indented area consisted of hard filler particles or of soft epoxy matrix. When increasing indentation depth, a gradual decrease in elastic modulus is observed for the unfunctionalized xSPs. This relates to crack formation at the SP-epoxy interface, as observed in the SEM micrograph in Fig. . For the amine functionalized and crosslinked material the elastic modulus remained nearly constant within the measured range (consistent with the observed reduced crack formation). In some cases still some crack formation was observed for these materials, but as can be seen in Fig. cracks were expanding also within the hard particle filler, indicating that a non-negligible portion of the load was also transferred to the secondary particles and thus confirming an improved bonding at the epoxy-SPs interface.