Once the the NC-NC interface is strengthened via ligand crosslinking, a significant boost in the mechanical response is observed. Now, the xSP-epoxy composites reach values of stiffness and strength up to 4.4 ± 1.2 GPa and 65 ± 12 MPa, respectively (Fig. ). In fact, the evaluation of fracture surfaces (Fig. ) reveals that the xSP fillers remain intact -no fractured SPs appear -but the material fails within the epoxy phase (2nd HL). Moreover, it can be seen that when single xSPs are pulled out of the epoxy matrix during fracture, the exposed xSPs do not show any significant polymer residues on their surface, indicating that the material presents an adhesive failure at the epoxy-SP interface. In contrast, when the material consists of amine-functionalized and crosslinked SPs (xSP-NH 2 ), a significant enhancement of the composites' elastic modulus and strength, up to 6.5 ± 0.8 GPa and 101 ± 7 MPa is achieved. Close-examination of the obatained fractured surfaces indicates that an increased number of fractured supraparticles appear (Fig. ), again confirming an improved bonding at the epoxy-SPs' interface and a promoted cohesive failure within the SPs.