Biomolecules can act as functional templates for the organization of inorganic particles. Here we use two protein containers, engineered with opposite surface charge, as building blocks for the construction of a new type of biohybrid material. Binary structures with crystalline order were obtained, adopting a tetragonal lattice. Moreover, the cavity of the engineered protein containers can be filled with inorganic nanoparticles. The controlled assembly of these protein-nanoparticle composites yields highly ordered binary nanoparticle superlattices as free-standing crystals, with up to a few hundred micrometers in size. Because the structure and lattice parameters of the protein-nanoparticle crystals are independent of their nanoparticle cargo, the binary protein material may serve as a generally applicable matrix for the assembly of a variety of nanoparticles types.

Designing nanoscaled hierarchical structures with increasing levels of complexity is challenging. Here we show that electrostatic interactions between two complementarily supercharged protein nanocages can be effectively utilized to create nested Matryoshka-type structures. Cage-within-cage complexes containing spatially ordered iron oxide nanoparticles spontaneously self-assemble upon mixing positively supercharged ferritin compartments with AaLS-13, a larger shell-forming protein with a negatively supercharged lumen. Exploiting engineered Coulombic interactions and protein dynamics in this way opens up new avenues for creating hierarchically organized supramolecular assemblies for application as delivery vehicles, reaction chambers, and artificial organelles.

A highly efficient strategy for the encapsulation of nanoparticles into protein containers using cargo-loading peptides.