Electrostatic Self-Assembly of Soft Matter Nanoparticle Cocrystals with Tunable Lattice Parameters

Atomic crystal structure affects the electromagnetic and thermal properties of common matter. Similarly, the nanoscale structure controls the properties of higher length-scale metamaterials, for example, nanoparticle superlattices and photonic crystals. Electrostatic self-assembly of oppositely charged nanoparticles has recently become a convenient way to produce crystalline nanostructures. However, understanding and controlling the assembly of soft nonmetallic particle crystals with long-range translational order remains a major challenge. Here, we show the electrostatic self-assembly of binary soft particle cocrystals, consisting of apoferritin protein cages and poly(amidoamine) dendrimers (PAMAM), with very large crystal domain sizes. A systematic series of PAMAM dendrimers with generations from two to seven were used to produce the crystals, which showed a dendrimer generation dependency on the crystal structure and lattice constant. The systematic approach presented here offers a transition from trial-and-error experiments to a fundamental understanding and control over the nanostructure. The structure and stability of soft particle cocrystals are of major relevance for applications where a high degree of structural control is required, for example, protein-based mesoporous materials, nanoscale multicompartments, and metamaterials.