Bio-Inspired Synthesis of Macroporous Single Crystal Particles as Scattering Agents

Introducing controlled porosity into crystalline materials endows them with properties not exhibited by their solid counterparts. However, producing microparticles that combine submicron macroporosity and uniform particle shapes remains a significant challenge. Here, we present a general strategy that yields unique single-crystal microparticles with closed-cell macropores (300–500 nm), a size ideal for generating structural colour. Using a bioinspired approach, we occlude three distinct additives, polystyrene microspheres, polymer vesicles, and amino acids, within 10–20 µm calcite (CaCO3) single crystals. Remarkably, after thermal annealing, all systems are transformed into particles with well-defined shapes and macroporosity, while retaining their single-crystal character. This is particularly surprising for the amino acids, which are originally distributed as individual molecules throughout the crystal lattice. Mechanistic studies using solid state NMR and in situ TEM directly track pore formation and reveal that co-occluded water associated with the amino acids is crucial in creating the pores. This simple method—where the occlusion of amino acids is scalable and compatible with industrial calcite synthesis—delivers a new class of macroporous microparticles suited to coatings, fillers and photonic applications, where their bright white appearance and intense broadband light reflection makes them an environmentally-benign alternative to titanium dioxide for whitening applications.