Direct Assembly of Magnetically Tunable Nanoallotropes as Photonic Inks

The creation of colloidal allotropes, structures with identical composition but divergent architectures and properties, represents a key aspect for controlled material fabrication yet it is highly challenging. Particularly, a general and efficient method for fabricating such nanoallotropes over large areas remains elusive. Here, we report a direct magnetic assembly strategy to construct nanoallotropes via controlling the Fe₃O₄@SiO₂ nanoellipsoid orientation during drying. By tuning the magnetic field direction, the electrostatic and magnetic dipolar interactions, along with a physical boundary, drive the formation of amorphous photonic glasses, hexagonal close-packed (hcp) crystals, as well as multiple monoclinic crystals whose lattice parameters are continuously tunable. Capitalizing on this tunability, we develop a revolutionary photonic ink (P-ink) that enables single-ink, multicolor writing and printing. This all-inorganic ink is fully recyclable and produces high-resolution, nonfading, and glare-free colors on virtually any substrate. This work not only provides insights into the controlled assembly of anisotropic colloids but also establishes a versatile platform for applications in anticounterfeiting, color printing, and functional photonic devices.