Carbon dot-loaded Fe₃O₄ as photonic pigments for multilevel anti-counterfeiting

Amorphous arrays assembled from colloidal microspheres are a way that obtains angle-independent structural colors. In order to obtain additional properties, colloidal microspheres, which are constituent units, can be modified with other materials. Here, we utilized the silane-functionalized carbon quantum dots (SiCDs) by incorporating them into the Stöber reaction to fabricate Fe₃O₄@SiO₂/SiCDs nanospheres with a core-shell structure. Amorphous colloidal arrays (ACAs) were constructed on commercial printing paper using Fe₃O₄@SiO₂/SiCDs nanoparticles as structural units by a simple permeation assembly. Macroscopically, the prepared ACAs exhibit the magnetic properties of Fe₃O₄, while under sunlight, they display bright, angle-independent structural colors. Under ultraviolet light, the array shows significant fluorescence. This enables the presentation of multidimensional information under varying magnetic and lighting conditions. By adjusting the thickness of the outer SiO₂/SiCDs composite layer, the optical properties and magnetism can be controlled easily. Moreover, due to the strong light absorption capability and high refractive index of Fe₃O₄, the digital patterns constructed with Fe₃O₄@SiO₂/SiCDs nanospheres demonstrate excellent multi-level anti-counterfeiting characteristics, even under water exposure. The magnetic properties of Fe₃O₄@SiO₂/SiCDs nanospheres, along with their distinct display characteristics under different optical environments, suggest their wide applicability in the fields of multifunctional anti-counterfeiting pigments, bioimaging, and sensing displays.