Invisible photonic prints shown by UV illumination: combining photoluminescent and noniridescent structural colors

Invisible photonic prints that become visible by UV light irradiation were prepared via the self-assembly of Y2O3:Eu colloidal particles into amorphous photonic crystals (APCs) with controlled fluorescent and noniridescent structural colors. Y2O3:Eu particles of tunable sizes were fabricated via a hydrothermal method, followed by calcination at high temperature. Since the pattern and background showed similar photonic structures, reflection wavelength, and structural color but different fluorescent intensities, the invisible pattern could be hidden and revealed in the absence/presence of UV light, respectively. Moreover, a multicolor invisible print was also prepared by introducing APCs of different structural colors, which was only limited to the different photoluminescence intensities between the pattern and background. The as-fabricated patterns could instantly and reversibly be revealed under UV light and hidden many times, which possess long lasting durability for practical use. The bright and noniridescent structural color combined with fluorescence makes it more convenient to hide and show the pattern by avoiding the confusion resulting from the alterable colors of the angle-dependent photonic crystals. The invisible printing method in this work is simple, convenient, and efficient for the fabrication of invisible photonic prints, which will promote its practical applications in the field of novel color displays, security-related labels, and information storage.