Highly Invisible Photonic Crystal Patterns Encrypted in Inverse Opaline Macroporous Polyurethane Film for Anti-counterfeiting Applications

Invisible photonic crystal (PC) pattern with encrypted and discoverable information is potentially useful for anti-counterfeiting labels, but it is still a big challenge to realize strict invisibility, fast response, and convenient triggering. Here, a new kind of soaking-revealed invisible PC pattern is fabricated by the regional coating of “ethylene glycol-ethanol” ink on a collapsed inverse opaline macroporous polyurethane (IOM-PU) film, followed by a quick thermal treatment. During the above process, the wet-heating retains the collapsed but recoverable IOM structure, but the dry-heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the “pattern” and the “background” different optical response to the solvent. In the dry state, the pattern was invisible because both the collapsed IOM-PU film and the adhesive PU film are colorless and transparent. Once the sample is soaked by ethanol-water mixtures, the invisible pattern appears immediately, because o...