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

Abstract

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, wet heating retains the collapsed but recoverable IOM structure, but dry heating disables the recovery of ordered IOM structure due to the adhesion of macropore walls, which render the “pattern” and the “background” with different optical responses 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 in ethanol–water mixtures, the invisible pattern appears immediately because only the “wet-heated” region recovers the ordered macroporous structure and shows color, which forms a significant contrast in color to the “dry-heated” region. Compared to the previously invisible PC pattern, the current material has many superior properties, such as high invisibility, large color contrast in showing, excellent recyclability, and good toughness in bending and stretching.

Macroporous Hydrogels for Fast and Reversible Switching between Transparent and Structurally Colored States

“…[ 26–28 ] The stimuli‐responsive colloidal crystals can be micropatterned to show condition‐dependent and regioselective color change, [ 12,13 ] which is able to disclose hidden optical and graphical codes, verifying the authenticity. [ 29–36 ] However, there is room to further improve the property of the photonic materials for practical applications. In most sensors composed of the colloidal array, for example, the color shift is linearly proportional to the magnitude of the external condition, which sometimes restricts the sensitivity although the measurable range of stimuli is wide.…”

“…In most sensors composed of the colloidal array, for example, the color shift is linearly proportional to the magnitude of the external condition, which sometimes restricts the sensitivity although the measurable range of stimuli is wide. [ 14,15,22,26–28 ] In many user‐interactive anti‐counterfeiting materials, the graphical code is not completely encrypted so that the code is partially exposed with no external stimuli, [ 29–31,33 ] the patterning resolution is not high enough to reproduce precise graphical codes, [ 34 ] or only single‐colored pattern is available. [ 34,35 ] Furthermore, the production of the regular colloidal arrays frequently requires time‐consuming and delicate evaporation‐induced self‐assembly.…”

“…[ 14,15,22,26–28 ] In many user‐interactive anti‐counterfeiting materials, the graphical code is not completely encrypted so that the code is partially exposed with no external stimuli, [ 29–31,33 ] the patterning resolution is not high enough to reproduce precise graphical codes, [ 34 ] or only single‐colored pattern is available. [ 34,35 ] Furthermore, the production of the regular colloidal arrays frequently requires time‐consuming and delicate evaporation‐induced self‐assembly. [ 29,32,34–36 ] That is, it is still challenging to simultaneously achieve high sensitivity to external stimuli, complete encryption of photonic patterns, and simple and reproducible fabrication scheme for practical applications as sensors and security materials.…”

“…[ 34,35 ] Furthermore, the production of the regular colloidal arrays frequently requires time‐consuming and delicate evaporation‐induced self‐assembly. [ 29,32,34–36 ] That is, it is still challenging to simultaneously achieve high sensitivity to external stimuli, complete encryption of photonic patterns, and simple and reproducible fabrication scheme for practical applications as sensors and security materials.…”

Dual stimuli-responsive lanthanide-based phosphors for an advanced full-color anti-counterfeiting system

“…In recent decades, uorescence-based materials including semiconductor nanocrystals, carbon dots, metal nanoclusters, up-conversion nanoparticles, and lanthanide-based (Ln-based) materials have been utilized in extensive anti-counterfeiting elds, where the colorful emissions of the uorophores are considered as security elements. [7][8][9][10] Fluorescent patterns in banknotes and documents such as birth certication and passport are well-known anti-counterfeiting samples. It is noted that most of information is encoded with single-color uorescence tags which is not safe enough since it can be feasibly replicated.…”