Innovative Color‑Changing Photonic Crystals: The Future of Smart Sensor Technology

Scientists have developed color-changing, flexible photonic crystals that could be used to develop sensors that warn when an earthquake might strike next. The wearable, robust, and low-cost sensors can respond sensitively to light, temperature, strain, or other physical and chemical stimuli, making them a promising option for cost-effective smart visual sensing applications in a range of sectors including healthcare and food safety.

The method to produce photonic crystals containing a minuscule amount of graphene results in a wide range of desirable qualities with outputs directly observable by the naked eye. Intensely green under natural light, the sensors change color to blue when stretched or turn transparent after being heated.

The mechanically robust yet soft, freestanding, and flexible polymer-based opals contain solution-exfoliated pristine graphene. The scientists took inspiration from the biomimicry abilities in butterfly wings, peacock feathers, and beetle shells where the color comes from structure and not from pigments.

Other applications include time-temperature indicators (TTI) for intelligent packaging in which the sensors give a visual indication if perishables, such as food or pharmaceuticals, have experienced undesirable time-temperature histories. The crystals are extremely sensitive to even a small rise in temperature between 20 and 100 °C.

The crystals' pressure-responsive shape-memory characteristics are attractive for biometric and anti-counterfeiting applications. Pressing the crystals with a bare finger can reveal fingerprints with high precision showing well-defined ridges from the skin. The photonic crystals also can be used as tissue scaffolds for understanding human biology and disease. If functionalized with biomolecules, they could act as highly sensitive point-of-care testing devices for respiratory viruses, offering inexpensive, reliable, user-friendly biosensing systems.

The sensors’ mechanochromic response allows for their application as body sensors that could help improve technique in athletes. The sensors could be used in a wristband that changes color to indicate to patients if their healthcare practitioner has washed their hands before entering an examination room.