Highly sensitive mechanochromic photonic gel towards fast- responsive fingerprinting

Mechanochromic photonic materials, which are periodic structures engineered with photonic stop bands, have attracted enormous interest because of their rapid change in color upon stimulation with mechanical force. Here, we develop a high-performance mechanochromic photonic gel based on magnetically assembled, carbon-encapsulated Fe₃O₄ nanoparticles embedded into a soft copolymer of N-hydroxymethyl acrylamide and N-vinylcaprolactam in which the content of carbon-encapsulated Fe₃O₄ is as low as 0.18 wt%. The photonic gel not only displays fast-dynamic and reversible color change from purple to red in a broad wavelength range (Δλ = 233 nm), but also maintains high mechanochromic sensitivity [Δλ/σ = 53.1 nm (kPa)^-1] and spatial resolution (less than 100 μm). By taking advantage of these remarkable merits, the photonic gel has been successfully applied in the identification of color fingerprints and ultra-fine hair resolution. Considering these outstanding achievements, we believe that our photonic gel will guide the development of mechanochromic materials and open an avenue towards the design of mechanical-optical devices.