A visually readable and highly stable self-display photonic humidity sensor has been fabricated through the fast magnetically induced self-assembly of carbon-encapsulated superparamagnetic colloidals, followed by an instant radical polymerization to fix the photonic crystal structures inside a polyacrylamide glycol gel matrix.

Because of the use of magnetic assembly and radical-polymerization, we can quickly and conveniently prepare the photonic humidity sensor on a large scale with arbitrary shape which has an important significance in large-scale industrial production. The photonic humidity sensor can self-display brilliant colors from navy blue to light red as the relative humidity changes from 11% to 97%. The whole shift of the reflection wavelength is nearly 160 nm, which almost covers the whole visual region and its intensity is readable by the naked eye.

In addition, cycle tests demonstrate that the photonic humidity sensor has a good stability and reproducibility in its diffraction signal. Therefore, coupled with low cost and no power consumption, the photonic humidity sensor system can realize colorimetric detection of humidity, similarly to pH indicator paper.

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