Combining functional response materials into colloidal photonic crystals is an accepted encryption strategy for information security. Here, bilayer inverse heterostructure photonic crystals that enable instantaneously transparentizing of the top layer and simultaneously releasing the reflected light of the bottom layer when exposed to ethanol are reported. The transition can quickly return to its original state after the evaporation of ethanol. In addition, the bilayer film is responsive to water, which shows redshift of the bandgap position. The mechanism of the design involves optical scattering and diffraction in the fabricated periodic nanostructures and uses the infiltration and capillary evaporation of fluids with low surface tension to realize the spectral diversity of reflectance. The effects of scattering and color superposition of the upper layer can be obliterated and re-established for the fact of the infiltration and capillary evaporation of fluids with low surface tension; meanwhile, it provisionally displays the pattern of the bottom layer. Multiple reversible ways to hide and display information could be easily realized by these characteristics. Reconfigurable bilayer inverse heterostructure photonic crystals simultaneously provide a simple and sensitive optical technique for investigating the intriguing encryption effects at the nanoscale.
Combining responsive colloidal photonic crystals into invisible wettability patterns is important in steganography and watermarking for information encoding and decoding. Using a bilayer photonic crystal as a regionally functionalized porous carrier, a dual‐responsive surface with tunable wettability, reversible switching between wetting and no wetting, and responsivity to pressure and solvents are reported. The mechanism of the design involves reversible switching of micropore shapes and optical diffraction in the fabricated periodic nanostructures, and uses the infiltration and capillary evaporation of fluids to realize the spectral diversity of reflectance. It generates different invisible wettability patterns that are regulated by induced pressure and the Donnan equilibrium osmotic pressure. Immersion of the sample in water induces the varying degrees of infiltration and immediately displays multiple colors. It enables instantaneously transparentizing the full infiltration parts of the top layer and simultaneously releasing the reflected light of the corresponding bottom layer. Moreover, those invisible patterns can be reset by wiping with ethanol. Multiple ways to encode and reveal invisible complex information can easily be realized due to these characteristics. This work opens a new avenue for encoding complex information in a single material platform and extends the design approach of invisible patterns for information coding.
Colorimetric
sensors, as a key branch of the application of photonic crystals (PCs),
brings enthusiasm to scientists to do research. Here, simple mesoporous
and structurally colored one-dimensional photonic crystals (1DPCs)
constructed by alternating assembly of poly(acrylamide-N,N′-methylene
bis(acrylamide))
(P(AM-MBA)) nanogels and TiO2 nanoparticles are reported
as high-performance colorimetric humidity sensors. The sensors with
bright colors display rapid response to relative humidity (RH) change
and reach sensing balance in 0.5 s. By varying RH from 47.0% to 89.3%,
stopband of a sensor changes from 426 to 668 nm, almost spanning the
whole visible range. Meanwhile, visual sensing of RH possesses good
reversibility and repeatability. Moreover, the sensors with delicate
patterns are facilely fabricated by partial UV photodegradation of
the polymer layers with nano TiO2 as catalyst. The delicate
patterns and backgrounds show different colors and change color simultaneously
and quickly by varying the ambient humidity. Accurate QR code pattern
is also realized on the PC sensor; it is found successful reading
of the data is only achieved by increasing RH to realize high color
contrast between the code and background. Given their excellent properties,
the porous hybrid PCs are promising as high-performance humidity sensors
with potential display, decoration, information-storage, and encryption
functions.
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