Colorimetric litmus tests such as pH paper have enjoyed wide commercial success due to their inexpensive production and exceptional ease of use. Many such techniques operate based on a chemical tag whose optical absorption or fluorescence spectrum changes in response to a specific analyte. Specificity is an advantage in this case, but limits the variety of substances for which such a sensor can be used. On the other hand, the use of structural color – derived not from molecular absorption but from coherent scattering from wavelength-scale roughness – has no inherent specific chemical requirements (e.g. any material with periodic roughness displays iridescence). Thus, tunable structural color carries the potential for broad applicability in colorimetric sensing.
We developed a technique for patterning multiple chemical functionalities throughout the inner surfaces of a highly ordered 3D photonic crystal, generating complex wettability patterns. When immersed in a liquid, the pores are selectively infiltrated in a unique spatial pattern. This creates an optical fingerprint of that liquid through the color contrast between wetted and non-wetted regions. Using this platform, we have illustrated multilevel encryption, with selective decoding by specific liquids. A remarkable selectivity of wetting is observed over a very broad range of fluid surface tensions. These properties, combined with the easily detectable optical response, allow us to also exploit this system as a colorimetric indicator for liquids based on wettability.
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