Combinatorial wetting in colour: an optofluidic nose.

We present a colourimetric litmus test for simple differentiation of organic liquids based on wetting, which achieves chemical specificity without a significant sacrifice in portability or ease-of-use. Chemical specificity is derived from the combination of colourimetric wetting patterns produced by liquids in an array of inverse opal films, each having a graded wettability, but using different surface groups to define that gradient.

Wetting in color: colorimetric differentiation of organic liquids with high selectivity.

Colorimetric litmus tests such as pH paper have enjoyed wide commercial success due to their inexpensive production and exceptional ease of use. Expansion of colorimetry to new sensing paradigms is challenging because macroscopic color changes are seldom coupled to arbitrary differences in the physical/chemical properties of a system. Here we present in detail the design of a "Wetting In Color Kit" (WICK), an inexpensive and highly selective colorimetric indicator for organic liquids that exploits chemically encoded inverse-opal photonic crystals to project minute differences in liquids' wettability to macroscopically distinct, easy-to-visualize structural color patterns.