Wetting in color (W-Ink)

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.

Publications

Burgess IB, Loncar M, and Aizenberg J. Structural Colour in Colourimetric Sensors and Indicators. J. Mater. Chem. C [Internet]. 2013;1 (38) :6075-6086. Publisher's VersionAbstract
Colourimetric sensors and indicators are widely used because of their low cost and simplicity. A significant challenge associated with the design of this type of device is that the sensing mechanism must be simultaneously optimised for the sensitivity of the response and a visually perceptible colour change. Structural colour, derived from coherent scattering rather than molecular absorption, is a promising route to colourimetric sensor design because colour shifts are tied to changes in one of many physical properties of a material, rather than a specific chemical process. This Feature Article presents an overview of the development of low-cost sensors and indicators that exploit structural colour. Building upon recent advances in structurally adaptive materials design, structural colour sensors have been developed for a wide variety of previously inaccessible physical (e.g. temperature, strain, electric fields) and chemical stimuli (e.g. small organic molecules, charged species, biomacromolecules and metabolites). These devices, often exceeding the state of the art in performance, simplicity or both, have bright prospects for market impact in areas such as environmental monitoring, workplace hazard identification, threat detection, and point-of-care diagnostics. Finding the ideal balance between performance (e.g. sensitivity, specificity, reproducibility, etc.) and simplicity (e.g. colourimetric vs. spectroscopic readout) will be one of the most critical elements in the further development of structural colour sensors. This balance should be driven largely by the market demands and competing technologies.

Media Coverage

Materials inspired by nature, Physics World, June 11, 2014. 

"Watermark Ink" device (W-INK) wins R&D 100 Award Harvard press release, July 8, 2013. 

Colourimetry: Combinatorial Wetting, Nature Photonics, July 31, 2012. 

Nanocrystal Liquid Identification, Cosmetics & Toiletries, October 5, 2011. 

Surface tension decoded, Chemistry World, September 2011. 

Channeled chips can spot substances, Scientific American, August 11, 2011. 

Devices Reveal Hidden Messages, Chemical & Engineering News, August 15, 2011. 

"Watermark Ink" device identifies unknown liquids instantly, Harvard press release, August 3, 2011. 

Inverse opal technology could help identify chemical spills, The Engineer, August 4, 2011. 

"Watermark Ink" device identifies unknown liquids instantly, Materials Today, August 10, 2011. 

"Watermark Ink" chip can instantly identify liquids, Gizmag, August 4, 2011.