Combinatorial Wetting in Colour: An Optofluidic Nose
Original Entry by Cheng Wang, AP225, Fall 2012
Authors: Kevin P. Raymond, Ian B. Burgess, Mackenzie H. Kinney, Marko Loncar and Joanna Aizenberg
Publication: Raymond et al., "Combinatorial wetting in colour: an optofluidic nose" (2012) 12: 3666-3669
Keywords: Wetting, colourimetric, chemical test, array
Colourimetry is powerful in chemical sensing and its biggest challenge is to couple colourimetric response sensitive to general physical or chemical property. This work is based on a previous platform for colourimetry called Wetting In Colour Kit (WICK) . In WICK, the macroscopic colour depends on the number of unfilled (unwetted) layers. As the total number of layers changes across the sample, the structural colour pattern is highly sensitive to the liquid's wettability. However, this WICK method is only sensitive to the surface property. It cannot give any chemical information without prior-given information. From Fig 1B we can see that n-octane gives the same result as 80% EtOH, while Acetone is able to mimick the colourimetric response of 90% EtOH.
In this paper, the authors use an array of WICKs with slightly different chemical responses. The combinatorial patterns in the array gives more chemical information about the liquid, as Fig. 1C shows. The system consists of 6 different WICKs. Different concentrated EtOH and IPA are used as reference liquids, and the test liquid is scored by comparing its array pattern with reference pattern. This paper demonstrated the ability to differentiate 17 organic solvents. The chemically sensing test result is demonstrated in Fig. 2, showing the specificity of detecting several different chemicals.
This novel "optofluidic nose" is not so highly selective compared to other previously reported "artificial nose". Its biggest advantage, however, lies in that it is easy to use, by simply comparing the colourimetric pattern with naked eyes, just like what we do in pH test paper.
Fig. 1 (A) Schematic depicting the colour response to partial infiltration of liquids in IOFs with vertically graded wettability. (B) Illustration of the chemical non-specificity of WICK. (C) Chemical specificity derived from a WICK array. (D) Using reference liquids to assign numerical values to the colourimetric response.
Fig. 2 A) 2D principal component analysis of the reference liquid scores of all six elements of an array, showing differentiation of the different liquid classes. (B) Estimation of inter-sample variability using the reference liquid scoring system.
The new concept of colourimetric wetting combined the concept of wetting as well as 3D photonic crystal. It has been well known that wetting is a very promising method for chemical sensing. This WICK device used the physical property (i.e. photonic property) difference between unwetted and wetted surface to develop a convenient and user-friendly device for chemical sensing. By comparing the different response of the sensing array using naked eye, a novel artificial nose is available as reproted here, which is as easy to use as a traditional pH test paper, but of course much more selective!