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.
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.
thumb|Fig. 1 (A) Schematic depicting the colour response to partial infiltration of liquids in IOFs with vertically graded wettability. Blue pores indicate pores filled with liquid, white pores indicate air-filled pores, and the colour of the top of the structure represents the macroscopic colour observed by eye. The colour in a given region is determined by the number of layers that remain air-filled. (B) Illustration of the chemical non-specificity of WICK: while the sample (3FSA13FS functionalization) shows distinct patterns in 80% and 90% ethanol (aq), these two colour patterns can also be reproduced in entirely different liquids (n-octane, and acetone, here). (C) Chemical specificity derived from a WICK array: Comparison of the colour responses of two WICKs (left: DECA13FS, right: 3FSA13FS) in methanol and octane. Methanol penetrates fewer layers than octane in the DECA13FS WICK, but penetrates more layers than octane in the 3FSA13FS WICK. (D) Using reference liquids to assign numerical values to the colourimetric response: The colour response of a test liquid (o-xylene shown) in an element in the array is quantified by identifying an ethanol-water mixture that produces the same degree of wetting. In this example, o-xylene is given the following scores: PTOLA13FS: 85, 3PPA13FS: 75, 3FSA13FS: 50. Scale bars: 5 mm.