Photonic crystal sensing of components of a liquid mixture using an optical fiber spectrometer

We report the application of a photonic crystal sensor for the components of a liquid mixture by a simple and easy technique. A closely packed colloidal crystal film is made of arrayed 202-nm PS particles and PDMS elastomer is used for filling the voids in the film. Bragg's diffraction in the visible wavelength region causes the formation of a structural color in the colloidal crystal. The structural color varies according to the solvents depending on the swelling ability of the PDMS elastomer. We can perform the quantitative analysis of the swelling phenomena by measuring Bragg's diffraction peaks. The peak shifts as a function of the mixing ratio of the solvents, i.e., methanol, ethanol, and propanol. The peak position is proportional to the solvent concentration. In the case of a water-ethanol system, there is little peak-shift up to 80 vol.% of ethanol concentration. Above this concentration, the rate of peak shift increases beyond 50 nm. A reflectance spectrometer enables the detection of the components of ethanol liquids with the volume concentration of water ranging from 0 to 10%. In this study, a commercially available optical fiber spectrometer detects the volume concentrations of water of the order of 1 vol.% in ethanol. The photonic sensor has the potential to be used in a rapid analysis method that employs a portable optical fiber spectrometer.