Selective Chemical Sensing Using Structurally Colored Core-Shell Colloidal Crystal Films

We demonstrate for the first time selective sensing of multiple vapors at low concentrations based on the structurally colored colloidal crystal film formed from composite core/shell nanospheres. Since color changes of sensing colloidal crystal films are negligible at relatively low vapor partial pressures (P/P₀ < 0.1), a straightforward detection of color changes cannot be applied. To overcome this limitation, we apply a differential spectroscopy measurement approach coupled with the multivariate analysis of differential reflectance spectra. The vapor-sensing selectivity is provided by the combination of the composite nature of the colloidal nanospheres in the film with the multivariate analysis of the spectral changes of the film reflectivity upon exposure to different vapors. The multianalyte sensing was demonstrated using a colloidal crystal film comprised of 326-nm diameter core polystyrene nanospheres coated with a 20-nm thick sol-gel shell. Discrimination of water, acetonitrile, toluene, and dichloromethane vapors using a single sensing colloidal crystal film was evaluated applying principal components analysis (PCA) of the reflectivity spectra. The polar and nonpolar vapors at different relative vapor partial pressures were well separated in PCA space. The best selectivity was obtained between toluene and dichloromethane vapors, while water and acetonitrile vapors were almost unresolved. Achieved detection limits were within the range of interest or better than those needed for determinations of these vapors for industrial applications.