Dye coloration of silica with completely incompatible organic dye using stably coexisting organic solvent

Abstract We carried out dye coloration of sol–gel derived silica with near-infrared absorbing organic dye which is completely incompatible with silica sol. Since the direct dye coloration was impossible due to the incompatibility, the dye was introduced into the silica matrix being mediated by dye-dissolving amphiphilic solvents such as acetonitrile and ketones with various carbon numbers. The structural conditions for the uniform coloration were discussed based on optical microscopic observations and other auxiliary methods. The dye coloration was revealed to be possible when the dye-dissolving solvent remained in the form of small droplets, otherwise, the dye gets completely separated from the silica matrix in a dried powdery state. This result shows that the incompatible dye was immobilized in the silica matrix in a dissolved state inside the droplets. Optically transparent dye coloration which is more desirable from practical viewpoints was obtained when the dye-dissolving solvent has volatility in an appropriate region. Excessively high and low volatilities were found to lead to the segregation of powdery dry dye and larger droplets which made the sample look strongly opaque, respectively. In the case where the silica matrix was successfully colored retaining optical transparency, microscopic observation revealed that the size of the minute droplet structure was comparable to the wavelength of visible light being in the order of 10 2  nm.