The detection of important biological thiols such as cysteine (Cys) is highly relevant to a variety of fundamental physiological processes in organisms. A conjugated polymer with pendant aldehyde group, which was used for reactive site for Cys, was synthesized via Suzuki coupling polymerization and the polymer was immobilized on the surface of amine-functionalized silica nanoparticles. The conjugated polymer-modified silica nanoparticles could be dispersed in alcoholic media, exhibiting various emission colors according to the alcoholic media, presumably due to the various polarity of the alcohols used. The organic-inorganic hybrid showed changes in emission color upon exposure to Cys with high selectivity over other amino acids, because of the specific reaction of aldehyde in the polymer and aminothiol group in Cys.