Color switching in nanocomposites comprising inorganic nanoparticles dispersed in a polymer matrix

Although nanoparticles have found application as dyes already for millennia and experiments in the area of color transformations with nanoparticles have been realized for more than 100 years, color switching in polymer nanocomposites by means of inorganic particles has been explored more systematically only in the last 10–15 years (yet there are still relatively few systems characterized so far). Particles employed for this purpose have been, e.g., metals (gold, silver), transition metal oxides (polyoxometalates, titanium oxides) or coordination compounds (Prussian Blue, Ruthenium Purple). Importantly, the color of nanoparticles, in particular of metal particles, can depend on their sizes and shapes, and also on the interparticle distances. Changing these characteristics in nanocomposites therefore results in optical switching, induced e.g. by heat (thermochromic nanocomposites) or exposure to solvents. Also, an anisotropic arrangement of nanoparticles in the polymer matrix can offer the possibility for color transformations with polarized light (dichroism). Color alterations can further be generated by chemical reactions in nanoparticles, in particular redox reactions initiated by application of a voltage or by light (electrochromic or photochromic nanocomposites, respectively). Notably, the color changes can occur immediately or take hours or days, and the corresponding color variations can be reversible or irreversible, depending on the system. While basically color switching in nanocomposites is suited for many applications, a challenge for their production is the transfer from the laboratory to the industrial scale.