Photoinduced degradation of fluorescence and formation of copper nanoparticles in sol-gel silica doped with flavins

Copper nanoparticles were formed by photoirradiation of doped sol-gel silica, which was prepared by mixing Cu²⁺ ions, ethylenediaminetetraacetic acid (EDTA), and riboflavin into sol-gel solutions of tetramethoxysilane. The doped silica exhibited broad absorption bands at 442 nm due to riboflavin and 740 nm due to Cu²⁺-EDTA complexes. After photoirradiation, the sol-gel silica showed a reddish brown color and the absorption peak around 580 nm due to the plasmon band of copper nanoparticles. Copper nanoparticles were also formed from other sol-gel silica doped with lumichrome or lumiflavin. The photostability of the flavins dyes obtained from the fluorescence intensities was in the order of lumichrome > lumiflavin > riboflavin in the sol-gel silica without Cu²⁺ ions. On the other hand, the fluorescence intensities were considerably reduced by photoirradiation of the sol-gel silica doped with Cu²⁺ ions, irrespective of the flavin dyes doped. Considering the absorption and fluorescence spectral changes during the photoirradiation, we concluded that copper nanoparticles are produced by the photoinduced electron transfer from the flavin dyes in the sol-gel silica.