1. Department of Polymer Engineering, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski Blvd., 1756 Sofia, Bulgaria
2. Department of Silicate Technology, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski Blvd., 1756 Sofia, Bulgaria
3. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 11, 1113 Sofia, Bulgaria
4. Faculty of Biology, Sofia University “St. Kl. Ohridski”, 8 Dragan Tsankov Str., 1164 Sofia, Bulgaria
5. Faculty of Electrical Engineering, Technical University of Sofia, 8 Kliment Ohridski Blvd., 1756 Sofia, Bulgaria