1. Departement of Biological, Geological and Environmental Sciences, University of Catania, C.so Italia 57, 95129 Catania, Italy
2. Department of Physics and Chemistry, University of Palermo, Viale delle Scienze, pad. 17, 90128 Palermo, Italy
3. National Interuniversity Consortium of Materials Science and Technology, Via G. Giusti 9, 50121 Firenze, Italy