1. Sensors and Biosensors Group, Department of Chemistry, Universitat Autónoma de Barcelona, Edifici Cn, 08193 Bellaterra, Catalonia, Spain tel: +34 93 5811017; fax: +34 93 5812477
2. Department of Physics, Universidade Federal do Amazonas, 3000 Japiim, 69.077‐000 Manaus, AM, Brazil