1. Laboratorio NEST, Scuola Normale Superiore, Pisa, Italy. E-mail:
2. Center for Nanotechnology Innovation @NEST, Pisa, Italy
3. Fondazione Pisana per la Scienza (FPS), Pisa, Italy
4. NEST, Istituto Nanoscienze-CNR, Piazza S. Silvestro, 12, I-56127, Pisa, Italy
5. Department of Molecular Medicine, Sapienza University of Rome, Rome, Italy
6. Laboratory for Fluorescence Dynamics, Department of Biomedical Engineering, University of California at Irvine, Irvine, California, USA