1. Laboratorio NEST, Scuola Normale Superiore, Pisa 56127, Italy
2. Center for Nanotechnology Innovation@NEST (CNI@NEST), Pisa 56127, Italy
3. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Pisa 56127, Italy
4. Laboratory for Fluorescence Dynamics, Department of Biomedical Engineering, University of California, Irvine, California 92697-2715, United States