1. M. Bruchez Jr. and A. P. Alivisatos, Department of Chemistry, University of California, Berkeley, CA 94720, and Materials Sciences Division, Lawrence Berkeley National Laboratory (LBNL), 1 Cyclotron Road, Berkeley, CA 94720, USA. M. Moronne and P. Gin, Life Sciences Division, LBNL, 1 Cyclotron Road, Berkeley, CA 94720, USA. S. Weiss, Materials Sciences Division and Physical Biosciences Division. LBNL, 1 Cyclotron Road, Berkeley, CA 94720, USA.