1. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
2. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Nanomaterials for Bioimaging Group, Departamento de Física de Materiales, Facultad de Ciencias, Universidad Autónoma de Madrid, Madrid 28049, Spain
4. Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States
5. Department of Nuclear Engineering and Chemistry, University of California, Berkeley, California 94720, United States
6. Division of Molecular Biophysics & Integrated Bioimaging, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States