1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
2. Fluidic Analytics Ltd., Cambridge, U.K.
3. Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, United States
4. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
5. Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, ETH Hönggerberg, HCI F 105, 8093 Zurich, Switzerland
6. Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.