Figure 14.

Overview of photonic quantification of single-molecule properties. Photonic measurements are typically based on observation of a fluorophore or larger optical probe covalently linked to the target molecule. Longer length scales enable fluorophore locations to be correlated with larger structures such as cells. Short distances (1−100 nm) can also be probed by measuring coupling between two optical labels on a single molecule; such experiments are used for observing conformational changes in proteins and DNA. Orientation changes have been probed using both fluorescence polarization measurements and larger probes. Fluorophores can also be chosen to be sensitive to pH, ion concentrations, or other factors, providing a readout of the local environment around a single molecule.