Is it true that fluorescence is sensitive enough to detect single atoms and molecules?
Yes, but only in particular circumstances which can not always be achieved in a useful manner. To understand the subtleties a very simple calculation is useful. Consider a fluorophore with the following typical properties: Quantum yield for fluorescence=0.1 (i.e. a photon is emitted once every ten excitations on average. Many labels have higher quantum yields than 0.1) Quantum yield for photobleaching=0.0001 (i.e. on average the molecule decomposes after 10,000 excitations. Many labels are substantially more photostable than this) These figures tell us that a single molecule of our hypothetical label will (on average) emit 1000 photons if it is repeatedly excited until it is bleached irreversibly. The emitted photons must be collected by a lens, optically filtered to reject exciting light and detected by a sensor. Each of these processes introduces significant losses. Combining these reduces the overall detected signal substantially. Overall efficiency is usually in the range 1-5% or m
