An Introduction to Fluorescence

Fluorochromes will only fluoresce if they are illuminated with light of the corresponding wavelength. The wavelength depends on the absorption spectrum of the fluorophore and it has to be ensured that an appropriate quantity of energy is delivered to elevate the electrons to the excited state. After the electrons are excited they can dwell in this high energy state for a very short time only. When the electrons relax to their ground state or another state with a lower energy level, energy is released as a photon. As some of the energy is lost during this process, light with an increased wavelength and lower energy is emitted by the fluorochrome compared to the absorbed light.

As phosphorescing molecules can luminesce for a much longer time than fluorochromes, there must be a difference in the way they store the excitation energy. The basis for this discrepancy is found in the two forms of excitation levels, the singlet excited state and the triplet excited state, which are based on different spin alignments.

Spins are an attribute of electrons. In simplified terms, the spin describes the angular momentum of the electron caused by its rotation. The orientation of an electron’s spin can be positive (+1/2) or negative (–1/2). Spin pairs of higher energy levels can either be parallel or antiparallel in their orientation to each other. In antiparallel spin pairs the individual angular momentums compensate each other and the total spin gets a value of zero. This spin alignment is called singlet state. Two parallel spins do not compensate and get a value different from zero. In this case the spins are said to be in a triplet state.

Fluorescence occurs when electrons go back from a singlet excited state to the ground state.  But in some molecules the spins of the excited electrons can be switched to a triplet state in a process called inter system crossing. These electrons lose energy until they are in the triplet ground state. This state is of higher energy than the ground state but also of lower energy than the singlet excited state. The electrons can therefore not switch back to the singlet state, nor can they easily go back to the ground state, as only total spins with a value of zero are allowed due to quantum mechanics. The molecules are therefore trapped in their energy state. 

But a few changes from the triplet ground state to the ground state are possible at a time. These changes give rise to the emission of photons and the phosphorescence. As only a few events are possible at a time, the triplet ground state presents a kind of energy reservoir, making phosphorescence possible over a longer time period.

For microscopy, fluorescence is the most useful kind of luminescence. Fluorochromes can easily be excited with their specific wavelength via specific light sources (e.g. lamps and filter systems or lasers) and the emitted light can be distinguished from the excitation light by the wavelength (Stokes’ shift).

Using fluorescence imaging, the experimenter can characterize the amount and the localization of a molecule inside a cell. Another advantage of fluorescence microscopy is that several fluorochromes can be used simultaneously. The fluorochromes only have to vary in their excitation and emission wavelength. Hence, different target molecules can be observed simultaneously, allowing a huge variety of experiments e.g. colocalization studies, to be performed.