fluorescing

Fluorescing, or fluorescence, is the emission of light by a substance that has absorbed light or other electromagnetic radiation. The emitted light typically occurs almost immediately after excitation and stops when the exciting source is removed. In most cases, fluorescence involves a molecule absorbing a photon and promoting an electron to an excited singlet state (S1), followed by a rapid return to the ground state with the emission of a photon. The emitted light has a longer wavelength than the absorbed light, a difference known as the Stokes shift. Fluorescence lifetimes are usually in the nanosecond range and the efficiency of the process is described by the quantum yield.

Fluorophores, the substances that fluoresce, can be intrinsic to a material (autofluorescence) or introduced as labels (extrinsic fluorophores). Intrinsic examples include certain amino acids and metabolic cofactors, while extrinsic fluorophores include dyes such as fluorescein and rhodamine, and fluorescent proteins like GFP. Fluorescence is characterized by excitation and emission spectra; the observed color is influenced by the detection setup and optical filters. The spectral properties of a fluorophore can be affected by its chemical environment, including pH, solvent, and interactions with other molecules, which can cause shifts in hue or brightness or lead to quenching.

Common applications span biology and medicine (fluorescence microscopy, immunofluorescence, flow cytometry, biosensing), chemistry (spectroscopy, sensor design), and materials science (fluorescent markers, OLEDs). Limitations include photobleaching, phototoxicity in live samples, autofluorescence from surrounding material, and spectral overlap between fluorophores, which requires careful experimental design.

Fluorescence of minerals and organic compounds was first described in the 19th century and has since become a fundamental tool in science for visualizing structure, processes, and composition.