Some substances, absorbing energy, are capable of producing a response glow. This phenomenon is called "luminescence" (from the Latin word "light"). It was first described in the 18th century. One of the varieties of luminescence is fluorescence.
Types of Luminescence
To understand how fluorescence differs from other types of luminescence, it is necessary to consider the types of luminescence.
The following types are distinguished:
- chemiluminescence - the ability of chemicals to emit light during chemical interactions;
- bioluminescence is manifested in the ability to glow in living organisms (fireflies, plankton, some species of fish, mushrooms, etc.);
- electroluminescence - the phenomenon of luminescence due to the transmission of electric current;
- thermoluminescence manifests itself during the heating of a substance;
- radioluminescence - luminescence caused by ionizing radiation;
- photoluminescence.
The latter species is divided into two types: phosphorescence and fluorescence. In both cases, the glow is due to exposure to a light source. The difference is that phosphorescence does not start immediately, but lasts longer, even after the cessation of light exposure. With fluorescence, this process is observed only during exposure.
Discovery story
The name fluorescence phenomenon is due to the mineral fluorite, in which it was first observed.
In 1833, the Scottish scientist David Brewster discovered an unusual phenomenon. When rays of white color hit a green chlorophyll solution, chlorophyll acquired a red tint. So, for the first time, fluorescence of this pigment was discovered.
In 1845, the English scientist John Herschel noticed that the colorless quinine solution turned bluish under the influence of sunlight.
Finally, seven years later, George Stokes, examining the mineral fluorite, noticed that under the influence of ultraviolet rays the mineral began to emit a glow. The scientist gave the phenomenon the name fluorescence.
What is the reason for this phenomenon?
Substances whose elements are capable of fluorescence are fluorophores. Electrons in atoms are at energy levels. When energy is absorbed, the electrons of the fluorophore atoms go into the so-called excited state and begin to move between energy levels. The phenomenon of fluorescence of a substance occurs when electrons return to their normal state.
Quantum output
The quantum yield of fluorescence is the efficiency indicator with which the absorbed energy is converted into radiated emission. In other words, this is the ratio of photons released and absorbed in the process. The more photons produced during the process, the higher the fluorescence intensity. Therefore, the choice of fluorophore in research and practical application depends on the magnitude of the quantum yield.
Fluorescent compounds
Compounds capable of this process are divided into organic and inorganic.
Organic fluorophores are fluorescein, equorin, green fluorescent protein, eosin, quinine, acridine dyes, etc.
As inorganic fluorophores, complexes of organic and inorganic compounds — bioconjugates — are used. Organic fluorophore transfers the glow to the atoms of inorganic substances, and they themselves begin to fluoresce.
Chlorophyll fluorescence
Chlorophyll is the main pigment of plants, giving them a green color. Absorbing light, chlorophyll molecules are able to go into an excited state. When the molecules return to their normal state, a glow appears. In ultraviolet light, the green pigment turns red. This process is called fluorescence of chlorophyll.
Photosynthesis is a process on which the life of plants, as well as entire ecosystems, depends. Slowing photosynthesis is an indicator of poor plant health. The reasons may be lack of water, nutrients, adverse temperature conditions, weeds, etc. Therefore, fluorescence analysis of plants is necessary to evaluate their health.
Application of fluorescence
Fluorescence and fluorescence analysis methods are used in many fields of activity.
Fluorescence is used in biology and medicine. In molecular biology, special fluorescent probes and dyes are used. They are used to detect some of the constituent components of biological structures. For example, some blood cells are counted during analysis due to the presence of eosin in their structure - an organic fluorophore.
The phenomenon is widely used in the manufacture of paints and pigments. Fluorescent substances are used to produce bright colors. This is due to the fact that fluorescent pigments are able to convert ultraviolet from natural and artificial sources into radiation of a certain spectrum. Due to this, the color of the material becomes more vivid.
An example of such a dye is optical blue, which converts ultraviolet radiation into a bluish tint. It is used in factory dyeing of fabrics, as well as in washing powders, giving clothes a snow-white color. In addition, this method is used to lighten paper.
Fluorophores can be used to detect crashed pilots in large bodies of water. They carry special containers with fluorescent substances. When the dye gets into the water, a characteristic spot appears, which helps to detect the crash site.
In the art, pigments are added to some technical fluids. When illuminated with ultraviolet light, the leakage of such liquids immediately becomes noticeable.
Fluorescence is also used in ecology, hydrology, and laser manufacturing.
Finally, the methods of fluorescence analysis are invaluable for studying the process of plant photosynthesis. Any deviations in their state of health can damage the ecosphere.
To summarize:
- fluorescence is a type of luminescence;
- it lies in the ability of substances and compounds to glow when exposed to a light source;
- Research and methods of this phenomenon are applied in many branches of knowledge.