Fluorescence

Fluorescence is a form of luminescence in which light is emitted by a substance that had absorbed electromagnetic radiation or light. However, the emitted light has lower energy due to the longer wavelength as compared to the absorbed radiation. There are various practical applications of fluorescence including gemology, mineralogy, fluorescent labeling, chemical sensors, biological detectors, dyes, cosmic ray and fluorescent lamps. The emission of light frequently occurs in nature such as in minerals and other biological states in the animal kingdom.

History

Fluorescence was observed earlier in 1560 and 1565 when infusion was made. The matlaline is the chemical compound that is responsible for the emission of light. The chemical compound is also the oxidation product of the flavonoids extracted from the wood of Eysenhardtia polystachya and Pterocarpus indicus.

The phenomenon of fluorescence was described in fluorites by Rene Just Hauy in 1822. It was also associated to chlorophyll by Sir David Brewster in 1833 and in quinine by Sir John Herschel in 1845. Fluorescence was described as the ability of the uranium glass and fluorspar to alter invisible light over the violet end into a blue light. Europium serves as the activator that facilitates emission of blue light.

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Physical Principles

  • Photochemistry

Fluorescence happens when the orbital electron of an atom or molecule relaxes to ground state through the emission of photon of light. The phenomenon occurs after the photon is elevated to higher quantum state.

 

Excitation:

Fluorescence/emission:

The hv refers to the photon energy in which h=Planck’s constant and v=light frequency.

 

  • Quantum yield

The quantum yield of fluorescence provides the efficiency of the process. It is explained as the ratio of the number of emitted photons to the number of absorbed photons as shown in the formula:

Fluorescence quantum yields are determined by comparison to standard. The quinine sulfate is the common standard in measuring the fluorescence.

 

  • Lifetime

The fluorescence lifetime pertains to the standard time in which the molecule remains in its excited state prior of emitting photon. It usually follows first-order kinetics:

The fluorescence lifetime is an essential factor for practical applications such as Fluorescence-lifetime imaging microscopy and fluorescence resonance energy transfer.

 

  • Jablonski diagram

This diagram explains the relaxation mechanisms of molecules on its excited state.

 

  • Fluorescence anisotropy

The polarization of the emission of light is dependable on the transition moment. Likewise, the transition moment also relies on the fluorophore physical orientation.

 

  • Fluorence

Fluorence is the unusual appearance of the fluorescent pigments that is also called as neon color. It is associated to the brightness of the color being a component of white. When the illumination wavelength changed, the fluorescent color seems brighter or more saturated as compared to the appearance of the reflection.

Rules

Fluorescence involved several rules and each rule serves as useful guidelines to understand the concept of fluorescence. Here are some of the general rules.

  • Kasha-Vavilov rule – It explains that the quantum yield of fluorescence does not rely on the wavelength of the exciting radiation.
  • Mirror image rule – The absorption spectrum is considered to be the mirror image of an emission spectrum. This rule is related to Franck-Condon principle stating that the electronic transition is vertical and the energy alters without changing the distance.
  • Stokes shift – The emitted fluorescent light has lower energy, yet longer wavelength. This phenomenon is called as Stokes shift and it occurs due to the loss of energy.

Fluorescence in nature

There are various natural compounds that show fluorescence. Likewise, greeneye and other deep-sea animals are exhibiting fluorescence.

  • Biofluorescence – It is the absorption of the electromagnetic wavelengths in living organisms. It is also the reemission of the visible light at lower energy level. This makes the absorbed light to be in different color than the re-emitted light.
  • Bioluminescence – This is different from biofluorescence in a way that the light is produced through chemical reactions.
  • Biophosphorescence – this is comparable to biofluorescence when it comes to the needed light wavelengths. The difference is apparent in terms of the relative stability of energized electron.

Mechanisms of biofluorescence

  • Epidermal chromatophores – These are the pigment cells that are showing fluorescence. The functions are analogous to regular chromatophores.

Aquatic biofluorescence

The water can absorbs lights having long wavelengths, yet less light coming from the wavelengths can be seen as it reflects back. Thus, warm colors seem less vibrant as the depth of the water increases. As the water spreads light having shorter wavelengths, the cooler colors control the visual field. The intensity of light decreases 10 times as the depth increases 75 meters. Here are some creatures that exhibit fluorescence:

  • Fish including lizardfish, sharks, wrasses, scorpionfish and flatfishes.
  • Coral serves different functions and contribute in the process of photosynthesis.
  • Jellyfish is a carrier of the green fluorescent protein or GFP.
  • Mantis shrimp carries yellow fluorescent along the shell and the antennal scales.
  • Siphonophores exhibit red and yellow fluorescence as the result of the bioluminescence.
  • Dragonfish emits blue light and generates red biofluorescence.

Terrestrial biofluorescence

  • Butterflies – Butterflies emit fluorescent light from their wings that have pigment-infused crystals. The crystals absorbed the wavelength of light that is why the more light absorbed, brighter light is emitted.
  • Parrots – The fluorescent plumage present in parrots is useful for mate signaling.
  • Spiders – Spiders possess wide range of fluorophores when it is exposed to UV light. The fluorescence in spiders is taxonomically widespread. It is used for interspecific and intraspecific signaling.
  • Flowers – Some flowers have fluorescent betaxanthins. Fluorescence in flowers is important in pollinator attraction, thus pollination is encouraged when there is stronger fluorescence.

Applications of fluorescence

  • Fluorescence is significant for different applications. A fluorescent lamp will not emit light without the fluorescence. The ultraviolet light emitted by mercury atoms is discharge in the tube making it to emit visible light.
  • Fluorescence is also being used in analytical procedures involving fluorometer. It works best by using Boltzmann distribution.
  • Fluorescence is also used in tracking or analyzing biological molecules.
  • Fingerprints can be detected through fluorescent compounds. Thus, fluorescence is important in forensics.
  • Dye tracing is utilized in determining cracks in the surface. Flourescence plays significant role in mechanical engineering.
  • Fluorescent colors are being used in road signs and signage. The colors can be easily recognized even in longer ranges.