Fluorescence is a phenomenon where certain materials rapidly (around 10-8 seconds) emit light when they are exposed to specific types of electromagnetic radiation, typically ultraviolet (UV) light. Fluorescent materials are those that can exhibit this characteristic. At a scientific level, fluorescence can be defined as the absorption of a photon by an atom or molecule, which raises its energy level to an excited state, followed by the emission of a lower-energy photon as the atom or molecule returns to its original state. Understanding fluorescence is important for diverse applications ranging from medical imaging and diagnostics to energy-efficient lighting and environmental monitoring.
Examples of Fluorescent Materials
Fluorescence is common occurrence in the natural world, as well as in everyday products. Here are some examples of fluorescent materials:
- Chlorophyll, the photosynthetic pigment in plants and algae, has its peak fluorescence in the red portion of the spectrum.
- Many minerals are fluorescent under UV light, including some types of fluorite, diamond, calcite, amber, rubies, and emeralds.
- Some coral species contain fluorescent proteins, which help them absorb and utilize sunlight used for photosynthesis.
- The green fluorescent protein (GFP) was first discovered in the jellyfish Aequorea victoria and is now widely used in research.
- Petroleum fluoresces in colors ranging from dull brown to bright yellow to blue-white.
- Tonic water fluoresces due to the presence of quinine.
- Banknotes and stamps use fluorescent inks for security.
- Some fluorescent markers and highlighters glow under a black light, usually due to the presence of pyranine.
- Fluorescent lamps are glass tubes that are coated with a fluorescent material (a phosphor) that absorbs ultraviolet light from a mercury vapor tube and emits visible light.
- Laundry detergent and paper often contains fluorescent brighteners that release blue light. This counteracts yellowing or dullness that occurs over time.
The discovery of fluorescence dates back to 1560 when the Italian mineralogist, Bernardino de Sahagún observed the phenomenon in an infusion called lignum nephriticum. Lignum nephriticum comes from the wood of trees that contain the compound matlaline, which has a fluorescent oxidation product. The term “fluorescence” was coined in 1852 by the British scientist Sir George Stokes coined the term “fluorescence” in 1852. Stokes observed and studied the emission of light by fluorite and uranium glass under UV radiation.
How Fluorescence Works
Fluorescence occurs when a material absorbs a photon and transitions from its ground state to an excited state. After a brief period, called the fluorescence lifetime, the material returns to its ground state, emitting a photon with lower energy in the process. The photon emission does not cause a change in electron spin (which it does in phosphorescence). The difference in energy between the absorbed and emitted photons corresponds to the energy lost during the excited state, often as heat.
This process occurs in steps:
- Absorption: An atom or molecule absorbs an incoming photon. Usually, this is visible or ultraviolet light because x-rays and other energetic radiation are more likely to break chemical bonds than get absorbed.
- Excitation: The photons boost the atoms or molecules into a higher energy level, which is called an excited state.
- Excited State Lifetime: The molecules do not remain excited for long. They immediately begin to decay from the excited state toward a relaxed state. But, there may be smaller energy drops from within the excited state called non-radiative transitions.
- Emission: The molecule drops all the way to one of the ground states, emitting a photon. The photon has a longer wavelength (less energy) than the absorbed photon.
A Jablonski diagram illustrates these processes as a graph showing energy absorption and emission for excited (S1) and singlet ground (S0) states.
Three useful rules in fluorescence are Kasha’s rule, Stokes shift, and the mirror image rule:
- Kasha’s Rule: This rule states that the quantum yield of luminescence does not depend on the wavelength of the absorbed light. In other words, the fluorescence spectrum is the same regardless of the color of incident light. However, simple molecules often violate this rule.
- Stokes Shift: The emitted photons have a longer wavelength than the absorbed light. This is because there is an energy loss, usually due to non-radiative decay or else from a fluorophore dropping to a higher vibrational level of the ground state.
- Mirror Image Rule: For many fluorophores, the absorption and emission spectra are mirror images of each other, reflecting the relationship between the electronic and vibrational transitions during the absorption and emission processes.
In nature, organisms use fluorescence for communication, mate attraction, luring prey, camouflage, and UV protection. Fluorescence has numerous practical, commercial, and research applications:
- Medical Imaging and Diagnostics: Fluorescent dyes and proteins help researchers visualize specific structures and processes within living cells and tissues.
- Energy-Efficient Lighting: Fluorescent lamps and LEDs are more energy-efficient compared to traditional incandescent bulbs due to their ability to convert more input energy into visible light.
- Environmental Monitoring: Fluorescent sensors detect pollutants or contaminants in air, water, and soil samples.
- Forensics: Fluorescent materials detect fingerprints, biological samples, or counterfeit currency.
- Research Tools: Fluorescent markers and tags are essential in molecular and cell biology for tracking and monitoring
Fluorescence vs Phosphorescence
Both fluorescence and phosphorescence are forms of photoluminescence. While fluorescence occurs immediately, phosphorescence releases light more slowly so that phosphorescent materials often glow in the dark for seconds to hours.
- Fluorescence: A material absorbs a photon, transitions to an excited state, and then quickly returns to its ground state, emitting a lower-energy photon in the process. The emitted light ceases almost immediately after the excitation source is removed, with the fluorescence lifetime typically ranging from nanoseconds to microseconds.
- Phosphorescence: In phosphorescence, the absorbed energy causes the electron to transition to a metastable state with a different spin multiplicity, known as a triplet state. The transition back to the ground state is spin-forbidden, which means it takes longer for the electron to return to its original state. As a result, phosphorescence lasts from milliseconds to hours after the excitation source is removed.
Difference Between Fluorescence and Bioluminescence
Both fluorescence and bioluminescence emit light, but they differ in duration and mechanism.
- Fluorescence: Fluorescence is a type of photoluminescence. It is a physical process where a material emits light after absorbing energy from an external source. The emission of light is almost immediate and does not continue once you remove the energy source.
- Bioluminescence: In contrast, bioluminescence is a form of chemiluminescence that occurs within living organisms. It involves the production and emission of light as a result of a chemical reaction. The reaction typically involves a substrate (e.g., luciferin) and an enzyme (e.g., luciferase) that catalyzes the oxidation of the substrate, releasing energy in the form of light. Bioluminescence does not require external energy sources like UV light. It releases light as long as the reaction continues. This process occurs in various organisms, including fireflies, certain marine creatures, and some fungi.
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