Above: Image © Alexander Klepnev, Wikimedia Commons

What do highlighters, jellyfish and road signs have in common? They all glow when light shines on them. This process is called fluorescence. It happens when a substance absorbs electromagnetic energy, such as visible and UV light, and then emits electromagnetic energy of its own.

Some substances can fluoresce, and others can’t. It depends on whether or not they contain molecules called fluorophores. Different fluorophores emit different colours.

From glowing minerals to the reflective strips on your winter coat, fluorescence is all around you. But did you know that fluorescence is also extremely important to medical research?

Did you know? Fluorescence in living things is called biofluorescence. Many organisms use the ability to glow for camouflage and communication. Coral, fish, seahorses and even parrots can fluoresce.

How does fluorescence work?

Fluorescence happens in three stages.

  1. Excitation: A fluorophore absorbs a photon (light particle). That energy gets transferred to the molecule.
  2. Excited-State Lifetime: Once it absorbs the photon’s energy, the fluorophore goes into a higher-energy or “excited” state. This only lasts for a very short time, usually nanoseconds! That’s because the excited molecule is highly unstable.
  3. Emission: When the fluorophore can’t hold on to the extra energy any longer, it releases the energy back into the environment. This is often in the form of visible light. That’s why the object looks like it’s glowing from within.

Did you know? Fluorescence was first documented in 1565, when Spanish scientist Nicolás Monardes described the blue glow produced by water that came in contact with Mexican kidneywood.

Finding tumours and proteins with fluorescence

Some of the most exciting uses of fluorescence are in medicine and biomedical research. One really cool example is using fluorescence-guided surgery to remove malignant (cancerous) ovarian tumours.

Ovarian tumour cells need vitamin B9 (folic acid) to grow and divide. As a result, they have a lot of receptors for this vitamin. Using techniques developed at Purdue University, doctors in the Netherlands took advantage of these receptors to detect cancer.

The researchers attached a fluorescent probe molecule called fluorescein iso-thiocyanate to folic acid. When they injected the folic acid into a cancer patient, it was absorbed by the cancer cells. Finally, when they shone light on these cells, the fluorescein iso-thiocyanate glowed. This showed the exact location of the tumours!

Scientists often use a similar technique called immunofluorescence to study specific proteins inside living things. First, they design antibodies that can target these proteins. Then, they attach fluorophores to them. Antibodies are made by the immune system. Their normal job is to eliminate bacteria and pathogens. But when they have fluorophores attached, researchers can use a fluorescent microscope with a powerful light source (usually a laser) to excite the fluorophores. This makes them glow. And since the antibodies will go directly to the protein they are made for, scientists can clearly see that protein they’re studying inside an organism.

Here are some cool images that I made by tagging neurons with fluorescent antibodies. This lets me target proteins that are only found inside these kinds of brain cells.

From glowing jellyfish to medical research

Scientists like me love using fluorescence any chance we get! Often, we use a green fluorescent protein (GFP) isolated from the jellyfish Aequorea Victoria. Researchers have been able to insert the GFP gene into the genomes of laboratory animals to make certain proteins glow inside their cells. Among other things, this can help with studying the development of embryos and fetuses.

Scientists have also introduced specific mutations into the GFP gene. This has created many different fluorescent markers (indicators), ranging from turquoise to orange. As a result, researchers can now see more than one protein at a time.

Another technique based on fluorescence is called Brainbow. It lets scientists tag the cells inside the brain of a mouse with different colours. Someday, this technique may help researchers discover how different parts of the human brain connect with each other.

Did you know? Green fluorescent protein (GFP), which is isolated from a type of glowing jellyfish, was first identified by Martin Chalfie, Osamu Shimomura and Roger Y. Tsien. This discovery earned them the 2008 Nobel Prize in Chemistry.

Will humans ever be able to glow?

As you can see, fluorophores can be very useful for scientists who study living things! For now, most of these techniques have only been tested on laboratory animals. But in the future, medical researchers may be able to use these glowing molecules to help keep humans healthy.

This article was updated by Let's Talk Science staff on 2017-09-12.

Learn More!

About fluorescence:

Basic concepts of fluorescence (2016)
Molecular Expressions

Microscopy Day 2011: Principles of Fluorescence (2011)
M. Spitaler, Imperial College London

The science of bioluminescence
Luminescent Labs

Fluorescence fundamentals
ThermoFisher Scientific

About how scientists use fluorescence:

Follow the Brainbow! Fluorescent Proteins and Neural Circuitry (2013)
L. Halvorsen, CurioCity by Let’s Talk Science

The green fluorescent protein (1998)
R.Y. Tsien, Annual Review of Biochemistry 67

Ekaterina Turlova


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