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When you look around the world, things seem to fall into two categories:. There’s stuff that produces light, like the Sun or your computer monitor, and there’s stuff that only reflects it, like the walls of your room or your face.

At a molecular level, that first kind of thing uses energy to create light. The second only absorbs and reflects certain wavelengths of light, which give it color. But there’s a type of molecule that bends these rules: fluorophores.

Fluorophores get their name from the process of fluorescence. That’s when a molecule absorbs incoming light and then emits it at a different wavelength. And since different wavelengths correspond to different colors, fluorophores are secret color-converting workshops.

But it’s more than just, like, cool in theory:. Fluorescence turns out to be a kind of chemical superpower that lets us tackle all kinds of problems from solving crimes, to saving lives, and even helping you look your best. First, in 1961, researchers at the University of Princeton were studying a jellyfish off the coast of North America that glowed green around its edge.

To learn more about this, they ended up taking samples from thousands of jellyfish. And they isolated a protein called aequorin. Except… aequorin produces blue light from a chemical process.

Which wasn’t the same as the green light they saw in the live jellyfish. So, something must have been changing that light from blue to green. The answer to the puzzle was green fluorescent proteins, or GFPs.

These glow green when exposed to blue or ultraviolet light. And in the 1970s, researchers figured out why:. It’s a combination of how these molecules absorb energy, along with the way they’re structured.

So, molecules are made up of atoms, and atoms are just a central nucleus surrounded by some number of electrons. And in an atom, there are different orbitals that electrons can occupy. For a rough analogy, you can think of them as floors of a building electrons can exist on one level or another, but they can’t hover in-between floors.

When GFP molecules absorb light, that energy pushes electrons up to a higher energy orbital. Then, eventually, the electrons relax and fall back down to their original state. But when they do, they release some light but it’s not exactly the same light the molecule first absorbed.

Some of that original energy might have gone into moving the whole molecule around a little, or twisting or rotating it. So, the light that’s released has less energy than before. Less energetic light has a longer wavelength, and that wavelength is what determines its color.

So, ultimately, high-energy blue light goes into the GFP, and less-energetic green light comes out. This is how all fluorescence works in general. But it specifically works in these GFP molecules because of how they’re shaped.

They’re essentially built like a soda can, with a combination of three amino acids hanging out inside. On their own, those three amino acids don’t seem to fluoresce. But when they’re inside a GFP, the surrounding “can” of proteins holds them in place and shields them from their environment.

That means when light falling into the can gets absorbed, it’s harder for these amino acids to convert that energy into movement, or transfer it to other molecules nearby. Instead, they’re more likely to lose the energy by releasing it as light. All in all, this allows GFPs to convert ultraviolet and blue light into visible green light.

And that was a huge breakthrough, it turns out, for biology. By the 1990s, scientists had managed to take the gene for producing GFPs and put it into E. coli bacteria. Then, when scientists observed the bacteria under a microscope and shined UV light on them, the bacteria glowed green!

This introduced a groundbreaking tool for studying life. See, at a molecular level, life is basically just a bunch of genes that switch on and off, and those genes tell the cell to make or not make proteins that control biological processes. So, by placing GFP genes alongside other genes, scientists can actually “see” cells switching certain processes on and off.

And when I say “see,” I mean with their eyes! In fact, by tweaking the segment of genes that produces GFPs, they even managed to get the proteins to shine in colors other than green which has been hugely helpful. Like, in 2007, a study used different-colored GFPs to tag the nerve cells in a mouse’s brain as it developed.

Afterward, the resulting nerve cells could be individually traced with different rainbow colors, which the researchers obviously called the “Brainbow”. The next year, the Nobel Prize in chemistry went to the researchers who discovered, isolated the genes for, and created new colors of GFPs. GFPs are used for a bunch of different stuff, too.

Since whole organs, like skin, can be made to produce GFPs, we’re also able to make genetically-modified animals that glow under the UV light from a blacklight. It’s all very retro. Nowadays, people can stock their aquariums with “GloFish”, which are pet fish genetically engineered to glow in different colours.

We still don’t quite know why that original jellyfish evolved to fluoresce the way it does, but we’ve definitely learned a whole lot from its ethereal glow. Fluorophores don’t just tell us about the mysteries of life. We can also use them to solve the mysteries of… death.

Fluorescein is a chemical that looks like reddish-brown brick dust to the naked eye. But as the name hints, deep down, it has fluorescent powers, too. It’s made of organic molecules whose whole structure consists of several rings.

In that arrangement, each of the atoms in fluorescein has strong chemical bonds with its neighbors, making the molecule rigid and stiff. And like the GFP soda-can structure, that stiffness gives fluorescein its abilities. When blue light gets absorbed by a fluorescein molecule, the energy is stored by the electrons.

But because the molecule is rigid, the electrons can’t easily give up their energy to the molecule through movement or deformation. So the molecule ends up releasing most of its energy in the form of light specifically, a bright, yellowish-green color. And that’s come in handy for medicine.

For instance, doctors occasionally need to inspect blood vessels in the back of the eye for damage that might diagnose certain conditions. Like, diabetic patients might have narrow or blocked vessels in their eye that could impair their vision or cause blindness. To inspect those blood vessels, they use a procedure called fluorescein angiography.

The first step is to inject fluorescein dye into a patient’s bloodstream. After a short while, the dye will be circulating in all of their blood vessels, including the ones in their retina. Then, the doctors can project blue light onto the retina to get those fluorescein compounds glowing, and then they can check out the shape of the blood vessels with a filtered camera.

Meanwhile, fluorescein can also detect blood even when it’s outside the body, which gives it a slightly more gruesome use in police work. Investigators can detect tiny droplets of blood using a modified form of fluorescein that doesn’t glow on its own. They mix it with hydrogen peroxide, then spray it over a suspected crime scene.

If their mixture comes in contact with even a small amount of blood, the hydrogen peroxide reacts with iron in the blood, which releases oxygen. And when the oxygen reacts with the fluorescein, it can convert it back to a form that fluoresces as usual. Then, an investigator can shine blue or UV light over everything and look for that telltale yellowish-green glow.

And luckily, the DNA within the blood won’t be affected by the process, either, which allows investigators to extract samples to identify suspects. Blood and crime scenes might sound grim, but fluorophores also play a role in saving lives. Daylight fluorescent pigments are a whole family of artificial molecules that fluoresce in different colors.

Like fluorescein, their general molecular structure consists of rings of molecules, which create strong chemical bonds between neighboring atoms and absorb UV light in a similar way. But they can also be made to absorb visible light, by adding rings of carbon atoms or a long chain of atoms. These extra atoms change how the electrons jump around inside the molecules, which ultimately changes what colors of light the molecules release.

So, this idea can be used to create fluorescent dyes in pretty much any color. Even in normal light, the colors of daylight fluorescent dyes seem brighter than normal objects of the same color because... they are! In addition to reflecting light, they add even more light to their natural color through fluorescence.

The fact that they shine visibly in daylight is what gives the dyes their name. And since the colors seem to almost leap off the material, daylight pigments make materials extremely visible. And that is useful in lots of scenarios.

Safety jackets, road signage, and lifeboats benefit from the vibrant, bright colors of fluorescent dyes. Fire trucks and ambulances are covered with fluorescent pigments so they can be spotted from far away, too. But bright colors aren’t just for safety.

Lots of clothing, face paint, and accessories are also made with daylight fluorescent dyes in them. As well as looking bright and colorful in the sun, those same clothes will shine just as brightly under a blacklight. So, file that away for a future trip to the nightclub.

I know we’re all going to go face paint and rave out as soon as the COVID’s over. Finally, while fluorescent pigments might bring some color to a Saturday night, it’s a different kind of fluorophore that shows up at the office on Monday morning. Optical brighteners are organic molecules with rigid, chemical bonds between their atoms.

Like other fluorophores, that allows them to absorb UV light. But brighteners tend to emit blue light, specifically. Now based on that, you might think we’d use them to make things look blue.

But actually, we more often use them to make things look white. On their own, materials like paper become yellow over time as the organic compounds in them break down, reacting with oxygen in the air to become yellow. White laundry can also turn yellow over time because of factors like nitrogen dioxide in the air reacting with the material of the fabric.

And it’s not easy to get rid of the yellow, but a bit of blue can cover it up. That’s because colors of light add together when you mix them, with white light being an even mixture of colors from across the spectrum. So you can compensate for the extra yellow by adding some extra blue.

The overall effect is that the material looks white again! And that’s why optical brighteners are added to laundry detergent and the paper-making process. The brighteners allow the material to appear brighter and whiter for longer.

Weirdly enough though, brighteners might not keep working as well as they once did. The brighteners rely on the presence of UV light to make the molecules fluoresce, but LED lights don’t produce UV. So the more we switch to efficient lights like LEDs, the less effective fabric brighteners will be.

The good news is that, if history is anything to go by, new innovations with fluorophores will continue improving different areas of life, including laundry, but probably also things like medicine and forensics as well. But for now, if your white shirt looks a little yellow in the office, try blaming science. If you like watching SciShow, you might also like our podcast, it’s called SciShow Tangents.

It’s a lightly-competitive knowledge showcase where the hosts rack up points for teaching everyone the most mind-blowing science facts. If you liked this video, you might want to start with our episode called “Glowing Things,” which includes talks of death fluorescence. I’m not going to tell you what that is.

You have to listen to the podcast to find out. If you like science, and laughing, and lighthearted, nerdy competitions, you can find SciShow Tangents anywhere you get your podcasts. [♩OUTRO].