Warning: Do not look into laser beam with remaining good eye.
Greetings fellow nerds.
In this video we’re going to explore the science and effects of fluorescence.
We’ve done a lot of fluorescence and related science over the years so there is a lot of stuff to cover.
Let’s start simple.
A really cool way to play with fluorescence at home is to get a dish or jar of water and get a fluorescent highlighter.
Pull out the cartridge and squeeze out a drop of the fluid into the water.
Stir it up to get a homogeneous solution.
Now get a violet or blue laser pointer and shine it through.
You can clearly see the beam as the solution fluoresces in its path.
This is a much clearer way to show a laser beam than using cloudy water or smoke.
This is also a great way for you teachers to demonstrate things like total internal reflection.
Now the common definition of fluorescence is something that glows a visible color when exposed to ultraviolet light.
Fluorescence is actually much broader than that and you don’t need ultraviolet light in particular.
Let me show you.
Here is a sample of yellow fluorescent dye and I’m going to shine a violet laser at it.
As expected it glows yellow.
Now here I have a green laser and it too can activate the dye and emit yellow light,
proving you don’t always need ultraviolet light.
Now I’m going to shine a red laser and we get no yellow fluorescence from it.
Why is that?
Let me show you with this Jablonksi diagram.
What’s happening in fluorescence is that the incoming light raises the energy of the electrons in the molecule to an excited state.
These electrons then lose a bit of energy due to vibrations of the molecules.
And finally the electrons return to the ground state by releasing light.
Now since energy cannot be created or destroyed
and a bit of energy was already lost as heat in the vibrations of the molecules,
the energy of light emitted must have lower energy than the light absorbed.
So since this yellow fluorescent dye emits yellow light,
we need to use light of higher energy like violet and green for it to glow.
Red is lower energy than yellow light so it can’t excite the dye.
This also explains why ultraviolet light is used so often.
Because it's the highest easily accessible light so it activates most dyes.
And because its almost invisible to human eyes so its cooler to use for fluorescence effects.
Note that you don’t get fluorescence if you use the same color of light as the fluorescent color.
This is because you almost always lose a bit of energy and therefore it has to emit a different color or none at all.
Now just because the light is higher energy doesn’t mean this will always work.
This is a sample of Europium Tetrakis (Dibenzoylmethide)Triethylammonium.
Under a violet light it will glow a bright orange color.
But under green light it doesn’t glow at all.
This is because the substance simply doesn’t absorb green light and so it can’t reach an excited state where it can fluoresce.
So you need both absorption and higher energy to get fluorescence.
Higher energy does not automatically imply absorption.
A really great way to demonstrate the concepts thus far is to use multiple dyes with multiple wavelengths of light.
Under ultraviolet light all the dyes emit their respective fluorescent colors as expected.
Now let’s use blue light.
The other dyes still look the same but the blue fluorescent dye on the left now looks clear.
Sure it’s reflecting blue light from the glass but it's not actually glowing.
It does not absorb blue light and it cannot emit fluorescent blue light.
So without absorption or emission it looks clear.
Now lets move onto green light.
Now the first two dyes go clear since they don’t absorb green and can’t emit light since green has a lower or equal energy.
But for the last two dyes, yellow and red, green light is still within their absorption band and has enough higher energy to cause them to fluoresce.
Let’s move on to yellow light.
Now only the red dye still fluoresces red while the other three dyes are crystal clear.
This goes to show that when working with fluorescent compounds under the right conditions
you can actually select and study a single compound with the right light sources.
Finally onto red light.
All the dyes neither absorb red light nor can they emit any light and they all look like crystal clear water.
This demonstration always amazes me.
Looking at them in this light you’d never know they are vividly different fluorescent dyes.
Now I get this question a lot: What happens when I mix fluorescent dyes?
Well it depends on the dyes in particular.
Here we have a blue fluorescent dye and over here I have a yellow fluorescent dye.
Let’s mix them together.
Now they both fluoresce at once and they both emit light which combines to give a white light.
I know a few of you are expecting green but fluorescent dyes work on additive color theory,
which is not the same as subtractive color theory that absorption based dyes work on.
Now here is a different set of dyes, a green dye and a red dye.
Although this particular dye glows a more yellow color rather than red so we’ll call them green and yellow dye.
Mixing these two dyes together we get... yellow again?
I know green and yellow are very close but we should have at least seen a change in hue or something,
these two yellows are exactly the same.
There is actually a reason for this.
The green dye is emitting green light as expected but the yellow dye immediately absorbs that green light and converts it to yellow.
So when mixing fluorescent dyes the final result depends on how much their absorptions and emissions overlap.
In the previous blue and yellow dye demonstration the yellow dye does absorb a bit of blue but not enough to totally block it.
Okay moving on now and going even deeper into fluorescence.
Remember when I said it worked by using light to excite electrons into higher energy levels then dropped down and emitted light?
Well if we could just push these electrons into their higher energy levels then we don’t necessarily need the light do we?
In fact we can. In a glow stick.
A glow stick reaction uses chemicals instead of light to excite the electrons in a fluorescent dye.
This is why glow sticks must be made with fluorescent dye.
They need to have the ability to emit light.
Now instead of using chemicals to excite those electrons and cause fluorescence we can use direct mechanical grinding.
A very special range of fluorescent compounds has this property called triboluminescence.
I prefer to call them smash glow crystals.
You can check them out in a separate video I made.
Ok next topic.
Remember when I said way back that after an electron is excited it loses a bit of energy due to molecular vibrations?
It follows that if we can alter or stop these vibrations then we can change the energy of fluorescence and thus its color.
For some dyes we can.
This is a sample of special dye called pyridine copper iodide.
It normally fluoresces yellow at room temperature but when dipped into liquid nitrogen it turns blue.
By cooling the dye we reduce the energy loss from molecular vibrations.
More energy is now available for fluorescence and we get blue instead of yellow.
Not all dyes change this much but it’s really impressive when they do.
This property of changing fluorescence with temperature is called fluorescence thermochromism.
You can see more of it in another video I made.
Another incredible property of fluorescence is its inherently low detection limit and sensitivity.
Let me show you.
Here I have several samples of solvent but to one of them I have dissolved 1 microgram of this yellow compound.
Yes I said that correctly 1 microgram, as in millionth of a gram, less than a grain of salt.
Now the stuff is yellow so let me ask you.
Can you tell by color which one of these samples has the yellow compound?
It’s almost impossible but this stuff is also flourescent so let me get the lights.
I think it’s painfully obvious now which one has the compound.
Fluorescence can be used to detect such tiny amounts.
It’s a fun demonstration but actually this has a life saving application.
In this scenario we have a bunch water samples and some of them are contaminated with toxic levels of E.Coli.
Now to each one we have added a specially engineered dye that fluoresces when it comes into contact with E.Coli.
Can you tell just by looking at it which ones are contaminated?
Well with the dye and fluorescence you can.
A highly advanced version of this technique is already being used in medical diagnostics to identify all sorts of diseases in people.
So there you have it, a cool effect that helps us to understand light, have fun and save lives.
That was a basic overview of the science of fluorescence.
Thanks for watching.
In this video we're going to demonstrate temperature sensitive fluorescence. Also known as fluorescence thermochromism.
In this video we're going make to crystals that glow a blue color when smashed.
In this video we're going to make flowers glow in the dark under ultraviolet light using a fluorescent dye.
In this video we're going to make fluorescein, a strongly fluorescent chemical.
In this video we're going to make cadmium selenide quantum dots. A type of nanoparticle with interesting properties.
In this video we're going to make glow sticks of various colors and explain a few interesting points about them.