THE SCIENCE OF FLUORESCENCE
The Strange Case of the Disappearing Fluorescence
You've prepared your sample, found the perfect field of view and finally got everything looking exactly right. Then the fluorescence starts fading. What on earth happened?
Welcome to photobleaching, one of fluorescence microscopy's more irritating party tricks. It's been frustrating researchers for decades, usually at precisely the moment they've found something interesting. And while we can't promise to make fluorescent molecules last forever, the chemistry behind their disappearance is fascinating.
Understanding what causes fluorescence to fade can also help researchers get more from their samples, particularly when observing living cells or recording images over extended periods. Sometimes the answer is surprisingly simple: give your sample a little less light.
Where Does the Fluorescence Actually Go?
Fluorescent molecules, known as fluorophores, absorb light at particular wavelengths and release some of that energy as light at a longer wavelength. It's the principle that allows fluorescence microscopes to reveal structures otherwise difficult to see. Unfortunately, fluorophores aren't indestructible. Every time they're excited, there's a small chance they'll undergo a chemical change that leaves them permanently unable to fluoresce.
Oxygen and other reactive molecules often play a part, damaging the fluorophore until it can no longer produce detectable light. Some dyes are more resistant than others, but given enough excitation, many will eventually fade. And unlike your phone battery, a permanently photobleached molecule isn't going to recover after a quick recharge.
The slightly frustrating irony of fluorescence microscopy is that the very light helping us see something can also make it disappear.
Sometimes, Less Light Really Is More
When a fluorescent signal looks weak, the temptation is to turn up the illumination. More excitation light generally produces more fluorescence initially, but it can also accelerate photobleaching. For live-cell microscopy, there's an additional concern: excessive light exposure can damage the cells themselves through phototoxicity. This is particularly troublesome during time-lapse experiments, where repeated illumination can affect both image quality and the biology under observation.
There's no universal setting that works for every fluorophore or sample, but researchers can often reduce unnecessary exposure without sacrificing useful information. A few practical habits are worth considering.
Use only as much excitation light as your imaging experiment needs. Maximum brightness isn't automatically better.
Consider shorter exposures and suitable camera settings, while keeping enough signal for reliable measurements.
Where possible, illuminate the sample only during acquisition rather than leaving it exposed unnecessarily.
Giving Your Samples a Break
This is one area where modern LED illumination is particularly useful. With appropriate electronic control, LEDs can be switched rapidly, their intensity adjusted precisely and their output synchronised with image acquisition. Instead of exposing samples continuously, researchers can deliver excitation light when it's actually needed. Traditional mercury and metal halide systems can also control exposure using shutters, but LEDs offer convenient electronic control directly at the light source.
At CoolLED, managing when and how much illumination reaches the sample is an important part of our fluorescence microscopy technology. But LEDs aren't immune to photobleaching. Run one at excessive intensity for long enough and you'll still cause damage. They're simply another tool for helping researchers control the light their samples receive.
A Little Light Goes a Long Way
Photobleaching is an unavoidable consideration for many fluorescence experiments, but careful illumination can help limit its effects. Whether that means reducing intensity, shortening exposure or preventing unnecessary illumination between images, the aim is to capture the information you need while preserving as much fluorescence as possible.
So the next time your beautifully labelled sample starts losing its sparkle, resist the temptation to immediately turn everything up to maximum. Those fluorescent molecules are working quite hard already, and they've only got so many photons to give.
Best not to waste them.
Take Control of Your Fluorescence Illumination
CoolLED's LED Illumination Systems provide precise intensity control, fast switching and wavelength selection for fluorescence microscopy.
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