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Analytical Chemistry

Jumpstarting nanoparticles with UV light

A zap from a laser can restore fluorescence in silicon dioxide particles

by Neil Savage
June 21, 2016

Illustration of SiO<sub>2</sub> nanoparticles illuminated by blue and UV lasers.
Credit: Nano Lett.
A beam of ultraviolet light (purple) strikes a silicon dioxide nanoparticle, creating a defect. A beam of blue light causes the defect to fluoresce.

Fluorescing nanoparticles are helpful tools for biological imaging, allowing researchers to spot hard-to-see phenomena through a microscope by lighting up when hit with laser light at the correct wavelength. However, their ability to fluoresce doesn’t last; after a few minutes, the light causes photobleaching, and the particles lose their usefulness. Now a team of researchers has proposed a way to reverse the photobleaching in silicon dioxide nanoparticles, allowing them to fluoresce anew (Nano Lett. 2016 DOI: 10.1021/acs.nanolett.6b01361).

Since small defects in a crystal structure are the source of nanoparticles’ fluorescence, breaking chemical bonds on the surface could potentially create new centers that bring back the glow. “It seems to be very simple,” says Alexey I. Chizhik, a postdoctoral fellow in Jörg Enderlein’s lab at Georg August University. “By exposing the silicon dioxide particles to UV light, we are trying to create new defects.”

To test this idea, the team spin-coated glass slides with SiO2 nanoparticles ranging from 11 to 166 nm in diameter. They hit the nanoparticles with pulses of blue laser light at 488 nm to stimulate any fluorescence. Once they’d observed photobleaching, they pulsed a 378-nm UV laser at the nanoparticles to create defects. With the blue laser still firing, they were able to see if any new fluorescence occurred.

They saw renewed fluorescence in 8% of the particles they treated. It may be, Chizhik says, that they also created defects that emit at other wavelengths, such as in the infrared, that would not be visible in the system they’d set up.

To his surprise, the smallest particles reactivated their fluorescence best. That may have to do with the curvature of the surface, which places strain on the chemical bonds and would be greater in smaller particles than in larger ones, he says. This strain may make the bonds easier to break, generating defects more easily.

They’d like to use the technique to restore fluorescence in particles placed in biological samples, but they’ll have to explore whether they can do so using low enough laser powers and wavelengths that won’t damage the samples.

Philip Tinnefeld of the Technical University Braunschweig praised the work: “The finding that smaller particles are activated more easily is very important for their potential use as fluorescent labels, which should be as small as possible to not interfere with the function of the labeled object.”

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