An artist’s rendering shows the layers of a new, onion-like nanoparticle whose specially crafted layers enable it to efficiently convert invisible near-infrared light to higher energy blue and UV light. Credit: Kaiheng Wei (Davidwei_loga@foxmail.com)
A dye-coated surface is 1 of 3 specially crafted layers that help the particle emit light ideal for bioimaging, solar energy harvesting and light-based security techniques. The particle’s innovation lies in its layers: a coating of organic dye, a neodymium-containing shell, and a core that incorporates ytterbium and thulium. Together, these strata convert invisible near-infrared light to higher energy blue and UV light with record-high efficiency, a trick that could improve the performance of technologies ranging from deep-tissue imaging and light-induced therapy to security inks used for printing money.
When it comes to bioimaging, near-infrared light could be used to activate the light-emitting nanoparticles deep inside the body, providing high-contrast images of areas of interest. In the realm of security, nanoparticle-infused inks could be incorporated into currency designs; such ink would be invisible to the naked eye, but glow blue when hit by a low-energy laser pulse – a trait very difficult for counterfeiters to reproduce.
“Our particle is about 100 times more efficient at ‘upconverting’ light than similar nanoparticles created in the past, making it much more practical,” says Jossana Damasco, a UB chemistry PhD.
Converting low-energy light to light of higher energies isn’t easy to do. The process involves capturing two or more “photons” from a low-energy light source, and combining their energy to form a single, higher-energy photon. The onionesque nanoparticle performs this task beautifully. Each of its 3 layers fulfills a unique function:
The outer layer is a coating of organic dye. This dye is adept at absorbing photons from low-energy near-infrared light sources. It acts as an “antenna” for the nanoparticle, harvesting light and transferring energy inside. The next layer is a neodymium-containing shell. This layer acts as a bridge, transferring energy from the dye to the particle’s light-emitting core. Inside the light-emitting core, ytterbium and thulium ions work in concert. The ytterbium ions draw energy into the core and pass the energy on to the thulium ions, which have special properties that enable them to absorb the energy of three, 4 or 5 photons at once, and then emit a single higher-energy photon of blue and UV light.
So why not just use the core? Why add the dye and neodymium layer at all? The core itself is inefficient in absorbing photons from the outside world. That’s where the dye comes in. Neodymium – whose excited state is in between that of the dye and thulium’s act as a bridge between the two, creating a “staircase” for the energy to travel down to reach emitting thulium ions. http://www.buffalo.edu/news/releases/2015/11/017.html
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