Onion-like layers help this efficient new nanoparticle glow

November 10, 2015, University at Buffalo
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

A new, onion-like nanoparticle could open new frontiers in biomaging, solar energy harvesting and light-based security techniques.

The particle's innovation lies in its layers: a coating of , a neodymium-containing shell, and a core that incorporates ytterbium and thulium. Together, these strata convert invisible to higher blue and UV 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.

"It opens up multiple possibilities for the future," says Tymish Ohulchanskyy, deputy director of photomedicine and research associate professor at the Institute for Lasers, Photonics, and Biophotonics (ILPB) at the University at Buffalo.

"By creating special layers that help transfer energy efficiently from the surface of the particle to the core, which emits blue and UV light, our design helps overcome some of the long-standing obstacles that previous technologies faced," says Guanying Chen, professor of chemistry at Harbin Institute of Technology and ILPB research associate professor.

"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 student who played a key role in the project.

A transmission electron microscopy image of the new nanoparticles, which converts invisible near-infrared light to higher energy blue and UV light with high efficiency. Each particle is about 50 nanometers in diameter. Credit: Institute for Lasers, Photonics and Biophotonics, University at Buffalo

The research was published online in Nano Letters on Oct. 21 and led by the Institute for Lasers, Photonics, and Biophotonics at UB, and the Harbin Institute of Technology in China, with contributions from the Royal Institute of Technology in Sweden; Tomsk State University in Russia; and the University of Massachusetts Medical School.

The study's senior author was Paras Prasad, ILPB executive director and SUNY Distinguished Professor in chemistry, physics, medicine and electrical engineering at UB.

Peeling back the layers

Converting low-energy light to light of higher energies isn't easy to do. The process involves capturing two or more tiny packets of light called "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 three layers fulfills a unique function:

  • The outermost layer is a coating of organic . 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, Ohulchanskyy says.
  • 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, four or five 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?

As Ohulchanskyy and Chen explain, the core itself is inefficient in absorbing photons from the outside world. That's where the dye comes in.

Once you add the dye, the neodymium-containing layer is necessary for transferring energy efficiently from dye to core. Ohulchanskyy uses the analogy of a staircase to explain why this is: When molecules or ions in a material absorb a photon, they enter an "excited" state from which they can transfer energy to other molecules or ions. The most efficient transfer occurs between molecules or ions whose excited states require a similar amount of energy to obtain, but the dye and ytterbium ions have excited states with very different energies. So the team added neodymium—whose excited state is in between that of the dye and thulium's—to act as a bridge between the two, creating a "staircase" for the energy to travel down to reach emitting thulium ions.

Explore further: Nanoparticles with a core-shell structure can minimize the overheating of cells during bioimaging experiments

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not rated yet Nov 10, 2015
the ingredients are too expensive to use on photovoltaic panels
not rated yet Nov 18, 2015
the ingredients are too expensive to use on photovoltaic panels

Looking at the picture, if the particles are 50 nm then the expensive (non-dye) layers look like ~40 nm spheres which would average ~20 nm thick, but a 50 nm spacing reduces that to ~13 nm, so that's ~1/75 micron, or 1/75 cm3/m2 and those materials average ~7.5g/cm3, or 0.1 grams per square meter per layer of spheres.

At ~~10,000/kg thulium is more than 10x the price of the others, but even if it were ALL thulium that would be ~~$1 of thulium per square meter per layer of spheres.

A square meter of a good panel (20%) generates 200 W(peak), so if this results in even a 1-point boost to 21% that's +10W/m2 and the cost of the boost is only ten cents per Watts.

So if it takes MANY layers, then it might well be too expensive, but if it take one layer then is is probably VERY affordable.

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