A new type of nanoparticle has been created by that converts invisible near-infrared light to higher energy blue and UV light with record-high efficiency. The multi-layered layered nanoparticle has potential for use in solar energy harvesting, bio-imaging, and light-based security techniques.

The feat of converting low-energy electromagnetic energy to light of higher energy isn't one that is simple to achieve. Involving multi-step cascade energy transformation, the technique requires the capturing of two or more photons from a low-power light source and then combining their energies to form a single, higher-energy photon.

In the new nanoparticle, this low-to-high energy transformation is achieved by transferring photoexcitation energy from an outer layer of infrared-harvesting organic dyes (similar to those used in dye-sensitized solar cells) to a shell of neodymium where sensitizer ions transfer energy to upconverting ion pairs in the core (consisting largely of ytterbium and thulium) at a quantum conversion rate of around 19 percent.

"Our particle is about 100 times more efficient at 'upconverting' light than similar nanoparticles created in the past, making it much more practical," says UB chemistry PhD student, Jossana Damasco.

In detail, when molecules or ions in the layer of organic dyes absorbs a photon, they go into an excited state from which they can move captured energy to other molecules or ions. Transferring this energy to the ytterbium and thulium core, however, is not directly possible due to the large energy level gap between dyes and the materials in the core. To achieve this, neodymium is incorporated in the outer shell as it has an energy level somewhere between that of the outer and inner layers. This, then, provides a "staircase" up which the energies may travel from the outer shell to the inner core.

A transmission electron microscopy image of the new nanoparticles(Credit: Institute for Lasers, Photonics and Biophotonics, 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.

When it comes to suggested uses, the researchers believe that the nanoparticles could be used in bio-imaging, where near-infrared light could be shone on the light-emitting nanoparticles surgically-implanted or ingested in the body, so that they glowed brightly to provide a light source for high-contrast medical imaging. The scientists also believe that inks containing the new nanoparticles could be used to secure printed currency by impressing invisible markings into the notes that are invisible, but glow blue when illuminated by a low-energy laser pulse, making such markings extremely difficult for counterfeiters to copy.

The project was led by the Institute for Lasers, Photonics, and Biophotonics at the University of Buffalo, and the Harbin Institute of Technology in China, with input from the Royal Institute of Technology in Sweden; Tomsk State University in Russia, and the University of Massachusetts Medical School. The findings were recently published in the journal Nano Letters.

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