Go with the glow: X-rayed nano onions

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  • Published: Nov 15, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Go with the glow: X-rayed nano onions

It's the dope

An artist’s rendition of the onion-like layers within specially crafted nanoparticles that can convert near-infrared to higher energy blue and ultraviolet light. Credit: Kaiheng Wei

Lanthanide-doped nanoparticles hold much promise in bioimaging, in solar energy conversion and for volumetric displays. Now, a new approach to making their emissions brighter and broadening their bandwidth by layering them up has been studied with X-ray diffraction, XRD, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP-MS).

The new 50-nanometre particles were created by a team in the Institute for Lasers, Photonics and Biophotonics, at the University at Buffalo, New York, USA and can convert near-infrared light to higher energy blue and ultraviolet light very efficiency. The team made their new nanoparticles with several layers: a coating of organic dye, a neodymium-containing shell, and a core that incorporates ytterbium and thulium. Their improved efficiency for energy conversion over other particles might allow them to be exploited in deep-tissue imaging and light-induced therapy and perhaps as novel security inks used for money that would be very difficult to copy.

"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," explains chemist and team member Guanying Chen. Tymish Ohulchanskyy adds that, "It opens up multiple possibilities for the future."

Upconversion

The new nanoparticles offer one hundred times the efficiency of other materials in "upconverting" light, according to graduate researcher Jossana Damasco. Writing in a recent issue of the journal Nano Letters, the team included scientists at the Harbin Institute of Technology, China, the Royal Institute of Technology in Sweden, Tomsk State University in Russia and the University of Massachusetts Medical School in Boston.

Senior author Paras Prasad of the ILPB points out that converting low-energy light to light of higher energies is no easy task. The process usually involves trapping two or more lower-energy photons, merging their energies and re-emitting them as a higher-energy photon. The three onion-like layers of the nanoparticles take care of this process. First, the outermost organic dye layer absorbs low-energy photons in the near-infrared part of the electromagnetic spectrum, in other words it acts as an "antenna", harvesting light and transferring its energy to the next layer in.

Bridging the gap

The next layer being the neodymium-containing shell, which acts as a bridge between outer antenna and the light-emitting core, the 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 the capacity to absorb the energy of three, four or five photons at the same time and to then emit a single higher-energy photon in the blue or even ultraviolet region.

The team points out that, superficially one might imagine that the core alone would be sufficient, but with the outer dye and the neodymium bridge, energy absorption and thence conversion is so much greater. 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, the neodymium bridges this gap by acting as an energy shuttle between states and so exploits what the team refers to as a multistep cascade energy transfer. This energy-cascaded upconversion works with different infrared dyes and various lanthanide emitters, the team says, as well as with different fluoride host lattices.

"These features enable unprecedented three-photon upconversion (visible by naked eye as blue light) of an incoherent infrared light excitation with a power density comparable to that of solar irradiation at the Earth surface, having implications for broad applications of these organic − inorganic core/shell nanostructures with energy-cascaded upconversion," the researchers conclude.

Related Links

Nano Lett 2015, 15, 7400-7407: "Energy-Cascaded Upconversion in an Organic Dye-Sensitized Core/Shell Fluoride Nanocrystal"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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