Nanoparticles tune-up

Nanotechnology holds enormous potential in medicine, with particles a few tens of nanometres across carrying out imaging, diagnostics and drug delivery. Ideal candidate materials are crystals doped with lanthanides, one of fifteen elemental rare-earth metals, which have low toxicity and, crucially, can perform a neat optical trick called photon up-conversion.

Lanthanides have a long luminescence lifetime, which means that, after exposure to light, they relax by emission of light over a long period of time. After the metal absorbs low energy near-infra red light, a higher energy visible light is emitted. However, only three lanthanide ions, Er3⁺, Tm3⁺ and Ho3⁺, were suitable for this photon up-conversion owing to ladder-like arranged energy levels in their structure.

Now, a novel design of nanoparticles by chemists Feng Wang and Xiaogang Liu of the National University of Singapore and co-workers in China and Saudi Arabia demonstrate up-conversion in several lanthanide-doped nanoparticles and covering most of the visible light range rather than the narrow ranges that worked before.

The breakthrough came when they incorporated an ion sub-lattice of the lanthanide gadolinium into the core of the nanoparticle structure. This eliminated deleterious effects that result from undesirable energy transfer between different energy states of two or more neighbouring lanthanide dopant ions, which suppresses the desired luminescence. This enabled the researchers to fine tune the light emitted.

Team members Yu Han and Qingxiao Wang of the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, imaged the nanocrystals at atomic resolution using scanning transmission electron microscopy, which helped clarify how energy migrates between gadolinium atoms occupied in the crystal lattice.

"Up-conversion tuning by modulating relative emission intensities has been well-studied, but our core-shell design and use of gadolinium for energy migration enables the emission wavelength to be tuned," says Liu. "High sensitivity multiplexed assays are expected by using these nanoparticles."

Liu and co-workers have filed a patent on their work through the US patent office, which they claim will open a new class of luminescent materials capable of tuning emissions of photons. Such technology could also boost the performance of solar cells and optical data storage by raising sensitivity to near-infra red light.

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