Design of ultra-strong and ultra-pure red upconversion materials for biomedical applications

(Nanowerk Spotlight) A large part of low-energy photons, such as in the deep-red and infrared, are lost during conventional photovoltaic or photochemical processes, i.e. the light that a luminescent particle emits is usually much less energetic than the light that it absorbs. However, about half of all the solar energy reaching the Earth's surface can be found in these longer wavelengths.

In order to harvest this light more efficiently, scientists make use of a process called photon energy upconversion (UC) where two or more weak captured photons are converted into a single strong, energy-rich photon (see for instance: "Upconverting synthetic leaf takes it cues from nature"). The resulting upconversion materials are photoluminescent materials which can emit high energy light by employing relatively low energy light as an excitation source.

Since UC luminesce materials are promising for widespread application ranging from optical devices to biodetection and cancer therapy, the near-infrared light excited UC materials are attracting much research attention.

"In general, different applications have their specific requirements on the size and morphology of UC materials," Luyi Sun, an Associate Professor in the Institute of Materials Science at the University of Connecticut, explains to Nanowerk. "For example, micron or submicron-sized UC materials with a uniform morphology could meet the requirements for traditional lighting and displaying devices; while mono-dispersed nanoparticles are needed for applications in some specific miniature/precise optical devices, and biological and medical related fields. Therefore, controllable synthesis of UC nano/micro-crystals with desirable dimension and morphology is of practical significance and has attracted much attention in recent years."

Most biological tissues show minimum absorption of light in the range of 650-1350 nm (this wavelength range is called 'optical window'). UC nanomaterials with both excitations and emissions in this 'optical window' are particularly required for high-contrast bio-imaging and bio-labeling applications, as they can reduce the auto fluorescence and light scattering from cells or organs.

For this reason, researchers in this field have been trying to realize highly efficient and biocompatible near-infrared excited UC nanomaterials with highly pure red or near-infrared emissions. So far, though, current related researches suffer either from low UC efficiency or high stray light ratio.

In the latest work, Sun and his co-workers achieved an ultra-strong and ultra-pure red UC in erbium and ytterbium co-doped lutetium oxyfluorides through size and morphology control. The team reported their findings in the April 22, 2015 online edition of Nanoscale ("Size/Morphology Induced Tunable Luminescence in Upconversion Crystals: Ultra-Strong Single-Band Emission and Underlying Mechanisms"). This work was completed in collaboration with Dr. Huidan Zeng's group in East China University of Science and Technology, and Dr. Yuhua Wang's group in Lanzhou University.

(a) Comparison of UC spectra of Lu 5 O 4 F 7 : Er 3+ ,Yb 3+ nanoparticles, nanorods, hexagon-1, and hexagrams with those of β-NaYF 4 : Er 3+ , Yb 3+ micro-crystals, irradiated at the 980 nm laser with a pump power of 920 mW; (b) Pump power-dependent UC emission spectra of Lu 5 O 4 F 7 : Er 3+ , Yb 3+ nanoparticles; (c) Comparison of the integrated red emission intensities of Lu 5 O 4 F 7 : Er 3+ , Yb 3+ nanoparticles to the overall UC intensities of β-NaYF 4 : Er 3+ , Yb 3+ , as a function of pump power. (Reprinted with permission by Royal Society of Chemistry) (click on image to enlarge)

Rare earth doped fluorides are the well-known UC materials which show much higher UC efficiency than that of oxides. However, the UC properties of fluorides are sensitive to oxygen surface contamination and exhibit less favorable chemical, thermal, and mechanical properties than those of oxides.

"Since lanthanide oxyfluorides could combine the advantages of both fluorides and oxides, they are suggested to be novel upconversion materials with efficient luminescence and excellent physical properties," notes Sun.

In addition to the UC efficiency derived from matrices, some stray emission lights – those not in the optical window – always exist in upconversion processes, which requires them to be eliminated by further structural design.

"Controllable synthesis of UC crystals with desirable dimension and morphology is of both theoretical and practical significance," Sun points out. "By selecting an appropriate UC host, and further adjusting the size and morphology, we realized the complete elimination of stray emission lights in Lu 5 O 4 F 7 : Er 3+ , Yb 3+ nanoparticles with size of ca. 20 nm. The ultra-pure red UC of the nanomaterial is ten times stronger than that of β-NaYF 4 : Er 3+ , Yb 3+ , the well accepted highly efficient UC luminescent materials. This kind of UC nanomaterial has long been sought by the researchers in this field."

The researchers explain that the mechanisms behind the prominent UC features can be attributed to the phonon energy distribution of lutetium oxyfluorides as well as the size/morphology effect.

"The UC process is usually assisted by phonons in the matrices and the ultra-strong emission intensity is ascribed to the high phonon density at 850 cm -1 ," Dr. Zhaofeng Wang, the lead author of this work, explains. "The decrease of the size can deplete the energy on intermediate levels, and therefore eliminate the stray emission lights. Meanwhile, when the size is reduced to ∼20 nm, the generated regulation of phonon modes can further increase the phonon ratio at 850 cm -1 , which is the key to maintain the ultra-high emission intensity."

He adds that the team also performed cell viability tests and confirmed the biocompatibility of these nanoparticles.