Lanthanide-based Upconversion Research Articles

Biodistribution of sub-10 nm PEG-modified radioactive/upconversion nanoparticles

The biodistribution of lanthanide-based upconversion nanophosphors (UCNPs) has attracted increasing attention, and all of the reported UCNPs display metabolism in the liver and spleen mainly. Herein, ∼8 nm poly(ethylene glycol) (PEG)-coated NaYF4 nanoparticles codoped with Yb3+, Er3+, and (or) radioactive 153Sm3+ ions were synthesized, through a hydrothermal synthetic system assisted by binary cooperative ligands with oleic acid and PEG dicarboxylic acids. The as-prepared PEG-coating NaYF4:Yb,Er and NaYF4:Yb,Er,153Sm are denoted as PEG-UCNPs and PEG-UCNPs(153Sm), respectively. PEG-UCNPs were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) analysis, and Fourier-transform infrared (FTIR) spectroscopy. The PEG-UCNPs showed excellent water solubility with a hydrodynamic diameter of ∼10 nm and displayed upconversion luminescence (UCL) under continuous-wave excitation at 980 nm. At the same time, the 153Sm-doped nanoparticles PEG-UCNPs(153Sm) displayed radioactivity, and time-dependent biodistribution of PEG-UCNPs(153Sm) was investigated, through single-photon emission computed tomography (SPECT) imaging and γ-counter analysis. Interestingly, PEG-UCNPs(153Sm) had a long blood retention time and were partly eliminated through urinary pathways in vivo. Therefore, the concept of fabricating PEG-coated, small nanosize (sub-10 nm) nanoparticles with radioactive property is a useful strategy for providing a potential method to monitor lanthanide nanoparticles renal clearable.

Micro-optical design of photochemical upconverters for thin-film solar cells

All presently available types of solar cells transmit light with energies below their band gaps, foregoing energy. An elegant way toward overcoming these subbandgap losses and using a larger fraction of the incident light is the re‐shaping of the solar spectrum by upconversion (UC) of photons. Recently, first results on solar cells augmented by either lanthanide-based UC or triplet-triplet-annihilation UC in organic chromophores were presented. Both of these UC strategies are characterized by a nonlinear response on the illumination density under conditions relevant to solar energy conversion, opening a route for increasing the UC yield by concentrating the light. While operation of the whole cell under concentrated sunlight is in most cases undesirable, application of micro-optical focusing of the transmitted light in the upconverting layer is a promising strategy. In the present work, a more than two-fold enhancement of the current gain by UC behind an amorphous silicon solar cell through optimization of the upconverter optical design is demonstrated, including employing a focusing microstructured back reflector. The experimental data is rationalized using a simple ray tracing modeling approach, highlighting a further enhancement potential of a microstructured UC unit.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Highly-sensitive multiplexed in vivo imaging using pegylated upconversion nanoparticles

Lanthanide-based upconversion nanoparticles (UCNPs) have been widely explored in various fields, including optical imaging, in recent years. Although earlier work has shown that UCNPs with different lanthanide (Ln3+) dopants exhibit various colors, multicolor-especially in vivo multiplexed biomedical imaging-using UCNPs has rarely been reported. In this work, we synthesize a series of UCNPs with different emission colors and functionalize them with an amphiphilic polymer to confer water solubility. Multicolor in vivo upconversion luminescence (UCL) imaging is demonstrated by imaging subcutaneously injected UCNPs and applied in multiplexed in vivo lymph node mapping. We also use UCNPs for multicolor cancer cell labeling and realize in vivo cell tracking by UCL imaging. Moreover, for the first time we compare the in vivo imaging sensitivity of quantum dot (QD)-based fluorescence imaging and UCNP-based UCL imaging side by side, and find the in vivo detection limit of UCNPs to be at least one order of magnitude lower than that of QDs in our current non-optimized imaging system. Our data suggest that, by virtue of their unique optical properties, UCNPs have great potential for use in highly-sensitive multiplexed biomedical imaging.