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Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infra-red excitation into visible and ultraviolet emission. Their unique optical properties have advanced a broad range of applications, such as fluorescent microscopy, deep-tissue bioimaging, nanomedicine, optogenetics, security labelling and volumetric display. However, the constraint of concentration quenching on upconversion luminescence has hampered the nanoscience community to develop bright UCNPs with a large number of dopants. This review surveys recent advances in developing highly doped UCNPs, highlights the strategies that bypass the concentration quenching effect, and discusses new optical properties as well as emerging applications enabled by these nanoparticles.

Thermal quenching, in which light emission experiences a loss with increasing temperature, broadly limits luminescent efficiency at higher temperature in optical materials, such as lighting phosphors1–3 and fluorescent probes4–6. Thermal quenching is commonly caused by the increased activity of phonons that leverages the non-radiative relaxation pathways. Here, we report a kind of heat-favourable phonons existing at the surface of lanthanide-doped upconversion nanomaterials to combat thermal quenching. It favours energy transfer from sensitizers to activators to pump up the intermediate excited-state upconversion process. We identify that the oxygen moiety chelating Yb3+ ions, [Yb···O], is the key underpinning this enhancement. We demonstrate an approximately 2,000-fold enhancement in blue emission for 9.7 nm Yb3+-Tm3+ co-doped nanoparticles at 453 K. This strategy not only provides a powerful solution to illuminate the dark layer of ultra-small upconversion nanoparticles, but also suggests a new pathway to build high-efficiency upconversion systems.

Super-resolution optical microscopy based on stimulated emission depletion effects can now be performed at much lower light intensities than before by using bright upconversion emission from thulium-doped nanoparticles. Breaking the law Improvements in super-resolution optical microscopy based on stimulated emission depletion (STED) effects have a problem: they are typically limited by a 'square-root law' regarding the number of photons required to achieve a gain in resolution. Yujia Liu and colleagues have found a way to bypass this troublesome law. As others have done before them, they adopt lanthanide-doped upconversion nanoparticles as the emitting species used to achieve high-resolution imaging. The difference this time is that the laser-like absorption and emission properties of these nanoparticles are engineered to facilitate STED-like microscopy at much lower light intensities. Lanthanide-doped glasses and crystals are attractive for laser applications because the metastable energy levels of the trivalent lanthanide ions facilitate the establishment of population inversion and amplified stimulated emission at relatively low pump power 1 , 2 , 3 . At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be made with precisely controlled phase, dimension and doping level 4 , 5 . When excited in the near-infrared, these UCNPs emit stable, bright visible luminescence at a variety of selectable wavelengths 6 , 7 , 8 , 9 , with single-nanoparticle sensitivity 10 , 11 , 12 , 13 , which makes them suitable for advanced luminescence microscopy applications. Here we show that UCNPs doped with high concentrations of thulium ions (Tm 3+ ), excited at a wavelength of 980 nanometres, can readily establish a population inversion on their intermediate metastable 3 H 4 level: the reduced inter-emitter distance at high Tm 3+ doping concentration leads to intense cross-relaxation, inducing a photon-avalanche-like effect that rapidly populates the metastable 3 H 4 level, resulting in population inversion relative to the 3 H 6 ground level within a single nanoparticle. As a result, illumination by a laser at 808 nanometres, matching the upconversion band of the 3 H 4  →  3 H 6 transition, can trigger amplified stimulated emission to discharge the 3 H 4 intermediate level, so that the upconversion pathway to generate blue luminescence can be optically inhibited. We harness these properties to realize low-power super-resolution stimulated emission depletion (STED) microscopy and achieve nanometre-scale optical resolution (nanoscopy), imaging single UCNPs; the resolution is 28 nanometres, that is, 1/36th of the wavelength. These engineered nanocrystals offer saturation intensity two orders of magnitude lower than those of fluorescent probes currently employed in stimulated emission depletion microscopy, suggesting a new way of alleviating the square-root law that typically limits the resolution that can be practically achieved by such techniques.

Leveraging the resonant modes of all-dielectric metasurfaces, specifically quasi-bound state in the continuum and Mie resonances, the precise orthogonal polarization control has been realized.Leveraging the resonant modes of all-dielectric metasurfaces, specifically quasi-bound state in the continuum and Mie resonances, the precise orthogonal polarization control has been realized.

Shell coating is known to suppress luminescence quenching in spherical upconversion nanoparticles. However, the emergence of anisotropic nanoparticles with facet-selective, directional growth complicates the coating process, and the use of traditional active, inert, or polymer coatings on such structures remains largely unexplored. Here, we synthesize a series of nanorods with designed geometries, enabling quantitative spectral analysis at the single-particle level. We observe that directional growth of inert or active shells at the rod tips enhances emission relative to the parent core, with their relative effectiveness governed by power density and shell thickness. Ligand presence—polymer or oleate—quenches upconversion relative to bare nanorods. Although local heating is observed at the single-particle level, it does not affect spectroscopic observations, ligand stability, or data reproducibility. Our findings reveal how directionally grown shells influence the optical properties of upconversion nanorods, providing essential insights for their future applications in bioimaging, sensing, and photonics. Shell coatings reduce luminescence quenching in spherical nanoparticles for upconversion, but anisotropic nanoparticles are less well studied. Here, the authors investigate the effects of coatings on individual nanorods and show that their effectiveness depends on power density and shell types.

Cross-relaxation among neighboring emitters normally causes self-quenching and limits the brightness of luminescence. However, in nanomaterials, cross-relaxation could be well-controlled and employed for increasing the luminescence efficiency at specific wavelengths. Here we report that cross-relaxation can modulate both the brightness of single upconversion nanoparticles and the threshold to reach population inversion, and both are critical factors in producing the ultra-low threshold lasing emissions in a micro cavity laser. By homogenously coating a 5-μm cavity with a single layer of nanoparticles, we demonstrate that doping Tm 3+ ions at 2% can facilitate the electron accumulation at the intermediate state of 3 H 4 level and efficiently decrease the lasing threshold by more than one order of magnitude. As a result, we demonstrate up-converted lasing emissions with an ultralow threshold of continuous-wave excitation of ~150 W/cm 2 achieved at room temperature. A single nanoparticle can lase with a full width at half-maximum as narrow as ~0.45 nm. Cross-relaxation between neighbouring emitters usually limits the brightness of luminescence due to self-quenching. Here, the authors present single upconversion nanoparticle lasing where the cross relaxation modulates the brightness and ultra-low threshold lasing is achieved.

Inertial microfluidics has been broadly investigated, resulting in the development of various applications, mainly for particle or cell separation. Lateral migrations of these particles within a microchannel strictly depend on the channel design and its cross-section. Nonetheless, the fabrication of these microchannels is a continuous challenging issue for the microfluidic community, where the most studied channel cross-sections are limited to only rectangular and more recently trapezoidal microchannels. As a result, a huge amount of potential remains intact for other geometries with cross-sections difficult to fabricate with standard microfabrication techniques. In this study, by leveraging on benefits of additive manufacturing, we have proposed a new method for the fabrication of inertial microfluidic devices. In our proposed workflow, parts are first printed via a high-resolution DLP/SLA 3D printer and then bonded to a transparent PMMA sheet using a double-coated pressure-sensitive adhesive tape. Using this method, we have fabricated and tested a plethora of existing inertial microfluidic devices, whether in a single or multiplexed manner, such as straight, spiral, serpentine, curvilinear, and contraction-expansion arrays. Our characterizations using both particles and cells revealed that the produced chips could withstand a pressure up to 150 psi with minimum interference of the tape to the total functionality of the device and viability of cells. As a showcase of the versatility of our method, we have proposed a new spiral microchannel with right-angled triangular cross-section which is technically impossible to fabricate using the standard lithography. We are of the opinion that the method proposed in this study will open the door for more complex geometries with the bespoke passive internal flow. Furthermore, the proposed fabrication workflow can be adopted at the production level, enabling large-scale manufacturing of inertial microfluidic devices.

The intracellular metabolism of organelles, like lysosomes and mitochondria, is highly coordinated spatiotemporally and functionally. The activities of lysosomal enzymes significantly rely on the cytoplasmic temperature, and heat is constantly released by mitochondria as the byproduct of adenosine triphosphate (ATP) generation during active metabolism. Here, we developed temperature-sensitive LysoDots and MitoDots to monitor the in situ thermal dynamics of lysosomes and mitochondria. The design is based on upconversion nanoparticles (UCNPs) with high-density surface modifications to achieve the exceptionally high sensitivity of 2.7% K−1 and low uncertainty of 0.8 K for nanothermometry to be used in living cells. We show the measurement is independent of the ion concentrations and pH values. With Ca2+ ion shock, the temperatures of both lysosomes and mitochondria increased by ∼2 to 4°C. Intriguingly, with chloroquine (CQ) treatment, the lysosomal temperature was observed to decrease by up to ∼3 °C, while mitochondria remained relatively stable. Lastly, with oxidative phosphorylation inhibitor treatment, we observed an ∼3 to 7°C temperature increase and a thermal transition from mitochondria to lysosomes. These observations indicate different metabolic pathways and thermal transitions between lysosomes and mitochondria inside HeLa cells. The nanothermometry probes provide a powerful tool for multimodality functional imaging of subcellular organelles and interactions with high spatial, temporal, and thermal dynamics resolutions.

Phosphor-converted white-light-emitting diodes (pc-WLED) have been extensively employed as solid-state lighting sources, which have a very important role in people’s daily lives. However, due to the scarcity of the red component, it is difficult to realize warm white light efficiently. Hence, red-emitting phosphors are urgently required for improving the illumination quality. In this work, we develop a novel orangish-red La4GeO8:Bi3+ phosphor, the emission peak of which is located at 600 nm under near-ultraviolet (n-UV) light excitation. The full width at half maximum (fwhm) is 103 nm, the internal quantum efficiency (IQE) exceeds 88%, and the external quantum efficiency (EQE) is 69%. According to Rietveld refinement analysis and density functional theory (DFT) calculations, Bi3+ ions randomly occupy all La sites in orthorhombic La4GeO8. Importantly, the oxygen-vacancy-induced electronic localization around the Bi3+ ions is the main reason for the highly efficient orangish-red luminescence. These results provide a new perspective and insight from the local electron structure for designing inorganic phosphor materials that realize the unique luminescence performance of Bi3+ ions.LEDs: Painting the town redPhosphorescent substances that give white-light-emitting diodes a more natural reddish hue can be made by tuning the electron structure of their activator ions. Jun Lin of China’s Changchun Institute of Applied Chemistry, Chinese Academy of Sciences and colleagues combined germanium, lanthanum and bismuth oxides to manufacture a reddish phosphorescent substance that, when added to blue and green phosphors on an LED chip, produced more natural-looking light compared to conventional phosphor-converted white LEDs. LEDs are constantly being improved as an energy-efficient light source, but the colour of the light they emit can be unnaturally white or blue. Fabricating high quality red-emitting phosphors to change this has been a challenge. The researchers developed a substance with a unique reddish photoluminescence caused by electrons localizing around bismuth ions. Their approach offers a new perspective for exploring luminescence in inorganic materials.