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Photon avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000-fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano-bio-technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two-color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super-resolution imaging is realized for the first time in 3% Tm3+ doped LiYF4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials.

Photon avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000‐fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano‐bio‐technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two‐color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super‐resolution imaging is realized for the first time in 3% Tm3+ doped LiYF4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials. [ABSTRACT FROM AUTHOR]

Colloidal, oil-core polymer-shell nanocapsules (NCs), loaded with both upconversion NaYF4:Yb,Er nanoparticles (NPs) and CdSe quantum dots (QDs), were prepared via nanoemulsification/solvent-evaporation methods. Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) techniques were used to comprehensively determine the structure, phase composition, and morphology of the NCs. Furthermore, we demonstrated that by simultaneously loading the hybrid fluorophores into the multimodal NCs, it is possible to combine their mechanistically distinct one- and two-photon luminescence properties. The two-photon absorption cross-section obtained at 850 nm for a single capsule was equal to 2.5 × 104 GM (Goeppert-Mayer units). Importantly, cytotoxicity studies indicated the potential of the co-encapsulated NPs and QDs for fluorescence imaging in biological specimens. We conclude that such non-toxic water-dispersible multifunctional nanohybrids represent a promising platform for advanced one- and two-photon induced fluorescence bioimaging and photonic sensing technologies. [ABSTRACT FROM AUTHOR]

Lanthanide‐doped nanoparticles (NPs) exhibit temperature‐dependent luminescence, enabling the design of luminescent nanothermometers for industrial and medical applications. This research demonstrates the temperature‐sensing properties of NaYF4:7.5%Er3+@NaYF4 and NaErF4@NaYF4 NPs, which have a hexagonal shape and average size of 17 nm. Their core@shell structure is confirmed using high‐resolution transmission electron microscopy, and they exhibit intense upconversion (UC) emission under 1532 nm excitation in H2O and D2O colloids. The recorded spectra show Er3+ emission bands with varying intensity ratios depending on the Er3+ concentration, chosen solvent, and temperature. The spectroscopic properties of the studied NPs allow for their excitation and observation of emission within biological windows, which makes them useful for bio‐related applications. The emission of prepared NPs is analyzed as a function of temperature from 298 up to 358/363 K in H2O and D2O. The ratios for thermally‐coupled levels  and non‐TCLs and their relative sensitivities are studied. For the high dopant concentration sample in water, the O─H vibrations and blue shift in the absorption spectrum lead to a record relative sensitivity of 2.50% K−1 (at 363 K) for the 2H11/2/4I11/2 ratio. The use of synthesized NPs for bioimaging under 1550 nm excitation is also demonstrated to observe their accumulation in the guts of Daphnia magna. [ABSTRACT FROM AUTHOR]