Lanthanide-based upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing and imaging. One of the best studied materials are hexagonal 𝛽-NaYF4 UCNPs doped with 20 % Yb3+ and 2 % Er3+, which efficiently convert 976 nm light to photons emitted at 540 nm, 655 nm, and 845 nm, respectively, reveal long luminescence lifetimes (> 100 μs), and are very photostable and chemically inert.[1,2] The properties of their upconversion (UC) luminescence (UCL) are, however, strongly influenced by particle size, concentration and spatial arrangement of dopant ions, surface chemistry, and microenvironment.[3,4] In addition, the multiphotonic absorption processes responsible for UCL render UCL dependent on excitation power density (𝑃).
The rational design of brighter UCNPs particle architectures encouraged us to assess systematically the influence of these parameters on UCL for differently doped UCNPs relying on the commonly used -NaYF4 matrix using steady state and time resolved fluorometry as well as integrating sphere spectroscopy for P varied over almost three orders of magnitude. This includes comprehensive studies of the influence of size and shell, Yb3+ and Er3+ dopand concentrations, and energy transfer processes from UCNPs to surface-bound organic dyes or vice versa . Our results underline the need for really quantitative luminescence studies for mechanistic insights, the potential of high P to compensate for UCL surface quenching, and the matrix- and P-dependence of the optimum dopand concentration.
Key words: upconverting nanoparticles, size, FRET, fluorescence, absolute fluorescence quantum yield, fluorescence decay kinetics, power density dependence
References.  Haase, M.; Schafer, H., Angew. Chem. Int. Ed. 2011, 50, 5808-5829.  Liu, G. K., Chem. Soc. Rev. 2015, 44, 1635-1652.  Würth, C.; Kaiser, M.; Wilhelm, S.; Grauel, B.; Hirsch, T., Resch-Genger, U.; Nanoscale 2017, 9, 4283-4294.  Kaiser, M.; Würth, C.; Hyppanen I.; Soukka T;, Resch-Genger, Nanoscale 2017, on line (10.1039/C7NR02449E).  Kraft, M.; Würth, C.; Muhr, V.; Hirsch, T.; Resch-Genger, U., Nanoscale, submitted.
Ute Resch-Genger and Tanja Knop, "Effect of particle architecture, dopant concentration, and excitation power density on the luminescence efficiency of upconversion nanocrystals (Conference Presentation)," Proc. SPIE 10507, Colloidal Nanoparticles for Biomedical Applications XIII, 1050702 (Presented at SPIE BiOS: January 27, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2291605.5752187211001.
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