Upconversion Nanocrystals with High Lanthanide Content: Luminescence Loss by Energy Migration versus Luminescence Enhancement by Increased NIR Absorption

Abstract Lanthanide‐doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background‐free luminescence. The progress in synthesis nowadays enables access to complex core‐shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core‐only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near‐infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF 4 (20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF 4 shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb 3+ and 2% Er 3+ . The impact of size, composition, and core‐shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.