Relaxation Processes in Rare-Earth-Doped α-NaYF4 Nanoparticles by Nuclear Magnetic Resonance Spectroscopy

NaYF4 nanoparticles (NPs) are among the most promising hosts for lanthanides in different applications as IR-to-UV upconversion and photodynamic therapy due to their crystal electric field properties and their low phonon energy. The efficiency in such applications depends on the dispersion of the ions and their local structure in the host. Paramagnetic nuclear magnetic resonance (NMR) has been one of the main tools to perform such studies, based on the comprehensiveness of the hyperfine interaction of the rare-earth (RE)'s unpaired electrons with the observed nuclei and its effects in NMR quantities, like paramagnetic shift, line broadening, and relaxation enhancement. However, it can be challenging to resolve these effects for spin systems with large dipolar or quadrupolar interactions and site heterogeneity. We hereby address these issues for RE3+-doped α-NaYF4 NPs of small sizes and low polydispersity [ϕ ≈ 7(1) nm] with 23Na and 19F NMR employing high-resolution techniques associated with relaxation measurements. In particular, remarkable fast relaxation rates are found for the Gd3+-doped NPs which are rationalized in terms of Solomon's relaxation mechanism enhanced by the long electronic relaxation time of the ion.