Abstract
We present a systematic study on the effect of surface ligands on the luminescence properties and colloidal stability of beta-NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs), comparing nine different surface coatings to render these UCNPs water-dispersible and bioconjugatable. A prerequisite for this study was a large-scale synthetic method that yields similar to 2 g per batch of monodisperse ...
Abstract
We present a systematic study on the effect of surface ligands on the luminescence properties and colloidal stability of beta-NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs), comparing nine different surface coatings to render these UCNPs water-dispersible and bioconjugatable. A prerequisite for this study was a large-scale synthetic method that yields similar to 2 g per batch of monodisperse oleate-capped UCNPs providing identical core particles. These similar to 23 nm sized UCNPs display an upconversion quantum yield of similar to 0.35% when dispersed in cyclohexane and excited with a power density of 150 W cm(-2), underlining their high quality. A comparison of the colloidal stability and luminescence properties of these UCNPs, subsequently surface modified with ligand exchange or encapsulation protocols, revealed that the ratio of the green (545 nm) and red (658 nm) emission bands determined at a constant excitation power density clearly depends on the surface chemistry. Modifications relying on the deposition of additional (amphiphilic) layer coatings, where the initial oleate coating is retained, show reduced non-radiative quenching by water as compared to UCNPs that are rendered water-dispersible via ligand exchange. Moreover, we could demonstrate that the brightness of the upconversion luminescence of the UCNPs is strongly affected by the type of surface modification, i.e., ligand exchange or encapsulation, yet hardly by the chemical nature of the ligand.
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