Surface engineering of small and bright upconversion nanoparticles providing chemical and colloidal stability in biological media

One of the main challenges for any applications of nanoparticles in biological media is to control the surface chemistry of the nanomaterials preventing chemical disintegration and agglomeration. When the surface effects dominate over the bulk properties, which is especially the case for small lanthanide doped NaYF4 nanocrystals, the surface capping significantly affects the brightness of the upconversion luminescence. Here, we present the influence of commonly used buffer systems such as N-(2-hydroxyethyl)piperazine-N’-(2-ethanesulfonic acid) (HEPES) and 2-(N morpholino)ethanesulfonic acid hydrate (MES) on the stability and surface chemistry of NaYF4:Yb,Er nanoparticles. The results indicate that surface modifications by ligand exchange provide a simple strategy for attaching many different ligands to the particle surface and render them water dispersible. Nevertheless, one has to take into account that particle surfaces are not fully covered and certain buffers, especially those with sulfo groups, may alter the surface chemistry with time.