Effect of Excess Ligand on the Reverse Microemulsion Silica Coating of NaLnF₄ Nanoparticles

Silica coating of inorganic nanoparticles (NPs) is widely employed as a means of providing colloidal stability in aqueous media and surface functionality for a variety of applications, particularly in biology. When the NPs are synthesized with a surface coating of an organic surfactant like oleic acid, silica coating is performed by using the reverse microemulsion method. There are many reports in the literature of the successful application of this method to NaYF₄ upconversion NPs (doped with Yb and Er), and we have used this method to coat NaHoF₄ NPs designed as a mass cytometry reagent. This method failed when we attempted to apply it to other NaLnF₄ NPs (Ln = Sm, Eu, Tb). In this report we describe an investigation of the problem and show how it can be overcome. To control size in the synthesis of NaLnF₄ NPs and at the same time maintain size uniformity, it is necessary to adjust the Na/F and F/Ln ratios. Problems with silica coating are associated with substoichiometric F/Ln ratios (F/Ln < 4) that leave Ln oleate salts as a byproduct, often as a phase-separated oily layer that could not be purified from the NPs by precipitation with ethanol and redispersion in hexanes. The nature of the oily byproduct was inferred from a combination of TGA, NMR, and FTIR measurements. We explored five different additional purification procedures, and by adopting the appropriate purification method, NaLnF₄ NPs with a variety of compositions and synthesized using different reaction conditions could be coated with a thin shell of silica.