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

Abstract

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.

Revisiting carboxylic group functionalization of silica sol–gel materials

“…37 Regarding the silica coating in the reverse emulsion method, it has been pointed out recently that unreacted Ln-oleate molecules act as secondary nucleation sites for SiO 2 nucleation interfering with the homogeneous deposition of SiO 2 uniform shells. 38 The design of multi-purpose luminescent and magnetic lanthanide(III) chelate labels relies on a variety of coordinating groups where carboxylate, phosphonate and amine moieties are among the most used ligands.…”

“…37 Regarding the silica coating in the reverse emulsion method, it has been pointed out recently that unreacted Ln-oleate molecules act as secondary nucleation sites for SiO 2 nucleation interfering with the homogeneous deposition of SiO 2 uniform shells. 38…”

“…Carboxyethylsilanetriol (CES) is by far one of the most used of the commercial silanes, exposing an acetic carboxylic group when anchored to SiO 2 and oxide surfaces and typically supplied as a salt and in 25% w/w aqueous solution, limiting its application in organic solvents. 18–48 Earlier reports of industrial production of this silane indicate a method based on the base-catalyzed addition of Cl 3 SiH to acrylonitrile in the presence of triethylamine. 49 The obtained cyano-derivatized chlorosilane is hydrolyzed exhaustively in a later step resulting in the salt of the carboxylated silane as presented in Scheme 1.…”

Visible–near-Infrared Light-Driven Photocatalytic Characteristics of Er3+/Yb3+-Codoped BiOBr Upconverting Microparticles for Tetracycline Degradation

“…As a proof of the above deduction, Er 3+ was selected as the activator on account of its emissions originating from intra-4f transitions, while Yb 3+ was used as sensitizer since it can not only absorb the NIR light efficiently but also transfer the energy to Er 3+ . − Moreover, BiOBr played as host and Er 3+ /Yb 3+ -codoped BiOBr microparticles were prepared with the aid of a high-temperature solid-state reaction method. The electronic structure of BiOBr impacted by Yb 3+ doping was explored on the basis of first-principles density functional theory (DFT).…”