A concept for the growth of silica shells with a thickness of 5-250 nm onto oleate-coated NaYF4:Yb3+/Er3+ upconversion nanoparticles (UCNP) is presented that enables precise adjustment of shell thicknesses for the preparation of thick-shelled nanoparticles for applications in plasmonics and sensing. This concept comprises the growth of an initial 5-11 nm thick shell onto the UCNP in a reverse microemulsion. This is followed by a stepwise growth of these particles without a purification step, where in each step equal volumes of tetraethyl orthosilicate and ammonia water are added, while the volumes of cyclohexane and the surfactant Igepal CO-520 are increased so that the ammonia and surfactant concentrations remain constant. Hence, the number of micelles stays constant, and their size is increased to accommodate the growing core-shell particles. Consequently, the formation of core-free silica particles is suppressed. When the negative zeta potential of the particles, which continuously decreased during the stepwise growth, falls below -40 mV, the particles can be dispersed in an ammoniacal ethanol solution and grown further by the continuous addition of tetraethyl orthosilicate to a diameter larger than 500 nm. Due to the high colloidal stability, a coalescence of the particles could be suppressed, and single-core particles are obtained. This strategy can be easily transferred to other nanomaterials for the design of plasmonic nanoconstructs and sensor systems.
Keywords: upconversion nanoparticles; silica coating; reverse microemulsion; stepwise growth; thick shells
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When a peer-reviewed version of this preprint is available in the Beilstein Journals, this information will be updated in the information box above. If no peer-reviewed version is available, please cite this preprint using the following information:
Kembuan, C.; Saleh, M.; Rühle, B.; Resch-Genger, U.; Graf, C. Beilstein Arch. 2019, 201962. doi:10.3762/bxiv.2019.62.v1
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