Sodium Hexametaphosphate: An Inexpensive Inorganic Polyelectrolyte “Primer” For Upconverting Nanoparticles/Mesoporous Silica Core‐Shell Synthesis

We introduce a method for depositing a uniform mesoporous SiO2 layer onto upconverting nanoparticles (UCNPs) following a two step approach. First, thermolysis synthesized hydrophobic oleate‐capped UCNP were subjected to a ligand exchange procedure with sodium hexamethaphosphate (HMP) in order to obtain hydrophilic and exceptionally stable colloidal particles. In a second step, a uniform mesoporous SiO2 shell was anchored to the HMP‐UCNP using cetyltrimethylammonium bromide (CTAB) as a porogen agent. These porous layers are mesosotructured, chemically reactive and do not change the original optical properties of the UCNP. Moreover, anchoring HMP to the nanocrystal surface favors the formation of single UCNP@mesoporous SiO2 core‐shell entities with yields up to 86%. Ligand exchange of oleic acid‐capped NaFY4:Er;Yb upconversion nanoparticles (UPCN) with sodium hexametaphosphate (HMP) facilitates the synthesis of mesoporous SiO2 shells. The phosphate ligand minimizes aggregation of particles during the SiO2 condensation while preserves the optical properties if the UCNP.