Polysulfonate Cappings on Upconversion Nanoparticles Prevent Their Disintegration in Water and Provide Superior...

The stability of organic cappings on hexagonal NaYF₄:Ln³⁺ upconversion nanoparticles (UCNPs) is crucial for their luminescence efficiency in aqueous solutions. The capping removal quickens as the acidity of the medium increases. We demonstrate here that polysulfonates, namely poly(2-acrylamido-2-methyl-1-propanesulfonate) (PAMPS) and poly(sodium 4-styrene sulfonate) (PSS), remain anchored to the surface of NaYF₄:Yb³⁺,Er³⁺/Tm³ UCNPs even at a pH as low as 2 due to strong acidity of the sulfonate anchoring groups (pK _a of ca. -3). Bare UCNPs progressively disintegrate into their compositional F^-, Na⁺, Y³⁺, and Ln³⁺ ions. Their disintegration is particularly worrying in highly diluted dispersions of nanoparticles because both the lanthanide ions and/or the bare UCNPs can cause undesirable interference in a chemical or biological environment. Remarkably, the UC@PSS nanohybrid is particularly chemically stable, exhibiting an amazingly low release of Y³⁺ and Ln³⁺ ions for up to 96 h in highly diluted water dispersions (10 μg/mL). Additional advantages of the use of PSS as capping layer are its biocompatibility and its high dispersibility in water, together with easy further functionalization of the UCNP@PSS nanohybrids.