The stability of organic cappings on hexagonal NaYF 4 :Ln 3+ 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 NaYF 4 :Yb 3+ ,Er 3+ /Tm 3 UCNPs even at a pH as low as 2 due to strong acidity of the sulfonate anchoring groups (p K a of ca. −3). Bare UCNPs progressively disintegrate into their compositional F – , Na + , Y 3+ , and Ln 3+ 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 Y 3+ and Ln 3+ 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.

Despite the significant advancements of developing upconversion nanoparticles (UCNPs) for high performance biosensing and bioimaging, the development of DNA‐functionalized UCNPs with thin coatings, efficient surface passivation, and fully functional DNAs for hybridization sensing in biological media remains extremely challenging. Here, a straightforward concept of labeling DNA to a thin polysulfonate polymer layer on UCNPs is presented. Both UCNPs and DNA preserve their full functionality as demonstrated by Förster resonance energy transfer (FRET) hybridization assays with a Cy3‐conjugated complementary DNA. The important bioanalytical benefits of the UCNP‐DNA nanohybrids are demonstrated by their implementation into rapid and wash‐free microRNA hybridization FRET assays. Using a simple fiber‐coupled USB‐spectrometer and 980 nm diode laser excitation in a 96‐well microplate, ratiometric FRET from UCNP‐to‐Cy3 can quantify miR20a in a 0.01–10 × 10−9 m concentration range with a detection limit of 30 × 10−12 m (4.5 fmol of miR20a), which is more than 20‐fold lower compared to quantum dot‐based FRET assays. The functional thin‐coating DNA‐UCNP nanohybrids have a strong potential for translating UCNPs into advanced DNA‐based biosensing, bioimaging, and high‐throughput and point‐of‐care clinical diagnostics.

Water-dispersible upconversion nanoparticles (β-NaYF4:Yb(3+),Er(3+), UCNP) coated with a thin shell of a biocompatible copolymer comprising 2-hydroxyethylmethacrylate (HEMA) and 2-acrylamido-2-methyl-1-propanesulphonsulphonic acid (AMPS), which we will term COP, have been prepared by multidentate grafting. This capping is remarkably resistant to strong acidic conditions as low as pH 2. The additional functionality of the smart UCNP@COP nanosystem has been proved by its association to a well-known photosensitizer (namely, methylene blue, MB). The green-to-red emission ratio of the UC@COP@MB nanohybrid exhibits excellent linear dependence in the 7 to 2 pH range as a consequence of the release of the dye as the pH decreases.

Sensitized emission of lead halide perovskite nanoparticles (LHPNPs) can be achieved by near-infrared (NIR) excitation of nearby lanthanide-doped upconversion nanoparticles (UCNPs) by using a low-cost diode laser. Here, the first preparation of linear assemblies of core and core-shell NPs, as well as linear coassemblies of LHPNPs and UCNPs, within an open peapodlike lead sulfate shell are reported. UCNPs with a NaYF 4 matrix doped with ytterbium and thulium or erbium, and with an inert shell of NaYF 4 in the case of core-shell, and all-inorganic CsPbX 3 NPs (X = halide) are chosen for these studies. Interestingly, the lead sulfate shell enhances the luminescence of the core/core-shell UCNPs in the polymers by ≈20-fold and it also plays a role in the efficiency of the sensitized emission of the LHPNPs under NIR excitation of the UCNP-LHPNP copolymers, as well as in the chemical stability of the LHPNPs in contact with water. The (co)polymers are prepared as colloids and deposited as solid films on a glass substrate. The lifetime of the sensitized LHP emission and the efficiency of the process wholly depends on the irradiance and on the sample state. These copolymers are promising candidates for the manufacture of photonic devices.