Monodisperse, oil-dispersible β-NaYF₄:Yb,Er(Tm) up-conversion (UC) luminescent nanospheres (NPs) have been successfully synthesized. The as-prepared NPs show high size-uniformity without any size-selection process, and the sizes of the NPs can be tuned in the range of 18 nm–45 nm, which are well suitable for biolabels. The NPs are thoroughly characterized and a growth mechanism is proposed. By co-doping Yb-Er or Yb-Tm, the as-prepared NPs can show bright green or blue UC luminescence under the irradiation of a 980 nm near-infrared (NIR) laser, and the UC luminescence can be obviously enhanced by constructing a NaYF₄:Yb,Er@NaYF₄ core/shell structure. Furthermore, the hydrophobic UC nanospheres (UCNPs) are converted into water-soluble by a robust ligand-exchange pathway with poly(acrylic acid). The presence of free carboxylic acid groups on their surfaces not only results in high solubility in water, but also allows further conjugation with biomolecules such as antigens, antibodies and oligonucleotides, which pave the way for potential bio-applications of the UCNPs.

Graphene oxide can act as an ultrahighly efficient quencher for upconversion nanophosphors and thus, an extraordinarily sensitive biosensing platform is constructed.