Rare-earth upconversion nanoparticles (UCNPs) exhibit great potential in luminescent biolabels and other multifunctional probes; however, their applications are limited by their low water solubility and the lack of binding groups. To address these problems, a clean and flexible strategy to modify hydrophobic monodisperse UCNPs into hydrophilic ones that are capped with functional groups is developed. The modification process is implemented by direct oxidation of oleic acid ligands with ozone under specific conditions, where the oleic acid (OA) ligands on the surface of the UCNPs can be converted into azelaic acid ligands (HOOC(CH2)(7)COOH) or azelaic aldehyde HOOC(CH2)(7)CHO, as is revealed by Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) measurements. This oxidation process has no significant side-effects on the morphology, phase, composition, or luminescent properties of the UCNPs. Free carboxylic acid groups on the surface endow the UCNPs with good water solubility, while aldehyde groups at the surface provide binding sites for amino-containing molecules via Schiff-base condensation, such as 2-(4-aminophenylethylyl)-5-methoxy-2-(2-pyridyl)thiazole (MPTEA) and 2-aminoethanethiol hydrochloride (NH2CH2CH2SH center dot HCl, HEMA). A Ce4+ sensor based on the dual-emission arising from the different spectral responses of MPTEA and the UCNPs. Facilitated by the covalent linkage between the terminal aldehyde group on the UCNPs and the amino group in HEMA, a hybrid structure of UCNPs and Au NPs is fabricated. The effective coupling between the aldehyde group and the amino group suggests that these functionalized UCNPs have potential in combining other functional units for simultaneous biolabeling, or other optical applications.