Surface modification of hydrophobic NaYF4:Yb,Er upconversion nanophosphors and their applications for immunoassay

beta-NaYF4:Yb,Er upconversion nanophosphor (UCNP) is known as one of the most efficient NIR-to-visible upconversion materials, which shows great potential in bioanalytical chemistry and bioimaging. However, its applications are greatly limited due to its low water dispersibility and thus poor biocompatibility. In this paper, poly(acrylic acid) (PAA)-based ligand exchange strategies are carried out to modify oleic acid-capped hydrophobic beta-NaYF4:Yb,Er UCNPs into hydrophilic ones. After efficient surface modification, the presence of free carboxylic acid groups on the surfaces of UCNPs results in high solubility in water, and also allows further conjugation with NH2-containing biomolecules. Facilitated by the covalent linkage between the -COOH groups on UCNPs surfaces and -NH2 groups in antigen/antibody, a sensitive immunosensor is constructed by using PAA-functionalized beta-NaYF4:Yb,Er UCNPs as biolabels. Through monitoring the upconversion fluorescence intensity or fluorescent imaging of the final immunocomplexes, high sensitivity is achieved for the proposed immunoassay and as low as 0.1 ng/mL goat anti-human immunoglobulin G (IgG) can be detected, which suggests that PAA-modified UCNPs may serve as an ideal candidate for use as bioanalysis and bioimaging probes.