Study Of Upconversion Fluorescent Biolabel-based Biosensors

Compared with traditional fluorescent materials, upconversion nanophosphor (UCNP) has lots of advantages as fluorescent biolabels, such as good chemical stability, low toxicity, no background fluorescence, high signal to noise ratio and so on.β-NaYF4:Yb,Er upconversion nanophosphors (UCNPs) are known as one kind of the most efficient NIR-to-visible upconversion materials, which show great potential in bioanalytical chemistry and bioimaging. However, their applications are greatly limited due to their low water dispersibility and thus poor biocompatibility.In this paper, poly(acrylic acid)(PAA)-based ligand ex-change strategies are carried out to modify oleic acid-capped hydrophobic β-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-NH2groups in antigen/antibody, a sensitive immunosensor is constructed by using PAA-functionalized β-NaYF4:Yb,Er UCNPs as biolabels. Through monitoring the upconversion fluorescence intensity and fluorescent imaging of the final immunocomplexes, high sensitivity is achieved for the proposed immunoassay and as low as0.1ng/mL goat anti-human immunoglobulin G (IgG) can be detected. Fluorescence quenching-based turn-on assay has high sensitivity, but it is quite difficult to find an effective quencher for UCNPs to entirely quench their anti-Stokes upconversion fluorescence. In the paper, we demonstrate for the first time that water-soluble graphene oxide (GO) can efficiently quench the fluorescence of UCNPs, and thus the complex of GO and UCNPs can offer a highly sensitive biosensing platform. Minewhile, in our work, pure β-NaYF4UCNPs with excellent water solubility and biocompatiblity have been directly synthesized by a facile one-pot solvothermal approach, which has been successfully applied to the high-contrast fluorescent imaging and detection of HeLa cells.All of these results suggest that carboxyl groups-functionalized UCNPs may serve as an ideal candidate for use as bioanalysis and bioimaging probes.