Up-converting rare-earth nanophosphors (UCNPs) have great potential to revolutionize biological luminescent labels, but their use has been limited by difficulties in obtaining UCNPs that are biocompatible. To address this problem, we have developed a simple and versatile strategy for converting hydrophobic UCNPs into water-soluble and carboxylic acid-functionalized analogues by directly oxidizing oleic acid ligands with the Lemieux-von Rudloff reagent. This oxidation process has no obvious adverse effects on the morphologies, phases, compositions and luminescent capabilities of UCNPs. Furthermore, as revealed by Fourier transform infrared (FTIR) and NMR results, oleic acid ligands on the surface of UCNPs can be oxidized into azelaic acids (HOOC(CH2)7COOH), which results in the generation of free carboxylic acid groups on the surface. The presence of free carboxylic acid groups not only confers high solubility in water, but also allows further conjugation with biomolecules such as streptavidin. A highly sensitive DNA sensor based on such streptavidin-coupled UCNPs have been prepared, and the demonstrated results suggest that these biocompatible UCNPs have great superiority as luminescent labeling materials for biological applications.

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Because of the ability to selectively reveal the objects of interest with subcellular resolution, fluorescence microscopy provides widespread applications from basic biological research to clinical diagnosis. However, challenges still remain in reducing the degree of photobleaching and increasing the contrast between signal and noise. Herein, we found that rare-earth nanophosphors exhibit a unique up-conversion luminescence mechanism and imaging modality and developed a new three-dimensional visualization method of laser scanning up-conversion luminescence microscopy (LSUCLM) with little photobleaching and no background fluorescence, by introducing a reverse excitation dichroic mirror and the confocal pinhole technique. Moreover, we demonstrated the up-conversion emission imaging of thin films containing embedded rare-earth nanophosphors and cells multilabeled with the nanophosphors and organic dyes. These data show that LSUCLM not only shares noninvasive benefits and deep penetration of two-photon microscopy but also offers some distinct advantages, such as little photobleaching of both organic dyes and rare-earth nanophosphors, no background fluorescence from either endogenous fluorophores or colabeled fluorescent probes, and excellent compatibility with conventional confocal microscopy.

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Uniform silica-coated NaYF(4): 20 mol % Yb, 2 mol % Er nanocomposites with good dispersibility, containing organic dye incorporated in the silica shell and folic acid conjugated on the surface of the shell, were prepared and characterized. The core-shell nanocomposites are 20-22 nm in size, water soluble, and buffer stable, with good photostability and biocompatibility. Folic acid (FA) offers a means of targeting human cells that greatly overexpress the folate receptor (FR). By the use of confocal microscopy and quantitative flow cytometry analysis, we demonstrate the receptor-mediated delivery of FA-conjugated nanocomposites targeting FR-positive cell lines, such as KB cells. The receptor-mediated targeting was confirmed by a comparison with the uptake of these nanocomposites in FR-negative cell lines, such as MCF-7. These results show that the silica-coated upconverting nanophosphor (UCNP) nanocomposites prepared by our strategy can potentially be useful as multimodal bioimaging agents.