| Bio-labeling is one of the most commonly used and important techniques in molecular biology. The extensive research and application of labelling materials have promoted the rapid development of biology and medical science. Lanthanide-doped nanocrystals (NCs) have gradually attracted more and more attention due to their distinct advantages such as high emission intensity, good chemical stability, long photoluminescence lifetime, low toxicity, which are acknowledged new class of luminescent nanomaterials. Amongst various lanthanide-doped NCs, rare earth (RE) fluorides co-doped with sensitizer and activator nanocrystals are of great interest. Rare earth elements possess both rich optical and magnetic properties, could gather photic and magnetic functions in a single material and may be applied to magnetic resonance images (MRI). In addition, compared to the conventional fluorescent biolabels, rare-earth doped upconversion luminescent nanoparticles (UCNPs) could emit visible light upon infrared light excitation, which effectively overcome the drawbacks such as autofluorescence from organisms and biological damage. Therefore, UCNPs have shown wide application prospects in biological labeling and bioassay fields.In this work, NaGdF4:Ce,Tb nanoparticles with excellent green fluorescence and good magnetic characteristics were synthesized in the oleic acid-ethanol-water system via a solvothermal approach, by using rare-earth stearate as the precursor. The morphology, crystal structure, magnetic, luminescent properties and fluorescence lifetime of the nanoparticles were characterized through transmission electron microscopy (TEM), X-ray diffraction (XRD), magnetic hysteresis loop (MHL) and fluorescence spectra. The possible luminescence mechanism and effects of reaction conditions on luminescent properties have been explored. The experimental results showed that the as-synthesized nanoparticles were almost spherical with the size of around15nm in hexagonal crystal system, presented good superparamagnetic and optical properties. During the fluorescence process, the energy mainly was transfered from Ce3+to Tb3+via Gd3+acting as an intermediate to allow energy to migrate over the Gd3+sublattice. The optimum reaction conditions were as follows:reaction temperature was120℃, reaction time was18h, the molar doped content for Tb3+and Ce3+were15%and10%respectively, with the amount of NaF of5mmol.The amino-modified NaGdF4:Ce,Tb nanoparticles were served as the energy donors while the gold nanoparticles were used as the energy acceptors, due to its absorption spectrum overlapped well with the emission spectra of NaGdF4:Ce,Tb nanoparticles. A luminescence resonance energy transfer (LRET) system was constructed through the covalent linking of the amino on NaGdF4:Ce,Tb surface and the carboxyl on Au nanoparticles. The LRET mechanism of this system was discussed in detail. When the concentrations of nano gold were in the range of0～7.2μg mL-1, the luminescence intensity of NaGdF4:Ce,Tb NPs descreased linearly with the increasing of nano gold concentration.The same solvothermal approach was employed to synthesize NaGdF4:Yb,Er upconversion nanoparticles. The as-synthesized small UCNPs with uniform size and regular shape were suitable for biological labeling. The possible upconversion luminescence mechanism has been studied. Due to the presence of amino groups on the surface of NaGdF4:Yb,Er nanoparticles, the UCNPs were well conjugated with a protein called transferrin, which could specifically recognize the transferrin receptors overexpressed on HeLa cells, and employed for biolabeling and fluorescent imaging of HeLa cells. |
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