Study On Preparation Of Upconversion Nanomaterials, Their Properties And Cell Imaging

Owing to their unique optical properties, upconversion nanocrystals (UCNs) showgreat potentials for applying in the fields of solid lasers, solar cells, biological labels,medical diagnosis and laser anti-counterfeits. However, most of the UCNs generallysuffer from low upconversion luminescence (UCL) efficiency due to structural defectsand fluorescence quencher existed in the surface of the UCNs, and low absorbance ofexcitation light. Thus the UCNs used for high sensitive biological and medical detectionsare still limited. To improve UCL efficiency and develop new UCNs with high UCL isimportant and challenge for the study of UCNs. Focused on this, we synthesized a seriesof UCNs with different matrixes, doping elements and core-shell structures in this work.In addition, a novel core-shell nanocomposite based on the UCNs was prepared. Theproperties of these products were characterized. The fluorescent properties, cytotoxicitiesand cell imaging applications of the UCNs and nanocomposite were investigated. Basedon this study, some important results have been achieved that can provide some valuableinformation for the study of UCNs.The main contents of this study are listed below:(1) Four kinds of UCNs of NaYF4:Yb/Er, NaYF4:Gd/Yb/Er, NaLuF4:Yb/Er andNaLuF4:Gd/Yb/Er were successfully synthesized by a facile solvothermal method. Themorphologies, crystalline phases, compositions, grain sizes and upconversionluminescence spectra were characterized by XRD、SEM、HRTEM、EDX and Fluorescencespectrometer, respectively. The results show that all of the UCNs are uniform inmorphology and size (272nm), and of high monodispersity. The typical UCL spectra ofEr3+can be observed in all of the prepared UCNs. The crystalline phases of the UCNs aredifferent. NaYF4:Yb/Er is mixed-phase including cubic and hexagonal phases,NaYF4:Gd/Yb/Er is pure hexagonal phase, while NaLuF4:Yb/Er and NaLuF4:Gd/Yb/Erare all pure hexagonal phases, indicating that Gd3+can induce a phase transformationfrom cubic to hexagonal phase and NaLuF4host is apt to form pure hexagonal. The UCLintensities of the UCNs are in the sequence (from weak to strong) of NaYF4:Yb/Er, NaYF4:Gd/Yb/Er, NaLuF4:Yb/Er and NaLuF4:Gd/Yb/Er. These results indicate thatdoping Gd3+is favorable for the UCL and the NaLuF4is a more excellent host for UCL incompared with NaYF4.The NaLuF4:Gd/Yb/Er was modified by sodium citrate and itscytotoxicity and cell imaging were measured. It was found that the sodium citrate-cappedUCNs were water-soluble and low cytotoxicity. Bright fluorescence was observed whenUCNs-labeled cells were excited with980nm light.(2) Based on the prepared hexagonal phase UCNs of NaLuF4:Gd/Yb/Er, a series ofcore-shell structural UCNs of NaLuF4:Gd/Yb/Er@NaLuF4，NaLuF4:Gd/Yb/Er@NaLuF4:Yb and NaLuF4:Gd/Yb/Er@NaLuF4:Yb/Ho weresynthesized through coating an inert-shell or active-shell onto the core ofNaLuF4:Gd/Yb/Er. The properties of the UCNs were investigated. The results show thatall of the UCNs are pure hexagonal phases, with uniform morphologies, sizes and highmonodispersities. After coating a NaLuF4-based shell, the sizes of the UCNs are enlargedto302nm, and UCL intensities of the UCNs are significantly enhanced. In contrast tothe core UCNs of NaLuF4:Gd/Yb/Er, the UCL intensities of the core-shell structuralUCNs of NaLuF4:Gd/Yb/Er@NaLuF4, NaLuF4:Gd/Yb/Er@NaLuF4:Yb andNaLuF4:Gd/Yb/Er@NaLuF4:Yb/Ho are enhanced by4.4,6.5and11.2times, respectively.The active-shell coated UCNs of NaLuF4:Gd/Yb/Er@NaLuF4:Yb/Ho presents the highestUCL among the4kinds of the as-prepared nanocrystals. Its UCL intensity is2.5timeshigher than that of inert-shell coated UCNs of NaLuF4:Gd/Yb/Er@NaLuF4,with a highquantum yield of3.9±1%. These results indicate that coating a same matrical shell canenhance the UCL of the UCNs remarkably, and the active-shell is better than theinert-shell. The enhancement can be attributed to the shell which can passivate the surfacedefects of the nanocrystals, protect the luminescence ions in the core from non-radiativedecay caused by surface defects and vibrational deactivation induced by solvents orsurface-bound ligands. In compared with inert shell, the active shell can not only protectthe luminescence core, but also transfer energy to the core after absorbing NIR light frompump source, leading a higher enhancement. When theNaLuF4:Gd/Yb/Er@NaLuF4:Yb/Ho was used in HeLa cells imaging with980nmexcitation light, strong green and red UCLs were observed, implying the as-prepared UCNs could be utilized as potential upconversion luminescence probe for sensitivebioimaging.(3) A novel UCNs-based nanocomposite of Ag@SiO2@Lu2O3:Gd/Yb/Er wasdeveloped. Its morphology, crystalline phase, size, composition, optics property and cellimaging application were investigated. The results show that the grain size of thenanocomposite is88nm it is composed of an Ag core (~20nm), a silica shell (~30nm)and a Lu2O3shell (~4nm). Due to the function of metal-enhanced fluorescence from Agcore, a maximum UCL enhancement of30-fold was obtained. This enhancement offers apotential increase in overall upconversion nanocrystals detectability. The nanocompositeexhibited bright UCL when it was used for HeLa cells imaging.