Synthesis And Properties Of Rare-earth Upconversion Nanoparticles As Luminescent Biological Probes

Researches on life science have deeply reached the levels of cells andbio-unimolecules. It has become a research focus in this field to perform fluorescentdetection and imaging on cells, virus and bio-molecules in vivo. Recently, with therapid development of nanotechnology, the nanomaterials-based biologicalfluorescent probes have played an increasingly important role that cross fields ofinformation science, chemistry, material science and biomedicine. Among thesenanomaterials, the luminescent rare earth nanomaterials show a wide applicationprospect in the field of biological information due to their unique luminescenceproperties. More and more attentions have been taken on this type of materials and ithas been hotspot for them to be studied in life science.There exist a great many endogenous fluorophores which will cause seriousinterference to fluorescence signals in the process of bio-labeling and bio-imaging,leading to a decreased signal-to-noise ratio. How to eliminate the interference ofbackground noise is a serious problem to be solved in the biological fluorescentdetection. Until now, the most widely used bio-probes mainly include organic dyeand semiconductor quantum dot which are absolutely pumped by ultraviolet, violetand blue light and the excitation light of these wavelengths rang could also excite theendogenous fluorophores in living organism to emit auto fluorescence. Thus, it hasalways been a serious problem to these conventional bio-probes for suffering frominterference of background noise. The light penetration “window” of the organismlies in the wavelength range of800–1200nm, which cause the minimal absorptionby the organism. It is know that rare earth doped upconversion nanomaterials havethe ability to convert low-frequency photons to high-frequency photons. Throughcontrolling the sort of doped sensitizer-activator ions, near infrared （NIR） light （980nm） lying in the “window” can be converted to visible or NIR light with higherfrequency. With usage of these materials as bioprobes, auto fluorescence can be avoided and the signal-to-noise ratio is enhanced. Owning the unique luminescenceproperties, rare earth doped upconversion nanomaterials have become promisingalternatives to organic dye and quantum dot. Broad attentions have been taken onsynthesizing upconversion nanoparticles （UCNPs） for luminescent biolabeling. Tomeet the needs for biolabeling, the UCNPs must have water solubility, small size（normally be sub-50nm）, high luminescence intensity, good biocompatibility. Basedon these issues, we prepared water soluble rare earth doped UCNPs, and furthermoreperform experiment of biolabeling on HeLa cells in vitro using these UCNPs.Details are as follows:（1） NaYF₄:20mol%Yb,0.5mol%Tm upconvertion nanoparticles （UCNPs） weresynthesized by solvothermal approach using polyvinylpyrrolidone （PVP） as asurfactant and ethylene glycol （EG） as solvent. These UCNPs were proved to bepure cubic phase NaYF₄nanocrystals with average size of about40nm. Theamphiphilic molecules of PVP which were attached on the surface of the UCNPsendowed the UCNPs good water dispersibility. Under the near infrared excitationof980nm, the UCNPs dispersed in deionized water emitted brightnaked-eyes-visible blue upconversion luminescence. The superior properties ofthe UCNPs, such as small size, good water solubility and strong upconversionluminescence, make them potential in the areas of biological applications.Furthermore, to test the cytotoxicity of the UCNPs, we performed MTTexperiment which indicated that the UCNPs almost had no cytotoxicity withoutthe excitement of NIR light. Then, HeLa cells were cultured together with theUCNPs to study the application of the UCNPs as biolabels. The performanceshowed that the as-prepared UCNPs successfully labeled the HeLa cells byluminescence in vitro.（2） We prepared Dual-mode luminescent core-shell nanocomposites. The coreparticles were Yb-Tm （or Yb-Er） co-doped NaYF₄UCNPs, while the shell layerswere silicon dioxide （SiO2） which embedded with Eu （or Td） complexes. Thenanocomposites had a small average size of50nm with shell thickness of about10nm. It is shown that the core-shell nanoparticles possessed superior water solubility to uncladded core nanoparticles. The core-shell nanoparticles dispersedin deionized water could be simultaneously excited by980-nm NIR and330-nmultraviolet （UV） light and presented dual-mode luminescence. The function ofdual-mode luminescence may allow the core-shell nanoparticles to dual label.The core-shell nanocomposites with dual-mode luminescence are expected to beapplied as dual labels with the function of improving the labelling sensitivity andaccuracy and as biological luminescent coding agents.