Preparation Of Water-soluble Rare Earth Fluoride Nanocrystals

Rapid advances in diagnostics, sensitive bio-analytics and photodynamic therapy（PDT） are the driving force for the development of more sensitive and efficientnanophosphors as bio-imaging contrast agents. Fluorescin, organic dye and quantumdot are widely used as luminescent probes in biological labeling and imagingapplications. However, the excitation sources of these traditional probes areultraviolet or violet light, which can also excite the biological tissue. It indicates thestrong background in biological labeling and imaging experiments. To overcome theselimitations, upconversion nanophosphors have been intensively studied to use as anew type of biological probes. Upconversion （UC） process is anti-Stokes typeemission process. During the UC processes, the sequential absorption of two or morephotons leads to the emission of light at shorter wavelength than the excitationwavelength. It means the UC nanophosphors can be excited by near infrared （NIR）light and emit visible light. Therefore, when these UC nanophosphors are used asluminescent biological probes, the detection sensitivity can be greatly improved dueto the lack of autofluorescence background.Among various upconversion nanophosphors, lanthanide （Ln） ions dopedsodium rare-earth （RE） fluoride （commonly tetrafluorides, NaRE₄） nanocrystals（NCs） are considered the most promising candidates due to their intensive UCluminescence （RE=Ln and Y）. Recently, in order to obtain high UC luminescenceefficiency, a large number of works have focused on the synthesis and investigation of high quality lanthanide doped cubic and hexagonal NaYF₄nanocrystals. However,besides the high UC luminescence efficiency, water solubility is necessary for theseNaRE₄materials used as bio-probes. To be suitable for biological labelingapplications, the NaYF₄nanocrystals must be compatible with biological substrates. Itmeans the nanocrystals must be soluble （dispersible） in aqueous media and thereshould be some specific groups on the surface of the nanocrystals, which can bind thenanocrystals to the biological targets. To link these UC nanocrystals to bio-molecules,reactive functional moieties such as amino or carboxylic acid groups are required. Inour research work, we have explored to synthesize water soluble NaYF₄nanocrystalspossessing carboxylic acid groups using the polyacrylic acid and polyacrylic acidsodium as surfactants.（1） Surface Modification of Hydrophobic NaYF₄Nanocrystals by LigandExchange. Oleic acid ligands coated NaYF₄:Yb3+,Tm3+nanocrystals weresuccessfully synthesized via a solution-based co-thermolysis of metal complexprecursors method. The XRD analysis confirmed that the products were pure cubicNaYF₄crystals. TEM images showed that the UCNPs were nano-shperes with aparticle size of10nm. Surface modification was carried out to render them watersoluble （dispersed） by ligand exchange method for future biomedical applications. Weused polyacrylic acid （PAA） as the surfactant for the ligand exchange. Throughsurface modification, the carboxyl groups of PAA were attached on the surface of theUCNPs, which were endowed the UCNPs good water solubility. Under the nearinfrared excitation of980nm, the NaYF₄: Yb, Tm/PAA UCNPsolution emitted bright violet blue light. The UCNPs has a strong ability that can convert infrared lightinto visible light upconversion luminescence.（2） Synthesis of water soluble upconversion nanocrystals by a facilesolvothermal method using an amphiphilic surfactant, polyacrylic acid sodium （PAAs）as the chelating agent. Water-soluble NaYF4:20mol%Yb,1mol%Tm upconvertionnanoparticles （UCNPs） have been synthesized directly through one-step reaction. ThePAAS was selected as the chelating agent and ethylene glycol （EG） was used assolvent. These UCNPs were proved to be pure hexagonal phase NaYF4nanocrystalswith an average size of about140nm. The UCNPs could be well dispersed indeionized water to form a transparent solution. Under the near infrared excitation of980nm, this solution could emit bright violet blue upconversion luminescence.FT-IR analysis indicated that PAAs had tightly linked to the surface of UCNPs., Thecarboxyl groups of the ligand made the UCNPs highly water-soluble andbio-compatible. The experimental results showed that the UCNPs were promisingnanophosphors for biological probes because of their good water dispersibility,biological compatibility and high UC efficiency..