Optimization of Luminescence Properties of Lanthanide-doped Upconversion Nanoparticles for Their Applications in Multi-modal Bioimaging
Lanthanide doped upconversion nanoparticles(UCNPs),as a kind of novel nano-fluorescent.probes,can emit visible or ultra-violet(UV)light under near infrared(NIR)light excitation(anti-stokes shift emission),comparing with conventional organic dyes or quantum dots(QDs).UCNPs have been paid extensive concerns because of their some advantages,such as long life-time emission,low signal-to-noise(SBR)for bio-tissues,high emission stability and better biocompatibility.However,some challenges still need to be conquered for UCNPs,for instance,complicated synthesis procedures,low emission quantum yield(QY)and poor efficacies in multi-modal imaging.In this thesis,we developed some novel works including optimization of synthesis procedures,upconversion emission,efficiency controlling and multi-modal bio-imaging.The main contents of this thesis were listed as following.(1)The upconversion emission mechanism,optical properties,methods for synthesizing and improving emission efficiency,and the applications in multi-modal bio-imaging and bio-detection fields were briefly summarized.Then,the existing problems for UCNPs in bio-imaging were reviewed.(2)We introduced a novel strategy to prepare UCNPs using lanthanide chelated oleate compound instead of conventional lanthanide oxides or lanthanide chloride as precusors(denoted as Ln-OA method).The high pure UCNPs were prepared using Ln-OA as precusors and oleic acid/1-octadecene(OA/ODE)as solvents.The prepared UCNPs showed advantages of high purity and size controllability by related characterizations.Moreover,we demonstrated that this novel method could be utilized to prepare core-shell UCNPs with different excitation and emission wavelengths.Therefore,we provided a novel strategy to prepare UCNPs with high quality.(3)Core-shell fabrication plays key roles in upconversion luminescence(UCL)emission of UCNPs.Core-shell UCNPs exhibit higher UCL intensity than core UCNPs,attributed by the defective surface lattice being protected.However,due to the anisotropic of shell growth and various doping rates of lanthanide ions for different lattice face,the prepared NaYF4:Yb:Er(Tm)@NaYF4 core-shell UCNPs tended to be rod-like with increased NaYF4 shell thickness.The UCL efficacy of core-shell UCNPs could be further improved by optimizing their shapes.In this chapter,we successfully prepared spherical core-shell UCNPs by a facile method of adjusting the OA/ODE volume ratio in reaction system.Moreover,we demonstrated the higher UCL efficiency of spherical than rod-like core-shell UCNPs,because of the similar shell thickness around different lattice face for spherical structure.Therefore,we introduced a new strategy to further improve the UCL intensity of core-shell UCNPs.The prepared spherical core-shell UCNPscould be promising in bio-imaging applications.(4)Upconversion matrices can provide an ideal environment for UCL process and greatly affect UCL performance.Multi-modal bio-imaging could be available by doping with different lanthanide ions in UCNPs.In recent years,NaLuF4 based UCNPs were paid more attentions for higher UCL efficacy over NaYF4 based UCNPs.Moreover,NaLuF4 could be utilized in CT imaging.However,conventional method for preparing NaLuF4 based UCNPs often suffer from large particle size(>200 nm),resulted in poor cellular uptake percentages and potential cytotoxicity to biological tissues.In this chapter,we designed a new strategy to optimize the phase and size of the NaLuF4 based UCNPs by Y3+ doping.The performance of size and phase control by Y3+ doping was extensively investigated under different reaction temperature.The particle size could be obviously decreased with increased Y3+ doping contents under high temperature(320 ℃).The phase transfer from cubic(a)to hexagonal(3)could also be available by increasing the Y3+ doping percentages under low temperature(300℃).Moreover,the controlling mechanism of Y3+ doping were discussed.Finally,we extended the phase and size control of UCNPs by related lanthanide ions doping.(5)The UCL intensities of NaLUr4 were gradually decreased with increased Y3+doping contents,because of the decreasing particle size.Also,the CT signal was decreased with Y3+ doping,due to reduced Lu3+ doping in NaLuF4.In order to improve both UCL and CT efficacies,we designed theβ-NaLuF4:Y:Yb:Tm@NaLuF4(denoted as Lu/Y/Tm@Lu)core-shell UCNPs.After hydrophilic modifications,the core-shell Lu/Y/Tm@Lu exhibited some advantages than Lu/Y/Tm core,such as higher UCL intensity and near infrared(NIR)optical depth for tissues,excellent UCL and CT imaging performances in vitro and in vivo.Finally,the bio-distribution of prepared Lu/Y/Tm@Lu core-shell UCNPs were investigated in mice using CT imaging system.The good biocompatibility was also demonstrated for UCNPs treated mice based on histological analysis of main organs.Therefore,by rational and elaborate design of NaLuF4 based UCNPs,we further improved the UCL and CT dual modal imaging efficacies of UCNPs.