| Over the past decades, quantum dots (QDs) have been studied extensivelybecause of their high extinction coefficient and quantum yield, easy tunability ofemission wavelength, and resistance to photobleaching. The major obstacle in usingthem as imaging agents for long-term cellular imaging is that the visible/UVexcitation can give rise to background autofluorescence signals, photobleaching andphototoxicity. Compared to conventional luminescent materials such asorganicfluorescent dyes and QDs, lanthanide (Ln3+)-doped, near-infrared(NIR)-excited, upconversion labels show superior features, including sharp emissionbandwidths (<10nm), large anti-Stokes shifts (up to500nm), long luminescencelifetime (μs–ms range), high chemical stability and low long-term toxicity. Thesefeatures coupled with the remarkable light penetration depth and the absence ofautofluorescence in biological specimens under infrared excitation make theseupconversion nanocrystals (UCNCs) ideal luminescent probes for biological labelingand imaging. In the first chapter of five, the concepts, research methods, history andnew developments of UCNCs are introduced. At the end of this chapter, theimportance of the project is described.In the second chapter, water-dispersible Tb3+/Dy3+doped CeF3colloidalnanocrystals with controllable morphology and high crystallinity have beensuccessfully synthesized through a solvothermal process. The TEM images illustratethat the Re3+doped CeF3nanocrystals are rectangular (or cubic) with a mean diameterof10nm. The excellent dispersibility in some of the polar solvents including water isachieved by using polyethyleneimine as the capping agent. The amine groups of the polymer chains on one hand bind to the nanocrystal surface, on the other hand the freeones could link to functional materials including bio-molecules. Monodispersewater-soluble LaF3:Ln3+NCs have been successfully fabricated via a fast, facile andenvironmentally-friendly microwave-assisted modified polyol process withpolyvinylpyrrolidone as an amphiphilic surfactant. The obtained NCs can be welldispersed in hydrophilic solutions with small sizes in the range of9–12nm. TheLaF33:Ln+NCs (Ln=Eu, Nd, Ce, Tb, Yb, Er, Yb, Ho and Yb, Tm) have the uniquefeature of up–down conversion from visible to NIR emission owing to the ladder-likearranged energy levels of Ln3+and in particular, the high efficiency upconversion ofthe two-photon (UCL), obtained from excitation by a continuous980nm laser.Furthermore, the three-dimensional PDMS rod-like fluorescence displays and a silicasurface modification by a core/shell structure on the obtained NCs can improve thebiocompatibility, indicating potential applications in optical3D devices and asbio-probes.In the third chapter, monodisperse water-soluble hexagonal phase Ln3+-dopedNaGdF4UCNCs have been successfully fabricated by means of a fast, facile, andenvironmentally friendly microwave-assisted route with polyethylenimine as thesurfactant. Fine-tuning of the UC emission from visible to near-IR and finally to whitelight has been achieved. Furthermore, studies of the magnetic resonance imaging aswell as the magnetization (magnetization–magnetic field curves) and the targetedrecognition properties of FA-coupled amine-functionalized NaGdF4@SiO2UCNCsindicate that the obtained NaGdF4UCNCs can be potential candidates for dualmodeoptical/magnetic bioapplications.In the fourth chapter, we demonstrate one-step synthesis of highly luminescentNaYF4:Yb,Er@NaGdF4core–shell UCNCs in the “aqueous” phase under mildconditions using innocuous reagents. A microwave-assisted approach allows forlayer-by-layer epitaxial growth of a hydrophilic NaGdF4shell on NaYF4:Yb, Er cores.During this process, surface defects of the nanocrystals could be gradually passivatedby the homogeneous shell deposition, resulting in obvious enhancement in the overallupconversion emission efficiency. In addition, the up-down conversion dual-mode luminescent NaYF4:Yb, Er@NaGdF4:Ce, Ln (Eu, Tb, Sm, Dy) nanocrystals were alsosynthesized to further validate the successful formation of the core–shell structure.More significantly, based on their superior solubility and stability in water solution,high upconversion efficiency and Gd-doped predominant X-ray absorption, theas-prepared NaYF4:Yb, Er@NaGdF4core–shell UCNCs exhibited high contrast invitro cell imaging and in vivo X-ray computed tomography (CT) imaging,demonstrating great potential as multiplexed luminescent biolabels and CT contrastagents.The last chapter provides the summary and conclusion of the research project.This thesis illustrates a comprehensive investigation of UCNCs and UCNCs-basedcomposite nanostructures for applications in cell imaging, MRI, and CT biologicalimaging. Our results contribute to further explorations of those functionalnanomaterials in biomedical research. |
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