Fabrication Of Upconversion Fluorescence Based Nanoprobes For Multimodal Bioimaging In Vivo

The integrated merits of different imaging modalities and complementary information from each imaging modality render multimodal imaging promising for accurate medical diagnosis. Rare-earth upconversion nanoparticles (UCNPs) show excitation/emission wavelengths located in the biologically "transparent window" and excellent anti-Stokes optical properties, therefore they have attracted broad attention in the optical bioimaging field, especially in the in vivo bioimaging field. UCNPs also show powerful multimodal bioimaging capacity by introducing rare-earth element with larger atomic number or paramagnetism, they are thus widely applied in multimodal bioimaging with the development of synthesis and surface modification techniques for UCNPs. However, structure design for the organic integration of different imaging modalities is still desired for multimodal imaging. This dissertation focused on structural design, preparation and in vivo bioimaging application of multimodal bioimaging nanoprobes based on UCNPs. The main contents and novelty are summarized as follows:(1) Hydrophilic and single chromatic NaYF4:Mn/Yb/Er(Tm) UCNPs were prepared using polyethylenimine (PEI) as the capping ligands via a simple and one-pot hydrothermal method. The morphology, structure, composition and upconversion luminescence (UCL) characteristics of the UCNPs induced by Mn2+ion doping were studied. The Mn2+doped (30mol%) NaYF4:Yb/Er UCNPs not only showed nearly single chromatic red UCL, which is beneficial for deep tissue bioimaging application, but also in vitro magnetic resonance (MR) contrast effect. They were further used for UCL and T1-based MR small-animal imaging, demonstrating promising for dualmodal bioimaging application.(2) Multishell structured NaYF4:Yb/Tm@NaLuF4@NaYF4@NaGdF4UCNPs were designed and fabricated as nanoprobe for in vivo trimodal UCL/X-ray computed tomography (CT)/MR bioimaging and targeted UCL imaging of tumor-bearing mice. Hexagonal phase NaYF4:Yb/Tm is used as the core to provide UCL, while the shell of NaLuF4is epitaxially grovn on the core not only to provide an optically inert layer for enhancing the UCL, but also to serve as a contrast agent for CT. The outermost NaGdF4shell is fabricated as a thin layer to give high longitudinal relaxivity (r1) desired for MR imaging. The transition shell layer of NaYF4not only provides an interface to facilitate the formation of NaGdF4shell, but also inhibits the energy transfer from inner upconversion activator to surface paramagnetic Gd3+ions. The fabricated multishell structured nanoprobe shows intense near-infrared UCL, high r1value of3.76mM-1s-1and in vitro CT contrast effect. The multishell structured nanoprobe offers great potential for in vivo UCL/CT/MR trimodal imaging. Further covalent bonding of folic acid makes the multishell structured nanoprobe promising for in vivo targeted UCL imaging of tumor-bearing mice.(3) Sandwich-structured rare-earth NaGdF4:Yb/Tm@NaLuF4:Yb/Tm@NaYF4upconversion nanocrystals (-16nm), as in vivo dualmodal UCL/CT bioimaging nanoprobe, were fabricated to meet the requirements of intense near-infrared fluorescence emission and efficient clearance characteristics for bioimaging nanoprobes. Ultra-small NaGdF4:Yb/Tm nanoparticles are used as UCL center as well as core nanoparticles to achieve epitaxial growth of NaLuF4:Yb/Tm layer. The NaLuF4:Yb/Tm layer not only provides the second UCL center for cooperative enhancement of UCL, but also supplies a potential functional CT contrast shell. Furthermore, the optically inert shell layer of NaYF4is used to improve the UCL intensity of the NaLuF4:Yb/Tm nanoparticles. The sandwich-structured design fully takes the advantages of NaLuF4in UCL/CT and ultra-small characteristic of NaGdF4. The as-prepared sandwich-structured UCNPs have great potential for in vivo dualmodal UCL/CT imaging.(4) The paramagnetic behaviors of NaGdF4MR functional shell layer on rare-earth nanoparticles were investeged for improving the positive MR contrast efficiency and dosage compatibility of multimodal rare-earth imaging nanoprobe. The paramagnetic behaviors of NaGdF4shell layer show dependence of surface ligand directly coordinated to Gd3+ions. The paramagnetic behaviors of thin NaGdF4shell epitaxially grown on rare-earth nanoparticles with different particle size (～25nm,～20 nm,～15nm) were further brought into comparison. Meanwhile, the ultra-thin NaGdF4shell is favorable for dosage compatibility of rare-earth multimodal imaging nanoprobe. The as-prepared UCNPs with an average diameter of～15nm and ultra-thin NaGdF4shell demonstrate excellent in vivo T1-weighted positive MR contrast effect under total contrast agent dosage which is compatible for CT imaging.