A study of multifunctionalization of fluorescent nanoparticle probes

Abstract

Multimodal bioimaging is an important area of research in biology and medicine. Magnetic nanoparticles are used in site-specific drug delivery vehicles, magnetic bioseparation, and magnetic resonance imaging (MRI); while fluorescent nanoparticles are used in cell labelling, biosensor, and in vitro and in vivo imaging. It can be foreseen that the combination of both fluorescent and magnetic properties into a single entity would provide us with multi-functional, multi-targeting and multi-treating fluorescent probes. Owing to the importance of these multifunctional nanophosphors, investigations of multifunctional lanthanide (Ln³⁺)-doped nanoparticles are the main subject of this thesis. All the multifunctional Ln³⁺-doped nanoparticles are synthesized with the facile hydrothermal method and novel solution-based method. The basic principles, advantages and disadvantages of the hydrothermal method and the solution-based method will be given. The fundamentals of the formation of nanoparticles, and the operating principles of photoluminescent spectroscopy and vibrating sample magnetometer measurement will be briefly introduced. Firstly, magnetic property is crucial to some applications. Therefore, studies on the physical mechanisms elucidating the intrinsic luminescent and magnetic properties are carried out, which assists in the understanding of their viability in some biological applications such as bio-separation and MRI. Multifunctional GdF₃:Eu³⁺ nanoparticles were synthesized using a hydrothermal method. Photoluminescent excitation and emission spectra, and lifetime were measured. The average lifetime of the nanoparticles is about 11 ms. The nanoparticle exhibits paramagnetism at both room temperature and low temperature, ascribing to noninteracting localized nature of the magnetic moment in the compound. The magnetic properties of GdF₃:Eu³⁺ is intrinsic to the Gd³⁺ ions, which is unaffected by the doping concentration of the Eu³⁺ luminescent centres. A measured magnetization of about 2 emu/g is close to reported values of other nanoparticles for bio-separation. Secondly, the light absorption coefficient of biological tissues is minimum in near-infrared (NIR) region (750 - 1000 nm). In this work, nearly pure NIR-to-NIR upconversion (980 nm excitation; 807 nm emission) in multifunctional GdF₃ :Yb³⁺,Tm³⁺ nanoparticles is reported for the first time. As the NIR excitation and emission are away from the visible region and the simultaneous visible emission at 478 nm is greatly suppressed, these are beneficial to deeper tissue penetration, decreased damage to biological tissues and reduced autofluorescence. Detailed mechanisms elucidating the remarkable NIR-to-visible emission intensity ratio (I₈₀₇nm/I₄₇₈nm) of 105 are provided. Raman spectroscopy measurements suggest the high probability of energy transfer leading to NIR emission in GdF₃ host. Paramagnetism of the multifunctional nanoparticles is favourable to bioseparation. Finally, the optimal size for bioimaging probes is reported to be less than 10 nm. Therefore, water-dispersible ultra-small (<10 nm) multifunctional Ln³⁺ -doped nanoparticles with intrinsic magnetic and luminescent properties are developed. In this work, ultra-small multifunctional KGdF₄:Tm³⁺ ,Yb³⁺ nanoparticles with NIR-to-NIR upconversion were synthesized. The average sizes of KGdF₄:Tm³⁺ 2%, Yb³⁺ 20% core-only and KGdF₄:Tm³⁺ 2%, Yb³⁺ 20%/KGdF₄ core/shell nanoparticles are ~3.7 nm and ~7.4 nm, respectively, which fall within the reported optimal sizes (<10 nm) for bioimaging probes. The weak upconversion luminescence of the ultra-small core-only nanoparticles is overcome by the use of the core/shell approach. The magnetic mass susceptibility and magnetization of the ultra-small core-only and core/shell nanoparticles have been determined, and are close to those of large nanoparticles (26 and 50 nm) used for magnetic resonance imaging and bio-separation. There is no variation in the magnetic properties with the nanoparticle sizes between the core-only (~3.7 nm) and core/shell (~7.4 nm) nanoparticles, differing from the known size-dependent luminescence. The oleate-capped core/shell nanoparticles have been further encapsulated with a PEG-phospholipid shell to endow them with dispersibility in water, which is indispensable for future biological applications.