Anisotropic surface functionalization and applications of upconversion nanoparticles

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Lanthanide-doped upconversion nanoparticles (UCNPs) step-wise convert near-infrared adsorbed light into visible emitted light. Monodispersed UCNPs have a large anti-Stokes shift, sharp emission and long excited-state lifetimes. The controlled synthesis of their heterogeneous nanostructures can create different morphologies with different properties. This makes it possible to use these particles to perform biomolecular assays, multi-scale and multi-modal imaging, as well as targeted delivery of drugs for nanoscale therapies. To make these inorganic nanomaterials practically useful, their surfaces require functionalization with active chemical groups that target important specific biomolecular and cellular structures within the body. Current literature assumes UCNPs are spherical in shape and that isotropic surface modification strategies such as ligand oxidation, amphiphilic ligand interaction (insert), ligand exchange, and silica coating techniques are sufficient for their surface modification. In this thesis, the author explores the surface coverage at the nanoscale of a library of non-spherical rods, discs, and dumbbell structures of UCNPs, and demonstrates that different facets of nanocrystals can be selectively modified by DNA and surfactant molecules, and silica coating layers. This results in the creation of anisotropic surfaces on these particles, i.e. one where nanocrystals can have either/both hydrophilic or/and hydrophobic surfaces depending on their facet arrangement. It is hoped that outcomes from this research advance understanding of nanoscale surface chemistry and introduce a series of surface modification techniques that could lead to new uses for these particles. The addition of anisotropic silica coating and controllable self-assembly of nanocrystal building blocks could also lead to tailored efficiency and behaviours in the cellular uptake of this new class of nanoparticles. Chapter 1 summarises the unique optical properties of UCNPs techniques used for their surface modification methods and their many uses. This chapter lists current literature that reports on the anisotropic surface properties of other nanoparticles such as gold nano-rods, and their potential usefulness. This leads to the inspiration for this thesis and a description of its four-specific research goals. To justify the feasibility of its hypothesis, that is, that different crystalline facets result in different anisotropic surface properties of UCNPs with unique behaviours, this chapter describes some preliminary studies that the author was involved that led to the work described in this thesis. Specifically, the author’s work on the controlled synthesis of non-spherical UCNPs and the computational simulation of their surface properties and their calculated interactions with common surfactant molecules. Chapter 2 provides a detailed description of the materials, preparation and characterization methods used in the work described in this thesis. Chapter 3 reports on a simple “mix-and-shake” method to selectively modify the facets of UCNPs by the addition of synthetic DNA molecule strands, with or without the phosphate group at the ends of these molecules. A range of characterization techniques were then described to verify that the different facets of these UCNPs were tailored to have hydrophobic and hydrophilic surface properties, enabling their usefulness in a range of interesting applications described in the three chapters that are next. Chapter 4 describes a method that enabled the addition of an anisotropic silica coating onto a range of UCNPs that have various shapes. The addition of this coating can be switched on/off by controlling the surface conditions of the particles using the facet-selective modification of DNA molecules. According to the results described, silica shell was only deposited onto UCNP facets that had not undergone DNA modification, regardless of the specific morphology of the UCNPs tested. In this chapter, the feasibility, mechanism, and reproducibility of this coating approach were provided. Chapter 5 describes the technique used for the controlled self-assembly of UCNPs and their hybrid structures. Both side-by-side and end-to-end self-assembly of rod-shape UCNPs structures were reported. Moreover, an end-to-end pattern of self-assembly technique was reported that created a hybrid structure with gold nanoparticles in between of pairs of UCNP, forming a one-dimensional chain structure. In this chapter, the conditions for obtaining these self-assembly structures were described. Chapter 6 describes the evaluation of the different behaviours of UCNPs that possess different isotropic and anisotropic surface properties with regard to their cellular uptake. An improved efficiency in this uptake was observed in UCNPs with anisotropic surface properties, and this suggests a new controllable feature for their use in drug delivery applications. The results reported in this thesis provide a new understanding of the surface properties of UCNPs. A possible outcome of this work is a new range of uses for this particle technology through the precise control of their surfaces.
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