Surface conjugation of upconversion nanoparticles via supramolecular host-guest self-assembly

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Surface functionalization and conjugation hold the key to driving many purpose-synthesized nanoparticles into real-world applications. Lanthanide (Ln) ions doped upconversion nanoparticle (UCNP) is a good example: while it delivers a great promise as molecular probes, sensors, drug carriers and light transducers for a library of application in the area of biological and physical science. However, the hydrophobic surface of UCNPs, sometimes, is one of the main challenges for further applications, especially in natural science. It remains a bottleneck for the community to explore practical strategies to functionalize the UCNPs surfaces for hydrophilicity conversion and further bioconjugation to meet many cellular and molecular specific needs. Supramolecular self-assembly is one of the leading topics in supramolecular chemistry. It is a process in which disordered molecular building blocks are specifically re-ordered and also a phenomenon where the components of a system assemble themselves spontaneously via interaction to form a larger functional unit. In the view of thermodynamics, self-assembly is an equilibrium process where the assembled components are in equilibrium with the individual components. In other words, it is driven by the minimization of Gibbs free energy. Compared with molecular chemistry, self-assembly, as the most critical component of the assembled method, was formed by intermolecular forces. This thesis shows a new surface conjugation approach for UCNP modification via supramolecular host-guest interactions leading as-made oleic acid (OA) stabilized UCNPs (OA-UCNPs) from hydrophobic into hydrophilic. At the same time, the modified host molecules on the surface of UCNPs can provide a considerable number of cavities, which can form the host-guest interaction with a series of molecules, such as anti-cancer drugs, biomolecules, and inorganic materials. The advantage of the modification approach is no further reaction formed for conjugation, which hugely increases the binding amounts and also shows the excellent binding behaviour by using the different binding systems. This thesis is aiming to conduct studies of: (i) the investigation of the surface of OA-UCNPs (in Chapter 3) To convert UCNPs with high efficiency and yield, the original cover needs to be investigated. We predict and proof the binding strength and binding mode between an OA and two ß-NaYF4 crystal surfaces of UCNPs. (ii) the binding behaviours of different organically functional groups (Chapter 4) Compared with the original of OA from the surface of UCNPs, we also choose several organic molecules which hold different functional groups. We discuss the effect of different functional groups and, most importantly, the binding mode and binding behaviour on the surface of UCNPs. (iii) the investigation of binding behaviours of macrocycles on the surface of UCNPs and their applications in binding with organic molecules, biomolecules, and inorganic materials (Chapter 4 ~ Chapter 6). We raise and design a new approach to convert the surface of UCNPs with different kinds of macrocycles. Then, we provide three applications in the areas of drug delivery, cell targeting, and energy transfer.
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