Bioconjugation study of upconversion nanoparticles for cancer detection

Over the past decades, nanotechnology and fluorescence imaging techniques have become increasingly attractive for early detection of cancer. The high sensitivity required for differentiating abnormal cells from normal cells presents a challenge to existing diagnostic protocols. Among other types of nanomaterials, lanthanide-doped upconversion nanoparticles (UCNPs) have unique optical properties and thus hold a great promise in cancer detection at a very early stage using fluorescence imaging. Large Stokes and anti-Stokes shifts, long luminescence lifetime and excellent photostability make UCNPs a good choice for biological and biomedical applications. On the other hand, biological detection based on UCNP-probes is seriously limited by aggregation of the UCNPs when interacting with biomolecules and by non-specific binding in cell-based detection assays. This thesis explores a series of novel UCNP bioconjugation strategies to overcome the current barriers in bioapplications of UCNPs by enhancing their water dispersibility and stability and decreasing the non-specific binding of UCNP-biomolecule conjugates. In this research, NaYF₄: Yb³⁺/Er³⁺ and NaYF₄: Yb³⁺/Tm³⁺ UCNPs were synthesized and a NaYF₄: Yb³⁺/Er³⁺@NaGdF₄ core-shell structure was used to improve the upconversion luminescence intensity (Chapter 2). Various surface modification methods including ligand attraction, ligand oxidation, ligand exchange and silica coating were tested and compared to identify a suitable approach for the subsequent bioconjugation study (Chapter 3). Two novel bioconjugation strategies will be presented in detail. Chapter 4 illustrates conjugation of UCNPs to the MIL-38 antibodies through a biotin-streptavidin bridge. Chapter 5 demonstrates a modified one step bioconjugation strategy between UCNPs and MIL-38 antibodies by using a poly(ethylene glycol) (PEG) linker and orientation controlled functionalization of antibodies. Both novel UCNP-biomolecule conjugation strategies produce water-stable and dispersible products and the biomolecules tested retained their biological activity during the conjugation process. In addition, a fabricated UCNP-antibody complex specifically targeting DU145 prostate cancer cells without detectable non-specific binding was achieved.