Exploring unique optical properties of upconversion nanocrystals towards a new library of molecular probes

This thesis explore the next-generation inorganic luminescent nanomaterial - upconversion nanocrystals, for advanced cell imaging and molecular sensing. Unlike traditional fluorochromes, e.g. fluorescent proteins and organic dyes, lanthanide-doped upconversion nanocrystals are able to efficiently convert the near-infrared excitation light to shorter wavelength emissions, e.g., the visible light. Upconversion nanocrystals are non-blinking, non-bleaching and immune to the autofluorescence interference, ideal for long-term background-free detecting trace amounts of target biomolecules or rareevent cells. The investigation of this thesis falls into two categories to advance upconversion nanocrystals to address needs of frontiers of bioimaging, biosensing, and biomedical diagnostics. The first part presents the explorartion towards lifetime-encoded multuplexed upconversion nanocrystals. We studied the impact of different nanocrystal sizes on optical properties of NaYF4:Er/Yb nanocrystals in both the spectral and temporal domains. We discovered the upconversion lifetime can be precisely tuned by controlling sizes of nanocrystals since the effect of surface quenching becomes increasingly dominating as the nanocrystals size shrinks. Combining with the fact that the lifetime can be engineered through varying dopants concentrations and tuning the fluorescence resonance energy transfer efficiency, respectively, we have established three independent mechanisms to produce non-crosstalking lifetimes and succeeded in demonstrating the lifetime multiplexing concept in the upconversion luminescence. In the second part, for the first time we discovered that the brightness of highly doped upconversion nanocrystals can be leveraged by a large dynamic range of excitation irradiance. We found that under the intense irradiance (unexplored regime previously), the concentration of Tm³⁺ emitters could be significantly increased from 0.5 mol% to 8 mol% in NaYF₄ nanocrystals without quenching the upconversion luminescence. This finding realized the remote tracking a single nanocrystal within the microstructured fibre dip sensor, which represents three orders of magnitude improvement than that of quantum dots. We also described that a combination of low-/high-Tm³⁺ doped upconversion nanocrystals holds the promise of being desirable luminescent nanomaterials for sensing, imaging, and security printing applications.