Single upconversion nanoparticle optical characterizations for biophotonic applications

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Lanthanide elements-doped upconversion nanoparticles (UCNPs) are of great interest in both biophotonics and nanophotonics. These nanoparticles possess many advantages including low background noise level, low excitation energy, high photostability, tunable wavelength and lifetime, and high tissue penetration depth. However, the development of UCNPs has been hindered by a lack of quantitative analysis technology. Such technologies must have the ability to distinguish a single particle; and the sensitivity to detect the emission lifetime and spectrum of a single particle, both of which are challenging to achieve with the current optical measurement instrumentation. In my PhD thesis, I will describe a novel single UCNPs characterization method, which offers a way to achieve highly precise, efficient, and quantitative measurements (Chapter 2). This technology will be applied in Chapter 3 to investigate the energy transfer process in the core shell UCNPs. The results revealed that the optimized shell thickness is 6.3 nm for fluorescence probe. In Chapter 4, I used this single particle characterization system to investigate the self-interference phenomenon of UCNPs on a mirror surface. Based on the observed intensity curves, I developed a nanosensor with the ability to detect axial position with 10 nm resolution. More importantly, I found an incomplete destructive interference (IDI) phenomenon, which may enable real-time super-resolution microscopy in the future. In Chapter 5, conclusions are drawn and the prospectives including single UCNP lasing, mirror enhanced super resolution, and interaction of multi-emitters in a UCNP are introduced.
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