Upconverting nanoparticles: pushing theory and technology towards biomedical applications

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Souza Matias, Jean
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University College Cork
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Biophotonics faces a significant challenge in developing non-invasive imaging and interrogation techniques with high spatial resolution and penetration depth for precise diagnosis and treatment. These techniques rely on non-ionising radiation and non-toxic contrast agents possessing excellent photo and chemical stabilities. Among the various non-ionising wavelengths, the near-infrared (NIR) optical window offers the highest tissue penetration due to its minimal scattering and absorption in living tissues. Upconverting nanoparticles (UCNPs) possess the desirable properties of absorbing and emitting NIR light, making them ideal contrast agents for biomedical applications. UCNPs have shown promise in deep tissue imaging, optogenetics, photodynamic therapy, temperature sensing, drug delivery, and super-resolution microscopy. However, the efficiency of upconversion (UC) in UCNPs, as quantified by the quantum yield (QY), remains a significant challenge, particularly at low excitation power densities (PDs) where the non-linearity of upconversion luminescence (UCL) dominates. Furthermore, the lack of commercially available devices and standardised protocols that account for the crucial parameters affecting PD-dependent QY further complicates accurate characterisation of these materials. To address these issues, this thesis presents the design and construction of a comprehensive, broad-band, multi-variable QY characterisation system. This opto-electronically engineered setup enables simultaneous measurement of absorption and luminescence at two selected wavelengths. In addition, the fully automated system incorporates capabilities for characterising excitation beam profiles, scattering, and the emission spectrum of luminescent compounds in aqueous solutions. Accurate characterisation of the excitation beam profile is of particular importance due to its influence on the PD-dependent QY behaviour of UCNPs. Varying beam profiles lead to distinct QY values, necessitating beam profile compensation to derive an intrinsic QY property of the material independent of measurement configuration. However, achieving this compensation requires a comprehensive understanding of the mechanisms governing each UC emission wavelength, which has not yet been extensively studied across a wide range of PDs. Consequently, this thesis also includes a detailed theoretical study based on the rates of populating and depopulating the electronic energy states involved in UC processes. The investigation successfully quantifies the transition points at which the UCL transitions from non-linear behaviour of different orders to linearity. The theoretical model is validated using experimental UCL data acquired from two distinct UCNP compounds at various wavelengths. These combined theoretical and experimental studies are of utmost importance for the accurate characterisation and engineering of optimal UCNPs, representing a crucial advancement in the development of high-resolution, deep-penetration biomedical techniques and devices.
Upconversion , Upconverting nanoparticle , UCNP , Quantum yield , Biophotonics , Power density
Souza Matias, J. 2023. Upconverting nanoparticles: pushing theory and technology towards biomedical applications. PhD Thesis, University College Cork.