Development of NaYF4:Yb,Tm Upconverting Nanoparticles as a Nano-Tool for Near-Infrared to Ultraviolet Photochemistry

Abstract

A major problem which stymies the use of some photochemical techniques for biomedical use is the requisite ultraviolet (UV) radiation to initiate chemical reactions. UV covers the range of the electromagnetic spectrum from 10 to 400 nm. The majority of UV initiated photochemistry occurs within the range of 280 to 380 nm which constitutes the UVA and UVB. However, this range of UV is affiliated with carcinomas, DNA damage, immune modulation and mitochondrial damage.[1–3] Upconversion nanoparticles (UNPs) have the ability absorb multiple near-infrared (NIR) photons to produce emission with a higher energy than any of the individual absorbed photons ranging from higher energy NIR to UVB emissions. By utilizing NIR excitation, one can drive photochemical reactions in biological environments at the site of the UNPs rather than direct UV radiation to the culture or tissue. The biological window includes the NIR between 700 and 1350 nm which fortunately includes the range of excitation wavelengths in UNPs occur. This presents a serendipitous opportunity to develop systems for NIR triggered photochemical reactions without the need for a continuous dose of UV irradiation. The major goal of the research presented in this dissertation was to synthesize functional upconversion nanoparticles for improved UV emission, explore their potential as a nano-tool for photochemistry while maintaining imaging capabilities, and developing a novel UNP that will increase the appeal of UNPs as biomedical technology. The first aim was to perform an exhaustive study of the available preparative mechanisms for controlling the UV emission of UNPs (chapter 2). The second aim was to successfully and reproducibly synthesize UV emitting UNPs, and to functionalize these nanoparticles for use in optical techniques (chapter 3) to understand their potential as both a bioimaging agent and photochemical tool (chapter 4). The third aim was to develop a new paradigm for UNPs which utilizes a kinetics-based approach to harvest UV energy from UNPs without UV emission (chapter 5). First, a broad analysis of how to prepare particles for maximizing UV emission. These preparative techniques were broken into three categories: hosts, dopants and architecture. Tuning of UV emission and mechanisms by which it is done were systematically analyzed. Biological and medical appeal of UV for photochemistry were observed. Lastly, the compatibility between different techniques as well as the compatibility between certain techniques and different purposes of UNPs were considered. Second, synthesis and functionalization of UNPs were performed so the optical properties and applications of the UNPs could be determined. This included the discovery of a post-preparative excitation technique to control the UV emission from UNPs by means of modulating the excitation pulse width. Third, a new type of UNP, intended specifically to advance photochemistry for biological environments was developed. By utilizing known rate constants from lifetimes, and theoretical rates of energy migration or transfer based on literature a kinetic model of UNPs was drafted. To achieve the parameters of the kinetic model a thorough investigation of UV quenching co-dopants were examined. Lastly, to verify the kinetic model UNPs were combined in suspension with fluorescent energy transfer acceptors. Energy transfer was measured by the compared luminescence spectra of UNPs with and without the energy acceptor.