Development of an Upconverting Nanoparticles Quantum Yield Measurement System

Abstract

Fluorescence imaging is a growing biomedical technique; it can be used to localize the luminescent biomarkers inside the tissue. Lanthanide-doped upconverting nanoparticles (UCNPs) are promising luminescent probes for multiple applications in biophotonics. They allow acquiring autofluorescence-free recordings with high spatial resolution. How- ever, upconverting nanoparticles have a low quantum yield, especially at low excitation power densities. In this thesis, an upconverting nanoparticles quantum yield measure- ment system is developed, the upconverting nanoparticles used are the sodium yttrium tetra fluoride doped with thulium and ytterbium ions, NaYF4: Yb3+,Tm3+. The quantum yield for UCNPs is dependent on the excitation power density. A well-characterized dye, DY-781, with a known quantum yield of 11.9%, is used as the reference dye in these measurements. Upconverting nanoparticles quantum yield is then obtained by using a comparative method. At the balance point, half way to completely saturating the excitation, the power density equals 3214 mW/cm^2 yield of approximately 0.33%. The quantum yield of upconverting nanoparticles can be presented as a function of the excitation power density, and the values for maximum attainable quantum yield is estimated to be 0.69%. The setup is successful for obtaining the saturation of thulium-doped upconverting nanoparticles as they saturate at low power densities.

Popular Abstract

I assume that you as a reader of this thesis are familiar with luminescence and fluorescence. Artists like to use luminous paint in drawings, and their beautiful artwork glows when UV light is directed at it; UV light is outside of the visible spectrum, so we only see the paintings themselves exhibit luminescence in the dark. Maybe you have used glow-in-the-dark stickers, a kind of fluorescent stickers used to label objects so that you can distinguish them in the darkness. Have you ever thought about making our cells glow as well? Of course, we are not going to make our entire body glow. The goal is to target specific cells inside our body; they would momentarily exhibit fluorescence so that we could trace the location of those cells by detecting the fluorescent light. In this way, we could mark the location of tumors or cancer cells as they have different tissue properties. And since one ideal fluorophore specifically targets one kind of cells, the fluorophore can also carry drugs directly to the cells and start the treatment. Imagine a future when, in order to treat tumors or cancer cells, you just need to go to a hospital, get one injection that illuminates the body, and get treated by a doctor. This method is way friendlier to the human body than the current tumor treatments such as surgery, radiation therapy, or chemotherapy. The processes of labeling cells with a fluorescent dye and visualizing the fluorescence image are called fluorescence imaging.

Fluorophore and light treatment do sound very attractive, but applying them to medical treatment is difficult. Our tissue exhibits high light scattering and absorption. This is why, when we place a strong flashlight against our hand, we cannot see the clear bone structure. The light cannot pass through the hand directly without being multiple-scattered, and it loses strength both due to multiple scattering and absorption. There is also obviously a limit to the amount of light that can be applied to our skin before it is damaged. Thus, increasing the light strength is not an ideal option. However, given that the optical properties are wavelength dependent, it is possible to find wavelengths where our tissue has relative low scattering and absorption, such that we can find the wavelengths that correspond to fluorophores. The fluorophores should have an emission wavelength in a region with low scattering and absorption.

Lanthanide-doped upconverting nanoparticles (UCNPs) show great promise in fluorescence imaging. UCNPs have a relatively long luminescence lifetime, and their emission photon energies are higher than their excitation photon energies. Unlike UCNPs, the fluorescence from biological tissue, autofluorescence, and its emission photons have lower energies than the ex- citation photons. UCNPs’ unique features allow the detection of weak signals and generate autofluorescence-free recordings. Also, UCNPs’ excitation and emission wavelengths are in the region where our tissues have low scattering and absorption. It has also been shown that UCNPs allow imaging with higher spatial resolution than conventional fluorophores. However, UCNPs have a relatively low fluorescence quantum yield, which means there are fewer emitted photons than absorbed ones, which increases the difficulty of detecting UCNPs’ fluorescence signals. A UCNPs quantum yield measurement system is developed in this thesis. Most of the work in this thesis presents the operation of the experiment. This system will help the development of fluorescence imaging enhancement.

• author: Li, Meng
• supervisor: Monirehalsadat Mousavi; Stefan Andersson-Engels
• organization: Atomic Physics; Department of Physics
• course: FYSK02 20161
• year: 2016
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: UCNPs, upconverting nanoparticles, QY, quantum yield, fluorescence, luminescence
• report number: LRAP-520 (2016)
• language: English
• additional info: LRAP-520 (2016)
• id: 8883472
• date added to LUP: 2016-06-22 09:34:26
• date last changed: 2016-11-15 13:58:39