Nd-doped Upconverting Nanoparticles for Deep Tissue Imaging

Abstract: During the last decade, upconverting nanoparticles (UCNPs) doped with rare-earth (RE) ions have been extensively studied in the field of biophotonics. Due to their unique properties of anti- Stokes shifted luminescence and with excitation and emission wavelengths optimal for biomedical imaging, they have become an interesting class of fluorescent contrast agents. An issue with downconverting fluorescent contrast agents for imaging in tissue is the inevitable autofluorescence that overlaps the signal from the added fluorophore, limiting the signal-to-background ratio. With UCNPs it is possible to achieve a near autofluorescence free signal due to their upconverting nature. Up to date, one of the most efficient UCNPs is the Yb/Tm NaYF4 UCNPs. With an excitation wavelength of 975 nm and emission at 800 nm they operate in the diagnostics window (600 - 1200 nm), allowing for substantial tissue penetration. Unfortunately, water has an absorption peak at 975 nm, leading to a loss in penetration depth as well as heat produced in the tissue. Very recently, Nd/Yb co-sensitized UCNPs have been synthesized, tuning the excitation wavelength to 808 nm. With a lower water absorption at 808 nm the new co-sensitized UCNPs are proposed to gain in penetration depth as well as reduce heat production in tissue. This thesis explores the advantages of using the newly proposed UCNPs as compared to Yb/Tm UCNPs. By using tissue phantoms with realistic optical properties, the penetration depth as well as the signal loss was evaluated experimentally. Additionally, simulations were performed in order to further explore the benefits of the new particles. Based on the results presented in this work, the new UCNPs may very well replace the traditional particles for certain applications, and may lead us one step closer to finding the optimal tools for biomedical applications.