2025 Theses Doctoral

Thulium-doped Avalanching Nanoparticles

Xu, Emma

Innovations in optics and photonics are crucial to propel forward the technologicaladvancement of our society. In particular, the phenomenon of photon upconversion using lanthanide ions holds great potential for many applications such as solar energy harvesting, bioimaging, and anti-counterfeiting technologies. During my journey of studying lanthanide- doped upconverting nanoparticles, I helped discover the first nanoparticles doped with Thulium (Tm) ions that exhibit the photon avalanching (PA) behavior. PA is a special case of photon upconversion; our discovery, called avalanching nanoparticles (ANPs), was exciting for the field because it exhibits highly optically nonlinear behavior and increases the quantum efficiency of the upconversion process by an order of magnitude compared to regular upconversion, thus opening up new applications. In this thesis, three major applications are explored: viral disinfection, photoswitching, and bioimaging.

To study how ANPs can be applied for viral disinfection, we used photoluminescence measurements to observe the UV emission from ANPs when pumped with a 1064nm laser. The emissions at 360nm and 340nm correspond to the 1D2 to 3H6 and 1I6 to 3F4 energy level transitions in Tm respectively. While the 290nm emission, corresponding to the 1I6 to 3H6 transition, was not observed due to limitations within the optical setup, it is safe to infer that they do exist since the 340nm emission comes from the same excited state. All three of the aforementioned emissions are within the UV spectrum and are germicidal. To understand how many ANPs, and how much pump power is needed to disinfect a surface area, we proposed to incorporate ANPs inside N95 masks, and disinfect them with NIR (1064nm) light. We modeled various parameters: laser power (10- 100W), ANP in polymer volume (1.5-15%) and the quantum yield of ANPs (0.3-10%). Most of the calculation yielded a faster disinfection rate than the current state-of-the-art. Disinfecting with NIR light compared to UV is thus not only more efficient, but is also better and safer for the materials and humans present.

While doing measurements on ANPs, I unexpectedly observed them blinking on and off mid-measurement. This is surprising because they were supposed to be extremely photostable according to all the literature on upconverting nanoparticles in the last couple of decades. To better understand this, we did various experiments to rule out any external possibilities with photoluminescence microscopy, atomic force microscopy, and temperature measurements; we concluded the blinking was an intrinsic property of the Tm ions. We learned to deterministically control the blinking process, In the end, we demonstrated several potential applications with the photoswitching property of ANPs, including super resolution imaging.

To demonstrate bioimaging with ANPs, we incubated ANPs in MDA-MB-231 cancer cells rich with receptors. We showed that just like single ANPs on a coverslip, ANPs in cells can also show sub-100nm resolution when imaged with a laser scanning confocal microscope. To better understand how ANPs are distributed within a given cell, 3D sectioning is performed on a cell with 11 frames in the XY direction, 1 um away from each other in the Z direction. With quantitative analysis of the distribution of ANP emission, we concluded that the ANPs were distributed mostly as monomers, with decreasing chance of forming small clusters of dimers, trimers, and quadruplets. This could infer the clustering tendencies of the cancer cell receptors. Imaging with ANPs as the probe thus offer the capability of conducting quantitative analyses that fluorescent probes don’t.