Nano-Scintillators as Next Generation Tools for Optogenetics

Optogenetics is a technique that is used to study neural pathways and has the ability to activate or silence synaptic behavior using visible light. Currently, the visible light sources used for optogenetics are surgically implanted into the brain tissue, but this harmful and invasive technique may be avoided if suitable scintillating nanoparticles can be inserted via injection. The proposed nano-scintillator particles must adhere to rigorous parameters including being under 100 nm, uniform, nontoxic, and dispersible to be successful in this biological system. The phase, crystallinity, and dopant concentration must be optimized to absorb X-ray radiation and emit photons of the desired energy necessary to activate the selected neurons. This work has led to the production of a number of rare-earth-containing species that are attractive candidates for such an application. A modified core-shell synthesis was developed for the production of rare earth orthosilicate (RE2SiO5) nanoparticles. Starting with a silica core allowed for the formation of spherical, uniform particulates. An inorganic shell capable of producing visible light upon radiation was then deposited onto the silica. Manipulations of the shell reaction chemistry and thermal profiles were conducted to find the conditions that produce the desired crystallinity, phase purity, and crystallite size of the inorganic shell while limiting the amount of particle deformation. Yttrium orthosilicate (Y2SiO5) formed as sub 100 nm spheres, proved to be an efficient host for cerium dopants, and produced bright blue emission under UV and X-ray excitation. In order to continue to try to increase radioluminescence heavier rare earth silicates, such as gadolinium orthosilicate, were targeted and are discussed in chapter 3. This exploration produced several phases including Ce:Gd4.67(SiO4)3O (Ce:GSAP), Ce:Gd2SiO5 (Ce:GSO), and Ce:Gd2Si2O7, (Ce:GPS) nano-scintillators. In the modified core-shell synthesis the silica core served as both a template to ensure uniformity in size and shape, and could be manipulated to elicit different silicate phases. The addition of lutetium into the gadolinium silicate nanoparticles was also investigated. The mixed gadolinium/lutetium materials were also engineered to produce different silicate phases and showed enhanced light output under X-ray stimulation. Next, europium-doped rare earth oxide (Eu:RE2O3, RE = Y, Gd, Lu) nanospheres were targeted as light sources for inhibitory opsins, such as the red-shifted cruxhalorhodopsin. Monodispersed, spherical RE2O3 nanoparticles were produced via an urea-assisted homogenous precipitation followed by annealing. Eu:Y2O3, Eu:Gd2O3, and Eu:Lu2O3 were produced as uniform, 90 nm spheres and exhibited bright emission under X-ray excitation. The effects of different annealing conditions on the particle morphology and radioluminescence intensity were investigated. Additionally, two luminescence-enhancing techniques applied to the Eu:Y2O3 nanoparticles were shown to significantly increase the radioluminescence of the nanoparticles. Multiple synthetic routes for the synthesis of terbium-doped yttrium oxysulfide (Tb:Y2O2S) nanoparticles, that would meet the constraints for noninvasive optogenetic studies, were compared. Of the three methods used to synthesize the Y2O2S phase, the co-precipitation synthesis produced particles that meet the requirements of uniformity, dispersity, and sub 100 nm size to be tested for noninvasive optogenetics. Luminescence measurements of the Tb:Y2O2S nanoparticles under both UV and X-ray excitation were collected. The nano-scintillators exhibited the desired bright green emission with an integrated intensity greater than that of a commercial cerium doped lutetium orthosilicate (Ce:LSO) standard. The Tb:Y2O2S nanoparticles presented herein offer the unique potential to activate green light absorbing opsins without surgical implantation. In addition, cell proliferation assays were run on human cells incubated with the various nanoparticles as a preliminary cytotoxicity assessment. Cells exposed to the tested nanoparticles did not show a statistically significant difference in viability compared to control cells. Furthermore, promising results on surface modification, toxicity, biodistribution, and immune response studies carried out on the Ce:YSO nanoparticles are also discussed. The ability to produce these particles in a straightforward, uniform, and dispersible manner as well as their relative chemical and biological inertness make them exciting candidates for the optogenetics toolbelt