NIR-emitting scintillation of YAG: Yb optical ceramics as testing platforms for medical bioimaging

Ionizing radiation, especially X-rays and γ photons, are essential in many tools and techniques developed for bioimaging. The most common application regards their use in direct medical imaging such as radiography and tomography. In such applications, the main building blocks of radiation detectors are scintillating materials, that absorb and down-convert the energy deposited by the incoming ionizing radiation to low-energy UV-Vis-IR light, which is then easily read out by common photodetectors. A valuable class of scintillating materials is represented by rare-earth doped garnets. Garnets in the form of single crystals are already used in some medical instruments and are also promising candidates for Time-of-Flight Positron Emission Tomography (TOF-PET) detectors where stringent requirements on fast time response are crucial to optimize the spatial localization of the tumours and detection accuracy. Moreover, they can be easily obtained as micro- and nano-sized powders and, in this form, they can be used as phosphors in optical bioimaging. The latter technique consists in the inoculation of the nanoparticles in the tissues to be treated and the imaging occurs through laser-stimulated luminescence in the biological window where tissue absorbance is minimal (700 – 1350 nm). However, this approach presents some drawbacks: the signal monitoring is difficult since excitation and emission lights are close in frequency, and more importantly, the required laser power is very high and may lead to skin damage and to autofluorescence [1]. Here we explore a new approach in optical bioimaging by exploiting the scintillation properties of specifically designed Y 3 Al 5 O 12 (YAG) nanoparticles doped with near infrared emitting rare-earth ions (Yb, Nd, or Er). The needed NIR emission is quite unusual for standard scintillating materials and not yet adequately investigated [2], since they have been optimized to match the sensitivity peak of conventional photodetectors in the visible region. An important advantage afforded by this approach is the use of low dose X-rays as excitation source instead of lasers, thus eliminating autofluorescence, tissue damages, and detector overexposure. These materials can also be easily produced in the form of ceramics with the desired size and shape, not achievable by their single crystal counterparts, but with comparable optical quality, and more affordable costs. Therefore, for the development of YAG nanoparticles, we study the properties of rare-earth doped YAG optical ceramics as a transparent testing platform to identify the best doping ion and its concentration, towards the optimization of the scintillation properties of rare-earth doped garnets for optical bioimaging.