Grant: Research Tools and Instruments

The X-ray excitation of materials can have dramatic impacts on their ensuing scattering and relaxation phenomena. In radiation therapy, sensitizers are materials that enhance the absorption of X-ray radiation, thereby increasing the local therapeutic effect of radiation in reducing/eliminating tumours. Metals, especially metal nanoclusters (MNCs), have high X-ray absorption cross sections and are thus being investigated as next-generation sensitizers for radiation therapy. At the heart of advancing this field is the need for a deeper understanding of the relaxation processes following X-ray excitation. The current proposal aims to build instrumentation capable of measuring both photoluminescence and X-ray excited optical luminescence using a single set of collection optics and spectrometers, enabling the quantitative characterization of emissions resulting from absorbed X-ray radiation in some exceptionally performing new materials. MNCs are nanomaterials comprising up to several dozen metal atoms at their "core" and have surfaces passivated by organic ligands; due to this distinctive structure, they are able to both absorb significant quantities of X-ray radiation and (among other processes) emit visible light while in an electronically excited state. To the best of our knowledge, the scintillator efficiency (i.e., the ability to convert X-rays to optical photons) has never before been quantified for such materials. The present work aims to measure scintillator efficiency for numerous, recently synthesized MNCs by using internal standards to compare their efficiencies with those of compounds with well known photoluminescence quantum yields. These new measurements will become the benchmark, or standard, against which other materials can be tested and, therefore, evaluated for their ability to generate emissive and/or reactive excited states by absorbing X-rays. This work will greatly enhance the ongoing research of several collaborating groups that have isolated more than a dozen new MNCs, each with structures and ligands specifically designed for use in radiation therapy. Some of these MNCs have already been shown effective, exhibiting unprecedentedly large dose-enhancement factors (DEFs, a measure of inducing cell death upon X-ray irradiation). The combination of highly emissive excited states and high DEFs make MNCs the perfect candidates to serve as benchmarks for scintillator efficiencies.