Energy-modulated x-ray fluorescence and luminescence emissions from therapeutic nanoparticles

In this study, we have investigated the possibility of modulating x-ray fluorescence (XF) and x-ray luminescence (XL) emissions from therapeutic nanoparticles (NPs) by fine-tuning the energy of incident x-rays from benchtop x-ray sources. We have carried out detailed experimental studies to determine the strength of XF and XL emissions from Y₂O₃:Eu³⁺ and LaF₃:Tb³⁺ NPs being irradiated with x-rays from benchtop x-ray sources operated with different tube-voltages and coupled to various filter configurations. These studies demonstrated that low-energy x-rays with average energy at around 10-15 keV are the most efficient to stimulate XL emission from the Y₂O₃:Eu³⁺ and LaF₃:Tb³⁺ NPs. The efficiency falls quickly when x-ray energies go above or below the optimum energy range. As one would expect, x-rays with average energy just above the corresponding absorption edge of the target metal would be the most efficient in inducing XF emission. In this study, we have also demonstrated that one could fine-tune the incident x-ray energy to modulate the XL and XF emissions, such as (a) selectively inducing either XL or XF emission from the same type of NPs, (b) inducing preferential XL activation of Y₂O₃:Eu³⁺ over LaF₃:Eu³⁺ or controlling the ratio of XL activation of these two types of NPs, and (c) introducing preferential XF emission from one type of NPs over the other. As a potential application, one could optimize the energy-characteristics of the incident x-rays to facilitate multiplexed combinatorial delivery of photodynamic therapy (X-PDT), where different agents could be administrated and then selectively activated in user-defined spatial and temporal patterns to fulfill combinatorial therapeutic effects. The understanding gained through this study could prove critical for enhancing the therapeutic delivery in X-PDT, and for attaining high-quality x-ray fluorescence computed tomography (XFCT) and x-ray luminescence computed tomography (XLCT) images while minimizing the x-ray dose to the sample.