1 Department of Chemistry and Biochemistry, Centre for NanoScience Research, Concordia University 7141 Rue Sherbrooke Ouest Montreal QC H4B 1R6 Canada John.capobianco@concordia.ca.

Radioluminescent nanomaterials have garnered significant attention in the past decade due to their potential to perform X-ray mediated photodynamic therapy (X-PDT). Many of these materials are assumed to produce singlet oxygen based on a single assay. Herein we demonstrate that multiple assays are required to confidently determine whether singlet oxygen or other reactive oxygen species are being produced through type I or type II PDT mechanisms. Rose Bengal and Merocyanine 540 photosensitizers were loaded into mesoporous silica-coated NaLuF₄:Dy³⁺,Gd³⁺ nanoparticles and the combination of ABDA, DPBF, and NaN₃ assays along with electron paramagnetic resonance were employed to determine that superoxide and hydroxyl radicals were exclusively produced from this system under X-ray excitation. Knowledge of the correct PDT mechanism is crucial for informing what types of disease may be best suited for treatment using PDT nanosystems.