The radiosensitizing effect of 5-250 nm diameter Au nanoparticles (AuNPs) in water was investigated under irradiations of diagnostic x-ray and UV light. Enhanced generations of hydroxyl radical ((·)OH) and superoxide anion (O(2)(-)) were confirmed from their dependencies on the absorbed energy, ethanol concentration and AuNPs' concentration. Two kinds of fluorescent probes revealed that the reactive oxygen species (ROS) generation rate under x-ray irradiation was enhanced by factors of 1.46 for (·)OH and 7.68 for O(2)(-). Photo- and Auger electron charge transfer is possibly relevant to generation of O(2)(-) near the particle surface, whereas fluorescent x-rays are involved in generation of (·)OH in the secondary water radiolysis. Smaller diameter AuNPs with larger surface area showed a greater yield of ROS. An inverse proportion of ROS generation to the AuNPs' diameter suggests a catalytic function of AuNPs' surface for enhanced ROS generation.
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