To enhance localization and precision of therapeutic radiation delivery, nano-radiosensitizers have been developed. A specific volume of comprising surface atoms is known to be the radiosensitizing region. However, the shape dependent local dose enhancement of nanoparticles is often underestimated and rarely reported. Here, a noble metal nanostructure, inspired by the photoperiodic day-flowers, was synthesized by metal reduction with bile acid molecules. The impact of high surface area of day-flower like nanoparticles (D-NP) on radiosensitizing effect was demonstrated with assays for ROS generation, cellular apoptosis, and clonogenic survival of human liver cancer cells (HepG2) cells. In comparison with lower surface area spherical night-flower like nanoparticles (N-NP), exposure of our D-NP to external beam radiation doses led to a significant increase in reactive oxygen species (ROS) production and radiosensitizing cell cycle synchronization, resulting in enhanced cancer cell killing effect. In clonogenic survival studies, dose enhancing factor (DEF) of D-NP was 16.5-fold higher than N-NP. Lastly, we demonstrated in vivo feasibility of our D-NP as a potent nano-radiosensitizer and CT contrast agent for advanced image guided radiation therapy.