Development of low cost, easy-to-use chemical sensor systems for low dose detection of gamma radiation remains highly desired for medical radiation therapy and nuclear sensor molecule, 4,4'-di(1H-phenanthro[9,10-d]imidazol-2-yl)- biphenyl (DPI-BP), which can be dissolved into halogenated solvents (e.g., CHCl3, CH2Cl2) to enable instant detection of gamma radiation down to the 0.01 Gy level. The sensing mechanism is primarily based on radiation induced fluorescence quenching of DPI-BP. Pristine DPI-BP is strongly fluorescent in halogenated solvents. When exposed to gamma radiation, the halogenated solvents decompose into various radicals, including hydrogen and chlorine, which then combine to produce hydrochloric acid (HCl). This strong acid interacts with the imidazole group of DPI-BP to convert it into a DPI-BP/HCI adduct. The DPI-BP/HCl adduct possesses a more planar configuration than DPI-BP, enhancing the ir rr stacking and thus molecular aggregation. The strong molecular fluorescence of DPI-BP gets quenched upon aggregation, due to the it it stacking interaction (forming forbidden low-energy excitonic transition). Interestingly the quenched fluorescence can be recovered simply by adding base (e.g., NaOH) into the solution to dissociate the DPI-BP/HCl adduct. Such sensing mechanism was supported by systematic investigations based on HCl titration and dynamic light scattering measurements. To further confirm that the aggregation caused fluorescence quenching, a half size analogue of DPI-BP, 2-phenyl-1H-phenanthro[9,10-djimidazole (PI-Ph), was synthesized and investigated in comparison with the observations of DPI-BP. PI-Ph shares the same imidazole conjugation structure with DPI-BP and is expected to bind the same way with HCl. However, PI-Ph did not show fluorescence quenching upon binding with HCl likely due to the smaller pi-conjugation structure, which can hardly enforce the pi-pi stacking assembly. Combining the low detection limit, fast and reversible fluorescence quenching response, and low cost of halogenated solvent composites, the sensor system presented may lead to the development of new, simple chemical dosirnetry for low dose detection of gamma radiation.