A pH, thermal, and redox potential triple-responsive expansile nanogel system (TRN), which swells at acidic pH, temperature higher than its transition temperature, and reducing environment, has been developed. TRN quickly expands from 108 nm to over 1200 nm (in diameter), achieving more than 1000-fold size enlargement (in volume), within 2 h in a reducing environment at body temperature. Sigma-2 receptor targeting-ligand functionalized TRN can effectively target head and neck tumor, and help Pc 4 targeting mitochondria inside cancer cells to achieve enhanced photodynamic therapy efficacy.