A subphthalocyanine substituted with nine tetra(ethylene glycol) chains on the periphery has been synthesised. This novel amphiphilic and cone-shaped compound can self-assemble in water into spherical nanoparticles with a hydrodynamic diameter of 154 nm. These nanoparticles can be taken up readily by cancer cells and localised predominately in lysosomes where they disassemble gradually, leading to activation in fluorescence emission and, photocytotoxicity, showing IC50 values of as low as 1.2 μM.