Disclosed are compositions that contain a plurality of biocompatible self-assembling molecules that transform from isolated molecules or spherical micelles in the circulation into cylindrical nanofibers in the acidic extracellular environment of tumors which can be used to achieve a higher relative concentration of imaging, drug delivery, or radiotherapeutic agents at the tumor site compared to non-tumor tissues. This transition is rapid and reversible, indicating the system is in thermodynamic equilibrium.
