An activatable photoacoustic nanoprobe based on a semiconducting oligomer with amplified brightness and pH-sensing capability is developed by taking advantage of nanodoping to simultaneously create both intraparticle photoinduced electron transfer and intramolecular protonation within a single particle. This organic nanoprobe permits non invasive real-time ratiometric photo acoustic imaging of pH in tumors in living mice through systemic administration at a relatively low dosage.