Figure 1. Ultra pH-sensitive (UPS) nanoprobes shed new light on tumors. The UPS nanoprobes stay 'off' at pH 7.4 in normal blood vasculature. After reaching tumors, the UPS nanoprobes are turned 'on' when the nanoparticles disassemble in the acidic extracellular pH (pHe 6.5-6.8) within the tumor environment or disassemble in the intracellular pH (pHi 5.0-6.0) of endocytic organelles within the endothelial cells of angiogenic tumor vasculature. (Figure based on Figure 1 in ref. 1.)