Semiconducting single-walled carbon nanotubes (SWCNTs) have been investigated as a promising intracellular and in vivo imaging probe due to their high photostability and near-infrared fluorescence. The conjugation of covalent sp3 defects on SWCNTs, organic color centers (OCC), enables a newly developing category of biosensors with their unique sensitivities and increased quantum yield. In this presentation, we show the development of an OCC-based optical nanosensor that is capable of quantitive and temporal measurement of lysosome activities in live cells and solid tumors in live mice. We report the optimization of the OCC-based nanosensor to improve biocompatibility, colloidal stability, and functionality for biomedical applications. We believe that the discovery of OCC sensitivities and new methods to control the SWCNT surface chemistry will lead to the development of novel biosensors and facilitate the understanding of the regulatory mechanisms of dysregulated biological systems in disease.