Tumor-specific fluorescence activation of rhodamine isothiocyanate derivatives

Fluorescence is routinely used for in vivo tracking and imaging of molecules and nanostructures with assuming that the fluorescence intensity is proportional to the dye concentration. Herein, we report the unique tumor-specific fluorescence character of rhodamine B isothiocyanate derivatives (RBITCs), which emits fluorescence selectively in cancerous tissues, including small metastatic tumors, but is quenched in blood and healthy tissues. A preliminary mechanism study shows that binding of the thiourea group in the RBITCs on hemoglobin quenches their fluorescence, but the oxidation of the thiourea by the elevated reactive oxygen species in tumor activates the fluorescence. Thus, the fluorescent intensity of RBITCs is associated with the microenvironment of tissues and positively correlates with the cancer stages. These findings suggest that the RBITCs are not suitable for tracking of cargos in the presence of red blood cells but may be useful for cancer imaging and early diagnosis, and probing the tumor microenvironment.