Herein, dual-emission carbon dots (CDs) were facilely prepared, CDs have a narrow size distribution, and the mean particle size is about 3.5 nm. CDs exhibit good water dispersibility and can emit intense green fluorescence under 365 nm UV light in an aqueous solution, which can be stable in different conditions. CDs used as ultrabright fluorescent probes for highly sensitive self-monitoring of pH on account of ratio of dual fluorescence intensities (I336 nm/I540 nm) against the pH variation from 2.5 to 12.0. The CDs exhibit cell permeable properties and a distinct pH-sensitive/excitation-independent PL emission feature, yielding significantly an optical probe for intracellular pH sensing and multicolor imaging of live HeLa and MDA-MB-231 cells with micron lateral resolution. Besides, MTT assay revealed the cell viability did not change upon treatment with CDs probe during 36 h.
