Carbon quantum dots with pH-responsive orange-/red-light emission for fluorescence imaging of intracellular pH.

N-doped carbon quantum dots (N-CDs) with polyaminobenzene hydrazine as precursor were prepared by solvothermal method for the monitoring of pH fluctuation in HeLa cells via fluorescence imaging. The N-CDs show two emission wavelengths at 582 and 640 nm under different pH with two excitation wavelengths. The fluorescence intensity at 640 nm (λ ex  = 520 nm) and the ratio of F 582 /F 640 (λ ex  = 470 nm) linearly increase with pH in the range of 2.4 ~ 3.6 (R 2  = 992) and 5.6 ~ 7.6 (R 2  = 0.987), respectively. The sensor exhibits high sensitivity and reversibility and anti-interference capability, thus enabling sensing pH change in intracellular environment in real time, as demonstrated by successful monitoring of intracellular pH fluctuation during H 2 O 2 stimulation in HeLa cells.