Low temperature synthesis of phosphorous and nitrogen co-doped yellow fluorescent carbon dots for sensing and bioimaging

Phosphorous and nitrogen co-doped carbon dots (P,N-CDs) with satisfactory quantum yield have been prepared through one-step acidic oxidation of pumpkin by H₃PO₄ at low temperature (90 °C). The as-prepared P,N-CD is relatively monodisperse with a narrow size distribution. The P,N-CD displays a remarkable emission enhancement in the yellow fluorescence region (λ_em = 550 nm) when the pH is increased from 1.5 to 7.4. The pK_a value of P,N-CDs was found to be 4.17 and it shows linear response to the physiological range of pH 4.7-7.4, which is valuable for near-neutral cytosolic pH research. It is observed that P,N-CDs are superior fluorescent bioimaging agents in animals and cells thanks to their excellent solubility and ultra-low toxicity. In addition, P,N-CDs display a notably large Stokes shift of 125 nm, good reversibility and could effectively avoid the influence of autofluorescence in biological systems. The confocal fluorescent microscopic images of subcellular distribution and the detection of pH in MCF-7 cells were achieved successfully, suggesting that P,N-CDs have excellent cell membrane permeability and are further applied successfully to monitor pH fluctuations in live cells with negligible autofluorescence.