pH-responsive green fluorescent nitrogen-doped carbon dots for visualization of intracellular pH with intuitive color changes

Designing pH-sensitive nanoprobes for visualization of intracellular pH with intuitive color change is of high significance for better understanding the pathogenesis of diseases. However, most of the reported intracellular pH imaging strategies are the single intensity-based strategies, which are susceptible to interferences from background effects. In this work, pH-responsive green fluorescent nitrogen-doped carbon dots were synthesized for visualization of intracellular pH with intuitive color change. The as-prepared green fluorescent N-CDs exhibited obvious red fluorescence when the pH value was less than 7, while only the green fluorescence intensity changed under neutral or alkaline conditions, which can be visually observed by the naked eye. In addition, the mechanism of the green fluorescence of the N-CDs was systematically investigated by density-functional theory (DFT) theoretical calculation together with X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared spectrophotometer (FT-IR), as well as the mechanism of the response of N-CDs to pH was systematically investigated by absorption spectrum and transmission electron microscope (TEM). Furthermore, the as-prepared green fluorescent N-CDs were used to visualize intracellular pH with intuitive color changes, demonstrating its promising potential in visual bio-imaging.