pH response mechanism of bifunctional fluorescent carbon quantum dots and application in cancer detection and bioself-targeting imaging

The development of fluorescent probes suitable for lysosome visualisation as well as real-time tracking is crucial because lysosomes play a crucial role in numerous metabolic activities and diseases. In this paper, nitrogen-doped carbon quantum dots (NCQDs) enriched with a large number of hydroxyl, amino, and carboxyl groups were prepared by a simple solvothermal method using tartaric acid and 4-aminosalicylic acid as raw materials. The fluorescence intensity of NCQDs decreased with increasing pH (1-13) due to protonation and deprotonation as well as hydrolysis of surface groups in alkaline environment and detachment of chromophores. This enabled the NCQDs to achieve label-free and rapid visualisation of pH in cells. The disparity in pH levels between cancer cells and normal cells resulted in varying fluorescence brightness of NCQDs observed in these two cells, which enabled the differentiation of cancer cells from normal cells. Most importantly, when NCQDs entered the cell, lysosomes could provide protons for NCQDs, and the protonation of NCQDs' surface groups led to stronger fluorescence after endocytosis of NCQDs in lysosomes. Thus, lysosomal targeted imaging could be achieved without the need to introduce lysosomal-targeted ligands, and NCQDs had a higher biocompatibility and stability compared to commonly used commercial dyes, which made it the ideal choice of lysosomal-targeted fluorescent probes.