Facile synthesis of multifunctional red-emissive carbon dots for fluorescent quercetin and pH sensing

A convenient and cost-effective hydrothermal route was developed for the synthesis of red-emissive carbon dots (R-CDs) from neutral red and catechol. Superior properties of the R-CDs including high photostability, outstanding biocompatibility, and desirable biological penetration were found, which could realize the monitor of quercetin and pH. The quenching fluorescence of R-CDs was observed upon the addition of quercetin on account of a static quenching mechanism, achieving the concentration of quercetin in the scope of 0.66–111.33 μM with a detection limit of 0.19 μM. Furthermore, a smartphone based laboratory device and color processing software could be used for accurate testing of quercetin. Besides, the proposed method has been successfully quantified trace amounts of quercetin in red wine, lake water and human urine samples. Meanwhile, the R-CDs presented a pH response by both fluorescent and colorimetric mode. As the pH value increased, an excellent linear relationship was obtained against pH in the range of 6.20–7.60 accompanied with a significant color variation of the R-CDs solution from deep pink to yellow. Finally, the proposed R-CDs have also been applied to monitor the quercetin and pH in living cells due to its low cytotoxicity, providing a great capacity for various targets molecules detection.