High-Yield, Stable, and Ultrasensitive Carbon Dots for pH Sensing and Fe²⁺ Detection Are Synthesized at Room Temperature

Fluorescent carbon quantum dots (r-CDs) were synthesized safely and readily by stirring at room temperature with a yield of 76.5%. The synthesized r-CDs exhibit bright green emission with an absolute quantum yield of 12.0% under the excitation of ultraviolet light. Within the pH range of 3–12, the r-CDs exhibit excellent fluorescence stability. Interestingly, the fluorescence intensity of r-CDs faded promptly when the pH value diminished to 2 and then regained when the pH value returned to 3. This means that r-CDs can be used as “on–off–on” fluorescent probes for pH sensing. The mechanism of this sensitive pH response behavior was discussed in detail, and a possible mechanism of protonation–deprotonation–protonation processes was proposed. In addition, we have proposed a green preparation route for eco-friendly filter paper using anticounterfeiting. Interestingly, the presence of Fe2+ significantly quenched the fluorescence of r-CDs. Experimental results demonstrated the r-CDs can also be used to detect the content of Fe2+ in water, and the detection limit was 8.82 μM/L.