Synthesis of rhodamine modified carbon quantum dots and application of ratiometric fluorescence detection for Cu2+ and triple-mode detection for 2,4-dichlorophenoxyacetic acid

s Background 2,4-Dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide whose environmental residues pose threats to ecology and health. Copper ions (Cu 2+ ), as common heavy metal pollutants, can exhibit synergistic toxicity with 2,4-D, exacerbating environmental risks. Traditional single-emission fluorescent probes are typically limited to detecting a single target or operating in a single mode, making it challenging to achieve simultaneous monitoring of multiple pollutants. Therefore, developing novel multifunctional fluorescent probes capable of concurrently detecting both Cu 2+ and 2,4-D holds significant importance. Results This work successfully synthesized Rhodamine-modified carbon quantum dots (R-CQDs) via a one-step hydrothermal method. The material exhibits dual emission fluorescence peaks at 464 nm and 544 nm. Cu 2+ selectively quenches the 464 nm fluorescence, while 2,4-D restores fluorescence intensity by competing with Cu 2+ for coordination via its carboxyl group. The detection limits for Cu 2+ and 2,4-D are 2.71 × 10 −3  μM and 1.86 × 10 −3  μM, respectively. Further integration with a smartphone platform combined photocurrent sensing and fluorometric colorimetry, enabling expansion to visual, portable triple-mode (fluorescence, photocurrent, colorimetry) detection. Detection limits for Cu 2+ and 2,4-D reached 0.01382 μM and 0.01925 μM, respectively. This achieved highly sensitive and selective cascade detection of 2,4-D and Cu 2+ . Significance: This multi-signal complementary strategy significantly enhances interference resistance and reliability, providing a portable, low-cost, and universal solution for on-site and rapid monitoring of heavy metal and pesticide residues in environmental and food matrices.