Fluorescence Turn-Off-On Sensing Strategy for the Detection of Pb²⁺ and Metalaxyl Fungicide Using Yellow Fluorescent Carbon Dots From Cordia dichotoma Fruits

In this work, a facile and rapid fluorescence turn-off-on sensing method was developed for the detection of Pb²⁺ and metalaxyl fungicide in environmental samples using yellow carbon dots (CDs) from Cordia dichotoma fruits. The prepared C. dichotoma-CDs showed the emission band (𝜆_Em) at 535 nm under 448 nm of excitation wavelength. The fluorescence intensity of C. dichotoma-CDs quenches by Pb²⁺ ion and then restores by metalaxyl fungicide, favoring to develop a fluorescence turn-off-on sensing strategy. The intensity ratios (I₀/I and I/I₀) are linear against Pb²⁺ (0.01-50 μM) and metalaxyl fungicide (0.025-10 μM) concentrations, offering the detection limits of 0.0054 and 0.0087 μM for Pb²⁺ and metalaxyl fungicide, respectively. Additionally, the potential application of C. dichotoma-CD-based fluorescence turn-off-on sensing approach was effectively demonstrated for Pb²⁺ and metalaxyl fungicide assays in real samples, which confirms that C. dichotoma-CDs act as "turn-off-on" fluorescent probe for assaying of Pb²⁺ and metalaxyl fungicide, respectively.