Fruit waste-derived carbon dots with rhodamine B for the ratiometric detection of Fe3+ and Cu2+†

A green and eco-friendly solvothermal approach is proposed for the synthesis of carbon quantum dots (CQDs) from watermelon rind. The as-prepared CQDs exhibited superior teal fluorescence in aqueous solutions, with a quantum yield of 13.9%. The CQDs and rhodamine B (RhB) were demonstrated to selectively react with Fe³⁺ and Cu²⁺, leading to a fluorescence (FL) quenching effect, which was successfully used for constructing “double-response-off” type ratiometric FL probes. A comparative study was conducted to assess the sensitivity and accuracy of ratiometric fluorescent probes, specifically those based on CQDs alone and in combination with RhB, for the selective detection of Fe³⁺ and Cu²⁺. By plotting the ratio of the differential fluorescence (ΔF) signals of CQDs to that of RhB against the practical application analyte concentration, the detection limits for Fe³⁺ (1.75 μM) and Cu²⁺ (0.43 μM) were markedly improved. The quenching mechanism was further explored, and the detection of Fe³⁺ and Cu²⁺ in surface water was demonstrated, showcasing the potential of efficient and effective nanosensors based on a static quenching effect. Futhermore, the addition of ascorbic acid can restore the fluorescence quenched by Fe³⁺. Therefore, in the presence of copper and iron, the ratiometric probe can demonstrate the ability to identify two different metals.