Ion-imprinted carbon dots: rationally designed fluorescent probes for the detection of selected metal ions from aqueous solutions

Photoluminescence properties of Carbon Dots (CDs) have been leveraged for their use as sensors for a variety of analytes, including inorganic ions, organic molecules, and biomolecules. The selective fluorescence response of CDs to specific analytes is generally not pre-designed. Rationally designed synthesis of CDs with pre-defined selectivity to specific analytes is a less explored avenue. This study presents a novel method for the customized synthesis of CD fluorescent probes and an ion-imprinting-based selective detection of metal ions using these CDs. Poly(sodium 4-styrenesulfonate) [PSS] treated with Cd(ii) ions was used as the precursor for preparing Cd-imprinted CDs, and a modified form of these CDs was used for the sensing of Cd(ii) in aqueous solutions. As synthesized CDs have Cd(ii) ions on their surface, which were subsequently removed through appropriate chemical treatment. This removal results in binding sites of Cd(ii) ions on the CDs. Formation of such binding sites results in alterations of the fluorescence of CDs. Exposure of these particles to analytes containing Cd(ii) ions leads to the re-occupation of the binding sites by the metal ions, resulting in a distinct fluorescence response, which serves as the sensing readout. Effectiveness of this 'ion-imprinting' approach is demonstrated by the selective and sensitive fluorescence response of the CDs towards Cd(ii) ions, with a limit of detection (LOD) of 3.62 nM. This strategy of Cd(ii) detection using ion-imprinted CDs represents a novel effort in CD-based sensors, and this can be extended to the sensing of other cations also.