Construction of an "ON-OFF" fluoroprobe using ionic liquids-modified orange peel-based carbon quantum dots for selective/sensitive permanganate assay in waters and the underlying quenching mechanisms.

Herein, we fabricated blue-fluorescence carbon quantum dots modified by ionic liquids (ILs-CQDs) with a quantum yield of 18.13% by employing orange peel as a carbon source and [BMIM][HPO] as a dopant. The fluorescence intensities (FIs) of ILs-CQDs were significantly quenched upon the addition of MnO with excellent selectivity and sensitivity in waters, and this phenomenon provided a feasibility for constructing a sensitive "ON-OFF" fluoroprobe. The prominent overlapping between the maximum excitation/emission of ILs-CQDs and the UV-Vis absorption of MnO implied an inner filter effect (IFE). The higher K value demonstrated that the fluorescence-quenching phenomenon was a static-quenching process (SQE). Coordination between MnO and oxygen/amino-rich groups in ILs-CQDs resulted in the alteration of zeta potential in the fluorescence system. Consequently, the interactions between MnO and ILs-CQDs belong to a joint mechanism of IFE and SQE. When plotting the FIs of ILs-CQDs vs. the concentrations of MnO, a satisfactorily linear correlation was obtained across the range of 0.3-100 μM with a detectable limit of 0.09 μM. This fluoroprobe was successfully applied to detect MnO in environmental waters with satisfactory recoveries of 98.05-103.75% and relative standard deviations (RSDs) of 1.57-2.68%. Also, it gave more excellent performance metrics as compared to the Chinese standard indirect iodometry method and other previous approaches for MnO assay. Overall, these findings offer a new avenue to engineer/develop a highly efficient fluoroprobe based on the combination of ILs and biomass-derived CQDs for the rapid/sensitive detection of metal ions in environmental waters.