A portable smartphone platform based on fluorescent carbon quantum dots derived from biowaste for on-site detection of permanganate

“Waste” is “wealth”: the transformation of biomass waste into high value-added substances holds significant environmental and economic importance. In our work, orange peel was effectively transformed into valuable fluorescent carbon quantum dots (CQDs) by solvothermal treatment. The resultant CQDs displayed favorable dispersion and a mean particle size of 3.14 ± 0.71 nm. Moreover, these CQDs exhibited a characteristic excitation-dependent fluorescence behavior, which can produce the maximum emission wavelength at 506 nm with excitation at 400 nm. Upon the introduction of permanganate (MnO₄⁻), the fluorescence intensity of CQDs diminished gradually caused by the inner filter effect (IFE) as well as the static quenching effect (SQE). The results revealed that the CQDs can serve as a fluorescent probe, which possessed high performance in terms of sensitivity for quantitative analysis of MnO₄⁻, boasting a low detection limit of 3.31 μM. Subsequently, the application of this fluorescent probe in actual water samples was further explored, resulting in a recovery rate between 98.18% and 101.69%, with relative standard deviations (RSDs) ranging from 1.85% to 2.68%, which was highly satisfactory. The portable paper-based sensor, fabricated by depositing CQDs onto filter paper, is capable of quantitatively and visually detecting MnO₄⁻ in combination with a smartphone, thus providing a rapid on-site detection tool for MnO₄⁻. As far as we know, this study provides the inaugural account of a smartphone-based portable sensor for visual determination of MnO₄⁻ by using biomass waste based CQDs, suggesting its immense potential for real-time monitoring applications.