Thiocyanate functionalized carbon quantum dots (SCN-CDs) were produced via microwave synthesis using tree leaves called feijoa as a green material. The products were analyzed using spectroscopy and microscopy techniques, including high-resolution transmission electron microscopy (HR-TEM), spectrofluorometry, and X-ray photoelectron spectroscopy (XPS). SCN-CDs with blue emissivity were used to detect ascorbic acid (Ascor) in aquatic environments. The selectivity of SCN-CDs was reasonable, and the sensitivity was excellent. Cu(II) interaction with the SCN-CDs via static quenching mechanism leads to the SCN-CDs’ fluorescence (FL) being quenched, and Ascor’s reduction capacity recovers the SCN-CDs/Cu(II) FL, resulting in a switch-off-on sensor for Ascor detection. As a rapid and sensitive turn-on sensor, the limit of detection (LOD) of SCN-CQDs/Cu(II) for Ascor detection was 0.69 lM.
