Converting agricultural waste into functional carbon quantum dots offers a sustainable approach to developing fluorescent nanomaterials for environmental monitoring. However, achieving desirable photophysical properties and sensing performance requires careful control of synthesis conditions and precursor composition. Herein, nitrogen- and sulfur-doped carbon quantum dots (N,S-CQDs) were synthesized in a one-pot microwave-assisted hydrothermal treatment from cherry-pit biomass using L-cysteine as a heteroatom source and evaluated in terms of their photophysical characteristics and sensitivity to chromium species. The N,S-CQDs demonstrated nanoscale dimensions and exhibited favorable photoluminescence properties, where the quantum yield increased from 19% to 30.5% with increasing L-cysteine content. The fluorescence response exhibited a linear pattern across the pH range of 5–9, with the highest emission recorded at pH 9. This observation establishes a foundation for subsequent chromium sensing. The N,S-CQDs demonstrated a concentration-dependent response toward Cr(VI), with a detection limit of 1.25 μM over 0–80 μM, whereas Cr(III) produced no appreciable response. Following oxidative conversion of Cr(III) to Cr(VI) using H₂O₂, indirect Cr(III) determination was achieved with a detection limit of 1.8 μM over 0–100 μM. The probe showed good tolerance toward representative interfering species and maintained its fluorescence response under storage, UV irradiation, varied ionic strengths, and various solvent conditions. Spectral overlap suggested a substantial contribution of the inner filter effect to the fluorescence quenching, while FT-IR, XPS, and zeta-potential analyses after Cr(VI) sensing proposed additional surface interactions and complexation between Cr(VI) and the CQD functional groups.
