Decoupling fluorescence attenuation from structural degradation of carbon dots in soil leachates using bispectral...

Accurate interpretation of fluorescence attenuation from carbon dots in soil systems remains challenging because optical signals are highly sensitive to matrix-induced quenching, surface-state perturbation, and background interference. Here, we developed a bispectral matrix-decoupling framework integrating fluorescence (FL), near-infrared (NIR) spectroscopy with operational partitioning, orthogonal organic carbon validation, and mechanistic quencher-addition tests to systematically evaluate sulfur-nitrogen co-doped carbon dots (S-N-CDs) in complex soil leachates. FL decreased rapidly, following apparent first-order attenuation with a rate constant of 0.127 day⁻¹ and a half-life of 5.48 days, whereas the monitored NIR windows at 1100-1200 nm and 1400-1450 nm displayed slower near-linear attenuation. Quantitative partitioning revealed that the leachate-associated mobile fraction decreased from 34.2% to 12.6%, mirror-imaged by an increasing unrecovered fraction. Quencher-addition tests confirmed that dynamic matrix components and co-existing metal ions directly drove severe fluorescence quenching. Crucially, NIR controls indicated that complementary spectral response windows provide robust empiric calibration rather than isolated structural tracking. These findings demonstrate that apparent optical signal loss must not be directly equated with complete material degradation. Ultimately, this transferable framework establishes a vital methodological cornerstone for distinguishing reversible optical attenuation from structural persistence, providing robust signal processing for active biophotonic sensing within complex, unstructured ecosystems.