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Fermentation is key to the quality of black tea, and traditional sensory evaluation methods are too slow and subjective for reliable, large-scale evaluation during fermentation. To address this problem, this study formulated a sensing system based on cobalt ions-doped carbon quantum dots emitting bright blue fluorescence. In evaluation experiments, this system was found to work stably for 90 min and accurately determine fermentation quality in real time. In fluorescence spectra results, fluorescence intensity decreased sharply with the concentrations of catechins, theaflavins, and gallic acid and this relationship was highly linear at concentrations from 0.01 to 50 μg/mL; R2 was >97% and the limit of detection was <0.046 μg/mL. A least-squares support vector machine model was established, and it had a 100% accuracy rate in discriminating between degrees of fermentation. Therefore, this quantum-dot-based system serves as an accurate and efficient means of measuring the fermentation quality of black tea.
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•A testing technique for in-process black tea products with dynamic quality change.•Co-CQDs respond stably to key substances in black tea fermentation in real time.•The developed model's accuracy in determining the degree of fermentation is 100%.•Stable CQDs provide reliable performance during black tea fermentation.•Standard curves for catechins, theaflavins, and gallic acid were established.