A simple and readily available fluorescent probe is needed for the real-time monitoring of endogenous cysteine (Cys) levels in living cells, as such a probe could be used to study the role of Cys in related diseases. Herein, we report the first fluorescent probe based on carbon dots (CDs-FITA) for the selective and ratiometric imaging of endogenous Cys in live cells. In this ratiometric fluorescent probe, a fluorescein derivative (FITA) that recognizes Cys is covalently linked to the surfaces of carbon dots (CDs); employing CDs greatly improves the water solubility of the probe. Acrylate on FITA is selectively cleaved by Cys in aqueous solution under mild conditions, leading to a dramatic increase in the fluorescence from fluorescein. The probe therefore allows the highly selective ratiometric fluorescent detection of Cys even in the presence of various interferents. The as-prepared CDs-FITA showed excellent performance when applied to detect Cys in blood serum. In addition, due to its negligible cytotoxicity, the CDs-FITA can also be utilized for the real-time monitoring of endogenous cysteine (Cys) levels in living cells.

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需要一种简单易用的荧光探针来实时监测活细胞中内源性半胱氨酸（Cys）的水平，因为这种探针可用于研究Cys在相关疾病中的作用。在此，我们报道了第一个基于碳点（CDs-FITA）的荧光探针，用于在活细胞中对内源性Cys进行选择性和比例成像。在这种比例荧光探针中，识别Cys的荧光素衍生物（FITA）与碳点（CD）的表面共价连接；使用CD可以大大提高探针的水溶性。在温和条件下，水溶液中的Cys选择性裂解FITA上的丙烯酸酯，导致荧光素的荧光显着增加。因此，即使在存在各种干扰物的情况下，该探针也可以进行Cys的高选择性比率荧光检测。所制备的CD-FITA当用于检测血清中的Cys时表现出优异的性能。此外，由于CDs-FITA的细胞毒性可忽略不计，因此还可用于实时监测活细胞中内源性半胱氨酸（Cys）的水平。

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