We present a green synthesis of fluorescent carbon quantum dots (CQDs) by using red cabbage (rc) and a one-step hydrothermal approach. The rcCQDs were characterized by various techniques such as UV-visible spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, Fourier-transform infrared spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. The synthesized rcCQDs exhibited an average size of 3 nm, measured by TEM, blue fluorescence, and a quantum yield of 8.3%. The highest emission band was observed at approximately 402 nm when excited at 330 nm. The rcCQDs exhibited strong antioxidant activities by scavenging 61, 56, and 91% against 2, 2-diphenyl-1-picrylhydrazyl, hydroxyl, and potassium permanganate radicals, respectively. The scavenging activity of rcCQDs was comparable with that of standard antioxidant L-ascorbic acid. Cell Counting Kit (CCK)-8 assay depicted superior bio-compatibility and negligible cytotoxicity of rcCQDs on SH-SY5Y neuroblastoma cells. They were used as a fluorescent probe for bio-labeling of Escherichia coli and Staphylococcus aureus. The viabilities of the labeled bacterial cells were analyzed by AFM and UV-visible spectroscopy. Furthermore, the rcCQDs were utilized as a fluorescent ink, an alternative to pens, and maybe suitable for paints and varnish agents. This study provides detailed mechanistic insights into the antioxidant activity of as-synthesized rcCQDs, which suggest the practical applicability of CQDs for bio-medical applications. Key points • Carbon quantum dots were prepared from red cabbage using the hydrothermal method. • The scavenging activity of rcCQDs was evaluated for DPPH, OH, and KMnO4radicals. • The rcCQDs were used for the labeling of foodborne bacteria. • The rcCQDs could be utilized as fluorescent ink. Graphical abstract Schematic representation of CQDs prepared from red cabbage (rc) with multifunctional applications.

中文翻译：

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红甘蓝的绿色合成多用途碳量子点及其抗氧化潜力和生物标记活性的估算。

我们提出了通过使用红甘蓝（rc）和一步水热法的绿色合成的荧光碳量子点（CQDs）。rcCQDs通过多种技术进行了表征，例如紫外可见光谱，透射电子显微镜（TEM），高分辨率TEM，傅立叶变换红外光谱，原子力显微镜（AFM）和X射线光电子能谱。合成的rcCQDs通过TEM测得的平均大小为3 nm，蓝色荧光，量子产率为8.3％。当在330 nm处激发时，在约402 nm处观察到最高发射带。rcCQD通过清除61％，56％和91％的2、2-二苯基-1-吡啶并肼基，羟基和高锰酸钾自由基表现出强大的抗氧化活性。rcCQDs的清除活性与标准抗氧化剂L-抗坏血酸相当。细胞计数试剂盒（CCK）-8分析显示rcCQD对SH-SY5Y神经母细胞瘤细胞具有优异的生物相容性和微不足道的细胞毒性。它们用作用于大肠杆菌和金黄色葡萄球菌生物标记的荧光探针。通过AFM和UV-可见光谱分析标记的细菌细胞的活力。此外，rcCQD被用作荧光墨水，是笔的替代品，并且可能适用于油漆和清漆剂。这项研究为合成的rcCQD的抗氧化活性提供了详细的机械原理，这表明了CQD在生物医学应用中的实际适用性。要点•碳量子点是使用水热法由红甘蓝制备的。•评价了rcCQD对DPPH，OH和KMnO4自由基的清除活性。•rcCQD用于标记食源性细菌。•rcCQD可用作荧光墨水。由红甘蓝（rc）制备的具有多功能应用程序的CQD的图形抽象示意图。