Green synthesis of carbon dots from mature green tea leaves for label-free fluorescence sensing of chromium(VI)

Hexavalent chromium (Cr(VI)) has been established as a highly carcinogenic and mutagenic entity. Therefore, there has been a persistent demand for affordable, sensitive, and quick sensors to monitor the presence of Cr(VI) in aqueous systems. A straightforward, cost-efficient, and eco-friendly approach has been reported in this work in terms of blue-fluorescent carbon dots (CDs, 5.2 nm) being realized in a single-step hydrothermal process. For this purpose, fully grown green tea leaves as the main carbon source were deployed due to being cheap, and environmentally sustainable and readily accessible sources. These CDs exhibit good water solubility, photo-stability and photo-emission (maximal emission at 438 nm for 360 nm excitation wavelength), anti-interference, and low cytotoxicity. The article demonstrates a straightforward fluorescent test for the precise detection and determination of Cr(VI). This was realized through the construction of a non-luminous complex which led to fluorescence quenching via the inner filter effect (IFE). Accordingly, the system can be employed as a label-free nanosensor. Under optimal condition (pH 4 and 5 min reaction time), the fluorescence quenching efficiency of CDs exhibited a well-defined linear correlation in the 0.01 to 8 mg/L concentration range of Cr(VI). A linear equation represents the system sensor as F0/F = 0.2101x + 1.0016 (R2 = 0.99). The limit of detection (LOD) has been as minimal as 0.004 mg/L (s/k = 3). These findings infer CDs as a potential cost-effective nanosensor for Cr(VI) identification in tap water specimens.