Unveiling heteroatom-containing carbon dots from soil bacterial extracellular metabolites and their application in toxic Cr6+ detection

Abstract

In this study, we present an eco-friendly approach for the synthesis of fluorescent carbon dots (CDs) using the extracellular supernatant of Bacillus sp. NBSG24, a soil-derived bacterium. The supernatant contains a diverse mixture of biomolecules, including proteins, enzymes, lipids, polysaccharides, and other soluble components of the bacterial secretome, which serve as natural precursors for carbon dot formation. The resulting NB24@CDs exhibit bright blue fluorescence, excellent water solubility, and high stability under rigorous conditions, including changes in pH, ionic strength, UV exposure, and prolonged storage. Notably, these CDs are intrinsically doped with nitrogen and sulfur atoms, originating from the bacterial extracellular metabolites, leading to enhanced optical properties and sensing capabilities. A key feature of NB24@CDs is their ability to rapidly and selectively detect hexavalent chromium (Cr⁶⁺), a highly toxic and environmentally persistent contaminant. Upon exposure to Cr⁶⁺, the fluorescence of NB24@CDs is quenched rapidly within 10 seconds, indicating a fluorescence “turn-off” response. The system exhibits a low detection limit of 30 nM (1.56 μg L⁻¹), substantially lower than the World Health Organization (WHO) permissible limit of 962 nM (50 μg L⁻¹) for Cr⁶⁺ in drinking water. Additionally, the sensing platform performs reliably in real water samples such as tap, river, and lake water, without interference from common coexisting ions. Spectroscopic analysis, including fluorescence lifetime, UV-vis, FTIR, and XPS, suggest that the detection mechanism involves static quenching due to complex formation with Cr⁶⁺, along with partial reduction of the metal ion. Overall, this work highlights the potential of microbial-derived, heteroatom-doped CDs as a sustainable, cost-effective, and scalable solution for environmental monitoring of toxic heavy metals.