Detecting Cr⁶⁺ at ≈100 pM Concentration with Fluorescence Enhancement Signatures in a Novel Eco‐Fluorophore: Matching WHO's 96 pM Recommended Standard for Drinking Water

Hexavalent chromium (Cr⁶⁺) ions in drinking water pose a significant risk to human health, being a leading cause for neurological disorders, organ damage, and infertility. This study introduces an ultrasensitive method for detecting trace Cr⁶⁺ over a wide concentration range (≈ 100 pM - 100 µM) through fluorescence enhancement signatures via integration of both covalent and non-covalent interaction strategies on carbon quantum dots (CQD). The covalent functionalization is achieved from dual-functionalized CQD (CQD-(NH₂, COOH)) derived from coffee-waste. Additionally, the covalent and non-covalent approach integrates CQD-(NH₂, COOH) with graphitic carbon nitride (g-C₃N₄) to form a 2D/2D heterostructure. The synergy between CQD-(NH₂, COOH) and g-C₃N₄ introduces a mid-gap band in their band structure, allowing multiple carrier excitation and recombination states, significantly enhancing the fluorescence quenching signal. This combination allows to achieve Cr⁶⁺ detection sensitivity down to ≈100 pM concentration-matching the World Health Organization's 96 pM permissible limit of total Cr in drinking water. Furthermore, a 70 pM detection limit is reported for Cr⁶⁺ in a mixture of twelve ions, including cations and anions, surpassing current state-of-the-art detection limits. These results highlight the potential of dual covalent and non-covalent modification strategy in nanomaterials to set new standards in ultrasensitive and wide-range fluorescent sensing applications.