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

Abstract Hexavalent chromium (Cr 6+ ) 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 Cr 6+ 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‐(NH 2 , COOH)) derived from coffee‐waste. Additionally, the covalent and non‐covalent approach integrates CQD‐(NH 2 , COOH) with graphitic carbon nitride (g‐C 3 N 4 ) to form a 2D/2D heterostructure. The synergy between CQD‐(NH 2 , COOH) and g‐C 3 N 4 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 Cr 6+ 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 Cr 6+ 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.