Deep eutectic solvent-assisted carbon quantum dots for nanomolar detection of 4-nitrophenol.

4-Nitrophenol (4-NP) is a toxic, persistent, and carcinogenic pollutant, classified by the U.S. EPA as a primary contaminant. Commonly released by the pharmaceutical industry, it poses serious health risks, damaging the liver, kidneys, central nervous system, and bloodstream, highlighting the need for eco-friendly detection methods. To solve the difficulty of 4-NP detection, the work offers a unique and sustainable detection approach based on nitrogen and chlorine co-functionalized carbon quantum dots (S-CQDs). The hydrothermal approach was used for the synthesis, with sucrose serving as a carbon precursor and a deep eutectic solvent (DES) composed of urea and choline chloride in a 1 : 2 molar ratio. The nanosensor exhibited strong green fluorescence, excellent water solubility, photostability and ∼56%. Quantum yield. HRTEM revealed spherical and monodispersed S-CQDs that averaged 3.06 nm in size. FTIR and XPS investigations revealed amino, hydroxyl, carboxyl, and chlorine groups on the surface of S-CQDs, confirming intrinsic nitrogen and chlorine functionalization. XRD, UV-vis spectroscopy, fluorescence spectroscopy, and TCSPC were used for further characterization. For 4-nitrophenol (4-NP), the nanoprobe demonstrated excellent sensitivity and selectivity with a detection limit of 10 nM. An inner filter effect (IFE) associated with a zwitterionic spirocyclic Meisenheimer complex was confirmed by mechanistic investigations because of spectrum overlap and unaltered lifespan values. To further elucidate the sensing process, photophysical metrics like binding constants and quenching efficiency were also assessed. This work paves the way for developing a sensitive, green fluorescent nanosensor for a rapid, cost-effective and environmentally friendly approach as well as on-site detection of 4-NP, offering a promising tool for pollution monitoring and control for environmental water samples.