Ultraviolet Light-Activated Carbon Dot Nanozymes as Pro-oxidants for Dual Colorimetric Sensing of Copper(II) Ions and Nabam

Copper(II) ions (Cu²⁺) and disodium ethylenebis(dithiocarbamate) (nabam) are critical environmental pollutants, requiring rapid, sensitive, and selective detection methods. This work presents a photoresponsive colorimetric sensor using a carbon dot (CD) with oxidase-mimetic activity for dual detection of Cu²⁺ and nabam. Hydrothermally synthesized from 1,2,3,4-butanetetracarboxylic acid and o-phenylenediamine, the CD exhibits high ultraviolet light-driven oxidase-like activity due to its energy band alignment with the redox potentials of photocatalytic oxygen reduction and water oxidation, facilitating the generation of reactive oxidizing species (hydroxyl radicals, superoxide radicals, and photogenerated holes). These species oxidize colorless 3,3′,5,5′-tetramethylbenzidine to a blue product. Cu²⁺ detection is achieved via its suppression of the CD activity through CD–Cu complexation and valence band negative shifting, yielding a linear range of 0.01–0.5 μM and a detection limit (LOD) of 3.3 nM. Subsequent addition of nabam restores the CD activity by sequestering Cu²⁺ through complexation, enabling nabam detection with a linear range of 0.05–1.5 μM and a LOD of 23 nM. The sensor demonstrates high selectivity against potential interferents and is successfully applied to quantify Cu²⁺ and nabam in environmental water and wheat grain matrices. This study advances nanozyme-based sensing platforms by integrating photoresponse modulation and chelation chemistry for multiplex environmental monitoring.