Dual fluorescence and absorbance sensing of Cu²⁺ and Pb²⁺ ions using plasma-engineered carbon quantum dots

Rapid and reliable detection of toxic heavy metal ions in water remains a critical challenge for environmental monitoring and public health protection, highlighting the growing need to develop environmentally friendly, scalable, and sustainable sensing materials that minimize ecological impact and support green analytical technologies. Here, we report a plasma-assisted approach for converting low-cost folic acid, serving as a single carbon and nitrogen source, into carbon quantum dots (CQDs) with intrinsic dual-mode optical sensing capabilities. The resulting CQDs exhibit strong blue fluorescence emission, well-defined absorption feature, and a large Stokes shift (110 nm), enabling highly sensitive and selective detection of copper (Cu²⁺) and lead (Pb²⁺) ions through both fluorescence quenching and absorbance modulation. Excellent linearity (R² > 0.99) was obtained for both Cu²⁺ and Pb²⁺ using fluorescence (0-10 ppm) and absorbance (0-20 ppm) responses. The proposed sensor achieved detection limits of 54.4 ppb and 47.4 ppb for Cu²⁺ and 100.0 ppb and 61.6 ppb for Pb²⁺ by fluorescence and absorbance detection, respectively, while exhibiting excellent selectivity against competing ions. Key features of the platform include its applicability to binary-mixture analysis and effective correction of matrix effects across diverse water matrices. Binary-mixture analysis of coexisting Cu²⁺ and Pb²⁺ was independently demonstrated using both fluorescence and absorbance measurements. Separately, matrix effects were systematically accounted for in individual-ion measurements, enabling reliable quantification of Cu²⁺ and Pb²⁺ in tap water, river water, and seawater, with recoveries of 94.07-114.74% and 94.05-116.11%, respectively, using both analytical modes. The results were further validated against the inductively coupled plasma (ICP) method, showing good agreement with the expected spiked values and demonstrating robust performance in complex real-world matrices. This work establishes a simple plasma-engineered CQD platform that combines green precursor conversion, dual-signal optical readout, binary Cu²⁺/Pb²⁺ analysis, and matrix-effect correction within a single sensing framework. By integrating these capabilities with low-cost and less hazardous materials, the platform offers a promising and potentially scalable approach to heavy-metal monitoring, with satisfactory analytical performance demonstrated in diverse real-water matrices.