Portable smartphone-assisted ratiometric fluorescent test paper based on one-pot synthesized dual emissive carbon dots for visualization and quantification of mercury ions

Dual-emissive fluorescent carbon dots (CDs) were prepared through the solvothermal method with citric acid and urea as raw materials and dimethylformamide as the solvent. Two emission peaks were observed at 465 nm and 630 nm. Hg²⁺ could selectively quench the fluorescence at 630 nm, but the fluorescence intensity at 465 nm was less affected. Accordingly, a ratiometric fluorescence sensor for Hg²⁺ detection was developed, with a linear detection range of 0.5-40 μM and a limit of detection (LOD) of 37 nM. The dual-emissive CDs were loaded on the surface of the filter paper to fabricate Hg²⁺ detection test paper. The color of the test paper could be changed from pink purple to blue by the addition of Hg²⁺, and thus the qualitative and quantitative detection of Hg²⁺ could be realized. The concentration distinguishable by the naked eye reached 50 μM, and the quantitative detection range was 5-10,000 μM. This method shows excellent selectivity for Hg²⁺ and can be used to detect Hg²⁺ in real water samples, providing a highly potential sensing platform for rapid on-site detection of mercury ions.