Synergistic regulation of surface O/N active sites and local conjugation in N-doped carbonized polymer dots for smartphone-assisted dual-mode sensing of Hg(2).

The development of portable, sensitive, and interference-resistant platforms for on-site Hg detection in aquatic food samples remains highly desirable for food safety monitoring. Herein, a smartphone-assisted fluorescence/colorimetric dual-mode sensing platform was constructed based on N-doped carbonized polymer dots (N-CPDs). The N-CPDs were prepared from ethylenediaminetetraacetic acid and m-phenylenediamine via one-step crosslinking polymerization. By regulating surface O/N active sites and local conjugated structures, the N-CPDs exhibited fluorescence quenching and colorimetric responses toward Hg without additional responsive reagents. The limits of detection were 40.91 and 38.97 nM for the fluorescence and colorimetric modes, respectively. Structural characterization and quantum chemical calculations indicated that the matching between key oxygen-containing recognition sites and local conjugated structures is critical to the sensing performance. Mechanistic analysis further revealed Hg-induced static fluorescence quenching and a coordination-driven colorimetric response. Coupled with smartphone-based RGB analysis, the platform enabled portable quantitative detection of Hg in aquatic food samples, including fish, shrimp, crab, and seaweed. The recoveries were 90.11-108.88%, with RSDs below 4.98%, and the results agreed well with ICP-OES measurements. This work provides an effective strategy for constructing high-performance CPD-based dual-mode sensing platforms and offers a promising tool for portable and reliable Hg monitoring in real food samples.