Synthesis of two nitrogen-doped carbon quantum dots to construct fluorescence probes for sensitive Hg detection with dual signal output.

The rapid and sensitive detection of heavy metal ions is of great importance in food safety and for the environment. Therefore, two novel probes, M-CQDs and P-CQDs, based on carbon quantum dots were utilized to detect Hg based on fluorescence resonance energy transfer and photoinduced electron transfer mechanisms. The M-CQDs were prepared from folic acid and -phenylenediamine (mPDA) using a hydrothermal method. Similarly, the novel P-CQDs were obtained according to the same synthetic procedure used to create M-CQDs except the mPDA was replaced with -phenylenediamine (PDA). Upon the addition of Hg to the M-CQDs probe, the fluorescence intensity reduced significantly with a linear concentration range between 5 and 200 nM. The limit of detection (LOD) was calculated to be 2.15 nM. On the contrary, the fluorescence intensity of the P-CQDs was enhanced greatly after the addition of Hg. The Hg detection was realized with a wide linear range from 100 to 5000 nM and the LOD was calculated to be as low as 52.5 nM. The fluorescence "quenching" and "enhancing" effect exhibited by the M-CQDs and P-CQDs, respectively, is due to the different distribution of -NH in the mPDA and pPDA precursors. Notably, paper-based chips modified with M/P-CQDs were established for visual Hg sensing, demonstrating the possibility for real-time detection of Hg. Moreover, the practicality of this system was confirmed through the successful measurement of Hg in tap water and river water samples.