Synthesis of two nitrogen-doped carbon quantum dots to construct fluorescence probes for sensitive Hg2+ 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 Hg2+ based on fluorescence resonance energy transfer and photoinduced electron transfer mechanisms. The M-CQDs were prepared from folic acid and m-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 p-phenylenediamine (pPDA). Upon the addition of Hg2+ 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 Hg2+. The Hg2+ 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 -NH2 in the mPDA and pPDA precursors. Notably, paper-based chips modified with M/P-CQDs were established for visual Hg2+ sensing, demonstrating the possibility for real-time detection of Hg2+. Moreover, the practicality of this system was confirmed through the successful measurement of Hg2+ in tap water and river water samples.