A Novel Pb²⁺and Cys Induced Switch Off-On Strategy and Its Application in Detection Based on the Platform of N-Methylmesoporphyrin IX Fluorescence Being Amplied by G-Quadruplex

Lead (Pb²⁺) ions give an imminent danger since they have been known to cause persistent damage to humans, plants, and animals, even at low concentrations, and cysteine (Cys) elevated levels are critical indicators for many diseases. Therefore, their detection is critical in pharmaceutical and environmental samples. This study tailored an innovative fluorescence switch off-on assay to detect Pb²⁺ and Cys based on the amplification of G-quadruplex (G-4) to N-methylmesoporphyrin IX (NMM). This assay operates on the fluorescence of NMM serving as a signal reporter which could be enhanced by an adenine-guanine-rich probes G-4. Initially, the fluorescence of NMM was increased after binding with G-4 and Pb²⁺ and effectively quenching fluorescence without altering the structure of G-4. As it was proved by Circular dichroism (CD). The number of binding sites for Pb²⁺ per NMM was determined to be 0.80 with a binding constant of 1.9 × 10⁴ mol /L. The presence of Cys may disrupt the interaction between Pb²⁺ and G-4/NMM due to its stronger binding affinity towards Pb²⁺ leading to high fluorescence recovery.The assay demonstrated the capability to detect Pb²⁺ within a concentration range of 0.4 to 1.6 μM, achieving a high correlation coefficient (R² = 0.985). with the detection limit of 0.45 μM was established. Similarly, Cys was effectively detected across a range of 1 to 6 μM, possessing correlation (R² = 0.973) with a detection limit of 1.51 μM, further confirming that the detection limit is not influenced by the starting point of the linear range. The assay detected these compounds among various other amino acids and heavy metals. Our approach is simple and innovative, enabling the accurate determination of Pb²⁺ and Cys concentrations in soil and medicinal samples, highlighting its potential in practical diagnostic and environmental applications.