Fluorescent Reversible Regulation Based on the Electrostatic Intercalation of N-Methyl Mesoporphyrin IX to...

G-quadruplex (G₄)-based biosensors have emerged as a promising platform for the rapid and sensitive detection of metal ions. This study reported a label-free (G₄)-based sensor using N-methyl Mesoporphyrin IX (NMM) as a fluorescent signal reporter for selective detection of zinc ions (Zn²⁺). A potassium (K +) induced fluorescent (G₄) probe was designed by a guanine-rich sequence. NMM fluorescence intensity was significantly increased upon binding with (G₄). The sensor catalyzed the insertion of Zn²⁺ into the NMM structure, leading to fluorescence quenching without altering the (G₄) structure. Circular dichroism analysis proved the stability of the (G₄) structure and the number of binding sites for Zn²⁺ per NMM was determined to be 0.77 with a binding constant of 5.0 × 104 L/mol. It exhibited a good linear correlation (R² = 0.985). The assay performance was optimized at pH 7 and (G₄) concentration of 5 × 10-6 mol/L, based on which detection range of 0.1 ~ 1.2 × 10-6 mol/L and excellent selectivity was reached. The practical applicability was tested through Zn²⁺ spiked recovery experiments from a pharmaceutical sample. Successful detection of spiked Zn²⁺ recovery (98.3-100%) from medicinal lysine glucosamine Zn²⁺ granules. Without the need for nanomaterial fabrication or unstable DNAzymes, this label-free assay provides a cost-effective, reliable quantitative method suitable for pharmaceutical analysis.