Fluorescent Reversible Regulation Based on the Electrostatic Intercalation of N-Methyl Mesoporphyrin IX to G-quadruplex: a Sustainable Approach of Zn Detection.

G-quadruplex (G)-based biosensors have emerged as a promising platform for the rapid and sensitive detection of metal ions. This study reported a label-free (G)-based sensor using N-methyl Mesoporphyrin IX (NMM) as a fluorescent signal reporter for selective detection of zinc ions (Zn). A potassium (K +) induced fluorescent (G) probe was designed by a guanine-rich sequence. NMM fluorescence intensity was significantly increased upon binding with (G). The sensor catalyzed the insertion of Zn into the NMM structure, leading to fluorescence quenching without altering the (G) structure. Circular dichroism analysis proved the stability of the (G) structure and the number of binding sites for Zn per NMM was determined to be 0.77 with a binding constant of 5.0 × 104 L/mol. It exhibited a good linear correlation (R = 0.985). The assay performance was optimized at pH 7 and (G) 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 Zn spiked recovery experiments from a pharmaceutical sample. Successful detection of spiked Zn recovery (98.3-100%) from medicinal lysine glucosamine Zn 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.