A sensitive and label-free sensor for melamine and iodide by target-regulating the formation of G-quadruplex

In this study, a novel, sensitive and label-free method was developed for melamine and iodide (I-) detection by target-regulating the formation of G-quadruplex structure. This assay was based on two basic phenomena: first, melamine could bind to thymine (T) through hydrogen bond to form thymine-melamine-thymine (T-M-T) hairpin structure. Second, combination force between mercury ions (Hg2+) and I- was much stronger than that between Hg2+ and thymine, therefore, I- could combine Hg2+ from the thymine-Hg2+ -thymine (T-Hg2+-T) hairpin structure. A T-rich single-strand DNA sequence was designed for the two targets detection, which can associate with N-methyl mesoporphyrin IX (NMM) and K+ to form G-quadruplex-NMM complex, producing a strong fluorescence signal. The concentration of free probe DNA was proportional to the amount of melamine or I-, enabling convert the target reaction events into detectable signal of G-quadruplex-NMM by fluorescence spectrometer. Under the optimized conditions, the limits of detections (LODs) were 0.08 nM and 4.6 nM for melamine and I-, respectively, which was comparable to that of other melamine and I- assays. The relative standard deviations (RSDs) (n = 7) were 1.8% and 0.3%, respectively. Besides, melamine and I- in real samples could be detected with satisfactory results. The proposed method retains several unique advantages, including simple, sensitive and selective for label-free analysis.