Input-dependent induction of G-quadruplex formation for detection of lead (II) by fluorescent ion logic gate

A label-free fluorescent AND logic gate has been developed utilizing ion-tuned configuration conversion of DNA probe with K(+) and Pb(2+) as two inputs. A well-designed hairpin DNA which is composed of a poly-G loop and a GR-5 DNAzyme stem serves as a recognition probe, and an derivative of aloe-emodin (AED) was designed and synthesized as signal probe. In the presence of Pb(2+), the substrate strand of DNAzyme is irreversibly and specifically cleaved at the cleavage site, which made the poly-G loop form G-quadruplex in the presence of a constant concentration of K(+). Such a structural change significantly affects the spectral behaviors of AED, which can be explored to ultra-sensitively detect Pb(2+) with a limit of detection of 22.8pM. By combing the high specificity of hairpin DNA and GR-5 DNAzyme, Pb(2+) can be highly selectively detected even when coexisted with other metal ions. Circular dichroism (CD), UV-vis absorption spectrometry and fluorescence polarization (FP) measurements further verified the reliability and reasonability of the sensing mechanism. Therefore, it provides a simple and label-free approach to detect ions with high sensitivity and specificity, and promises to provide a solid sensing platform for the detection of targets by altering the specific sequence of nucleic acid probe.