Recyclable Target Metal-Enhanced Fluorometric Naked Eye Aptasensor for the Detection of Pb²⁺ and Ag⁺ Ions Based on the Structural Change of CaSnO₃@PDANS-Constrained GC-Rich ssDNA

Abstract

Reliable, label-free, and ultraselective detection of Pb²⁺ and Ag⁺ ions is of paramount importance for toxicology assessment, human health, and environmental protection. Herein, we present a novel recyclable fluorometric aptasensor based on the Pb²⁺ and Ag⁺-induced structural change of the GC-rich ssDNA (guanine cytosine-rich single-strand DNA) and the differences in the fluorescence emission of acridine orange (AO) from random coil to highly stable G-quadruplex for the detection of Pb²⁺ and Ag⁺ ions. More interestingly, the construction and principle of the aptasensor explore that the GC-rich ssDNA and AO can be strongly adsorbed on the CaSnO₃@PDANS surface through the π-π stacking, hydrogen-bonding, and metal coordination interactions, which exhibit high fluorescence quenching and robust holding of the GC-rich ssDNA. However, in the presence of Pb²⁺, the specific G-rich ssDNA segment could form a stable G-quadruplex via G4-Pb²⁺ coordination and capture of AO from the CaSnO₃@PDANS surface resulting in fluorescence recovery (70% enhancement). The subsequent addition of Ag⁺ ion induces coupled cytosine base pairs in another segment of ssDNA to get folded into a duplex structure together with the G-quadruplex, which highly stabilizes the G-quadruplex resulting in the maximum recovery of AO emission (99% enhancement). When the Cys@Fe₃O₄Nps are added to the above solution, the sensing probe was restored by complexation between the Cys in the Cys@Fe₃O₄Nps and target metal ions, resulting in the fabrication of a highly sensitive recyclable Pb²⁺ and Ag⁺ assay with detection limits of 0.4 and 0.1 nM, respectively. Remarkably, the Cys@Fe₃O₄Nps can also be reused after washing with EDTA. The utility of the proposed approach has great potential for detecting the Pb²⁺ and Ag⁺ ions in environmental samples with interfering contaminants.