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“Aggregation-to-Deaggregation” Colorimetric Signal Amplification Strategy for Ag⁺ Detection at the Femtomolar Level with Dark-Field Microscope Observation

Li, Jingjing, Xi, Hongyan, Kong, Caiyun, Liu, Qingyun, Chen, Zhengbo
Analytical chemistry 2018 v.90 no.19 pp. 11723-11727
DNA damage, aptasensors, colorimetry, exodeoxyribonucleases, heavy metals, metal ions, microscopy, nanogold, nanosilver, river water, silver, single-stranded DNA, thymine
Robust but ultrasensitive aptasensors with an ability to detect lower concentrations of heavy metal ions enable the detection of serious environmental and health issues. We herein develop a label-free aptasensor for ultrasensitive detection of the silver ion (Ag⁺) utilizing gold nanoparticle (AuNP) intensity measurement methodology by dark-field microscopy, which is based on target Ag⁺ and exonuclease III (Exo III)-dependent DNA cleavage recycling amplification. In the presence of target Ag⁺, thymine (T) bases at two termini of hairpin DNA bind with Ag⁺ through C–Ag⁺–C coordination to form a DNA duplex, Exo III can recognize the blunt 3′ end of the DNA duplex and digest it from the 3′ end to the 5′ direction. The released target Ag⁺ then binds with another hairpin DNA via C–Ag⁺–C pairs. After many cycles of the digestion of the DNA duplex by Exo III, numerous remaining single-stranded DNA (ssDNA) are generated. These ssDNA are absorbed on the surface of AuNPs, enhancing the repulsion force between AuNPs, which further promotes the dispersion of AuNPs, leading to a significantly decreased intensity of yellow and red dots (aggregated AuNPs) under dark-field microscopy observation, in contrast to that of the blank solution (without target Ag⁺). On this basis, the detection limits of 41 and 39 fM were achieved for Ag⁺ in Tris-HCl buffer and river water, respectively.