"Aggregation-to-Deaggregation" Colorimetric Signal Amplification Strategy for Ag⁺ Detection at the Femtomolar Level...

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.