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

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.