Ultrasensitive electrochemical biosensor for silver ion based on magnetic nanoparticles labeling with hybridization chain reaction amplification strategy

Silver ion (Ag + ) is a highly toxic heavy metal ion to aquatic organisms and accumulates in the human body via the food chain. Therefore, fast and accurate detection of Ag + in water and food resources has become a critical issue within the scope of human health. Herein, we developed an ultrasensitive electrochemical biosensor for detection of Ag + based on magnetic Fe 3 O 4 @gold core-shell nanoparticles (Fe 3 O 4 @Au NPs) labeling with hybridization chain reaction (HCR) amplification strategy. In this sensing strategy, the magnetic Fe 3 O 4 @Au NPs were selected for labeling with HCR product and enrichment on the surface of magnetic gold electrode. Thiolated-oligonucleotide (S1) was firstly immobilized on the surface of Fe 3 O 4 @Au NPs through Au–S chemical bond. In the presence of Ag + , cytosine-rich DNA oligonucleotide S2 hybridized with S1 to form an intramolecular duplex, in which Ag + can selectively bind to cytosine–cytosine mismatches forming C–Ag + –C complex. The exposed stem of the C–Ag + –C complex opened two alternating ferrocene-labeled DNA hairpins (H 1 and H 2 ) in turn and triggered HCR to form a supersandwich DNA structure on the surface of Fe 3 O 4 @Au NPs. The HCR products modified Fe 3 O 4 @Au NPs were brought to the surface of magnetic gold electrode for direct electrochemical measurements. The proposed strategy led to a low detection limit of 0.5   fM and a wide dynamic range of 1   fM–100   pM for target Ag + . The developed biosensor was highly selective and its practical applicability in tap water and lake water samples was also investigated with a satisfactory result.