Triple signal amplification electrochemical sensing platform for Hg²⁺ in water without direct modification of the working electrode

Abstract

● MrGO-AuNPs, Exo III-ATC and HCR co-amplify the signal.

● MrGO-AuNPs increases the cDNA loading to improve the Hg²⁺ capture efficiency.

● The utilization of magnetism improves the mass transfer efficiency.

● The working electrode doesn’t require direct modification, simplifying operation.

● Ultrasensitive to Hg²⁺ with a LOD of 3.14 pmol/L.

An ultrasensitive electrochemical biosensor to detect trace Hg²⁺ in environmental samples was developed utilizing nanogold-decorated magnetic reduced graphene oxide (MrGO-AuNPs), exonuclease III-assisted target cycle (Exo III-ATC) and hybridization chain reaction (HCR) synergistic triple signal amplification. The MrGO-AuNPs is a superior carrier for capture DNA (cDNA) and acts as magnetic media for automatic separation and adsorption. This innovative utilization of the magnetism and improved sensing efficiency obviates the need for direct modification and repeated polishing of the working electrode. Additionally, the three DNA hairpins (cDNA, methylene blue (MB) labeled HP1 and HP2) further contribute to biosensor specificity and selectivity. When cDNA captures Hg²⁺, it activates Exo III-ATC due to the formation of a sticky end in the cDNA stem via thymine-Hg²⁺-thymidine (T-Hg²⁺-T), this leads to the hydrolysis of self-folded cDNA by Exo III-ATC to form “key” DNA (kDNA). The kDNA subsequently initiates HCR, resulting in massive super-sandwich structures (kDNA-[HP1/HP2]_n) carrying signaling molecules on MrGO-AuNPs, and this overall structure serves as a signal probe (SP). Leveraging magnetic adsorption, the SP was automatically adsorbed onto the magneto-glass carbon electrode (MGCE), generating an amplified signal. This biosensor’s detection limit (LOD) was 3.14 pmol/L, far below the limit of 10 nmol/L for mercury in drinking water set by the US EPA. The biosensor also showed excellent selectivity when challenged by interfering ions, and the results of its application in actual samples indicate that it has good potential for practical applications in environmental monitoring.

Graphical abstract

Keywords

Electrochemical sensing / Hg ²⁺ / Amplification / Graphene / Hybridization chain reaction / Exonuclease III

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