Synergistic interface engineering of a Co(OH)₂@AgNPs heterostructure for trace electrochemical Hg²⁺ detection

The on-site detection of toxic mercury ions (Hg²⁺) at trace levels is critical for environmental monitoring and compliance with stringent environmental regulations. However, developing electrochemical sensors that simultaneously achieve high sensitivity, rapid electron transfer, and specific recognition in complex matrices remains a challenge due to the poor conductivity and interface passivation of traditional electrode materials. In this work, a hierarchical Co(OH)₂@AgNPs heterostructure is in situ constructed on Fluorine-doped Tin Oxide (FTO) substrates via a one-step electrochemical co-deposition route. Experimental characterization confirms the formation of Ag-O-Co interfacial bonding, where AgNPs are anchored on Co(OH)₂ nanosheets. This structure reduces the charge transfer resistance to 10 Ω and facilitates a specific "soft acid-soft acid" alloying mechanism between Ag and Hg²⁺, while the Co(OH)₂ layers provide steric exclusion of larger interfering ions. The sensor exhibits a linear response from 10 to 140 nM with a high sensitivity of 0.02972 µA nM^-1 and an ultra-low limit of detection (LOD) of 0.85 nM (limit of quantification, LOQ = 2.83 nM). Furthermore, it demonstrates an exceptional selectivity against 13 co-existing ions (signal deviation <5%) and robust stability (decay <3% over 30 days) in real river water samples. Therefore, this work provides a cost-effective, enzyme-free interface engineering paradigm for the precise, scalable, and on-site analysis of heavy metal pollutants.