Highly effective target converting strategy for ultrasensitive electrochemical assay of Hg²

An electrochemical sensing system based on a highly effective mercuric ion (Hg²⁺) converting strategy and rolling circle amplification (RCA) is developed for the ultrasensitive detection of Hg²⁺. The Hg²⁺ converting strategy is implemented based on Hg²⁺ specific recognition thymine-Hg²⁺-thymine (T-Hg²⁺-T)-induced DNA strand displacement. First, polystyrene magnetic microspheres coated by AuNPs (Au@PSC) as a magnetic separator were labeled with ssDNA D1 (thymine-rich) and S1/D2 DNA duplex (guanine-rich S1). In the presence of Hg²⁺ and long ssDNA D3 (thymine-rich at the 5' end), the formation of a stable T-Hg²⁺-T structure between D2 and D3 pushes S1 out from the S1/D2 DNA duplex, realizing the conversion of input target Hg²⁺ into output S1. Thus, the total amount of output S1 is proportional to the amount of input Hg²⁺. Thereafter, the output S1 serves as the primer to perform RCA to obtain long guanine-rich ssDNA, which could be further hybridized with the capture DNA on the electrode surface. Subsequently, methylene blue (MB) as an electron mediator interacts with the ssDNA polymers via electrostatic binding to produce a detection signal. The electrochemical biosensor exhibits a wide linear range of 1 pM to 1 μM with a low detection limit of 0.684 pM. Importantly, this sensor can be successfully applied in water samples with good accuracy and excellent recovery, which indicates its potential for Hg²⁺ detection in the environment.