Sensitive label-free oligonucleotide-based microfluidic detection of mercury (II) ion by using exonuclease I

Mercury is a highly toxic metal that can cause significant harm to humans and aquatic ecosystems. This paper describes a novel approach for mercury (Hg ²⁺) ion detection by using label-free oligonucleotide probes and Escherichia coli exonuclease I (Exo I) in a microfluidic electrophoretic separated platform. Two single-stranded DNAs (ssDNA) TT-21 and TT-44 with 7 Thymine-Thymine mispairs are employed to capture mercury ions. Due to the coordination structure of T-Hg ²⁺-T, these ssDNAs are folded into hairpin-like double-stranded DNAs (dsDNA) which are more difficult to be digested by Exo I, as confirmed by polyacrylamide gel electrophoresis (PAGE) analysis. A series of microfluidic capillary electrophoretic separation studies are carried out to investigate the effect of Exo I and mercury ion concentrations on the detected fluorescence intensity. This method has demonstrated a high sensitivity of mercury ion detection with the limit of detection around 15nM or 3ppb. An excellent selectivity of the probe for mercury ions over five interference ions Fe ³⁺, Cd ²⁺, Pb ²⁺, Cu ²⁺ and Ca ²⁺ is also revealed. This method could potentially be used for mercury ion detection with high sensitivity and reliability.