Scheme 1 shows the design of the Hg2+biosensor. The PA con-tains a thiol group at the 5terminus, a MB tag at the 3terminus,and a T-rich sequence. At the same time, the nicking endonucle-ase recognition sequence is embedded into the loop portion of thehairpin capture PA. DNA nicking endonucleases are a special fam-ily of restriction endonucleases and occur either naturally or viagene engineering. NEase can cut one strand of a double-strandedDNA (ds-DNA) at a specific recognition nucleotide sequence knownas a restriction site and produce DNA molecules that are nicked.The nicking endonuclease used in this design is Nt.AlWI (NewEngland Biolabs), which can recognize the specific sequence 5-GGATC-3/3CCTAG-5in ds-DNA and catalyze a single strand break4 bases beyond the 3end of the recognition sequence 5-GGATC-3. The PA is covalently attached to the Au electrode through Au-Scovalent bond. Then the PA/Au electrode is blocked with MCH. Thecoverage of MCH can effectively prevent the non-specific adsorp-tion of hairpin capture PA on the electrode surface and displace theweaker adsorption contacts between the hairpin capture PA andthe Au electrode