biosensorElectrochemical impedance spectroscopy (EIS) is a power-ful technique for investigating the features of surface-modifiedelectrodes. The change of resistance could reflect the differentmodification processes that occur on the electrode surface. So thepreparation processes of the probe B/PA/MCH/Au electrode werecharacterized by EIS and the corresponding results are shown inFig. 2. In the 5 mM [Fe(CN)6]4−/3−solution containing 0.1 M KCl,the bare Au electrode exhibited very small impedance (curve a),indicating excellent electrochemical conductivity of the treated Auelectrode. The association of PA on the electrode enhanced theelectronegativity of the Au electrode and blocked the interfacialelectron transfer, which resulted in high impedance of 7200 cm2(curve b). There is a very large semicircle domain that implies a very Fig high electron-transfer resistance of the redox probe and indicatesthat the PA has been grafted on the Au electrode through the Au–Scovalent bond. MCH was added into the PA/Au electrode to pre-vent the non-specific and the weaker adsorption of hairpin capturePA (curve c). In the presence of Hg2+, the PA modified electrodereacted with probe B and the semicircle further increased (curved), indicating that PA hybridized with probe B through T–Hg2+–Tstructure. It could be clearly observed that the semicircle diameterdecreased obviously on the PB-Hg2+-PA/MCH modified Au elec-trode after incubating with nicking-enzyme (curve e). Because ofnicking reaction, the nicking enzyme nicked the hybridization atthe recognition sites. Then the probe B and the MB tag fragment ofthe cleaved PA dissociated from the Au electrode surface, leading todecrease in electrochemical impedance of the Au electrode surface.