Pb2+ induced DNA conformational switch from hairpin to G-quadruplex: electrochemical detection of Pb2+.

Conformational switch from hairpin DNA to G-quadruplex induced by Pb(2+) is studied by electrochemical impedance spectroscopy (EIS) in the presence of [Fe(CN)(6)](3-/4-) as the redox probe. In the presence of Pb(2+), the G-rich hairpin DNA opens the stem-loop and forms G-quadruplex structure, which gives rise to a sharp increase in the charge-transfer resistance (R(CT)) of the film reflected by the EIS. This structural change is also confirmed by circular dichroism (CD) measurements and UV-Vis spectroscopic analysis and calculated by density functional theory (DFT). On the basis of this, we develop a label-free electrochemical DNA biosensor for Pb(2+) detection. With increasing concentrations of Pb(2+), the differences in the charge-transfer resistance R(CT) before and after the Pb(2+) incubation is linearly dependent on the logarithm of Pb(2+) concentration within a range from 50 μM to 0.5 nM. The biosensor also exhibits good selectivity for Pb(2+) over other metal ions. This is a simple and label-free electrochemical method for Pb(2+) detection making use of the G-quadruplex.