Label-Free, Electrochemical Quantitation of Potassium Ions from Femtomolar Levels.

In this communication, a label-free and sensitive electrochemical method to detect potassium ions is proposed. The conducting polymer polypyrrole was used as both an anchor for the probe and a transducer of the detection event. A K(+)-specific G-rich aptamer was applied as a recognition element, which folded into the G-quadruplex structure in the presence of K(+), and this resulted in an increase in the electrode impedance. The combination of the K(+)-selective aptamer and the porous conducting polymer as a signal transducer afforded a successful sensor platform. The sensor responded approximately logarithmically over a wide dynamic range of K(+) concentrations from 20 fM to 1 mM, with a very low detection limit of 14.7 fM and excellent discrimination against other ions. Additionally, electrochemical impedance spectroscopy was used to study the kinetics of K(+) binding at the conducting polymer-immobilized aptamer surface, which indicated strong binding between the two. This work demonstrates a powerful approach for the sensitive, selective, and direct electrochemical detection of metal ions based on the switching conformation of G-rich aptamers attached to a porous conducting polymer surface. This assay scheme can be expanded to the detection of a wide range of targets by modifying the aptamer structure as a recognizing moiety.