A simple, rapid, highly sensitive electrochemical sensor for potassium ion(K+) based on the conformational change of DNA sequence containing guanine-rich segments is presented. In the presence of K+, guanine-rich DNA sequence folds to G-quadruplex structure, allowing a ferrocene tag to transfer electrons to the electrode. Gold nanoparticles(AuNPs), which are self-assembled on the surface of a bare gold electrode by using 4-aminothiophenol as a medium, offer a big surface area to immobilize a large number of aptamers and improve the sensitivity of the sensor. The square-wave voltammetry peak current increases with K+ concentration. The plots of peak current against K+ concentration and the logarithm of K+ concentration are linear over the range from 0.1 to 1.0 mmol·L-1 and from 1 to 30 mmol·L-1, respectively. A lower detection limit of 0.1 mmol·L-1 K+ is obtained for AuNPs-modified sensor, which greatly surpasses that(100 mmol·L-1) of the sensor without AuNPs modification by three orders of magnitude. Thus, the sensor with AuNPs amplification is expected to open new opportunities for highly sensitive detection of other biomolecules in the future. A simple, rapid, highly sensitive electrochemical sensor for potassium ion (K +) based on the conformational change of DNA sequence containing guanine-rich segments is presented. In the presence of K +, guanine-rich DNA sequence folds to G-quadruplex structure. a ferrocene tag to transfer electrons to the electrode. Gold nanoparticles (AuNPs), which are self-assembled on the surface of a bare gold electrode by using 4-aminothiophenol as a medium, offer a big surface area to immobilize a large number of aptamers and improve the sensitivity of the sensor. The square-wave voltammetry peak current increases with K + concentration. The plots of peak current against K + concentration and the logarithm of K + concentration are linear over the range from 0.1 to 1.0 mmol·L-1 and from 1 to 30 mmol·L-1, respectively. A lower detection limit of 0.1 mmol·L-1 K + is obtained for AuNPs-modified sensor, which greatly surpasses that (100 mmol·L-1) of the sensor without AuNPs modification by three o rders of magnitude. Thus, the sensor with AuNPs amplification is expected to open new opportunities for highly sensitive detection of other biomolecules in the future.