Herein, a biosensor based on quadruplex-forming aptamer for the determination of K+ is presented. The aptamer was used as a molecular recognition element; it was adjacent to two arm fragments and a dual-labeled oligonucleotide serving as a signal transduction probe (STP) which is complementary of arm fragment sequence. In the presence of K+, the aptamer were displaced from STP, which was accompanied by decrease signal. The quenching percent of fluorescence intensity was proportional to the concentration of K+ in the range of 0.05 mM to 1.4 mM. A detection limit of 0.014 mM was achieved. Furthermore, other metal ions, such as Na+, Li+,NH4 +, Mg2+ and Ca2+ caused no notable interference on the detection of K+.

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