We further studied the effect of K + concentration on the stability of G-quadruplex. In Figure 4b , addition of 0.1 mM K + in phosphate buffer dramatically increased the CD intensity at 240 and 260 nm. CD absorption intensity enhanced with the increase of K + concentration and reached a plateau when K + concentration was higher than 20 mM. The binding affinity of C14B1 at different concentrations of K + was also investigated. As the flow cytometry results shown in Figure 4c , very weak binding of C14B1 towards AGR2-beads was observed when there was no K + in the buffer, and the binding affinity kept increasing with the addition of K + . The binding constants of C14B1 were measured and compared in the presence or absence of K + (Figure 4d ). The K d value in the presence of K + was determined to be 6.261.9 nM. The results demonstrated that formation of G-quadruplex is important for the binding capability of aptamer C14B1 to its target, which is highly K + dependent.