Newly found K-Thioflavin T competitive binding to DNA G-quadruplexes and the development of a label-free fluorescent biosensor with extra low detection limit for K determination in urine samples.

The determination of potassium ion K in body fluids is important in health monitoring and diagnoses. One of the interesting and simple methods for K detection is the use of label-free biosensors based on DNA G-quadruplexes (GQs) coupled with a specific fluorescent probe, such as Thioflavin T (ThT), which lights up when bound with K-stabilized GQs. However, these biosensors are not generally sensitive. In this work, we found a solution: at a low concentration, K competes with ThT in binding to a bimolecular GQ or a tetramolecular GQ, resulting in a decrease in ThT fluorescence emission with increasing K. Therefore, we developed a label-free turn-off fluorescent K sensor. The sensor provides a very low detection limit of 21.87 ± 0.59 nM. Other possible interfering components in urine did not exert any effect even at quantities that were 10-fold greater than their upper limit of normal concentrations found in urine samples. With its only requirement of diluting samples, the developed low-cost label-free probe and simple sensor was successfully applied to the direct detection of K in normal urine samples with high accuracy (recoveries ranged from 90% to 100%).