SELEX-derived potassium aptamers for robust detection in a physiological sodium background

Potassium (K + ) is a vital metal in biology. So far, DNA-based potassium sensors all relied on G-quadruplex (G4) sequences that were not selected for K + binding. Instead they were originally selected to bind thrombin, hemin or from other sources. Here, we report the first SELEX experiment to isolate DNA aptamers using K + ions as the target. Based on deep sequencing results, only guanine-rich sequences were enriched and circular dichroism (CD) titration revealed a sequence named K-R17-36 with a K d of 0.08 mM, whereas a truncated sequence K-R17-36a had a similar K d of 0.06 mM. These affinities are over 7-fold tighter compared to commonly used K + -binding sequences such as the thrombin-binding aptamer (TBA) and human telomeric sequence (HTS). Using thioflavin T (ThT) fluorescence assays, we discovered sequences displaying both fluorescence enhancement and quenching responses upon K + binding. This dual-mode response allowed improved selectivity for K + detection. In the presence of 150 mM Na + ion and in artificial urine, the K-R17-36 aptamer still showed approximately 9-fold higher affinity than HTS. This work provides a comprehensive understanding of K + aptamers and a high affinity and robust binder for K + in physiological conditions.