LAUSR

Uric acid is the end‐product of purine metabolism in humans and an important biomarker for many diseases. To achieve the detection of uric acid without using enzymes, we previously selected a DNA aptamer for uric acid with a Kd of 1 μM but the aptamer required multiple Na+ ions for binding. Saturated binding was achieved with around 700 mM Na+ and the binding at the physiological condition was much weaker. In this work, a new selection was performed by alternating Mg2+‐containing buffers with Na+ and Li+. After 13 rounds of selection, a new aptamer sequence named UA‐Mg‐1 was obtained. Isothermal titration calorimetry confirmed aptamer binding in both selection buffers, and the Kd was around 8 μM. The binding of UA‐Mg‐1 to UA required only Mg2+. This is an indicator of successful switching of metal dependency via the salt‐toggled selection method. The UA‐Mg‐1 aptamer was engineered into a fluorescent biosensor based on the strand‐displacement assay with a limit of detection of 0.5 μM uric acid in the selection buffer. Finally, comparison with the previously reported Na+‐dependent aptamer and a xanthine/uric acid riboswitch was also made.