Capture-SELEX for a short aptamer for label-free detection of salicylic acid.

Salicylic acid (SA) is a hydrolysis product and an active form of aspirin, and SA is found in a range of fruits and other food products. For food and drug and analysis there is a strong desire to detect SA. Since SA is a very small molecule, aptamers have advantages over antibodies for its detection. In this work, we used the library-immobilization capture-SELEX method to isolate aptamers for SA. After 17 rounds of selection, two main families of aptamers were isolated. The SA1 aptamer from family 1 has a K d of 5.8 μM from a thioflavin T (ThT) fluorescence assay and 26.7 μM from isothermal titration calorimetry. The binding of other sequences was weaker compared to SA1. Based on mutation studies, the two conserved regions of SA1 were connected by two stems. Using ThT as a stain, a label-free fluorescent sensor was tested for the detection of SA with a detection limit of 2.2 μM. A few similar molecules were tested including aspirin, and only p-hydroxybenzoic acid showed a weak binding, indicating the high specificity of the SA1 aptamer. Finally, the SA1 aptamer was also tested in tomato juice and a similar binding performance was achieved.