ELAKCA: Enzyme-Linked Aptamer Kissing Complex Assay as a Small Molecule Sensing Platform.

We report herein a novel sandwich-type enzyme-linked assay for the "signal-on" colorimetric detection of small molecules. The approach (referred to as enzyme-linked aptamer kissing complex assay (ELAKCA)) relied on the kissing complex-based recognition of the target-bound hairpin aptamer conformational state by a specific RNA hairpin probe. The aptamer was covalently immobilized on a microplate well surface to act as target capture element. Upon small analyte addition, the folded aptamer was able to bind to the biotinylated RNA hairpin module through loop-loop interaction. The formed ternary complex was then revealed by the introduction of the streptavidin-horseradish peroxidase conjugate that catalytically converted the 3,3',5,5'-tetramethylbenzidine substrate into a colorimetric product. ELAKCA was successfully designed for two different systems allowing detecting the adenosine and theophylline molecules. The potential practical applicability in terms of biological sample analysis (human plasma), temporal stability, and reusability was also reported. Owing to the variety of both hairpin functional nucleic acids, kissing motifs, and enzyme-based signaling systems, ELAKCA opens up new prospects for developing small molecule sensing platforms of wide applications.