Interactions of the Cocaine and Quinine Aptamer with Gold Nanoparticles under the Dilute Biosensor and Concentrated NMR Conditions.

The cocaine aptamer was later found to bind quinine with an even higher affinity. In this work, we used a fluorescently labeled aptamer named MN4 to study its adsorption by gold nanoparticles (AuNPs), and the subsequent displacement by the nonlabeled aptamer and by quinine. Without washing, 14% of the preadsorbed MN4 strands were displaced by 4000-fold excess of free MN4, whereas no displacement was observed after washing, suggesting that washing removed weakly adsorbed aptamers. In a previous paper, rapid exchange was observed with NMR by directly mixing AuNPs and concentrated MN4, and our work has unified the dilute and concentrated aptamer conditions. The difference is attributable to the conformation of the adsorbed aptamer, where dilute aptamers are adsorbed in a collapsed state with a much higher affinity to AuNPs. In addition, the preadsorbed MN4 aptamer cannot be desorbed by adding quinine, indicating that direct desorption-based fluorescent sensors cannot be made. Finally, based on the similar color responses to both the aptamer and its nonbinding mutants, the label-free colorimetric detection method cannot be directly applied for the detection of quinine. This work indicated that different experimental conditions need to be carefully compared to have a unified understanding of aptamer/AuNP systems.