Sensing Adenosine and ATP by Aptamers and Gold Nanoparticles: Opposite Trends of Color Change from Domination of Target Adsorption Instead of Aptamer Binding.

The 27 mer DNA aptamer for adenosine and adenosine 5'-triphosphate (ATP) is a popular model system for designing biosensors. Various strategies have been reported for label-free colorimetric detection using gold nanoparticles (AuNPs). It is generally accepted that free aptamers can protect AuNPs against salt-induced aggregation, whereas target-bound aptamers cannot. However, these studies only considered the aptamer binding to its target, and the adsorption of the aptamer on AuNPs, but none considered the adsorption of target molecules by AuNPs. We herein report that the adsorption of adenosine destabilized citrate-capped AuNPs with an apparent Kd of just 7.7 μM adenosine, whereas that of ATP stabilized the AuNPs because of the negative charges from the triphosphate group. The adsorbed ATP inhibited the adsorption of DNA. Using the aptamer and a nonbinding mutant, ATP and guanosine 5'-triphosphate (GTP) had the same colorimetric response, and so did adenosine and guanosine, regardless of the DNA sequence, indicating that the color change mainly reflected the adsorption of the nucleosides and nucleotides instead of aptamer binding. The related literature examples using this aptamer were classified into three types and individually analyzed, where the reported color changes can all be explained by the adsorption of target analytes.