Interactions Between Hemin-Binding DNA Aptamers and Hemin-Graphene Nanosheets: Reduced Affinity but Unperturbed Catalytic Activity

We demonstrate herein that hemin-graphene hybrid nanosheets (H-GNs) can differentiate hemin-binding DNA aptamers (i.e., guanine-rich DNA sequences that form G-quadruplex structures upon binding hemin) and nonspecific single-stranded oligonucleotides colorimetrically. By exploiting the tendency of the H-GNS to aggregate under high ionic strength and their inherent catalytic activities, excess H-GNS were removed and the amount of H-GNs remaining in the supernatant was quantified. Subsequently, the binding affinity of hemin-binding DNA aptamers to the H-GNS was determined upon titration with the DNA. With the conformity of the experimental data to the Langmuir adsorption isotherm, hemin is hypothesized to be sandwiched between the G-quadruplex DNA and graphene forming a ternary complex as a result of the specific binding of the aptamer with the H-GNS. The binding affinity of the aptamer is not as strong as that of hemin/G-quadruplex in solution, which is attributed to electronic effects imparted by the graphene surface. The catalytic activity of the resulting complex, however, was confirmed to be higher than that of H-GNs alone.