Influence of divalent cations on the adsorption of DNA origami nanostructures at TiO2 surfaces

The controlled adsorption of DNA origami nanostructures (DONs) at oxides surfaces is an important step in many applications, ranging from lithographic patterning to sensor fabrication. It is usually facilitated by divalent cations, which screen electrostatic repulsion between the DONs and the surface and may even form salt bridges that firmly immobilize the adsorbed DONs. Here, we investigate DON adsorption at TiO2(110) surfaces mediated by Mg2+, Ca2+, and Sr2+. DON surface coverage is found to increase with cation species in the order Mg2+ < Ca2+ < Sr2+, while cation adsorption at the TiO2(110) surface follows the opposite trend. At the same time, these three cations interact differently with DNA. While Mg2+ undergoes strong coordinative binding with the backbone phosphates, Sr2+ shows weaker binding and forms a large ion cloud around the DNA duplex. Exposing the adsorbed DONs to EDTA leads to the chelation of the bound cations and thus to rapid DON denaturation. However, even for complete DON denaturation, the denatured scaffold remains adsorbed at the TiO2 surface, which suggests that electrostatic repulsion between DON and the surface is overcompensated by additional attractive interactions such as van der Waals interactions. Our results thus provide new insights into the complex interplay of cation-DNA, cation-surface, and DNA-surface interactions and their roles in DON adsorption at TiO2 surfaces, which may result in surprising and counterintuitive behaviors.