DNA origami adsorption at single-crystalline TiO2 surfaces†

The immobilization of DNA origami nanostructures on solid surfaces is an important prerequisite for their application in many biosensors. So far, DNA origami immobilization has been investigated in detail only on a few surfaces such as mica, SiO₂, and graphite. TiO₂ is a conductive oxide with extensive applications in photocatalysis, energy conversion, and (bio)sensing. Despite its great importance, however, TiO₂ has not been investigated as a substrate for DNA origami immobilization yet. Here, we systematically investigate the adsorption of 2D DNA origami triangles on single-crystalline TiO₂ surfaces under various experimental conditions. Interestingly, the effect of the Mg²⁺ concentration on DNA origami surface coverage is found to depend on the orientation of the TiO₂ surface. On TiO₂(110) and TiO₂(111), 10 mM Mg²⁺ yields a higher surface coverage than 5 mM. However, the inverse is observed for the TiO₂(001) surface, where the lower Mg²⁺ concentration leads to an increase in surface coverage by up to 75%. This is explained by the interplay between Mg²⁺ binding to the DNA phosphates and Mg²⁺ adsorption at the TiO₂ surfaces, which in the case of TiO₂(001) results in a maximum density of Mg²⁺ salt bridges already at a low Mg²⁺ concentration. At higher concentrations, both the surface and the DNA phosphates are getting saturated with Mg²⁺ ions, which introduces electrostatic repulsion at the TiO₂–DNA interface and thus lowers the surface coverage. Our results demonstrate that DNA origami surface coverage at different TiO₂ surfaces can be controlled by the Mg²⁺ concentration. However, the same mechanism may also play a role in DNA origami adsorption at other single-crystalline oxide surfaces.