Electron Transfer in Spacer-Free DNA Duplexes Tethered to Gold via dA Tags.

Electrical properties of DNA critically depend on the way DNA molecules are integrated within the electronics, particularly on DNA-electrode immobilization strategies. Here, we show that the rate of electron transport in DNA duplexes spacer-free tethered to gold via the adenosine terminal region (a dA tag) is enhanced compared to the hitherto reported DNA-metal electrode tethering chemistries. The rate of DNA-mediated electron transfer (ET) between the electrode and methylene blue intercalated into the dA-tagged DNA duplex approached 361 s at a ca. half-monolayer DNA surface coverage Γ (with a linear regression limit of 670 s at Γ → 0), being 2.7-fold enhanced compared to phosphorothioated dA* tethering (6-fold for the C-alkanethiol linker representing an additional ET barrier). X-ray photoelectron spectroscopy evidenced dA binding to the Au surface via the purine N, whereas dA* predominantly coordinated to the surface via sulfur atoms of phosphothioates. The latter apparently induces the DNA strand twist in the point of surface attachment affecting the local DNA conformation and, as a result, decreasing the ET rates through the duplex. Thus, a spacer-free DNA coupling to electrodes via dA tags thus allows a perspective design of DNA electronic circuits and sensors with advanced electronic properties and no implication from more expensive, synthetic linkers.