An electrochemical study based on thymine-Hg-thymine DNA base pair mediated charge transfer processesElectronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06238a

A DNA monolayer composed of a double strand of 25 base pairs was adsorbed onto a Au surface. Matched base pairs were present at the gold surface and distal part, while (TT) n mismatched pairs were present at the middle of DNA monolayer. Methylene blue was attached to the end DNA monolayer via a C 6 alkyene linker and acted as the electrochemical probe. Upon the addition of mercury ions (Hg( ii )), thymine-Hg-thymine ((T-Hg( ii )-T) n ) was formed through metal coordination between DNA duplexes. The charge transfer (CT) properties of the DNA monolayer were studied using Laviron's theory, and it was found that when n ≤ 6, the kinetics of CT followed the order: matched DNA base pairs < DNA duplexes with (TT) n < DNA duplexes with (T-Hg( ii )-T) n , and the CT kinetics increased with increasing n . The conformation of the DNA monolayer adsorbed onto the Au (111) was investigated using atomic force microscopy (AFM), and it was found that a duplex structure was retained when n ≤ 6. The formation of (T-Hg( ii )-T) n brought about a smoother DNA monolayer surface than that composed of matched DNA base pairs. However, an interesting phenomenon was found when n ≥ 12, that is the (T-Hg( ii )-T) n complex was formed between different DNA strands, and this inter-DNA (T-Hg( ii )-T) n structure caused DNA monolayer deformation, so CT could not occur along the DNA base pairs. The (TT) n might have more π-overlapping than the corresponding matched base pairs, and the intercalation of Hg( ii ) into TT may further increase this overlapping, causing faster CT kinetics.