An electrochemical study based on thymine–Hg–thymine DNA base pair mediated charge transfer processes

Abstract

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 C₆ 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.