Comparative Study on Electron Transfer of Methylene Blue Modified Stem-Loop and Linear DNA Self-Assembled Monolayers on Gold Electrodes

Understanding the electron transfer performance of DNA self-assembled monolayers (SAMs) on a transducer substrate was important for constructing DNA electrochemical sensors with high sensitivity. Here we immersed the gold electrodes in mixed solutions comprising thiol-modified stem-loop DNA (SL-DNA) or linear DNA (L-DNA) terminally bound with methylene blue (MB), and aliphatic thiols (HOC6SH, C6SH, HOOCC5SH, H2NC6SH, HOC11SH, C12SH, or HOOCC10SH) to obtain DNA/thiols mixed self-assembled monolayers (SAMs). MB electron transfer performances through SL-DNA/thiols and L-DNA/thiols mixed SAMs were investigated by cyclic voltammetry (CV). Our results showed that the mixed assembly of thiols with DNA was conducive to MB electron transfer. The electron transfer rate (ks) values of MB through L-DNA SAMs were bigger than those through SL-DNA SAMs, indicating that the flexibility of DNA chains was important to MB electron transfer. Thiols chain length or terminal groups influenced ks of MB significantly, that increasing NaCl or MgCl2 concentrations in solution around DNA availed MB electron transfer, and Mg2+ advanced MB electron transfer more than Na+, and that the stability of L-DNA/thiols SAMs was much better than SL-DNA/thiols SAMs. These conclusions might provide a reference for studying DNA electron transfer mechanisms or designing DNA or RNA electrochemical sensors with optimal performance.