Size-fitting effect for hybridization of DNA/mercaptohexanol mixed monolayers on gold

In this article we proposed a simple hexagonal model for exploring the hybridization of thiol-modified probe DNA self-assembled monolayers (SAMs) on gold with target DNA molecules in solution. The size-fitting coefficient d(c)/d(t) from the model was used to discuss the principle for DNA optimal hybridization, where d(c) was the channel diameter among three adjacent probe DNA molecules on gold and dt was the gyration diameter of the target DNA molecules in solution. Experimentally we investigated the hybridization effect (hybridization efficiency H(E) and hybridization density H(D)) of thiol-modified probe DNA (DNA base amount m = 15, 25 or 35)/mercaptohexanol (MCH) mixed SAMs on gold in 1 M electrolyte solution by chronocoulometry (CC) and electrochemical impedance spectroscopy (EIS). The surface coverage Γ(m) of the probe DNA on gold was adjusted by changing the mixed concentration ratio of probe DNA with MCH (C(DNA)/C(MCH)) in the assembly solution. Results indicated that with the increase of C(DNA)/C(MCH), H(E) decreased gradually; H(D) first increased and then decreased, which arrived at the biggest at C(DNA)/C(MCH) = 1 for all the probe DNA/MCH mixed SAMs (m = 15, 25, 35). The optimal Γ(m) for achieving the biggest H(D) in DNA hybridization decreased with the increase of m from 15 to 35. The experimental conclusions obtained by CC and EIS measurements verified each other. Combining the simple model with our experimental results, we ascertained that d(c)/d(t) decreased with the increase of m, which showed a good linear relationship. These conclusions provided an important reference and guidance for controllably constructing DNA sensors with optimal performance.