Effect of surface pretreatment on self-assembly of thiol-modified DNA monolayers on gold electrode

In this article we investigate the effect of gold pretreatment procedures on self-assembly of thiol-modified double-stranded DNA (ds-DNA) and single-stranded DNA (ss-DNA) on polycrystalline gold electrodes through cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS), chronocoulometry(CC) and scanning electron microscopy(SEM). The pretreatment procedures include M + E, M + C(piranha), M + C(dilute aqua regia), M + C + E(piranha), M + C + E(dilute aqua regia) and RM + C + E(piranha), where M is mechanical polishing, C is piranha or dilute aqua regia dipping, E is electrochemical polishing and RM is roughly mechanical polishing. Results indicate that electrochemical characteristics (ions penetration, charge transfer and surface coverage) of ds-DNA or ss-DNA self-assembled monolayers (SAMs) on gold pretreated by different procedures except M + C(dilute aqua regia) are almost the same, although surface roughness and configuration of the pretreated gold electrodes differ. This conclusion differs from literature reports that pretreatment procedures have a big influence on the compactness of simple alkanethiols SAMs. We consider that the disagreement might be due to the special structural characteristics of DNA molecule, as compared with simple alkanethiols. The bigger size of DNA molecule and the existence of electrostatic or hydration forces among adjacent DNA molecules (Van der Waals force for simple alkanethiols) lead to the formation of sparse DNA-SAMs, which make much bigger selectivity for thiols adsorption sites on gold surface, therefore electrochemical characteristics of DNA-SAMs is almost independent of the pretreatment procedures. This work might provide a useful reference for constructing DNA-SAMs on gold.