Voltammetric Study On The Influences Of Surfactants On Thiols Modified Gold Electrodes

The investigation on SAMs (self-assembled monolayer) is of interest and important for the researches of electron transferring process, bioelectrocatalysis, molecules recognition and detection, development of chemical sensor and so on. Furthermore, it is a perfect technique to study interfacial phenomenon and control its characteristics accurately at molecule level. Surfactants can adsorb to the surface of SAMs and change its structure and properties. So they have been the focus in electrochemistry and electroanalysis, especially the new types of surfactants, gemini surfactants. In this paper, electroactive species were used to investigate the adsorption mode and influence of gemini surfactants on SAMs. The following aspects were mainly studied:(1) The adsorption of several kinds of gemini surfactants at dodecanethiol (DDT) self-assembled monolayer (SAM) modified gold electrodes was investigated by electrochemical methods with K₃Fe(CN)₆ as probe. It was found that K₃Fe(CN)₆ can exhibit redox peak at DDT/Au electrode via geminis adsorption on DDT SAMs. Geminis can intercalate into the SAM and form a second layer on the hydrophobic DDT film. The intercalation made channels occur in the DDT monolayer for Fe(CN)₆^3-/4- to penetrate and reach the bare gold, while the second layer acted as a media. The adsorption amount of gemini surfactants was closely correlated with their concentration and structure. For the geminis with similar molecular structure but different alkyl chain, their influence followed such order: C₈-s-C₈ > C₁₂-s-C₁₂ > C₁₆-s-C₁₆.(2) 4, 4'-thiobisbenzenthiol (TBBT), which formed mixed self-assembled at gold electrodes in virtue of the formation of S-S coupling, were studied electrochemically. In strong acidic solution the end group of the TBBT self-assembled multilayers exists in the form of -SH; in strong alkaline solution the end group dissociated thoroughly (-S^-) with large amount of negative charge on the film. In neutral or weak acidic solution the film structure is similar to that in basic solution but the -SH group dissociates to different extent. TBBT/Au electrode can promote the electrode reaction of MB, especially in the presence of gemini surfactants. This resulted form the adsorption of them on TBBT SAMs, which enhanced the hydrophobicity of the film. For geminis containing the same alkyl chain C₁₆, their enhancement degree to MB peak follows such order: C₁₆-ph-C₁₆ ≈ C₁₆-C₄-C₁₆> C₁₆-C₆-C₁₆ > C₁₆-C₁₂-C₁₆ > CTAB> C16-C4OH-C16 > DCAB; for geminis containing the same spacers, the order is Ci6-ph-Ci6 C,6-C4-Ci6 > Ci6-C6-Ci6 > C16-C12-C16 > CTAB > Ci6-C4OH-Ci6 > DCAB.(3) Electrochemical polymerization of 4-aminothiophenol (PATP) was examined. Firstly, PATP was adsorbed on gold surface by forming S-Au bond, and then polymerized during the scan process; if a high potential was applied (i.e. 0.8 V), polymerization had occurred during accumulation. In fact, the polymerization of PATP in HCIO4 solution was a competitive process of two polymerization modes, which was closely correlated with the pH and the solution conditions. At the same time, the presence of surfactants had also influences on polymerization process of PATP at gold electrode. The polyaminothiophenol modified gold electrode obtained by this method exhibited electro catalysis action on the electrode reaction of Na2S2C<3.(4) The carboxylic multi-walled carbon nanotubes (MWNT) were immobilized on glassy carbon (GC) electrode to prepare MWNT/ GC electrode. It was found that Epinephrine (EP) can exhibit an anodic peak at bare glassy carbon electrode, but the peak is very small. However, at MWNT-coated GC electrode and activated GC electrode the peak becomes much higher and the peak potential shifts negatively, resulting from the increase of electrostatic attraction and electrode area. Thus, the two electrodes can be used for the determination of EP. The two electrodes exhibit some different characteristics. The activated GCE was featured with electrostatic action, while the CNT/GCE was by amplifying effective electrode area.