Regulating The Electrochemical Reversibility Of Fe?CN?₆^3-/4- By Altering The Surface Potential Of The Compact Layer

The electrochemical process of the electroactive molecules takes place within the electric double layer at electrode surface.The interfacial potential distribution at the electrode surface can determine the rate of the heterogeneous electron transfer?ET?of the electroactive molecules.Especially,the surface potential of the compact layer?₂determines the driving force for the heterogeneous ET.Self-assembled monolayer?SAM?are highly ordered molecular structures that form by the spontaneous adsorption of molecules on a solid surface,and is a new type of organic ultra-thin films developed in the last three decades.Because of the good stability and easy preparation,SAM has generated wide interest in the fields of theoretical study,such as the study of the electron transfer kinetics.The alkanethiol SAM terminated with the carboxylic group?-COOH?,are much more important for many applications.And we can regulate the surface potential of the compact layer by adjusting the dissociation equilibrium of the terminal carboxyl groups,to control the heterogeneous electron transfer of redox species at the electrode.Based on the dissociation equilibrium of the terminal group,a negatively charged compact-layer surface was designed by modifying the 3-mercaptopropionic acid self-assembled monolayer?3-MPA SAM?on the Au electrode in this work.And we have study the effect of the compact-layer potential on the electrochemical reversibility of the classics redox species,Fe?CN?₆^3-/4-.The main experimental contents and conclusions are as follows:1.Cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?were used to compare the electrochemical reversibility of Fe?CN?₆^3-/4-at the bare Au and the3-MPA SAM electrode.And the effect of pH on the electrochemical reversibility of Fe?CN?₆^3-/4-was also studied.The results illustrated that the compact-layer surface potential,which can be adjusted by the dissociation equilibrium of–COOH,can control the electrochemical reversibility of Fe?CN?₆^3-/4-.The dissociation equilibrium of–COOH can build a negatively charged compact layer,which could make a negative surface potential of the compact layer.Thus will lead to a slow electronic transfer process of Fe?CN?₆^3-/4-and a poor electrochemical reversibility.And the electrochemical reversibility of Fe?CN?₆^3-/4-will be worse,with the increase of the pH.2.CV and EIS were used to study the effect of the divalent cations(Ba²⁺,Ca²⁺,Mg²⁺)on the electrochemical reversibility of Fe?CN?₆^3-/4-.It was found that the divalent cations can form ion-pairs with the dissociated carboxylic group?-COO^-?,which decrease the negative charge on the compact layer surface and lead to a better electrochemical reversibility of Fe?CN?₆^3-/4-.And the electrochemical reversibility of Fe?CN?₆^3-/4-enhances with the increase of the concentration of the divalent cations.It illustrates that the ability to promote the electrochemical reversibility of Fe?CN?₆^3-/4-at 3-MPA SAM modified Au electrode is Ba²⁺>Ca²⁺>Mg²⁺in order.So it can be concluded that the strength order of the ion-pairs is Ba²⁺>Ca²⁺>Mg²⁺.Differential pulse voltammetry?DPV?was used to compare the influence of the mixed divalent cations and the single-specie divalent cation on the electrochemical reversibility of Fe?CN?₆^3-/4-.It demonstrates that the two kinds of cations will simultaneously form ion-pairs with the terminal–COO^-,and their ratio decides the surface potential of the compact layer.So the electrochemical reversibility Fe?CN?₆^3-/4-can be adjusted by the different divalent cations,and determined by the ratio of the different cations.3.Base on the cyclic voltammetry curve changes with the pH and the concentration of the divalent cations,the logarithm of ion-pair formation constants?log K?between the-COO^-and the H⁺,Ba²⁺,Ca²⁺,Mg²⁺on the monolayer can be determined to be 5.24±0.13,2.78±0.03,2.75±0.04 and 2.69±0.04 for Ba²⁺,Ca²⁺,Mg²⁺by electrochemical titration method,respectively.Their ability on promoting the electrochemical reversibility of Fe?CN?₆^3-/4-is H⁺>Ba²⁺>Ca²⁺>Mg²⁺in order,according with the stability of the ion-pairs between the terminal carboxylate group?–COO^-?and the divalent cations.