Tuning the apparent formal potential of covalently attached ferrocene using SAM bearing ionizable COOH groups

• Covalent attachment of ethynylferrocene on the mixed SAM using “click” chemistry.
• Control of redox potential at the plane of electron transfer by double layer effect.
• Effect of pH of the medium on the redox potential of covalently attached ferrocene.
• Change of redox potential of ethynylferrocene with variation of chain length of the diluent and the surface coverage.

A method of tuning the apparent formal potential at the interface between the electrode surface covered with carboxylate thiol self assembled monolayer (SAM) and the solution containing electrolytes is reported. Redox active ferrocene moiety has been covalently attached to the terminal azido groups in the SAM using “click” chemistry. Cyclic voltammetry experiments show that the reduction potential of this ferrocene moiety can be tuned easily from 0.345 V to 0.200 V, i.e. by 145 mV by varying the chain length of the thiol carboxylic acid diluents, changing the pH of the electrolytic solution and by changing the coverage of the surface. Using the (applying the theory of interfacial potential distribution by Smith and White and including the Stern layer effect on the potential of the redox species at the interface proposed by Fawcett) modified Nernst equation the shift of apparent formal potential of redox species at the interface has been analyzed when the chain length of the diluents, pH of the solution and coverage of the surface are changed. The data indicate that the phenomenon is governed by the ionizable polar head groups SAM, i.e. COOH which changes the interfacial microenvironment around the redox active ferrocene centre.

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