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| - Influence of the Solution pH in the 6-Mercaptopurine Self-AssembledMonolayer (6MP-SAM) on a Au(111) Single-Crystal Electrode
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| - Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) have been prepared on a Au(111) single-crystalelectrode by immersion of the metal surface in a 100 μM 6MP and 0.01 M HClO4 solution. The 6MP-SAM Au(111)single-crystal electrodes were transferred to the cell and allowed to equilibrate with the different aqueous workingsolutions before the electrochemical experiments. The influence of the solution pH was studied by cyclic voltammetry,double layer capacitance curves, and electrochemical impedance spectroscopy. The electrochemical behavior of the6MP-SAM in acetic acid at pH 4 presents important differences in comparison to that obtained in 0.1 M KOH solutions.Cyclic voltammograms for the reductive desorption process in acid medium are broad and show some features thatcan be explained by a phase transition between a chemisorbed and a physisorbed state of the 6MP molecules. Thelow solubility of these molecules in acid medium could explain this phenomenon and the readsorption of the completemonolayer when the potential is scanned in the positive direction. The variation of the double-layer capacitance valuesin the potential range of monolayer stability with the pH suggests that the acid−base chemistry of the 6MP moleculesis playing a role. This fact has been studied by following the variations of the electron-transfer rate constant of thehighly charged redox probes as are Fe(CN)6-3/-4 and Ru(NH3)6+3/+2 as a function of solution pH. The apparent surfacepKa value for the 6MP-SAM (pKa ≈ 8) is explained by the total conversion of the different 6MP tautomers that existin solution to the thiol species in the adsorbed state.
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