Influence of the Solution pH in the 6-Mercaptopurine Self-Assembled Monolayer (6MP-SAM) on a Au(111) Single-Crystal Electrode

Abstract

Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) have been prepared on a Au(111) single-crystal electrode by immersion of the metal surface in a 100 microM 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 working solutions 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 the 6MP-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 that can be explained by a phase transition between a chemisorbed and a physisorbed state of the 6MP molecules. The low solubility of these molecules in acid medium could explain this phenomenon and the readsorption of the complete monolayer when the potential is scanned in the positive direction. The variation of the double-layer capacitance values in the potential range of monolayer stability with the pH suggests that the acid-base chemistry of the 6MP molecules is playing a role. This fact has been studied by following the variations of the electron-transfer rate constant of the highly charged redox probes as are Fe(CN)(6)-3/-4 and Ru(NH3)(6)+3/+2 as a function of solution pH. The apparent surface pKa value for the 6MP-SAM (pKa approximately 8) is explained by the total conversion of the different 6MP tautomers that exist in solution to the thiol species in the adsorbed state.

Thiol with an Unusual Adsorption−Desorption Behavior: 6-Mercaptopurine on Au(111)

“…Blank voltammograms recorded for clean Au(111) in the same electrolyte are also included as a reference (dotted lines in Figure ). For 6MP, a sharp reductive desorption peak can be found at −0.68 ± 0.01 V (vs SCE), as previously reported. , The peak charge is 56 ± 4 μC cm −2 , which corresponds to a surface coverage θ = 0.25 ± 0.02, in good agreement with that obtained from XPS measurements. As is well known, HT yields a desorption peak at −1.07 ± 0.03 V and a charge of 75 ± 7 μC cm −2 , in accordance with θ = 0.33.…”

Dissociation and Degradation of Thiol-Modified DNA on Gold Nanoparticles in Aqueous and Organic Solvents

“…Therefore, it is also important to know the stability of the purified DNA–AuNP hybrid. While desorption of thiol compounds has been extensively studied in the context of electrochemistry and SAMs, − ,,− few reports can be found on the dissociation of DNA from nanoparticles . In this work, we systematically studied the kinetics of DNA dissociation as a function of ionic strength, temperature, pH, and organic solvent.…”

Influence of Patterning in the Acid–Base Interfacial Properties of Homogeneously Mixed CH3- and COOH-Terminated Self-Assembled Monolayers

“…The reason is that the more negatively charged the SAM interface is, the greater surface potential difference between the outer plane of closest approach (OHP) and the bulk solution, Φ 1 . Under these conditions, the negatively charged redox probes are prevented from approaching the surface due to electrostatic repulsion, which in turns causes the depletion of the Fe­(CN) 6 3‑/4‑ surface concentration at the OHP zone in the electrical double layer. − Then, in the presence of double layer effects, R CT can be expressed as , where z is the charge of the redox probe, β = 1 – α, with α the electron transfer coefficient, and R CT 0 is the charge transfer resistance value for Φ 1 = 0 (i.e., uncharged monolayer). Similarly, the interfacial capacitance in the double layer, C , should be influenced by the dielectric properties of the SAM and its acid–base surface properties.…”