6-Mercaptopurine-coated gold nanoparticles (6MP-AuNPs) have been prepared by modification of the nanoparticle surface with 6MP upon displacement of the protective layer of citrate anions. The modification has been studied by UV-vis and FTIR spectroscopies. A study of the stability of these 6MP-AuNPs in aqueous solutions as a function of ionic strength and pH has shown the importance of the charges on the stabilization. The protonation of N9 of the 6MP molecules brings about a sudden flocculation phenomenon. However, the flocculation is reversible upon changing the pH to values where the molecules become newly charged. Evidence of the competence between the interaction of capping solvent molecules and the attractive forces between particles is also shown in this paper.

The synthesis of gold nanoparticles by using the one-phase method of Brust has been carried out in this work to obtain 6-mercaptopurine monolayer protected gold nanoclusters (6MP-MPCs). The 6MP-MPCs are somewhat polar clusters that are soluble in polar solvents such as DMF and DMSO and in neutral and alkaline aqueous solutions. The average size of the clusters is 2.4 ( 0.5 nm and a stoichiometry of Au 459 (6MP) 62 has been established by taking into account high resolution transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis results of the 6MP-MPCs. The mode of binding of the 6MP molecules to the gold surface has been studied by FT-infrared and X-photoelectronic spectroscopies that evidence the existence of the S-Au interaction. Moreover, the different tautomers of the 6MP molecules in homogeneous solution suffer the transformation into the thiol N(9)H tautomer in the adsorbed state. Finally, taking together the size of these MPCs with the low effective dielectric constant of the 6MP monolayer in contact with the DMF solution makes the observation of the quantized charging of the double layer of these 6MP-MPCs possible. A capacitance value of 1.06 aF consistent with the core dimension and protective monolayer dielectric properties has been determined for the individual 6MP-MPCs.

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.

Nanosized metal particles are currently used in the development of high-capacity metal-based negative electrodes for Li-ion batteries. Here, we report on a new strategy that uses 6-mercaptopurine-monolayer protected gold clusters (6MP-MPCs) as an electrode for a lithium battery. The higher performance of the Li/6MP-MPC as compared to the Li/2D-Au cell (the latter consists of naked gold microparticles composed of nanocrystallites of around 17 nm in size) is explained on the basis of the three-dimensional organization of the 6MP-MPCs in the powder sample. The cell specific capacity of the Li/6MP-MPC is kept on extended cycling in contrast to the behavior observed for the Li/2D-Au cell that shows an abrupt decay of this magnitude after a few cycles. The reorganization of the 6MP molecular layer on the lithium-gold alloying process allows the accommodation of the increased cores in the same space volume, avoiding the cracks in the electrode, thus, keeping the electronic conduction.