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.

Atomistic molecular dynamics calculations of self-assembled monolayers of alkanethiol molecules on gold nanoparticles are carried out to determine the surface per ligand molecule as a function of the size. The molecular footprint is determined by calculating the surface tension starting from its thermodynamic definition in the canonical ensemble. We use the method of Chiu et al. (Chiu, C. C. et al. J. Chem. Phys. 2010, 132, 054706) that makes use of a parametric dependence on the system size of the potential energy between the gold surface and the ligand molecules. The role of the different groups in the molecule in the surface tension is studied. An analysis of the dependence of the surface per thiol molecule on the molecular length is carried out showing that, for molecules larger than hexanethiol, this value is independent of the number of carbon groups in the molecule. The surface occupied per molecule on spherical nanoparticles as a function of the curvature is obtained, and the method is applied to flat surfaces, obtaining a very good agreement with the experimental results. A simple model for the surface density per molecule in curved and flat surfaces is developed.

The protein−gold nanoparticle bioconjugates are playing an important role in the studies of biological systems. The nature of the interaction and the magnitude of the binding affinity together with the conformational changes in the protein upon binding are the most addressed topics in relation to the uses of the bioconjugates in different organisms. In this work, we study the human serum albumin (HSA) protein−gold nanoparticle (AuNP) interactions focusing on the nature of the gold nanoparticle surface modification. We have found that the interactions of the HSA with the AuNPs are mainly electrostatic and that the concentration of protein necessary to stabilize the conjugates decreases when the overall negative charge on the nanoparticle surface increases. The changes in the localized surface plasmon resonance (LSPR) signals of the gold nanoparticles (13 nm diameter) are used to determine the number of protein molecules necessary to stabilize the conjugates in a high ionic strength medium. Fluorescence spectroscopy (stationary and time-resolved) is used to characterize the different bioconjugates and determine the binding constants under different experimental conditions. Moreover, the use of an extrinsic fluorescence probe (1-anilino-8-naphthalenesulfonic acid, ANS) gives us some information about the existence of partial unfolding of the protein upon binding to the nanoparticle.

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A study of the reductive desorption process of 6MP-coated Hg and Au(111) electrodes in alkaline media has been carried out by means of such electrochemical techniques as cyclic voltammetry, chronoamperometry, and capacity−potential curves. The cyclic voltammograms show a single reduction peak whose peak potentials are strongly dependent on the nature of the substrate, Hg and Au(111) in this study. The full widths at half-maximum (fwhm) for Hg and Au(111) electrodes respectively are 19 and 21 mV at a scan rate of 0.1 V/s. These values are generally indicative of the existence of strong lateral interactions between molecules.The chronoamperometric curves have been recorded by single potential step experiments from a potential where the monolayer is stable to different final values within the range of potential where the monolayer is being desorbed. The curve shapes resemble those of systems that follow nucleation and growth mechanisms in the formation or dissolution of two-dimensional layers. In fact, the curve analysis using the nucleation and growth model of etching centers which was carried out in this report is actually in agreement with a mechanism of this kind for the dissolution of the 6MP monolayer at Hg and Au(111) electrodes. This study compares the properties of the 6MP monolayer at the two substrates Hg and Au(111) as well as with other thiol derivative monolayers.

We present results of the formation of 1,8-octanedithiol (ODT) monolayers on the Au(111) single-crystal surfaces by oxidative deposition from alkaline solutions under electrochemical control. Cyclic voltammetry shows the presence of two well-separated oxidative peaks that are assigned to the formation of the S-Au bond (peak A1) and the oxidation of the thiolate species to give the disulfide dimer either in solution or in the adsorbed state (peak A2). The formation of a disulfide species can take place with the participation of two neighboring ODT molecules or through the formation of a bilayer in the adsorbed state. The reductive desorption of the layers formed under these conditions gives us some information about its nature and allows us the choice of the experimental conditions to carry out a potentiostatic method to build the layers of ODT with determined properties. Electrochemical techniques, such as cyclic voltammetry, differential capacitance-potential, and chronocoulometry curves, are used to discriminate between the monolayer and bilayer formation. Moreover, XPS data are used to confirm the electrochemical results. It is concluded that very reproducible layers that contain mainly standing-up ODT molecules are formed by the potentiostatic method and that they are built in a shorter time than those formed by the spontaneous assembly from an ethanolic solution.

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.