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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