Kinetic Approach for the Study of Noncovalent Interaction between [Ru(NH 3 ) 5 pz] 2+ and Gold Nanoparticles E. Grueso, † D. Alcantara, ‡ J. Martinez, ‡ M. Mancera, § S. Penades, ⊥ F. Sanchez, † and R. Pradogotor* ,† Department of Physical Chemistry, Faculty of Chemistry, UniVersity of SeVilla, C/Profesor Garcı ´a Gonza ´ lez s/N 41012 SeVilla, Spain, Grupo Carbohidratos, Laboratory of Glyconanotechnology IIQ, CSIC-UniVersity of SeVilla, Ame ´ rico Vespucio nr. 49, 41092 SeVilla, Spain, Departamento de Quı ´mica Orga ´ nica y Farmace ´ utica, UniVersity of SeVilla, C/Profesor Garcı ´a Gonza ´ lez s/N 41012 SeVilla, Spain, and CICsBiomacGUNE, Parque tecnolo ´ gico de San Sebastia ´ n, Paseo Miramo ´ n 182, San Sebastia ´ n 29009, Spain ReceiVed: May 10, 2007; In Final Form: July 23, 2007 Gold nanoparticles (AuNPs) capped with N-(2-mercaptopropionyl)glycine have been used to study the strength and character of the binding of a cationic metal complex, [Ru(NH 3 ) 5 pz] 2+ (pz ) pyrazine), at pH ) 8, to these nanoparticles. The strength of the binding has been studied using a kinetic approach consisting of the study of the kinetics of the oxidation of this ruthenium complex by S 2 O 8 2- at different NaCl concentrations. When the ionic strength increases, the strength of the binding decreases, as a consequence of the partial neutralization of the charge on the AuNPs which, at pH ) 8, has the tiopronin residue negatively charged. The increase of the ionic strength also produces a change in the character of the binding, which changes from anticooperative to noncooperative when the ionic strength increases. The nonelectrostatic and electrostatic components of the free energy of binding are determined. From the latter, we have obtained the values of the electrostatic potential differences at the AuNPs/solutions interface. Introduction A great number of applications in the nanotechnology research area are directly related to metal nanoparticles linked with sugars, 1 proteins, 2 dendrimers, 3 surfactants, 4 small ligands, 5 or DNA. 6 These nanoparticles are being used for assembling new materials, developing bioassays, and as multivalent systems for interaction studies. These interactions correspond in most cases to noncovalent interactions, that is, interactions between chemical species other than covalent bonds. Nanoparticles are characterized by the properties of the metal cluster core but also by the organic molecules that constitute the monolayer. The variety of these nanometer-sized metallic particles depends not only on the metal nature but also on the capping agents. Among them, alkanethiolate nanoparticles have received considerable attention due to their advantages of stability, suspendability in different solvents, and facile char- acterization by standard analytical techniques. 7 In this sense, gold nanoparticles (AuNPs) capped with tiopronin are really stable and present a surface charge density which is function of the pH of the medium. That is, the tiopronin ligand permits one to control the surface charge on the gold nanoparticle and therefore to modulate the interaction between the AuNPs and ligands. Although direct applications of these Au@tiopronin nano- particles have been recently described, for example, as cell targeting, 8 there are no systematic studies in relation to the strength and character of the binding of AuNPs to small charged ligands through noncovalent interactions. Generally speaking, these noncovalent interactions between two species produce a change in their properties. So, the union of a substrate, S, to a receptor, R, promotes a change in the free energy of the substrate given by 9 Here, the activity coefficient of the substrate, γ s , is defined with respect to a reference state in which [R] ) 0. On the other hand, K in eq 1b represents the equilibrium constant for the process Measuring some properties at different receptor concentrations, for example, changes of the rate constants of a given reaction in which S participates, it is possible to obtain K and, from this, the standard free energy corresponding to the union substrate/receptor. Following this approach, we have done a systematic study of the interaction between a cationic metal complex, [Ru(NH 3 ) 5 pz] 2+ , and AuNPs capped with tiopronin ligands. The equilibrium binding constant K and, therefore, the free energy of binding of the ruthenium complex (positively charged) to the AuNPs were obtained following the changes in the kinetics of the electron-transfer reaction between the [Ru- (NH 3 ) 5 pz] 2+ and S 2 O 8 2- . From these kinetic data, a two-state model allows us to evaluate not only the strength of the binding but also its character as function of the AuNPs concentration and the ionic strength of the medium. On the other hand, working at different ionic strengths it is possible to separate * Author to whom correspondence should be addressed. Tel.: 34-954 557177. Fax: 34-954557174. E-mail: gcjrv@us.es. † Department of Physical Chemistry. ‡ Laboratory of Glyconanotechnology IIQ. § Departamento de Quı ´mica Orga ´nica y Farmace ´utica. ⊥ CICsBiomacGUNE. ∆G S ) RT ln γ S (1a) γ S ) 1 1 + K[R] (1b) S(free) + R h S/R (substrate linked to the receptor) (2) 9769 J. Phys. Chem. A 2007, 111, 9769-9774 10.1021/jp073577c CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007