Published: May 12, 2011 r2011 American Chemical Society 7084 dx.doi.org/10.1021/la2007827 | Langmuir 2011, 27, 7084–7090 ARTICLE pubs.acs.org/Langmuir Dielectric Relaxations of Ionic Thiol-Coated Noble Metal Nanoparticles in Aqueous Solutions: Electrical Characterization of the Interface C. Cametti,* ,† I. Fratoddi, ‡ I. Venditti, ‡ and M. V. Russo ‡ † Department of Physics and INFM CRS-SOFT, “La Sapienza” University of Rome, Piazzale A. Moro 5, I-00185 - Rome, Italy ‡ Department of Chemistry, “La Sapienza” University of Rome, Piazzale A. Moro 5, I-00185 - Rome, Italy 1. INTRODUCTION Metallic nanoparticles (NPs), mainly gold and silver NPs, stabilized by organic or organometallic ligands, have attracted considerable attention in recent years because of the wide fields of applications envisaged for these materials, ranging from both fundamental and applied research, including optoelectronics and catalysis, 13 drug delivery, 4 sensors, 5 and biomedicine and diagnostics. 68 In this framework, gold NPs stabilized by charged ligands (polyelectrolytes) have been extensively investigated. These materials are suitable to form NP self-assembled multilayers, with a controlled thickness and a structure made by covalently attached organic/inorganic hybrids. 9 Silver NPs capped by mercaptosulfonic acid [Ag-3MPS] have been synthesized for the fabrication of nanostructured layer by layer films, which represents a great potential for broadening the selectivity of the matrix in surface enhanced Raman spectroscopy experiments. 10 Gold and silver NPs, prepared with mercapto sulfonic acid as stabilizer [Au-3MPS, Ag-3MPS, respectively], were also em- ployed as precursors of polyallylamine-chlorophyllide derivatives in sensor applications, 11 and Ag-3MPS NPs were widely em- ployed in view of their biocidal activity. 12 In this paper, we focus on the electrical characterization of the interface of Au and Ag NPs in aqueous solution stabilized by 3-mercapto propane sulfonate [3MPS], by means of radiowave dielectric relaxation spectroscopy. NPs with a metal core sur- rounded by a dielectric ligand shell, as those considered here, represent an interesting model from a dielectric point of view, since their properties can be modified by means of attachment of different stabilizing thiol ligands, displaying unique electrical, magnetic, and optical properties. 13 Due to the difference in the dielectric properties between the NP core, the coating external shell, and the aqueous phase, even if the fractional volume of the dispersed phase is very small, as in the present case, observable dielectric relaxations occur, whose dielectric parameters experience the electrical properties of the interface. The dielectric properties of these systems have been interpreted in light of the standard electrokinetic model for charged colloidal particles, as due to the polarization of the ionic atmosphere in the neighborhood of the particleaqueous inter- face. The effective charge that each NP bears has been de- duced from the dielectric parameters (dielectric strength Δε and relaxation frequency ν) and further substantiated by both ζ-potential and direct current (dc) electrical conductivity mea- surements. The combined use of these different techniques furnishes a full characterization of composite dielectricmetallic NPs in aqueous solution. 2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Hydrogen tetrachlor- oaurate (III) trihydrate (HAuCl 4 3 3H 2 O, Aldrich, 99.9þ%), silver nitrate (AgNO 3 , Aldrich, 99.5%), and 3-mercapto-1-propanesulfonic acid sodium salt (C 3 H 7 S 2 O 3 Na, [3MPS], Aldrich, 98%) were used as Received: March 1, 2011 Revised: April 27, 2011 ABSTRACT: The radiowave dielectric properties of organothiol monolayer- protected Au and Ag metallic nanoparticles have been investigated in the frequency range of 10 kHz to 2 GHz, where a dielectric relaxation, due to the polarization of the ionic atmosphere at the aqueous interface, occurs. The simultaneous measurement of the particle size, by means of dynamic light scattering technique, and of the particle electrical charge, by means of laser microelectrophoresis technique, allow us to describe the whole dielectric behavior at the light of the standard electrokinetic model for charged colloidal particles. Au and Ag metallic nanoparticles experience a large charge renormalization, in agreement with the counterion condensation effect for charged spherical colloidal particles. The value of the effective valence Z eff of each nanoparticle investigated has been evaluated thanks to the dielectric parameters of the observed relaxation process and further confirmed by direct current electrical conductivity measurements. All in all, these results provide support for the characterization of the electrical interfacial properties of metallic nanoparticles by means of dielectric relaxation measurements.