Electrochemical and Morphological Characterization of New Architectures Containing Self-Assembled Monolayers and Au-NPs Virgı ´nia C. Ferreira, †,‡ A. Fernando Silva, ‡ and Luisa M. Abrantes †, * CQB, Departamento de Quı ´mica e Bioquı ´mica, Faculdade de Cie ˆncias da UniVersidade de Lisboa, Campo Grande, 1749-016 Lisboa and CIQ-UP, Linha 4, Departamento de Quı ´mica, Faculdade de Cie ˆncias da UniVersidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal ReceiVed: December 23, 2009; ReVised Manuscript ReceiVed: February 12, 2010 Pure 1,10-decanedithiol (C 10 -SH) and mixed (1-decanethiol:1,10-decanedithiol) self-assembled monolayers (SAMs) prepared from ethanolic solution on Au(111) surfaces have been used in order to investigate the effect of the SAM organization and the availability of free -SH groups at the SAM/solution interface on the development of layer-by-layer architectures containing SAMs and gold nanoparticles (Au-NPs). The SAM modified electrodes have been electrochemically characterized by cyclic voltammetry in alkaline medium (reductive desorption) and in the presence of an electroactive species, Fe(CN) 6 3- , in KNO 3 solution, enabling the evaluation of the stability and organization of the SAMs. Enhanced stability, organization, and hindrance to the electron transfer were found for the mixed SAMs with increasing thiol content, when compared with the pure dithiol SAM. The mixed SAM prepared from solution containing the thiol to dithiol proportion of (50:1) and pure C 10 -SH SAMs have been selected for further modification; the electrochemical quartz crystal microbalance (EQCM) enables the detection of different amount of citrate stabilized Au-NPs attachment to the selected SAMs modified electrodes due to distinct availability of free -SH groups at the SAM/solution interface and the electrochemical characterization of the layer-by-layer assemblies (based on pure C 10 -SH and mixed SAMs) showed that the electron transfer (ET) properties of the such architectures strongly depend on the nature of the base SAM and amount of immobilized Au-NPs. Atomic force microscopy (AFM) morphological characterization of the C 10 -SH SAM upon layer-by-layer modification was performed ex situ in air. Introduction Self-assembled monolayers (SAMs) have been widely used in the past decades due to their ease of preparation, organization, and versatility. 1,2 The self-assembling process enables the possibility of tuning the surface properties by simple modifica- tion of the adsorbed molecules characteristics, namely chain length, conjugated/insulating properties, and terminal functional group. On gold surfaces, the strong interaction between sulfur and gold is the driving force for self-assembling of sulfur- containing molecules, such as thiols and disulfides, and justifies the extensive use of this type of compounds. 1,2 Investigation has been devoted to R,ω-dithiol SAMs (aliphatic or aromatic) since these molecules, presenting two -SH functionalities, allow the surface modification displaying free -SH groups at the SAM/solution (or air) interface. 3 However, some difficulties have arisen from the self-assembling process of dithiol com- pounds due to the possibility of loops formation (both -SH functionalities binding to the gold surface) and consequent loss of organization and compactness; 4 the possibility of inter- and intralayer disulfide bonding has also been considered for this lack of organization. 5–12 In this case, the presence of oxidizing species in the self-assembling solution, such as oxygen, which is a highly interfering component, may contribute to the oxidation of the -SH terminal groups at the SAM/solution interface. 13 It has been reported in the literature that, depending on the self-assembling conditions (adsorbate properties, con- centration, solvent, and temperature), the dithiol molecules can adopt different configurations, parallel to the surface, looped or with an upright orientation. 4,13–18 However, these modified surfaces, allowing the attachment to other functionalities such as metallic ions, 19,20 metals (e.g., by vacuum deposition or electrochemical reduction of metallic ions in solution), 3,9,21–24 and presynthesized gold nanoparticles, 15–18,25,26 reinforce the possibility of a single -SH group attachment to the gold surface. Some approaches have been used envisaging the formation of more compact and organized SAMs displaying free -SH groups at the SAM/solution interface and for the control over the SAM composition and nanopatterning. 3,27 A promising one- step procedure is the simultaneous coadsorption of alkanethiol and alkanedithiol molecules, in which advantage is taken from the stability and organization conferred by the van der Waals interactions between alkyl chains of the alkanethiol component, which increases with the chain length, 28,29 facilitating the attachment of dithiol molecules through a single -SH group to the gold surface. 27 Thiol-functionalized SAMs have been applied for the devel- opment of 2D and 3D architectures with gold nanoparticles (Au- NPs). Distinct routes have been used: Au-NPs surface modifi- cation with dithiol molecules, in solution, and immobilization on gold electrodes, 16,30,31 attachment of Au-NPs on dithiol SAMs through Au-S bonding, 15,17,18,26 and multilayer formation by alternate immersion in dithiol solution and colloidal suspen- sion. 16,25,32,33 * To whom correspondence should be addressed. Phone: +351 21 7500890. Fax: +351 21 7500088. E-mail: luisa.abrantes@fc.ul.pt. † CQB, FCUL, Universidade de Lisboa. ‡ Present address: CIQ-UP, Linha 4, FCUP, Universidade do Porto. J. Phys. Chem. C 2010, 114, 7710–7716 7710 10.1021/jp912123m  2010 American Chemical Society Published on Web 04/13/2010