Self-Assembled Multilayers of Gold Nanoparticles with Redox Polyelectrolytes: Ellipsometric and Optical Characterization

N.F. FERREYRA, M.A. PÉREZ , E. CORONADO, P. LABBÉ, M. LÓPEZ TEIJELO.

México, Cancún

Congreso; 210th Meeting of The Electrochemical Society and XXI Congreso de la Sociedad Mexicana de Electroquimica; 2006

Nanoparticles based nanostructured films are currently under intense investigation because their potential applications in various fields such as semiconductors, molecular electronics, photovoltaic, chemical and biological sensing and catalysis [1]. The assembly of metal nanoparticles into bi- or tridimensional superstructures has been reported using various kinds of electroactive linkers. Recently, the layerby- layer growth of polyelectrolyte/gold nanoparticle films has also been reported [2]. Depending on the olyelectrolyte structure and the nanoparticles properties as well as the self-assembling conditions, the final properties of the film can be modified. In this work we describe the electrostatic LbL assembling of negatively charged gold nanoparticles (NP6: 6.4±0.6 nm and NP11: 11.0±0.7 nm) with a viologen-based cationic redox  olyelectrolyte (PV) on quartz slides negatively charged by activation in basic medium or gold electrodes modified by 3-mercapto-1- propane-sulfonic acid. The growing of PV-NP assemblies was demonstrated from UV-visible experiments showing an increase of the NP6 or NP11 surface plasmon resonance absorption band intensity as a function of the number of layers. Red shifting of the maximum corresponding to the surface plasmon resonance band as compared with the absorption spectrum of the NP colloidal gold is obtained for either increasing adsorption times or number of layers.Results for increasing adsorption times are explained using an effective medium theory which takes into account that gold nanoparticles are embedded in a host matrix and, consequently, subject to an average polarization field due to both the matrix and the surrounding particles. On the other hand, red shifting with number of layers is explained in terms of electromagnetic coupling of gold nanoparticles in the different layers. In addition, in situ ellipsometric studies were performed to follow the step-by-step growth of the PVNP assemblies on MPS-thiolated gold surfaces. The ψ - ∆ trajectories obtained at different wavelengths (546.1 and 632.8 nm) for assemblies ended either in PV or NP using different NP time adsorption (15 or 60 minutes), were fitted based on a single layer model with uniaxial optical anisotropy. Optical properties as well as thickness for the different LbL assemblies were  obained. UV-visible spectra and ellipsometric results are correlated with the structure of the PV-NP assemblies and the electrochemical response of the viologen-based cationic redox polyelectrolyte. References [1] M.Ch. Daniel, D. Astruc, Chem. Reviews, 104 (2004) 293. [2] N.F. Ferreyra, L. Guérente, J. Fatisson, M. López Teijelo, P.Labbé, Chem. Comm., 16 (2003) 2056.6: 6.4±0.6 nm and NP11: 11.0±0.7 nm) with a viologen-based cationic redox  olyelectrolyte (PV) on quartz slides negatively charged by activation in basic medium or gold electrodes modified by 3-mercapto-1- propane-sulfonic acid. The growing of PV-NP assemblies was demonstrated from UV-visible experiments showing an increase of the NP6 or NP11 surface plasmon resonance absorption band intensity as a function of the number of layers. Red shifting of the maximum corresponding to the surface plasmon resonance band as compared with the absorption spectrum of the NP colloidal gold is obtained for either increasing adsorption times or number of layers.Results for increasing adsorption times are explained using an effective medium theory which takes into account that gold nanoparticles are embedded in a host matrix and, consequently, subject to an average polarization field due to both the matrix and the surrounding particles. On the other hand, red shifting with number of layers is explained in terms of electromagnetic coupling of gold nanoparticles in the different layers. In addition, in situ ellipsometric studies were performed to follow the step-by-step growth of the PVNP assemblies on MPS-thiolated gold surfaces. The ψ - ∆ trajectories obtained at different wavelengths (546.1 and 632.8 nm) for assemblies ended either in PV or NP using different NP time adsorption (15 or 60 minutes), were fitted based on a single layer model with uniaxial optical anisotropy. Optical properties as well as thickness for the different LbL assemblies were  obained. UV-visible spectra and ellipsometric results are correlated with the structure of the PV-NP assemblies and the electrochemical response of the viologen-based cationic redox polyelectrolyte. References [1] M.Ch. Daniel, D. Astruc, Chem. Reviews, 104 (2004) 293. [2] N.F. Ferreyra, L. Guérente, J. Fatisson, M. López Teijelo, P.Labbé, Chem. Comm., 16 (2003) 2056.