Self-Assembled Structures of Derivatised Gold Nanoparticles and Polymer : Electrochemical, Ellipsometrical and Quartz Crystal Microbalance Characterization

M. BELLINO; A. WOLOSIUK; M. OTERO; E. FORZANI; G. GORDILLO; E. CALVO

Viña del Mar, Chile

Workshop; International Workshop “ Frontiers in Material Science ”; 2002

CIMAT

Characterisation of modified gold electrodes by arrays of a redox polymer and colloidal derivatised particles is performed by electrochemical, quartz crystal microbalance (QCM) and ellipsometry studies. The build up of structures electrostatically self-assembled by alternate adsorption of cationic Os-complex modified poly(allylamine) (PAH-Os) and anionic modified  gold particles on 3-mercapto-1-propane sulphonic acid (MPS) modified gold surfaces is presented. Gold colloidal particles of ca. 20 nm diameter are prepared and derivatised with MPS. When derivatisation is completed the surface of gold nanoparticles are covered by a monolayer of MPS adsorbed molecules with their sulfonate group pointing towards the solution.  As result of this a well characterised negative colloid surface is obtained. The progress of the MPS adsorption on gold nanoparticles is monitored by UV-visible spectrophotometry, following the absorption peak intensity at l = 533.5 nm observed for colloidal gold in aqueous suspension, which reaches a minimum value when MPS adsorption is completed.   Gold electrodes are first modified with MPS in order to have negatively charged surfaces to attach the positive polymer (PAH-Os). Subsequent PAH-Os assembly yields positively charged surfaces where negative colloids are adsorbed in a following adsorption step. The mass of attached MPS-derivatised colloid obtained by use of a QCM reaches a constant value after adsorbing during ca. 10 min. Cyclic voltammetry allows to characterise the redox behaviour of the modified electrode. A redox peak system corresponding to PAH-Os trapped within the self-assembled structure has a reversible electrochemical behaviour. The mass of assembled PAH-Os is related to the reduction or oxidation voltammetric charge. Thickness changes and optical properties after each adsorption step are determined by ellipsometry. While optical properties remains nearly constants, thickness values increase with the nanoparticles adsoption step number. From electrochemical, ellipsometric and QCM results we conclude that the coverage of attached MPS-derivatised colloid after any adsorption step is very low, ca. 5 %. During the following PAH-Os adsorption step only the colloidal nanoparticles surface is covered. It is not found a layer-by-layer self assembly mechanism: when a new colloidal nanoparticles adsorption step is carried out this nanoparticles are adsorbed between other nanoparticles already covered by PAH-Os in the previous step.