Ferrocenated Au Nanoparticle Monolayer Adsorption on Self-Assembled Monolayer-Coated Electrodes

The robust, irreversible adsorption of ω-ferrocene hexanethiolate-protected gold nanoparticles (composition ca. {Au<sub>225</sub>(SC6Fc)<sub>43</sub>}) on electrodes provides an opportunity to investigate their submonolayer and monolayer films in nanoparticle-free solutions. Observations of nanoparticle adsorption on unmodified electrodes are extended here to Au electrodes having more explicitly controlled surfaces, namely self-assembled monolayers (SAMs) of alkanethiolates with ω-sulfonate, carboxylate, and methyl termini, and in different Bu<sub>4</sub>N<sup>+</sup><i>X</i><sup>−</sup> electrolyte (<i>X</i><sup>−</sup> = C<sub>7</sub>H<sub>7</sub>SO<sub>3</sub><sup>−</sup>, ClO<sub>4</sub><sup>−</sup>, CF<sub>3</sub>SO<sub>3</sub><sup>−</sup>, PF<sub>6</sub><sup>−</sup>, NO<sub>3</sub><sup>−</sup>) solutions in CH<sub>2</sub>Cl<sub>2</sub>. The nanoparticle surface coverage (Γ<sub>NP</sub>) and the stability of the adsorbed nanoparticle film to repeated ferrocene/ferrocenium redox cycling decrease in the order of sulfonate > carboxylate > methyl terminated SAM, with increasing hydrophobicity of <i>X</i><sup>−</sup> and with increasing alkyl chain length. The results are consistent with the proposal that the strong surface adsorption is jointly associated with the polyfunctional character of the nanoparticles, analogous to entropically driven adsorptions of polymeric ions on charged surfaces, and with lateral, ion-bridged nanoparticle−nanoparticle interactions.