Cyclic control of the surface properties of a monolayer-functionalized electrode by the electrochemical generation of Hg nanoclusters

Hg²⁺ ions are bound to a 1,4-benzenedimethanethiol (BDMT) monolayer assembled on a Au electrode. Electrochemical reduction of the Hg ²⁺-BDMT monolayer to Hg⁺-BDMT (at E°= 0.48V) and subsequently to Hg⁰-BDMT (at E° = 0.2V) proceeds with electron-transfer rate constants of 8 and 11 s^-1, respectively. The Hg⁰ atoms cluster into aggregates that exhibit dimensions of 30 nm to 2 μm, within a time interval of minutes. Electrochemical oxidation of the nanoclusters to Hg⁺ and further oxidation to Hg²⁺ ions proceeds with electron-transfer rate constants corresponding to 9 and 43 s ^-1, respectively, and the re distribution of Hg²⁺ on the thiolated monolayer occurs within approximately 15 s. The reduction of the Hg²⁺ ions to the Hg⁰ nanoclusters and their reverse electrochemical oxidation proceed without the dissolution of mercury species to the electrolyte, implying high affinities of Hg²⁺, Hg⁺, and Hg⁰ to the thiolated monolayer. The electrochemical transformation of the Hg²⁺-thiolated monolayer to the Hg ⁰-nanocluster functionalized monolayer is characterized by electrochemical means, surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact-angle measurements. The Hg⁰-nanocluster-modified surface reveals enhanced hydrophobicity (contact angle 76°) as compared to the Hg²⁺-thiolated monolayer (contact angle 57°). The hydrophobic properties of the Hg0-nanocluster-modified electrode are further supported by force measurements employing a hydrophobically modified AFM tip.