3 years ago

STM Characterization of Self-Assembled Monolayers of Cysteine Betaine on Au(111) Electrode in Perchloric and Sulfuric Acids

STM Characterization of Self-Assembled Monolayers of Cysteine Betaine on Au(111) Electrode in Perchloric and Sulfuric Acids
Weicheng Liao, Chun-Jen Huang, Shuehlin Yau
A bioderived zwitterionic molecule, cysteine betaine (Cys-b), can be used as a biomaterial coating to evade fouling and damage by light radiation. In situ scanning tunneling microscopy (STM) has been used to study the structures of the cysteine betaine (Cys-b) molecule adsorbed on an Au(111) electrode in 0.1 M HClO4 and H2SO4. A number of Cys-b structures have been identified in 0.1 M HClO4 before adsorbed Cys-b is irreversibly oxidized, including (4 × 8), (6 × 6), and (√19 × 3√3). By contrast, very different Cys-b structures, including (√7 × 4), which is an incommensurate structure, and disordered structures, are seen in the same potential region in H2SO4. These results are reconciled by a coadsorption scheme involving the Cys-b cation and ClO4 (or HSO4). The coverages of Cys-b are 1.31 × 1014 and 2.32 × 1014 molecules/cm2 at the same potential in HClO4 and H2SO4. Although Cys-b molecules are tethered to the Au(111) substrate via their S-ends, their spatial structures are influenced greatly by the interactions with the coadsorbed anions. As ClO4 and HSO4 anions are hydrated in the aqueous electrolyte, their hydrated shells can affect their interactions with the Cys-b cation, leading to different ordered structures as seen by STM.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04786

DOI: 10.1021/acs.jpcc.7b04786

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