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

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.