Effects of Potential and Anions on the Adsorption of Cytosine on the Au(111) Electrode in Neutral Media

The study of molecular adsorption at an ordered metallic electrode can yield a fundamental understanding of the chemical bonding at an electrified interface. The current study explored the adsorption of cytosine (C) on an ordered Au(111) electrode in neutral media of phosphate buffer solution (PBS), KClO₄, and K₂SO₄, as a sequel to our previous work conducted in acids (Electrochim. Acta, 2021, 390, 138871). The adsorption state of C on Au(111) was characterized by voltammetry and scanning tunneling microscopy, showing that C admolecules transformed from the physisorbed to chemisorbed state as potential was made positive. This transition resulted in a notable charge flow to the Au(111) interface and produced distinct peaks in the current–potential profiles with the morphology varying with the chemical identity of the supporting electrolyte. The first stage of chemisorption resulted in ordered adlattices, which transformed into a compact but less ordered structure with a more positive potential. These two chemisorbed states were poorly resolved in potential and coupled with the coadsorption of anions. Chemisorption of C occurred at the most negative potential in PBS and was fastest in the media used in the current study. Chemisorbed C molecules were arranged in Au(111) – (2√3 × 20) – C + H₂PO₄^– and (2√3 × 6) – C + ClO₄^– and SO₄^2–. The effect of pH on the adsorption of C on Au(111) was also discussed.