The adsorption of Dopamine species on Ag(111): a theoretical study of the protonation degree and the effect of electric field

A.C ROSSI FERNÁNDEZ; P.BELELLI; N.DOMANCICH; L.MEIER; N.J. CASTELLANI

Santa Fe

Conferencia; VI San Luis Conference on Surfaces,Interfaces and Catalisys; 2018

INTEC - UNL

The Dopamine (DA) molecule is a catecholamine that binds and activates cell surface receptors in humans and mammalians1. As neurotransmitter it plays an important physiological role in the central nervous, renal and hormonal systems. Then, it is of great medical and pharmacological interest to study chemical interactions in which this molecule is involved. Experimentally is known that, according to the pH value of the solvent medium, different forms of catecholamine molecules have been reported, including neutral, zwitterionic, cationic and anionic species. Recently, several proposals for DA detection have been published based on cyclic voltammetry and surface-enhanced Raman spectroscopy where the use of colloidal Ag nanoparticles as substrate for DA adsorption allows a better detection of this molecule2. In the present work, we study theoretically the adsorption of the neutral (DA), zwitterionic (Z-DA), protonated (P-DA) and deprotonated (DP-DA) species on the surface of Ag(111). This has been performed in the context of the Density Functional Theory (DFT) formalism as implemented in the Vienna Ab Initio Simulation Package (VASP)3,4. The repeated slabs representing the Ag(111) surface contain four layers of atoms, and a vacuum gap in the normal direction. The adsorbed species and at the same time the first atomic layer for Ag(111) were allowed to relax. As initial orientation for the adsorption of different DA species the aromatic ring was taken parallel to each Ag surface. The calculations show that the adsorption of P-DA on Ag(111) is a more favored process than that of Z-DA on Ag (111) whereas the adsorption of DP-DA is largely depressed in comparison with Z-DA. The adsorbate-substrate interaction was evaluated by computing the electron charge-density difference and performing the DEEC6 atomic-charge analysis5. The corresponding results indicate that an important reorganization of electronic charge is produced. While the neutral, zwitterionic and protonated DA species acquire a net positive charge, the deprotonated one, a net negative charge. The effect of an homogeneous electric field on the adsorption of DA species on Ag(111) was also studied. In particular, it was observed that a negative electric field stabilizes the ZDA/Ag(111) system and the Z-DA molecule is more positively charged. On the other hand, a positive electric field destabilizes the ZDA/Ag(111) system and the Z-DA molecule is less positively charged.