Neutral and zwitterionic dopamine species adsorbed on silver surfaces: A DFT investigation of interaction mechanism

Abstract

Biomolecules nondissociative adsorption on noble metals is a key process in metallic biosensors implying several questions related to the stability and orientation of such molecules. Here, the neurotransmitter dopamine (DA) adsorption on silver surface is investigated in the context of density functional theory (DFT). Two different dopamine isomers, the neutral (NDA) and zwitterionic (ZDA) species, and two different silver surfaces, Ag (110) and Ag(111), were considered. NDA shows relatively large binding energies, compared to previously studied π-π bonded systems. ZDA adsorbs even much more strongly although this species is less stable than NDA in vacuum. To elucidate the nature of the interaction between adsorbate and substrate, an electronic structure analysis was performed. Adsorbed NDA species suffers the loss of electronic charge, accompanied by a downshift of its molecular levels and the appearance of an attractive interaction of coulombic nature between adsorbate and substrate. The significant ZDA binding can be related to larger electron transfer and coupling between ZDA and Ag orbitals. Moreover, for both species, an important contribution of attractive noncovalent interactions of different degrees can be observed. The Ag substrate produces several modifications on NDA and ZDA vibrational frequencies. Noticeably relevant are the large red/blue shifts undergone by the N-H/O-H stretching bands of zwitterionic species, of up to −670/+430 cm−1.