Adsorption Study of L-Cysteine Self-Assembled Monolayers on Au (100) Surface by Scanning Tunnelling Microscopy and Ab-Initio Methods

VANINA G. FRANCO; SINDY JULIETH RODRÍGUEZ SOTELO; GUSTAVO RUANO; MARIO PASSEGUI; FLORENCIA CALAZA

Charlotte, North Carolina

Simposio; AVS 67th International Symposium & Exhibition (AVS 67); 2021

The functionalization of metal surfaces by biological molecules has a growing number of applications in various fields. For this, it is essential to understand the chemical and physical properties of the organic /inorganic interface and the formation process of self-assembled monolayers (SAM´s) of biological molecules such as amino acids. L-cysteine (Cys) is a particularly interesting amino acid as a macromolecule binder and chemical sensor. In this work, we carry out a theoretical and experimental study of the formation of SAM´s of Cys molecules on monocrystalline surfaces of Au (100), in order to clarify the molecular adsorption mechanism. The adsorption of molecules was performed by dipping the substrate in methanolic solutions of Cys at different coatings and characterized by scanning tunneling microscopy (STM) in air and at room temperature. Additionally, the substrate-adsorbate interaction was studied with ab-initio calculations using the Density Functional Theory (DFT) in three systems: (i) Cys on Au (100), (ii) Cys trimers on Au (100) and (iii) Methanol on Au (100), to determine the equilibrium configurations, adsorption energies and distances, state densities and charge transfer between the substrate and the molecule. By scanning tunnel microscopy, the molecular structure, order and reconstruction that induce the chemisorption and /or physisorption process of these molecules in the substrate were examined. It is revealed a pattern in the form of rectangular bands of Cys molecules, adsorbed as multilayers on Au (100). The calculations showed that the physisorption processes of Cys on Au (100) are mainly directed by the interaction between the thiol group and gold. The most stable configuration and the difference in charge density induced by the interaction, for this system, indicate that the amino acid is in its zwitterionic form. Furthermore, it was determined that the amino functional group moves away from the gold surface with a positive charge density, exposing the carboxyl group with a negative charge density closer to the surface; which would explain the chemical composition of SAM´s. The latter possibility was tested by ATR-IR (Attenuated Total Reflectance-IR) to try identifying which groups of the molecule are interacting more strongly with the surface Au atoms.