Linkage, charge state and layer of L-Cysteine on copper surfaces

The control of linkage and charge state between biomolecules and metals represents a key issue for the architect of bioactive systems. In this paper, the linkage, charge state and layer of L-Cysteine (L-Cys) self-assembled films were handled on copper surfaces at pH=6.86. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to measure the film quality and the details of self-assembled progress. X-ray photoelectron spectroscopy (XPS) and quantum chemical calculations of density functional theory (DFT) were used to characterize the linkage, charge state and layer of the L-Cys molecules on copper surfaces. The results indicate that, from 0s to 24h, the self-assembled process can be classified as three steps, fast adsorption at the beginning, and then rearrangement to form a monolayer, and then the formation of double layer. And L-Cys molecules link to the copper surface through CuS bond, not CuN bond. The thickness of monolayer is 10.5Å. Then the L-Cys molecules of second layer recline on the first layer. Finally, by the interaction of amine group and carboxylic acid group between the two layers, the second self-assembled film stands uprightly, and the -S^- group of the second layer point outward. The thickness of the double layer is 19.7Å. All the Cu/L-Cys films have negative charges because the pH (6.86) of the self-assembled solution is more than the isoelectric point of the L-Cys (5.05).