Novel mixed self-assembled monolayers of l -cysteine and methanol on gold surfaces under ambient conditions

Abstract

In this work, we carry out an experimental and theoretical study on the formation ofself-assembled monolayers of L-cysteine molecules on gold surfaces in presence ofmethanol as solvent. We report for the first time, L-cysteine and methanol orderedstructures forming a mixed self-assembled mono-layer on Au(100) surfaces underambient conditions. Finger-like ordered structures were observed using ScanningTunneling Microscopy under room temperature and ambient pressure conditions, theirdimensions range between 0.10-0.20 nm in relative height, 2.0 nm in average width andvariable lengths. By X-ray photoemission spectroscopy, it was determined that Lcysteine molecules bind to the gold surface through the sulfur atom of its thiol-group, intwo molecular configurations: neutral and zwitterionic. We found that the finger-likestructures are the result of complex interactions between L-cysteine molecules with goldsurfaces, L-cysteine molecules with methanol molecules each other and all threecomponents of the system simultaneously (L-cysteine + methanol /gold surfaces). Theseinteractions were detected by Attenuated Total Reflection-Fourier Transformed Infraredspectroscopy. Furthermore, adsorbate/substrate interactions were studied by ab-initiocalculations using the Density Functional Theory, resulting in molecular arrangementsbeing formed by chains of L-cysteine pairs surrounded by physisorbed methanol molecules.