Self-assembled monolayers of 6-amino-1-hexanethiol on gold as a potential form of surface functionalization in the construction of biocompatible platforms

Self-assembled-monolayers (SAMs) of amino-terminated alkanethiols and, in particular 6-amino-1-hexanethiol (AHT), are usually integrated in functional platforms as the presence of the amino groups in the interface facilitates the derivatization reactions. The chain length of AHT is very appropriate as building block because possess enough methylene units to direct the organization of the molecules in the SAM and, at the same time, is short to allow the communication of the electrode with the rest of the system. However, most of the studies on the characterization of these SAMs are dedicated to amino-alkanethiols with two or eleven methylene units and, no systematic electrochemical studies have been directed to the AHT-SAM. The use of Ru (NH 3 ) 6 Cl 3 redox probe to monitor the changes in the SAM interface with the solution pH, allows us to observe two inflections in the variations of either the current density, or the peak potentials as well as in the heterogeneous rate constants whether they are determined by cyclic voltammetry or electrochemical impedance spectroscopy. These inflections cannot correspond to apparent dissociation pK of different chemical species, but they should inform about different distribution and organization of the amino groups in the interface depending on the local solution pH. This complexity is also found in the behavior of the reductive desorption signals that do not follow the peak potential variation with pH according to the Nernst equation, but show a dependence on the crystallinity of substrate and variations of ∂ E p RD / ∂ pH that would indicate a kinetics effect on the ion H + penetration into the layer to compensate for the thiolate anion protonation, and the existence of regions with intra-layer hydrogen bonds that coexist with regions of protonated amino groups. A decrease in reductive desorption charge density with decreasing pH is thought to be due to the catalytic reaction of hydrogen evolution that masked the complete reduction of the monolayer. Contact angle measurements define a SAM that becomes less hydrophilic at high pH. No multilayer films are observed as stated by XPS that shows a S 2p peak containing only the signal corresponding to the S Au bond. These results can help in the design of new functional platforms that use the AHT-SAM as the first step in their construction.