Self-assembled monolayers of NH₂-terminated thiolates: Order, pK_a, and specific adsorption

Self-assembled monolayers (SAMs) of amino-terminated alkanethiols on Au were characterized by a combination of electrochemical (LSV, CV, and EIS) and spectroscopic (XPS and SER) techniques. Clear correlations were obtained between the apparent surface pK_a values determined by impedimetric titrations and order parameters such as the content of trans conformers in the SAMs. These results contrast with previous studies that exhibit dispersions of up to 6 pH units in the reported pK_a values. In addition, we determined that inorganic and organic phosphate species bind specifically to these SAMs mediating adsorption and heterogeneous electron transfer of positively charged macromolecules such as cytochrome c.