Switching Properties of Mixed Carboxylic Acid Terminated Self Assembled Monolayers on Au(111)

DANIELA K. JACQUELIN; MANUEL A. PEREZ; FERNANDO P. COMETTO; PATRICIA P. OLIVERA; EDUARDO M. PATRITO

Niza

Congreso; 61st Annual ISE Meeting; 2010

International Society of Electrochemistry

In this work we show that mixed self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) prepared by immersion in ethanolic solutions behave as a pH-sensitive switch. Despite the chain length difference, the MPA-MUA SAMs form homogeneous mixtures. In a previous work we found that the driving force leading to the formation of the homogeneous mixture is the strong hydrogen bond interaction between the adjacent carboxylic groups of the MPA and MUA molecules [1]. This motivated the present work which shows that the switching properties arise from the formation (acid pHs) and breakage (alkaline pHs) of the hydrogen bond interactions between the terminal carboxylic groups of adjacent MPA-MUA molecules. The mixed SAMs were immersed successively for a few minutes in HCl or NaOH solutions and the procedure was repeated for several cycles. After the immersion in each solution, the surface was rinsed with Milli-Q water and it was dried with nitrogen. The average monolayer thickness was then measured in air with ellipsometry. Two well defined and very reproducible monolayer thicknesses were observed. This indicates that the transformations of the mixed monolayer after immersion the alkaline and the acidic solutions are reversible. The lowest thickness was observed after immersion in HCl and the highest thickness was observed after immersion in NaOH. Immersion in an acidic solution protonates the terminal carboxylic groups of MPA and MUA and the hydrogen bond interaction between these groups produces a folding of the long chain MUA molecule over the short chain MPA molecule. This leads to a decrease of the average monolayer thickness. On the contrary, in an alkaline electrolyte, the repulsion of the negatively charged carboxylate groups produces the unfolding of the long chain MUA molecules with the corresponding thickness increase. This interpretation was corroborated by investigating the MPA-MUA interaction with density functional theory calculations performed on a gold slab constructed by replicating (3√3 x √3) R30o unit cells with an MPA-MUA pair in each cell.