Surface Plasmon Resonance Determination of the Binding Mechanisms of l-Cysteine and Mercaptoundecanoic Acid on Gold

Abstract

Surface plasmon resonance was used for the real-time monitoring of the formation of self-assembled monolayers of l-cysteine and 11-mercaptoundecanoic acid (MUA) on gold surfaces. We provide comparative details on the kinetics of the assembly of short thiols with multiple functional groups, as opposed to longer alkanethiols with fewer functional groups. Our results indicate that the adsorption of l-cysteine is a rapid process, involving both amino- and thiol-Au interactions, followed by the exchange of amino-Au to thiol-Au species and the physisorption of a second cysteine layer. The formation of MUA is also rapid, followed by a slower structural rearrangement of the monolayer. We find that monolayer formation, for both l-cysteine and MUA, is described by the Langmuir isotherm at low concentrations only. Numerical models are introduced to describe the assembly of both higher and lower concentrations of thiolated molecules on gold.