AVS 46th International Symposium
    Biomaterial Interfaces Group Friday Sessions
       Session BI-FrM

Paper BI-FrM7
Properties of Thiol Monolayers in Contact with Liquids: An In Situ Investigation by IR-vis Sum Frequency Spectroscopy

Friday, October 29, 1999, 10:20 am, Room 613/614

Session: Interface, Properties, and Modification
Presenter: M. Zolk, University of Heidelberg, Germany
Authors: M. Zolk, University of Heidelberg, Germany
F. Eisert, University of Heidelberg, Germany
M. Buck, University of Heidelberg, Germany
M. Grunze, University of Heidelberg, Germany
Correspondent: Click to Email
In the vast majority of cases the analysis of the structure and properties of self-assembled monolayers (SAM) takes place ex situ, e.g. in air or even under vacuum. In contrast, the application in fields relevant to biotechnology requires a detailed understanding of their properties in contact with liquids, in particular aqueous solutions. This raises the question of the relevance of ex situ investigations to conditions relevant for applications. We report experiments on SAMs of thiols on gold and metal substrates. The behavior of pure and end group modified thiols in contact with different liquids is studied. In particular, we focus on SAMs which consist of a methoxy terminated ethylene glycol (EG) unit attached to an alkane thiol. Whereas SAMs of pure alkane thiols are affected relatively little by solvents, EG-SAMs which are important for the preparation of protein resistant surfaces, exhibit a drastic dependence on the environment. All solvents examined penetrate the layer and induce significant conformational changes. Compared to air the signal from the methoxy end group is strongly reduced in contact with solvents. This indicates a transition from a well-ordered to an amorphous state. Analogously, the methylene vibrations gain intensity and thus indicate an increase of gauche conformations. Depending on the solvent the degree of penetration is different. A polar solvent such as water mainly interacts with the EG part of the layer whereas non-polar solvents interact as well with the hydrocarbon chain and thus penetrate deeper into the SAM. The experiments clearly demonstrate the need to investigate SAMs under conditions relevant for applications.