Tuning of Solid Surface Properties by Self-Assembled Monolayers

The zeta potential measurement assists in tailoring the properties of technical and biomedical surfaces using self-assembled monolayers.

Nanoscale coatings of organic silanes or thiols with functional end-groups are a smart procedure to tune material surface properties without changing the material thickness. As an example, alkane thiol anchored self-assembled monolayers (SAMs) on gold are widely used to immobilize and detect molecules including DNA and proteins. Such molecules get covalently bonded to the SAM on the gold surface and cannot be easily released.

To improve the selectivity of the SAM coating, the surface charge can be tailored by mixing alkane thiols with different functional end groups.
Glass slides of 20 mm x 10 mm with a gold layer were coated with different ratios of alkane thiols with acid-terminated C 6 and amine-terminated C 8 entities (see figure below). Acid dissociation (resulting in negative charges) and amine protonation (formation of positive charges) determine the pH dependence of the surface charge and thus the isoelectric point (IEP) of the SAM-coated gold surface.

The pH dependence of zeta potential of gold surfaces coated with self-assembled monolayers com-posed of different ratios of 16-mercapto hexadecanoic acid and 8-amino-1-octane thiol is shown below.

By tuning the net surface charge the affinity of the biosensor surface to specific proteins or other biomolecules can be enhanced.

W.-C. Lin, S.-H. Lee, M. Karakachian et al, Phys Chem Chem Phys 11 (2009) 6199-6204

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