Paper BI-TuM6
Switching the Force between a Hydrophobic Probe and Self-Assembled Monolayers on Gold by Changing the Ionic Strength

Tuesday, October 16, 2007, 9:40 am, Room 609

Alkanethiol based self-assembled monolayers (SAMs) have seen an ever increasing interest since they were first introduced more than two decades ago. Part of their attraction is due to the ease of their preparation in combination with the great flexibility they offer to create concentrated planes of functionality by modification of the surfactant molecules. One prominent example are oligo(ethylene glycol) (OEG)-terminated alkanethiol SAMs. They have been shown to resist the non-specific adsorption of some proteins and hence are of significant interest in the life sciences and related areas. Inspired by their protein repelling properties several studies were carried out to determine the underlying mechanisms and the interactions involved. The forces measured with a hydrophobic probe (attraction or repulsion) on these films were found to correlate with the protein adsorption properties (adsorption or resistance to adsorption). The forces indicated the existence of an electrical double layer, suggesting that an electrostatic component is involved. This was confirmed by theoretical calculations. We report on surfactant films based on OEG modified alkanethiol SAMs that can switch between an attractive and a repulsive force with hydrophobic probes depending on the ionic conditions of the environment. The ionic strength is an external parameter that can be easily controlled. The films are interesting for the reversible immobilization of hydrophobic (nano)particles and in colloidal chemistry. Similar surfactants might become important in bio-related fields and in connection with biosensors since they have the potential to adsorb/immobilize proteins reversibly if the switch can be tailored to occur in an ionic range that is compatible with proteins.