We report the preparation and characterization of low-density self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) on gold and silver. We describe the interactions of these low-density monolayers with lipids and hydrophobic small molecules, as measured by resulting changes in monolayer impedance and FTIR spectra. Novel patterned surfaces with defined low- and high-density monolayer regions are prepared and characterized by imaging ellipsometry. We also demonstrate the influence of substrate crystal lattice structure on low-density monolayer structure. These studies demonstrate the potential usefulness of low-density SAMs in sensor device applications.
The SAMs possess two unique properties which enhance their attractiveness for our target applications. The first is their low surface density, which we achieve by attaching a space-filling chlorotrityl protective group to MHA or MUA prior to monolayer assembly, and by removal of the protective group following monolayer assembly. This increases the free surface area available for accepting intercalation of compatible molecules (e.g., lipids) from the surrounding solution.
The second unique property of low-density SAMs discussed in this paper is the ability of its constituent molecules to undergo concerted conformational transitions when a positive electrical potential is applied to the gold surface, which attracts the negatively charged terminal carboxyl groups. The molecules can thus be controllably and reversibly switched from a “straight” conformation to a “bent” conformation by application or removal of the electrical stimulus.
These properties may be exploited for molecular sensing applications. We describe the influence of applied potential on the ability of the monolayers to accept intercalation of molecules from the surrounding medium. We also examine the influence of actively switching molecular conformation in order to expel intercalated molecules from the surface. These properties may ultimately result in rechargeable sensor surfaces, forming the foundation for a new class of biomedical diagnostic devices.