Hydrophobic interaction microscopy: mapping the solid/ liquid interface using amphiphilic probe molecules.

The adsorption, interfacial mobility, and desorption of amphiphilic molecules are extremely sensitive to the chemical properties of the interface at which they adsorb; we demonstrate here that this sensitivity can be used to map subtle spatial variations of surface hydrophobicity. We have used total internal reflection fluorescence microscopy to observe the dynamic behavior of individual fluorescently labeled fatty acid molecules at the interface between water and a hydrophobically modified fused silica surface. Patterned surfaces were prepared by photodegradation of trimethylsilane-modified surfaces using a contact photomask; the degree of hydrophobic contrast was varied by controlling the dose of ultraviolet radiation. Cumulative images of single-molecule fluorescence, integrated over various exposure times, exhibited structural features consistent with the photopattern, and the fluorescence contrast was systematically related to the hydrophobic contrast. Lateral force microscopy was also used to characterize the patterned surfaces and provided qualitative images when the hydrophobic contrast was relatively high. However, the fluorescent probes provided more sensitive, reproducible, and reliable images of the lateral hydrophobic variations.