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Surface Science
Microcontact Printing of Self-Assembled Monolayers
We are interested in organic surface science and its applications across science and technology. We have studied the conversion of alkanethiols into self-assembled monolayers (SAMs) on surfaces (Figure 1) and patterned SAMS for microcontact printing. In this technique, a PDMS stamp is constructed using soft-lithography. The stamp is then wetted with an alkanethiol and placed in contact with a gold (or other noble metal) for several seconds. SAMs form on the surface only in the areas that had been in contact with the stamp (Figure 2).
Microcontact printing of SAMs has a number of applications. By patterning one SAM with a hydrophobic terminus and then filling in the rest of the area with a SAM with a hydrophilic terminus, it is possible to create hydrophobic (or hydrophilic) patterns on surfaces with micron dimensions (Figures 3,4). Patterning specific areas with cell-friendly (protein terminated) and cell-unfriendly (polyethylene-glycol terminated) SAMs can be used to pattern endothelial cells on surfaces and to even force these cells to take on specific shapes (Figure 5).
Electrochemical Desorption of Self-Assembled Monolayers
We have also shown that alkanethiol SAMs can be released from surfaces when a small (less than 1 V) potential is applied across the surface; this process is called "electrochemical desorption" (Figure 6). In one application of electrochemical desorption, polyethylene-glycol terminated SAMs are pattered around islands protein-terminated SAMs. Applying the potential releases the polyethylene-glycol SAMs from the surface and allows the cells to spread out from confinement. This technique has allowed us to tune the inertness of surfaces in real time and to design cell motility assays (Movie 7).
Surface Analytical Techniques
We also use a number of surface analytical techniques to characterize the surface coatings of PDMS and glass; such techniques include x-ray photoelectron spectroscopy (XPS), reflectance IR and ellipsometry. Controlling surface properties of these materials is important for biological applications. These properties are particularly important in the field of electrokinetic injections and separation, because they affect adsorption of proteins as well as surface charge, which determines the magnitude of electroosmotic flow (EOF). In turn, measurements of EOF allow us to infer the density of surface charge and its surface uniformity. Figure 8 shows an XPS signal describing the presence of nitrogen from polyacrylamide photopolymerized inside a sealed PDMS channel.
Select Publications
1. Laibinis, P. E. et al. "Orthogonal self-assembled monolayers: alkanethiols on gold and alkane carboxylic acids on alumina." Science (1989), 245(4920), 845-7.
2. Abbott, N. L., Folkers, J. P. and Whitesides, G. M. "Manipulation of the wettability of surfaces on the 0.1- to 1-micrometer scale through micromachining and molecular self-assembly." Science (1992), 257(5075), 1380-2.
3. Kumar, A., Biebuyck, H. A. and Whitesides, G. M. "Patterning Self-Assembled Monolayers: Applications in Materials Science." Langmuir (1994), 10(5), 1498-511.
4. Wilbur, J. L. et al. "Microcontact printing of self-assembled monolayers: applications in microfabrication." Nanotechnology (1996), 7(4), 452-457.
5. Kane, R. S. et al. "Patterning proteins and cells using soft lithography." Biomaterials (1999), 20(23/24), 2363-2376.
6. Jiang, X. et al. "Electrochemical desorption of self-assembled monolayers noninvasively releases patterned cells from geometrical confinements." JACS (2003), 125, 2366-2367.
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