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Braun Group People

Carla Heitzman and Huilin Tu

Former Graduate Students in Materials Science and Engineering

Traditionally, the applications associated with Self-Assembled Monolayers (SAMS) are related to protecting a surface or altering the surface chemistry of a substrate. In accordance with these goals, the self-assembling process and molecules used to make the layers have been optimized with regard to high surface coverage and regular, ordered, often crystallized layers. For this purpose, relatively short, mono-dispersed, linear alkane-thiol molecules are most suitable and most studied.
By altering the structure of the self-assembling molecule, we hope to create chemically attached amorphous layers with sufficient free volume and mobility such that (unattached) small molecules and ions can be dissolved in the layer and laterally diffuse. By altering the “pore size” in these layers, as well as the chemistry (e.g. polarity) of the molecules composing the layers, we hope to choose the molecule(s) and/or ion(s) that are to be transported. Eventually, by creating pseudo-one-dimensional pathways, we would like to enable transport of the selected species to a specific destination.
There are two main components to our research. The “chemistry” component involves synthesizing various appropriate molecules and characterizing these molecules and the layers into which they assemble. Huilin Tu works mainly on these aspects of the project. The second component is concerned with measuring diffusion through these layers. Carla Heitzman works mainly on this aspect.

Creating SAMs for molecular transport
Various SAM-forming molecules were chosen with chemical and physical properties that enhance the probability that selected probe molecules or ions can diffuse through the monolayers. For example, thiol-terminated oligomers containing ethylene oxide repeat units were synthesized for the transport of polar species; molecules containing dimethylsiloxane repeat units were selected for the transport of nonpolar species. After monolayers are self-assembled on gold, salts or fluorescent molecules are deposited onto them, to form samples for ionic and molecular transport studies. Techniques including ellipsometry, contact-angle analysis, and atomic force microscopy were used to characterize the properties of the monolayers.

Detecting Diffusion
The number of molecules diffusing through a monolayer, that one must track in order to verify and quantify this diffusion, is exceedingly small; it is on the order of less than a single monolayer of diffusing species if the molecules are, in fact, diffusing laterally through the chemically attached layer. We are hoping to use fluorescence, in combination with Beckman’s Leica confocal microscope, to image this diffusion. To date, we have looked mainly at either diffusing molecules that fluoresce themselves or diffusing ions that increase the fluorescence of an ion-sensitive dye when they come in contact with it. Another option is the use of a layer which itself fluoresces to image the diffusion of a quenching species. Initially, consecutive pictures of an area will be used to directly track diffusion. Methods such as fluorescence recovery after photo-bleaching (FRAP) and fluorescence correlation spectroscopy (FCS) will subsequently be used for more quantitative analysis.

 

 

 

 

 

 

 

 

 

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