Yaling Cheng

Imaging of micropatterned self-assembled monolayers with adsorbed liquid crystals

We report a novel method of imaging micropatterned self-assembled monolayers (SAMs) using adsorbed films of thermotropic (smectic or nematic) mesophase which can then be studied by optical microscopy. Three alkylthiols, functionalized with CH3, OH and COOH groups, were used in various combinations to form patterned SAMs. Two alkylcyanobiphenyls (7CB and 9CB) were used as the liquid crystal imaging reagents. The images are formed by the contrast generated by the different alignments of adsorbed smectic or nematic films induced by different regions of the pattern. The spatial resolution is at least to 4 μm.

Synthesis of novel biotin anchors

Abstract Details are reported of the synthesis of biotinylated thiol and cholesteryl derivatives. The biotin derivative has been used to form multilayer films on gold through interaction with streptavidin.

Lipid bilayers suspended on microfabricated supports

The plasma membrane, that exists as part of many animal and plant cells, is a regulator for the transport of ions and small molecules across cell boundaries. Two main components involved are the phospholipid bilayer and the transport proteins. This paper details the construction of a micromachined support for bilayers (MSB) as a first step towards the development of highly selective and highly sensitive ion-channel based biosensors. The device consists of a ~100 micrometer hole in a polymeric support above a cavity that can hold ~25 nL of electrolyte. Electrodes attached to the structure allow the resistance of the membranes to be measured using d.c. conductivity. The MSB is made in two halves, using SU8 ultra-thick resist, which are subsequently bonded together to make the final structure. A layer of gold, surrounding the aperture, enables self-assembled monolayers of alkanethiols to be used to make the polymeric structure biocompatible. Lipid membranes have been formed over these holes with resistances comparable with those of natural membranes >10 MOhmcm^2. The ion-channel gramicidin has successfully been incorporated into the bilayer and its activity monitored. It is proposed that this type of device could be used not only for studying membrane transport phenomena but also as part of an ion-channel based biosensor.

Wetting transitions of simple liquid films adsorbed on self-assembled monolayer substrates: an ellipsometric study

We report on an ellipsometric experimental study designed to explore the relevance of the wetting phase diagram predicted by liquid state physics of basic models, to the wide class of simple organic liquid films that adsorb from saturated vapour onto planar substrates at room temperature. The wetting properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of specially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, which in our case is continuous, and then study its properties in detail. The general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the substrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. This conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adsorption isotherms is mesoscopic, suggesting that equilibrium statistical mechanics of simple models of inhomogeneous fluids cannot explain the data.