Cellular membranes define the boundries between various cellular compartments (such as the nucleus, mitochondria, endoplasmic reticulum, etc.) as well as enclosing the cell from the external environment. Similar to soap bubbles, cellular membranes are made of specific types of surfactants that tend to form bilayers. The primary surfactants are phospholipids, steroids (primarily cholesterol), and glycolipids. Two nanoscale bodies of water can be effectively separated from one by the formation of a bilayer of these molecules. The main barrier is the hydrophobic region in the center of a lipid bilayer membrane. Water and polar molecules dissolved in water can not pass through such a nonpolar barrier without the assistance of ion channels or molecular pumps.

Langmuir-Blodgett (LB) films are formed when synthetic or naturally occurring surfactants are gently placed (gently to avoid the formation of micelles) on the surface of water such that they form a layer that is only one molecule thick. The polar part of the surfactant molecule tends to be attached to the surface of the water while the nonpolar hydrophobic regions of neighboring molecules will stick together and orient themselves away from the surface. As the lateral pressure (2 dimensional pressure created by reducing the surface area of the water) is increased, various levels of self-assembly occur within the film.

Thiol self-assembly can be achieved by reacting thiol containing compounds with clean gold surfaces. Sulfhydryl groups on molecules will covalently bind to gold, thus allowing molecules to be arranged two dimensionally over a gold surface. This is extra useful because gold conducts electricity and makes for excellent electrical contacts, thus Scanning Tunneling Microscopy (STM) measurements can be made on such samples. For example, redox proteins can be mutagenically modified with cysteine residues (an amino acid that contains a thiol group). Exposing a gold surface to such engineered molecules results in self-assembled monolayers of protein.

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Keywords: bilayer lipid membranes thiol self-assembly monolayer SAM surfactants sulfhydryl langmuir blodgett thin organic films cysteine mutagenesis stm cellular membranes hydrophobic barrier scanning tunneling microscopy electrical contacts nanoelectronics nanotechnology