Adhesion Between Surfaces Coated with Self-Assembled Monolayers: Effect of Humidity


Sungsoo Kim Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign

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Self-assembled monolayers (SAMs) are monomolecular layers that are spontaneously formed on immersing a solid substrate into a solution containing active surfactant molecules. The best studied examples are silanes, which are used to modify, for example, silica and mica surfaces, and alkylthiols, which have an affinity for coinage metals. Organic SAMs can be used to alter and control the chemical nature of surfaces. For instance, SAMs provide a robust base layer for immobilizing biological molecules in biosensors. The bonding of enzymes to alkanethiol SAMs on gold has received attention as a method of constructing enzyme electrodes. Lee et al. are developing a surface plasmon resonance-based monosaccharide sensor by immobilizing a small thiol-based receptor molecule on gold containing a boronic acid group that is known to covalently bind the 1,2 or 1,3-diol of sugars. The success of SAM-based sensor applications depends on the stability of the underlying SAM as a function of aging and on environmental conditions such as humidity and temperature.

Self-assembly is simple and widely applicable in areas such as coatings, lubrication, templating, optoelectronics, and microelectromechanical systems/nanoelectromechanical systems (MEMS/NEMS). MEMS offers great promise for system integration of sensors, actuators, and signal processing. The miniaturization and integration offered by MEMS devices is attractive in applications where smaller size and weight are desirable. However, as devices become smaller, the surface area-to-volume ratio becomes larger and surface forces become more important. Factors determining the performance, reliability, and durability of MEMS devices are not well understood. In many applications, it will be impractical or impossible to protect MEMS devices from the operating environment. For instance, robust MEMS actuators must operate in a variety of conditions, some of which may be extreme (e.g., high temperature, a wide range of climatic conditions, and high vacuum). In MEMS, nanoscale dimensions potentially separate device components where water can capillary condense, giving rise to stiction. Many applications for MEMS/NEMS are not really practical, as many studies have revealed the profound negative influence of stiction, friction, and wear on the efficiency, power output, and steady-state speed of microdynamic/nanodynamic devices. Komvopoulos and Yan have shown that liquid bridging at intermediate humidity is the dominating short-range attraction mechanism prevailing at MEMS interfaces. It is also known that head/disk stiction occurs at humidities greater than 80%. The ubiquitous presence of water vapor makes the study of the effect of relative humidity (RH) on the performance and reliability of MEMS devices crucial. Depositing a hydrophobic SAM on a hydrophilic surface can reduce surface wettability. Organosilane SAMs are being investigated as boundary lubricants and antiwetting coatings in MEMS devices to reduce the effect of humidity on friction and wear.

In order for SAMs to be useful as boundary lubricants, in biosensors, as MEMS coatings, and in a variety of other applications, it is important to understand the effect of environmental conditions such as humidity and temperature on SAM robustness and stability. In this chapter, we present current research on the effect of humidity on the adhesiveness of alkyl-based SAM coatings on gold, silica, and mica. The wide variety of SAMs designed to be immersed in water (for instance functionalized SAMs for sensor applications) is not specifically discussed here, but this information should be pertinent for those interested in the effect of water on underlying SAM stability and adhesion.