Surface Stress Generation During Formation of Alkanethiol Self-assembled Monolayer (SAM)
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0951-E12-05
Surface Stress Generation During Formation of Alkanethiol Self-Assembled Monolayer (SAM) Kanaga Karuppiah K.S., Ruqin Zhang, Pranav Shrotriya, Abhijit Chandra, and Sriram Sundararajan Mechanical Engineering, Iowa State University, Ames, IA, 50011 ABSTRACT Micro-machined cantilevers coated with self-assembled monolayers (SAM) of alkanethiols are being utilized as sensing elements for new generation of high-sensitivity chemical and biological sensors. Presence of chemical species is detected by resolving the surface stress change associated with absorption/adsorption of analyte molecules on the sensitized cantilever. Challenges to widespread use of micromechanical cantilever sensors are: susceptibility to vibrations, integration in a single device and understanding the mechanism governing surface stress generation. In the current work, surface stress development associated with formation of self-assembled-monolayers of alkanethiols was characterized using curvature interferometry. In order to understand the molecular mechanism underlying the surface stress generation, a multiscale model is developed to predict the surface stress generated during absorption of the alkanethiols on a gold film. INTRODUCTION Cantilever deflection based chemical sensors are on the rise ever since Thundat et.al. [1] reported the deflection of atomic force microscope cantilevers due to changes in relative humidity and thus opened a myriad of possibilities for the use of atomic force microscope cantilever deflection technique for chemical and biological sensing. They predicted possibilities of adsorbate detection of the order of picograms and immediately followed up with another study in which they detected mercury adsorption on cantilever from mercury vapor in air with picogram resolution [2]. Berger et.al. [3] reported the generation of compressive stresses on an AFM cantilever during the formation of alkanethiol self-assembled monolayer on the cantilever’s surface. They showed a surface stress on the order of 0.1-0.5 N/m and also reported that the magnitude of surface stress increased linearly with the carbon chain backbone of the monolayer. Since the first report by Berger et.al. [3], SAMs have been used as test system for almost all cantilever based sensing techniques [4-7]. This is because they are relatively easy to prepare, form well-ordered close packed films and offers limitless possibilities of variations in chain length, end group and ligand attachments [8]. One of the commonly studied SAMs are alkanethiol SAMs (HS-(CH2)n-1CH3). Godin et.al.[4] have shown that the kinetics of formation of self-assembled monolayers on gold-coated cantilevers and the resulting structure are dependent on the structure of the gold grain itself and also the rate at which the SAM reaches the surface. They showed a surface stress value on the order of 0.5 to 15 N/m. The surface stress generated was also shown to be dependent for different surface density (coverage) of the monolayer on the substrate.
In this study, the surface stress genera
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