Aromatic Hydrocarbon Detection Using Self-Assembled Monolayer Coated Cantilevers
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Aromatic Hydrocarbon Detection Using Self-Assembled Monolayer Coated Cantilevers Andrew E Riley, Alan M Schilowitz, Dalia G Yablon, and Mark M Disko ExxonMobil, 1545 Route 22 East, Annandale, NJ, 08801 Abstract: The presence of toluene and xylene is sensed via surface stress induced deflection of microcantilevers functionalized with self-assembled monolayers (SAMs). Monolayers are formed on gold coated cantilevers using alkanethiols, mercaptanols, and aromatic thiols. These coatings create a variety of chemical functionalities at the cantilever surface, which impact the interactions between target molecules and the cantilever. The differential responses of the cantilevers are investigated as a means to selectively detect aromatic vapors at parts per thousands (ppth) levels. I. Introduction Micro-cantilever based sensors couple the mechanical sensitivity of a nanomechanical device with the molecular recognition of a selective adsorber. [1,2,3] These devices have the capacity to show fast responses to trace amounts of target analytes. The analyte adsorbs to the surface of the cantilever, usually through a surface coating that is designed to specifically adsorb a certain chemical target. Adsorption changes the surface stress of the cantilever, which leads to a measurable deflection of the cantilever. [4,5] Various surface coatings can be used to sense for analytes from bacteria to explosives and beyond. Self-assembled monolayers based on thiol adsorption to gold are an attractive route to functionalizing the surface of a micro-cantilever, generating cantilevers with a wide range of chemical properties. [6,7] Gold coated cantilevers, when exposed to organic thiols, form an organic surface coating that can be modified with organic functionalities. The properties of the surface then determine its interaction with analyte molecules. [8] Benzene, toluene, and xylene are three analytes of industrial interest due to their importance in commercial processes and toxicological properties. Their chemically inert nature and small difference in size makes these aromatics a challenge to detect at trace levels on fast time scales. At toxicologically relevant concentrations, many detection methods require the use of a pre-concentrator, which slows their response and renders them insensitive to rapid fluctuations in concentration. In this work, the use of self-assembled monolayer coated cantilevers is described for the detection of toluene and xylene. Micro-cantilevers are functionalized with one of three monolayer functionalities (aromatic, aliphatic, or alcoholic) to probe the response of these surfaces to small aromatic molecules. These cantilevers deflect with fast response times in the presence of saturated vapors. Humidity is also used to modulate cantilever response.
II. Experimental Contact mode micro-cantilevers (Si, 0.2 N/m nominal force constant, Budget Sensors) and silicon wafers (111 polished, Aldrich) were used as received. Approximately 25 nm of gold was sputtered directly onto one side of the silicon (waf
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