Microfabricated array of cantilevers coated with sensitive layers represent ultrasensitive devices for the detection of chemical and biochemical reactions in both gas-phase and liquid environment. In liquid cantilevers allow the detection of unlabeled biomolecules.
In order to address a fundamental problem of the transduction of a physical or chemical process on the sensitive layer of the cantilever into its mechanical bending one can use a model system sensitive to the properties of the solvent. Self-assembled monolayers (SAM) on gold offer a convenient model of the sensitive layer. The parameters of these monolayers such as the length of the molecules forming the monolayer and the nature of the tail-groups can be systematically varied.
Understanding of the sensor function of the SAM functionalised stress sensors requires the understanding of the main in- and out-of-plane forces acting in the monolayers. We address the influence of the solvent properties, such as pH and salt composition, on the nature of the interactions in CH3- and COOH- terminated SAM on gold (111) surface. In order to model the stress response in cantilevers, functionalised with these monolayers, we have considered two contributions: the elastic and the electrostatic. On the Debye-Huckel theory, we have found that the electrostatic contribution dominates the stress response on pH changes for SAMs with long chains. While for SAMs with short molecules the elastic contribution becomes more important.
A detailed comparison of theoretical and experimental results on the stress response in SAM covered cantilever sensors as a function of the chain-length of the molecules and the nature of salts in solution will be presented. |