4. Microcantilever Sensors: Electrochemical Aspects and Biomedical Applications

Microfabricated cantilevers, similar to those used in Atomic Force Microscopy (AFM), are generating growing interest as a sensor platform for label-free detection of chemical and biological molecules [1–17]. Using recent advances in surface microfabrication, it is possible to design and fabricate cantilevers and cantilever arrays with extremely high sensitivity for mass or surface stress. These cantilevers are generally fabricated from silicon or silicon nitride by top-down micromachining methods and can be produced efficiently and affordably. Although the cantilevers have micrometer dimensions, their response are in nanometer-scale, which lends itself to their reference as nanomechanical transducers. The cantilever can be made in different shapes and sizes allowing for flexibility in the design, which renders the resulting cantilevers ideal candidates for the possible incorporation in microfluidic and miniaturized lab-on-a-chip devices. Generally, these cantilevers are operated in either the static deflection mode or the dynamic resonant mode. The basic principle for the static mode is that a chemical or physical event occurring at the functionalized surface of one side of the cantilever generates a surface stress difference (between the active functionalized and passive non-functionalized sides) that causes the cantilever to bend away from its resting position. Whereas in the resonant mode, a binding event occurring on the cantilever increases the overall mass thus decreasing the resonant frequency, which is similar to quartz crystal microbalances. In general, when a force is applied to the end of a free standing cantilever a vertical bending will result. As described by Hooke’s Law (\( (F=-k_{spring}\Delta z),\)), the bending or deflection (Δz) of the cantilever is directly proportional to the applied force F, and the cantilever spring constant k _spring is the proportionality factor. The cantilever spring constant dictates the flexibility and sensitivity of the cantilever and is defined by its dimensions and material constants. For a rectangular-shaped cantilever, k _spring, is given by [18]