Nanomechanical sensors

The miniaturization of sensors is permitting the creation of very small nanomechanical sensors at IBM that could create very high sensitivities, e.g. the detection of a few molecules on the surface. Also, at the moment the use of an optical transducer looks promising. Optical sensors based on the physical vibration bands of molecules (e.g. infra-red spectrometers) are not particularly sensitive but combined with new materials the sensitivity can be impressive. For example, a recent report on colour-sensitive dyes based on Lewis acid ase reactions offers considerable promise [8]. Figure 8 shows an electronic nose based on a set of dyes that changes colour when the target analyte is introduced and this is detected using a CCD array chip. The unit has been shown to have ppb sensitivities to certain compounds. 

The use of polymer-coated cantilevers such as microfabricated beams of silicon is becoming more popular as the basis of nanomechanical sensors [11]. These devices detect physical and chemical interactions between the reactive layer on the surface and the environment [8]. When the polymer interacts with a gaseous species, it swells and causes the cantilever to bend as a result of surface stresses when used in the static mode. In the dynamic mode, the cantilever acts as a microbalance, which responds to changes in resonance frequency. Savran s group at Purdue University has been researching the micromechanical detection of proteins by use of aptamer-based receptor molecules [12]. 

A nanomechanical sensor is a mechanical structure that transduces analyte-induced stimuli into a signal via its structural change with nanometer precision. The definition of a nanomechanical sensor can also cover a mechanical transducer with nanometer scale. In either sense, a cantilever sensor is a representative example among various geometries. 

As demonstrated by various groups, nanomechanical sensors are applicable to a wide variety of targets. To take advantage of their attractive feature, it is important to understand the basics of nanomechanical sensors. In the following sections, we will briefly review working principles and readout methods of cantilever sensors [10]. Then, recent developments in the field of nanomechanical sensing will be highlighted. 

For solid coating layers, a simple analytical model is proposed. It provides general reference values in terms of the strain induced in the coating layer [11,12]. It will help toward analyzing the static behavior of cantilever sensors and various nanomechanical sensors in conjunction with physical properties of coating films as well as optimizing the films for higher sensitivity. The details of the analytical model will be discussed later. 

B.2.4 Structural Optimization of Nanomechanical Sensors for Improved Sensitivity... 

According to a recent study [11], deflection (signal intensity) of a nanomechanical sensor strongly depends on the thickness of a receptor layer. In this section, we focus on the analytical model that describes the relationship between deflection of a cantilever and various physical parameters of a cantilever itself and receptor layer on it. This analysis provides a practical guideline to optimize the thickness of a receptor layer. 

FIGURE 4.3.11 (a) Visualization of the two systems of two-dimensional stress induced on a cantilever-type nanomechanical sensor, (a) Type A system in which the stress is induced on the top surface of the coating layer, (b) Type B system in which the stress is induced at the interface between the silicon cantilever and the coating layer. Note that the dimensions and deflections of the cantilever and the coating layer are exaggerated. Thickness dependence on the deflection of the cantilever in the case of (c) Type A and (d) Type B. Poisson s ratio of coating layers is 0.30 for all plots. Reprinted from Ref [12], with permission from the American Scientific Publishers. 

Table 4.3.1 Reported Studies of the Many Biomolecules and Microorganisms that have been Detected by Means of Nanomechanical Sensors [40]...

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