Cantilever microcantilever

Mass-produced cantilever sensors, however, have the potential to satisfy the conditions of selectivity, sensitivity, miniature size, low power consumption, and real-time operation [5, 6], Microcantilevers are micromachined from silicon or other materials and can easily be fabricated in multiple-element arrays. They resemble miniature diving boards measuring 100 to 200 pm long by about 20 to 40 pm wide by 0.3 to 1 pm thick and having a mass of a few nanograms. Their primary advantage originates from their sensitivity, which is based on the ability to detect their motion with subnanometer precision. 

Microcantilever sensors can be operated in modes in which receptor-based coatings are not needed for example, deflagration of adsorbed energetic molecules can induce a measurable response [10]. Since cantilevers can be made extremely sensitive to temperature using bimaterial effect, calorimetric methods can be carried out on cantilevers with adsorbed molecules [11, 12], Exposing the temperature-sensitive cantilevers with adsorbed species to different infrared (IR) wavelengths in a sequential fashion creates mechanical signatures that mimic the IR absorption spectra of the analyte [13]. 

Microcantilever deflection changes as a function of adsorbate coverage when adsorption is confined to a single side of a cantilever (or when there is differential adsorption on opposite sides of the cantilever). Since we do not know the absolute value of the initial surface stress, we can only measure its variation. A relation can be derived between cantilever bending and changes in surface stress from Stoney s formula and equations that describe cantilever bending [15]. Specifically, a relation can be derived between the radius of curvature of the cantilever beam and the differential surface stress ... 

Fig. 15.3. Microcantilever for atomic-force microscopy, (a) A glass substrate with four cantilevers, (b) One of the cantilevers, (c) Close-up view of the tip. (After Albrecht et al. 1990.)...

Microcantilevers can be coated with DNA or antibodies to respond to biological molecules or even a single virus.1-2-3 Bound material can be detected by the change in resonant frequency, as above, or by measuring nanometer-scale static bending, shown at the left, caused by stress on the surface of the cantilever when molecules bind. 

M. Sepaniak, P. Datskos, N. Lavrik, and C. Tipple, Microcantilever Transducers A New Approach in Sensor Technology, Anal. Chem. 2002, 74, 568A C. Ziegler, Cantilever-Based Biosensors. Anal. Bioanal. Chem. 2004, 379, 946. 

Silicon microcantilever sensors that can be mass-produced using currently available microfabrication techniques, however, have the potential to satisfy the conditions of sensitivity, miniature size, low power consumption, and real-time operation [2], Microcantilevers are generally micromachined from silicon wafers using conventional techniques. Typical dimensions of a micromachined cantilever are 100 p,m in length, 40 p,m in width, and 1 xm in thickness. The primary advantage of a cantilever beam originates from its ability to sensitively measure displacements with sub-nanometer precision. Sensitive detection of displacement leads to sensitive detection of forces and stresses. 

Microcantilever deflection changes as a function of adsorbate coverage when adsorption is confined to a single side of a cantilever (or when there is differential adsorption... 

In Fig. 8 are shown the chemical signature patterns for five known analytes and one unknown analyte. The patterns were extracted from the response of an array of four coated microcantilevers to exposure to each analyte vapor. The four cantilever coatings... 

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Microcantilevers are associated with AFM, which basically are gold-coated surfaces based on a silicon core, which are associated with nanomechanics for biomolecular recognition (1,2,20,25). In AFM technology, a cantilever is in direct contact with the sample surface then the bending of the cantilever is determined by optical detection of the position of a laser beam (33). In fact, this method is a versatile tool for surface characterization and provides information concerning topological variations at the molecular level. 

The working principle of the family of biosensors based on nanomechanical transducers, and specifically on microcantilevers, involves the translation of biochemical reactions into a mechanical movement in the nanometer range. In microcantilever sensors, the biochemical receptor layer is directly in contact with one of the cantilever surface. The biomolecular recognition process between the receptor layer and its corresponding analyte induce... 

The cantilevers can be fabricated of any shape and from substantially any material utilized in microelectronics industry, i.e. crystalline or poly-silicon, silicon nitride, silicon oxide, polymer materials (see Note 2). The rectangular shape beams are the most frequently used in biological research. In biological sensors based on the bending method, it is important to have the cantilevers flat and in plane with the base surface. Initial offset or curvature of the beams complicates adjustment of the experimental setup, especially, if working with arrays of cantilever. For this reason, the most common material for cantilevers fabrication nowadays is single crystalline silicon. A large variety of biomolecular interactions have been detected with silicon microcantilevers. 

The cantilever deflection, Az, generated by a surface stress change, Act, (due to a biorecognition process, for example) depends on the microcantilevers dimensions (length L and thickness h) and its material s properties (Young s modulus E and Poisson coefficient v) following the Stoney s equation ... 

The structural materials that conform the cantilevers can have intrinsic stresses and stress gradients because of the fabrication processes. These can change the mechanical performance of the microcantilevers. If stresses and stress gradients are high, they must be previously measured by using specific test structures and their values should be included in the simulations. As monocrystalline silicon will be used in the fabrication of our devices, these stresses can be neglected. 

Fig. 3. (A) Static method for determining the cantilever spring constant. Force-distance curves obtained over a hard surface and over a fabricated microcantilever. The inset shows the method configuration scheme. (B) A profile of silicon cantilevers fabricated by the technology described above. The cantilever is flat, but the stresses at the anchoring area produce initial cantilever displacement. The profile has been measured by an optical confocal profilometer.

With this method, a spring constant value of0.0067N/mwas obtained for microcantilevers of 200-p.m length, 40-p.m width, and 0.334-pm thick. The difference respect to the expected value (0.0079 N/m) is attributed to the compliance of the chip region where the cantilever is clamped (due to a thinning of this region, produced during the fabrication process) (2). 

Further examples of acoustic sensors modified with zeolites include a QCM sensor with silver-exchanged ZSM-5 that responds selectively to acetone (in diabetic s breath) in the ppm-range,ll 16] principal component analysis of multiple QCM-sensor responses (with LTA, MFI, SOD) for the detection of NO/SO2 mixtures,[117] MFI-zeolite-coated microcantilevers with ppm-sensitivity for Freon detection [118,119] and other zeolite-coated cantilevers for humidity sensing.[120]... 

Recently Micro Electro-Mechanical Systems (MEMS) have been emerging as sensor platform for the development of sensors with extreme high sensitivity [8-14]. Micromachined silicon cantilevers are the simplest MEMS sensors that can be micromachined and mass-produced. Microcantilever sensor technology is an upcoming sensing technique with broad applications in chemical, physical, and biological detection. With their compactness and potential low cost, detection techniques based on silicon-based cantilevers provide a path for the development of miniaturized sensors. 

The typical dimensions of these microcantilevers are 50-200 pm long, 10-40 pm wide and 0.3-2 pm thick. The mass of the microcantilever is a few nanograms. The primary advantage of the microcantilever method originates from its sensitivity that is based on the ability to detect cantilever motion with sub-nanometer precision as well as the ease with which it may be fabricated into a multi-element sensor array. 

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