Microcantilevers

Figure 5.18 Scanning electron microscopy image of a microcantilever, electromachined into a stainless steel sheet by ultrashort voltage pulses (100 ns, 2 V, 1 MHz repetition rate) in 3 M HCI + 6 M HF. The tool electrode was a tiny loop of a 10 pm thick Pt wire. (Reproduced with permission from Ref. [80].)...

J.H. Lee, K.S. Hwang, J. Park, K.H. Yoon, D.S. Yoon, and T.S. Kim, Immunoassay of prostate-specific antigen (PSA) using resonant frequency shift of piezoelectric nanomechanical microcantilever. Biosens. Bioelectron. 20, 2157-2162 (2005). 

EXPLOSIVE VAPOR DETECTION USING MICROCANTILEVER SENSORS... 

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 ... 

The key to achieving chemical selectivity using microcantilevers is the ability to functionalize one surface of the microcantilever with receptive molecules so that explosive molecules will preferentially bind to the treated surface. Choosing receptor molecules that can provide highest affinity, therefore, can control the selectivity of detection. Another important requirement for a sensor system is fast regeneration (recovery), so that the sensor can be used repetitively. 

Deflagration experiments were conducted only for TNT vapors. For deflagration experiments we have used a piezoresistive microcantilever the piezoresistive... 

Chapter 12, Explosive Vapor Detection Using Microcantilever Sensors, is representative of a general class of sensors Surface Effect Microsensors that use the change in some mechanical or electrical property of an ultra-small structure to sense and identify a wide variety of molecules. 

Fig. 15.2. Fabrication of silicon nitride microcantilevers with integrated tips, (a) A...

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.)...

Akamine, S., Barrett, R. C., and Quate, C. F. (1990). Improved atomic force microscope images using microcantilevers with sharp tips. Appl. Phys. Lett. 57, 316-318. 

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. 

New boxed applications include an arsenic biosensor (Chapter 0). microcantilevers to measure attograms of mass (Chapter 2), molecular wire (Chapter 14), a fluorescence resonance energy transfer biosensor (Chapter 19), cavity ring-down spectroscopy for ulcer diagnosis (Chapter 20), and environmental mercury analysis by atomic fluorescence (Chapter 21). 

Mercury binding leads to an increase of mass of the gold layer which can be detected by electro-acoustic transducers based on quartz microbalance (QMB the abbreviation QCM = quartz crystal microbalance is also widely used), surface acoustic waves (SAW)—devices [20] or microcantilevers [21,22], Adsorption of mercury vapour increases resonance frequency of shear vibrations of piezoelectric quartz crystals (Fig. 12.2). This process can be described by Sauerbrey equation [23]. For typical AT-cut quartz, this equation is... 

Chemical sensors for mercury vapour, based on QMB, provide sensitivity of 0.7-5 ng/1 [24]. Reversibility is reached by heating the sensor a microheater can be integrated on the sensor surface. This detection principle was also used by several other groups, for example [7,9,25]. Another realization of this transducing principle is based on exploitation of gold-coated microcantilevers for mercury detection [21,22]. 

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