Cantilever-based sensors

Ziegler, C. (2004). Cantilever-based sensors. Anal. Bioanal. Chem. 379,946-959. 

There are several techniques suitable for the cantilever response read-out as the optical beam deflection, piezoresistivity, piezoelectricity, interferometry and capacitance, among the most important. However, the majority of the biochemical applications carried out with the cantilever-based sensor are based in the optical beam deflection method, because of its high sensitivity and its simplicity. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Cesium ions have been shown to have a high affinity for derivatives of l,3-altemate-calix[4]arene-benzocrown-6. This chemistry was utilized in designing a microcantilever sensor to detect Cs ions in a flowing solution (15). The high affinity of this ion-selective SAM-coated microcantilever for Cs allows it to be capable of detecting cesium ions in the presence of high concentrations of potassium or sodium ions. Our data show that the sensitivity of this cantilever-based sensor for in situ measurements is several orders of magnitude better than the sensitivities of currendy available ISE. 

Temporal response of the cantilever-based sensor to water vapor diluted in is shown in Fig. 11.6. 

Low-temperature sensors surface plasmon resonance sensors pellistors biosensors High-temperature electrochemical gas sensors sensors for tough conditions electronic nose RT electrochemical sensors Membranes filters for aU types of sensors SAW sensors cantilever-based sensors... 

Low-temperature sensors SAW sensors biosensors humidity sensors conductometric gas sensors membranes electrochemical sensors cantilever-based sensors optical and fiber-optic sensors electronic noise... 

Other transduction methods include mass-sensitive methods like the cantilever-based sensors [30]. Cantilevers are usually made of small silicon plate (length 100 pm) with a low force constant. They are fixed at one end and can vibrate at the other end. Silicon can easily be functionalized with bioelements such as antibodies or aptamers. Upon binding of the analyte to the bioelement at the surface of the cantilever, the resonance frequency of the cantilever is changing, which can be detected with help of a laser beam shining the cantilever surface [31]. 

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. 

This system can be easily scaled up allowing the detection of different concentrations and/or different analytes simultaneously. For biological experiments, arrays of cantilevers are normally employed (5) as one cantilever can be used as reference. Then the expected biointeraction can be diferenciated from any change of the experimental conditions. Actually, there are several commercial platforms available, based on cantilever-array sensors. 

The sensor is illustrated schematically in Fig. 1. The piezoelectric-excited millimeter-sized cantilever (PEMC) sensor is a macro-cantilever that comprises piezoelectric layer (lead zirconate titanate PZT) layer bonded to a nonpiezoelectric layer of a few millimeters in length and 1 mm in width (9,21). We use the direct piezoelectric effect to excite the cantilever, and the same PZT film senses the resulting response. PZT film is bonded to a base glass... 

In the best-case situation, both types of microcantilever sensors would be grouped in an array to provide a cross platform of sensitive and selective explosive detection system. Additional co-funded (TSA/ATF) work is eurrently on going in materials development for novel microeantilevers. This involves R D of silicon carbide (SiC) based cantilevers, for improvements in material properties (e.g., less fragile eompared to silicon) and to provide a platform for wide band gap type materials, like aluminum nitride (AIN). With an AlN/SiC based cantilever, the sensor can now work in the piezoeleetric resonator mode, providing enhance response and henee sensitivity to the analyte, along with a direct measurement by frequeney/resistance response, versus the more complex optieal deteetion... 

Capacitive membranes sensor typically is based upon the deformation of the membrane due to the interaction between bio-probe molecules at the functionalized surface and the corresponding bio-targets. The working mechanism is similar as how cantilever surface stress sensors operate. Surface stress-based sensor is comprised of stretchable membrane over which surfaces is functionalized by bio-probes. The interaction with the corresponding bio-targets results in surface stress and finally makes the membrane deform. Figure 3 shows the schematic structure... 

Integrated Optical Fiber Cantilever-Based Flow Sensors... 

QCM-, SAW-, and cantilever-based gas sensors are adsorption-type sensors in which the change in weight of the sensing element is the determining factor (Houser et al. 2001). Therefore, polymers applied in such devices should have the ability to sorb selectively and reversibly an analyte of interest from sampled air and to concentrate it so that lower concentrations can be detected. How specific adsorption properties of polymers can be, one can estimate on the base of results presented in Table 3.9. We need to note that for other analytes, polymers optimal for application will be different. 

Monchev B, FUenko D, Nikolov N, Popov C, Ivanov T, Petkov P, Rangelow IW (2007) Investigation of the sorption properties of thin Ge-S-AgI films deposited on cantilever-based gas sensor. Appl Phys A 87 31-36 Monroy E, Omnes F, CaUe F (2003) Wide-bandgap semiconductor ultraviolet photodetectors. Semicond Sci Technol 18 R33-R51... 

Piezoelectric-based or acoustic wave (AW) sensors such as surface acoustic wave (SAW), quartz crystal microbalance (QCM) or bulk acoustic wave (BAW), and cantilever-based devices create a specific class of gas sensors widely used in various applications (Ippolito et al. 2009 Korotcenkov 2011) (see Fig. 13.1). Virtually all acoustic wave-based devices use a piezoelectric material to generate the acoustic wave which propagates along the surface in SAW devices or throughout the bulk of the structure in BAW devices. Piezoelectricity involves the ability of certain crystals to couple mechanical strain to electrical polarization and will only occur in crystals that lack a center of inversion symmetry (Ballantine et al. 1996). 

Experiments have shown that capacitance-based gas sensors can detect vapors and gases with concentration in the ppm range. For example, Kitsara et al. (2006) reported on detection limits for PDMS-covered cantilevers below 50 ppm for toluene and 10 ppm for octane. Rodriguez et al. (2004), using frequenqr multiplication and PDMS as the active layer, established that the sensors had sensitivity to concentrations of toluene as low as 25 ppm. For gases the sensitivity is lower. Sivaramakrishnan et al. (2008) found that capacitance-based sensors with membranes coated by a single-walled nanotube (SWNT) film can detect CO with concentrations from 3.2 to 10.4 %. 

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