Cantilever microbalance

In contrast to SPFS, SPR, and SPDS are tools that can study biomolecular interactions without external labels. They share the same category of label-free biosensors with the reflectometry interference spectroscopy (RIfS) [46], waveguide spectroscopy [47], quartz crystal microbalance (QCM) [48], micro-cantilever sensors [49], etc. Although the label-free sensors cannot compete with SPFS in terms of sensitivity [11], they are however advantageous in avoiding any additional cost/time in labeling the molecules. In particular, the label-free detection concept eliminates undue detrimental effects originating from the labels that may interfere with the fundamental interaction. In this sense, it is worthwhile to develop and improve such sensors as instruments complementary to those ultra-sensitive sensors that require labels. 

The three major new atomic-scale experimental methods developed in the last decade are the quartz crystal microbalance (QCM) [2 4], atomic and friction force microscopes (AFM/FFM) [5,6], and the surface force apparatus (SEA) [7,7a,8]. These new tools reveal complementary information about tribology at the nanometer scale. The QCM measures dissipation as an adsorbed him of submonolayer to several monolayer thickness slides over a substrate. AFM and FFM explore the interactions between a surface and a tip whose radius of curvature is 10 100 nm [9]. The number of atoms in the contact ranges from a few to a few thousand. Larger radii of curvature and contacts have been examined by gluing spheres to an AFM cantilever [10,11]. SEA experiments measure shear forces in even larger-diameter ( 10 pm) contacts, but with angstrom-scale control of the thickness of lubricating hlms. 

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. 

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]. 

Various designs of microbalance are commercially available including beam, spring, cantilever and torsion balances (Figure 4.3). The microbalance should accurately and reproducibly record the change in mass of the sample, under a range of atmospheric conditions and over a broad temperature range. 

Various designs of microbalance are commercially available, including beam, spring, cantilever and torsion balances, as shown schematically in Figure 2.4. 

Figure 2.4 Various types of microbalance (I) beam (II) cantilever (III) spring. (IV) torsion wire...

Mass sensitive Changes in the weight, amplitude, phase or frequency, size, shape, or position Quartz crystal microbalance Surface acoustic wave propagation Cantilever... 

Kg. 1.9 Schematic diagrams of mass-sensitive gas sensors (a, b) quartz crystal microbalance (QCM) device (c) surface acoustic wave (SAW) device (d, e) microcantilever - (d) dynamic mode absorption of analyte molecules in a sensor layer leads to shift in resonance frequency, and (e) static mode the cantilever bends owing to adsorption of analyte molecules and change of surface stress at the cantilever surface (Reprinted with permission from Battison et al. (2001). Copyright 2001 Elsevier)... 

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). 

The microbalance technique is shown in Fig. 2-8. This method is based on the use of quartz fibers, which can form a cantilever spring or torsion balance. Sensitivities to 0.1 jug are possible with an electromicrobalance. Several investigators have used this apparatus (for example, Corn, 1961 Hamaker, 1957 Overbeek and Sparnaay, 1954 Stone, 1930). As with any sensitive technique, extreme care must be taken for accurate results. 

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