Acoustic transduction

These transducers have also been applied in immunosensors for food or water analysis [94]. The resonant frequency of an oscillating piezoelectric crystal can be affected by a change in mass at the crystal surface. Piezoelectric immunosensors are able to measure a small change in mass. Novelty contribution is the use of magneto elastic transduction, an example is a mass-sensitive magneto elastic immunosensor for detection of Escherichia coli [95]. However, most of the past publications have been based on immunosensors using quartz crystal microbalance (QCM) for the detection of trace amounts of chemical compounds, such as dioxins [96]. Other examples of application are focused on the determination of pathogens such as the work presented by Liu et al. [97], in this work the detection of E. coli O157:H7 was accomplished using a QCM immunosensor with nanoparticle amplification. The quartz crystal is a highly precise and stable oscillator. It has been used widely in electric circuits as a frequency standard clock in computers, communication systems and frequency measurement systems. The quartz crystal is the crucial component of the QCM because it registers and reports the mass deposited on its electrodes quantitatively: the mass changes its oscillation frequency. It is also used for immunosensor techniques, for film thickness metres and for chemical sensors.

A variety of surface-based detection principles are employed for immunosensors based on QCM. Preparation and characterization of such layers for useful deposition of detector molecules require sophisticated surface science input. Furthermore, commercialization for use in the food analysis, medical diagnostics and environmental monitoring fields necessitates stability and reliability of the bio-interface. Therefore, antibody immobilization method and its stabilization are very important to prepare the bio-functional interface of QCM immunosensors [98,99].

These methods are classified into three main categories

• Immobilization of the antibody on the crystal precoated with a suitable material

• Immobilization via entrapment in polymer membranes

• Immobilization via glutaraldehyde cross-linking.

The prerequisites for the active surface of a QCM immunosensor include that it be chemically active of or toward the immobilized antibody or antigen. Furthermore, the coatings achieved using the immobilization method must be as uniform and thin as possible. These features are particularly important in QCM immunosensors because high sensitivity can only be achieved using active, thin and rigid layers. Additionally, leakage of the immobilized materials must not occur to any extent during the immunosensor use. Although many immobilization methods have been tried with QCM immunosensors, no one ideal method exists that gives high immobilization yield and good stability.

Furthermore, the complexity and diversity of antibody or antigen that is useful for different purposes render it difficult to devise one optimum method. Therefore, it is necessary to find a suitable immobilization method for an antibody or antigen for a particular application.

The development of QCM immunosensors for food analysis is in an initial phase of development, but it is one of the most promising fields. Up to now most of the applications have been centred in the analysis of pathogenic microorganisms [100].

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