Transducer mass-sensitive

The immobilization of the hyperbranched spherical structures onto physical transducers greatly increases the binding capacity of the surface and leads to enhanced sensitivity and extended linearity of biosensors. Nucleic acid dendrimers were prepared and their amplification properties for the detection of DNA were examined using mass-sensitive transducers [45, 46]. Antibodies... 

Fig. 21.1. Schematic structure of a chemosensory system. Various physical or optical properties can be used for sensory detection. Typical mass-sensitive transducer (QCM and SAW) are shown.

This device employs single-stranded poly(adenylic acid) [poly(A)] as the chemical recognition agent. This species selectively recognizes its complementary polymer, poly(U), through hybridization to form a double-stranded nucleic acid. The poly(A) is immobilized onto the activated surface of a quartz piezoelectric crystal, which is a mass-sensitive transducer. Electric dipoles are generated in anisotropic materials (such as quartz crystals) subjected to mechanical stress, and these materials will... 

Keywords Mass-sensitive transducer QCM SAW Molecular imprinting Chemical sensor... 

With bioanalytes, different aspects have to be taken into accoimt in this case even an analyte monolayer covering of the transducer surface leads to highly appreciable sensor effects. But on the other hand it is absolutely useless to design a sensor layer with interaction centres distributed throughout the bulk of the material, for two reasons First, the sensor layers would become very bulky, which can lead to problems with mass-sensitive transducers. Second, such layers would require very long diffusion pathways, thus making real sensor detection in a reasonable time impossible. 

Aptamer-based biosensors, also called aptasensor have gain a wide interest in the last years due to the advantages of aptamers compared to antibodies. Similar to antibodies, a variety of immobilization methods is available to bind aptamers to the sensor element. Aptasensors can be coupled to an electrochemical, optical or mass-sensitive transducer [13]. One of the successful examples for aptasensor was the detection of thrombin which was widely investigated [14]. Xiao et al. [15] have made an interesting development a redox compound (methylene blue) was inserted into the thrombin aptamer. When the target bound to the aptamer, the induced conformation change inhibited the electron transfer from the methylene blue to the electrode. This change could be detected amperometrically. 

Chemical sensors may be classified according to the operating principle of the transducer as optical, electrochemical, electrical, mass sensitive, etc. 

The mass sensitivity of an ST-quartz APM device was determined by depositing metal onto the unelectroded quartz surface, i.e., the side opposite the transducers. The plate mode velocity shift is plotted vs the surface mass density of deposited silver in Figure 3.35. As expected from the discussion above, the device is approximately twice as sensitive when higher-order modes (n 1) are excited than with the lowest-order (it = 0) mode. The mass sensitivity measured... 

Chemical sensors are by definition small, inexpensive and preferably hand-held devices, capable of continuously monitoring chemical constituents in liquids or gases. MIP sensors usually consist of an imprinted sensitive layer and a transducer to convert the chemical information, in real time, into an electrical or optical signal which is further evaluated electronically [12]. Figure 21.1 shows the set-up of chemosensors and two typical mass-sensitive devices. 

A slurry contains crystals of copper sulfate pentahydrate [CuS04-5H20(s), specific gravity = 2,3] suspended in an aqueous copper sulfate solution (liquid SG = 1.2). A sensitive transducer is used to measure the pressure difference, A/ (Pa), between two points in the sample container separated by a vertical distance of h meters. The reading is in turn used to determine the mass fraction of crystals in the slurry, Xc(kg crystals/kg slurry). 

The reaction between the analjrte and the bioreceptor produces a physical or chemical output signal normally relayed to a transducer, which then generally converts it into an electrical signal, providing quantitative information of analytical interest. The transducers can be classified based on the technique utilized for measurement, being optical (absorption, luminescence, surface plasmon resonance), electrochemical, calorimetric, or mass sensitive measurements (microbalance, surface acoustic wave), etc. If the molecular recognition system and the physicochemical transducer are in direct spatial contact, the system can be defined as a biosensor [76]. A number of books have been published on this subject and they provide details concerning definitions, properties, and construction of these devices [77-82]. 

Abstract Brief historic introduction precedes presentation of main types of transducers used in sensors including electrochemical, optical, mass sensitive, and thermal devices. Review of chemical sensors includes various types of gas sensitive devices, potentiometric and amperometric sensors, and quartz microbalance applications. Mechanisms of biorecognition employed in biosensors are reviewed with the method of immobilization used. Some examples of biomimetic sensors are also presented. 

The research for chemosensors began as a branch of analytical chemistry and is now an approved and independent field of activities at the interface between research and application. A chemosensor can be considered as a small unit for the acquisition of analytical data. It has been optimized for one distinct application includes a sensitive layer, whose physico-chemical properties are affected by the interaction with the substance to be detected. These effects are translated into electronic signals by microelectronic devices and can be processed by data acquisition systems [11]. In most cases, mass-sensitive or optical transducers are used, and some of them are listed in Table 10.1. 

A wide range of physical parameters are suitable for chemical sensing applications, consequently, there is a very wide variety of different transducers. Some examples of frequent transducing techniques are metal oxide semiconductor devices (MOS diodes and field effect transistors) relying e.g. on changes in electrical fields or opt(r)odes concerning optical phenomena such as absorbance and fluorescence, but also miniaturised capacities [1]. Mass-sensitive, or acoustic, devices constitute another very popular class of transducers. Within this chapter we will focus on this transducing technique and introduce its abihties and properties in combination with selective artificial interaction materials. 

But why are acoustic transducers referred to as mass-sensitive Evidently, the plate thickness determines the resonance frequency of the quartz, but it... 

Template-directed generation of nanostructures in polymers thus indeed leads to antibody-like interaction abilities and thus highly appreciable selec-tivities. Sensitivity is added to the systems mainly by the transducer device, where the mass-sensitive strategy offers the big advantage of detecting a property that is inherent for any analyte. On a laboratory scale, sensor systems for a variety of analytes ranging from small molecules (volatile organic compounds) up to entire cells have been proposed. 

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