Transducers surface plasmon resonance

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

All the most relevant transducers such as MOSFET, CMOS, Surface Plasmon Resonance device, Optical Fibre, ISFET, will be covered in some detail including their intrinsic operating mechanisms and showing their limitation and performance. Shrinking effects of these transducers will also be commented on. 

The use of optical immune biosensors based on surface plasmon resonance (SPR) for the diagnostics of human and animal diseases as well as for environmental pollution monitoring, is one of prospective directions in biosensorics. The sensitivity of immune biosensors is similar to the ELIS A-method but the simphcity of obtaining results in the real time regime and the speed of the analysis are the main advantages of the biosensor approach. Performance of optical biosensors based on SPR depends on the state of the metallic surface as well as on the density, structure and the space volume of the immobilized molecules. It was demonstrated that the application of intermediate layers between the transducer surface and the sensitive biological molecules can optimize the working characteristics of the immune biosensor [7-14]. 

Starodub NF, Dibrova TI, Shirshov YuM, Kostiukevich KV (1997) Development of sensor based on the surface plasmon resonance for control of biospecific interaction. In Proceedings of eurosensors 12 and 11th european conference on solid-state transducers, Warsaw, Poland, 3 1429-1432,21-24 Sept... 

Optical devices have also been used as transducers. Laser fiber-optics allows high intensity light to travel a long distance using fibrous size carrier. The stable and intense light beam not only provides calibration stability but also makes all the detecting techniques faster and more sensitive. In addition to the UV-VIS absorbance and fluorescence intensity, measurements of multiple reflections, surface plasmon resonance, and total internal reflection fluorescence had recently been used (12, 13, 14). 

The transducers most commonly employed in biosensors are (a) Electrochemical amperometric, potentiometric and impedimetric (b) Optical vibrational (IR, Raman), luminescence (fluorescence, chemiluminescence) (c) Integrated optics (surface plasmon resonance (SPR), interferometery) and (d) Mechanical surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [4,12]. 

The attachment of protein to the transducer surface for immunoassay is a difficult problem, since it must be achieved without interfering with the active site. If an unlabeled assay is to be performed it must also be such that subsequent non specific interaction with the surface can be inhibited. This requirement is often contrary to those for effective antibody-antigen complex formation. A surface close packed with antibody will be sterically hindered and its reaction with antigen inhibited. On the other hand, a suitably spaced packing allows non-specific interactions to occur and large false positive signals to be recorded. As mentioned earlier, this was demonstrated by Cullen and Lowe who used the surface plasmon resonance technique to probe specific and non-specific protein interactions at metal surfaces. (23). 

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

Electronic and optical transduction of the formation of antigen-antibod/ affinity compleK-es on transducers (A) Amperometric transduction at an electrode (R /R is a redox label in the riec-trolyte solution). (B) Tnuisduction by faradic impedance >ectro cc. (C) hterogravimetric tpiartz crystal microbabnce (QCM) transduction in the presence of a piezoelectric quartz crystal. (D) Surface plasmon resonance transduction. 

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]. 

Planar supported lipid membranes were first prepared and studied as simplified structural models of cell membranes [4,6, 32], and more recently as biocompatible coatings for sensor transducers and other synthetic materials [33-37], A major advantage of the planar geometry relative to vesicles, and a major contributor to the expansion of this field, is the availability of powerful surface-sensitive analyti-cal/physical techniques. Confining a lipid membrane to the near-surface region of a solid substrate makes it possible to study its structural and functional properties in detail using a variety of techniques such as surface plasmon resonance, AFM, TIRF, attenuated total reflection, and sum frequency vibrational spectroscopy [38 -2]. 

Recently, piezoelectric sensors have received growing attention as a tool for label-free detection. Because of the progress made in the field of microelectronics and microfluidics, the piezoelectric transducers have become a competitive alternative to surface plasmon resonance (SPR) devices and grating couplers. 

The minor changes occurring at an antibody or antigen loaded surface resulting from an immune reaction with the respective counterpart (antigen or antibody, resp.) can be monitored as a mass change using a piezo-electric transducer [41] or optically, eg, by surface plasmon resonance [42]. 

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