Luminescence transducers

The technology involving light and other forms of radiant energy with a fundamental unit, the photon, is known as photonics. Transducers which have visible light as output are widely used. Examples are phosphors, which emit light. Uses of lanthanides in luminescence transducers are listed in Table 12.15. The list includes CRT Phosphors and fluorescent lamps. 

Some phosphors are self-generating and others are stimulable phosphors involving modulating transducers to delay the output. Some examples of lanthanides in luminescence transducers are fisted in Table 12.16. 

Horseradish peroxidase (HRP) is also used for the detection of toxic compounds. A chemiluminescence test based on the reaction of luminol and an oxidant in the presence of the enzyme HRP has been developed to indicate the presence of toxins in a sample. The HRP-catalyzed reaction produces light that is measured by a lumi-nometer or a luminescence transducer. This enzyme has been used to detect a range of compounds such as phenols, amines, heavy metals, or compounds that interact with the enzyme, reduce light output, and indicate contamination. Test kits such as the Eclox Water Test Kit (Seven Trent Services, U.K.) is based on the use of HRP in the test format described earlier. This type of test is designed for the qualitative assessment of water samples for a range to compounds that inhibit the HRP activity. 

The optode transduces the non-optical signal from the environment to the optical one, readable by the photodetector. Various indirect optical sensors and theirs applications are described in literature35. The optode can work as a chemical sensor that detects certain analytes in aqueous solutions or in air on chemical way. It means that changes in the environment cause the changes in the photosensitive material, which is immobilized in the optode matrix. These chemical changes influence the observed light intensity (for example, due to absorption) or one can analyze the intensity or time decay of luminescence. There are numbers of publications devoted to the family of optical chemical sensors36. 

The concept is extendable to templated sensors made of protein templated xerogels in which a luminescent reporter group is further added in close proximity to the template site so as to effectively transduce the protein-molecularly imprinted polymer binding event (Figure 6.12).11... 

The fluorescence and phosphorescence of luminescent materials are modulated by the characteristics of the environment to which these materials are exposed. Consequently, luminescent materials can be used as sensors (referred also as transducers or probes) to measure and monitor parameters of importance in medicine, industry and the environment. Temperature, oxygen, carbon dioxide, pH, voltage, and ions are examples of parameters that affect the luminescence of many materials. These transducers need to be excited by light. The manner in which the excited sensor returns to the ground state establishes the transducing characteristics of the luminescent material. It is determined by the concentration or value of the external parameter. A practical and unified approach to characterize the luminescence of all sensors is presented in this chapter. This approach introduces two general mechanisms referred as the radiative and the nonradiative paths. The radiative path, in the general approach, is determined by the molecular nature of the sensor. The nonradiative path is determined by the sensor environment, e.g., value or concentration of the external parameter. The nonradiative decay rate, associated with the nonradiative path, increases... 

The apphcation of an optical transducer based on the fluorescence quenching effect of oxygen was described by Preininger et al. [3]. Another interesting technique is represented by the use of the luminous bacterium Photobacterium phos-phoreum [34]. This device is based on the correlation of the intensity of luminescence to the cellular assimilation of organic compounds from the wastewater. 

The application of semiconductor luminescence to chemical sensing can rely on the chemical, electrical, and optical properties of II-VI and III-V semiconductors [1]. These properties provide the binding capability, transducing mechanism, and signal required for chemical sensing. The diverse chemical compositions of semiconductor materials provide a range of surfaces for molecular binding. 

Customarily, semiconductor surfaces are chemically or physically prepared to optimize their chemical and/or electro-optical properties. For chemical sensing applications, a freshly etched surface often provides greater chemical sensitivity. A Br2/MeOH etch of n-CdSe, for example, has typically yielded larger luminescence responses to analytes than have polished samples. Additionally, transducing films have been used to modify semiconductor surfaces to enhance the selectivity of CdSe for particular analytes [2]. 

The transducing mechanism of semiconductor luminescence involves the modification of the semiconductors surface electrical properties through molecular adsorption. Changes in solid-state electro-optical properties result from adsorption of the molecule of interest onto the semiconductor surface. 

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]. 

A combination of a highly sensitive transducer based on luminescence excitation in the evanescent field with fluidic systems gives detection limits of 100 femtomoles of analyte. Such systems are well suited for antigen-antibody detection and DNA analysis [29]. 

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]. 

Optical biosensors can be defined as sensor devices which make use of optical principles for the transduction of a biochemical interaction into a suitable output signal. The biomolecular interaction on the sensor surface modulates the light characteristics of the transducer (i.e., intensity, phase, polarization, etc.), and the biosensing event can be detected by the change in diverse optical properties such as absorption, fiuorescence, luminescence or refractive index, among others. 

An optical transducer is an attractive option in that selectivity can be enhanced through the use of optical filters to select specific wavelengths. When this is combined with a very specific optical sensing material, e.g. photo-luminescent quenching polymers, the possible limit of detection is extremely impressive (see chapter 23 of reference [5]) and [11]. 

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