Fibre Sensors Without Chemical Receptors Mediators

Fibre Sensors Without Chemical Receptors (Mediators) 

The optical properties of analytes can be evaluated in many cases to obtain concentration-dependent chemical signals without a mediator. The term mediator is used here to characterize a chemical receptor which forms an optically active reaction product in contact with the sample. If the analyte itself is optically active, i.e. if it appears to be coloured in visible Ught, then the concentration of the coloured substance can be evaluated by means of Beer s law (also Beer-Lambert law) given by Eq. (8.2). The law is valid for monochromatic light only. Discrepancies occur if the spectral width is too broad to speak about monochromatic radiation  

Quality requirements for optical sensor components are not lower than for classical spectrophotometers. It is possible to utilize classical photometers 

It is not necessary in every case to make use of a complete instrument with elaborate spectral dispersion. Often the set-up can be restricted to the demands of the actual measurement. A useful arrangement is the combination of a light-emitting diode (LED) with a photodiode possessing an absorption spectrum congruent with the emission spectrum of the LED. Such a small and cheap device is mobile and fulfils the demands of sensor use. An approach in this matter has been devised by Smardzewski (1988). 

By appropriate choice of materials, such microrefractometers can be adapted to the requirements of the specific analytical task. Arrangements of this kind played a certain role as detectors in liquid chromatography but have been displaced by more sensitive detectors. 

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