Extrinsic optical biosensors

Extrinsic. Many of the traditional bioassays are based on optical measurements, but these are solution assays without immobilised reagents. Optical biosensors are concerned with interrogation of reactions at surfaces, but often the reagents are the same the optical fibres acting, in the extrinsic mode as light guides to transmit information to and from a remote target reaction at its terminus. 

Optical biosensors based on the use of fiber optics can be classified into two categories intrinsic sensors where interaction with the analyte occurs within an element of the optical fiber, and extrinsic sensors in which the optical fiber is used to couple light, usually to and fi om the region where the light beam is influenced by the measurand. Moreover, biosensors become attractive because they can be easily used by non-specialist personnel and they allow accurate determination with either none or little sample treatment. Therefore, fiber-optic biosensors may be especially useful in... 

Fig. 5.4. Optical Fiber biosensor, (a) Extrinsic optical fiber is used for the guiding the light to and from the sensor area, (b) Intrinsic the receptor molecules are immobilized on the fiber core after decladding of the fiber. The detection is based on fluorescence labels.

It was mentioned before that fiber-optic chemical and biosensors are broadly classified into two categories extrinsic- and intrinsic-type sensors. In the extrinsic-type sensors, the fiber is acting as a link connecting optical signals to (and from) the active material (medium) positioned at the end of the fiber, such as the Optode case. In the intrinsic-type sensors, the fiber is modified in different ways, through construction of the sensing component, which will be explained next. 

Biosenscx are usually based on extrinsic sensors. The biocatalyst is immobilized on the tip of an optical fiber, and the biosensor measures the absorption, fluorescence, or chemi/bioluminescence produced. 

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