Introduction to Biosensor Technology - Sensor Technology in Neuroscience - page 29

Hall. 99 Both these processes have been employed extensively in the devel-
opment of biosensors and in this context the term 'optrode' has appeared, as
spawned by the older concept of an electrode in the field of electrochemistry. In
the first methodology often termed the 'extrinsic' device, the fiber is simply used
as a delivery system for light interaction with an optical configuration place at
the distal end of the structure. Unsurprisingly, this arrangement has been
employed, for example as the remote correlation of conventional,
Beer-Lambert absorption photometry, 100,101 (Figure 1.21(a)). In the earlier
days, the advantages of this sort of configuration were stated as lying in the
possibilities for removal of the sensing area from the source, correction for
variation of the latter and the introduction of a convenient reference system.
These features have, for some time, been considered to be superior to some
aspects of electrochemical devices such as those described above.
In the second scenario called the 'intrinsic' structure, some light energy finds
its way under certain conditions into the medium outside the fiber in the form
of a penetrative evanescent wave (Figure 1.21(b)). Importantly, the interaction
of surface chemistry at this interface with the evanescent wave can result in
perturbation of the light phase, intensity and polarization. There are many
examples of such an arrangement in biosensor detection. 102-104 One such
elegant case is the fluorescence-based system pioneered by Krull and his various
co-workers 105,106 at the University of Toronto at Mississauga. Here, for
example, nucleic acid duplex formation at the fiber-liquid interface can be
monitored very sensitively though various aspects of luminescent spectroscopy
(Figure 1.22). This type of device had been employed successfully in the assay
of 'real' samples such as detection of pathogens. 107
Finally, various configurations of optical fiber based systems have emerged
generated with the overall aim of increasing sensitivity (e.g. Figure 1.23). These
are summarized nicely in ref. 108.

d n 4 t 3 n g | 1

d n 3 .

Figure 1.21

(a) Schematic of extrinsic fiber optic delivery of radiation to a cell for
absorption measurement. (b) Evanescent radiation penetrating to the
exterior of a fiber in an intrinsic configuration.