Biophotonics_Concepts_to_Applications

Sect.7.4.1, an opticalfiber-based device can sense perturbations in the evanescent
field of the propagating modes in an opticalfiber. These perturbations arise through
the use of an opticalfiber that is specially coated with an antibody layer. In a biosensor
the antibody coatedfiber attracts a specific class of molecules when it is immersed in a
fluid containing these molecules, which are calledantigens. The adherence of these
antigen molecules to thefiber then causes a change in the effective index of thefiber
cladding,whichresults in a variation of an optical signal in thefiber. This change in the
signal characteristics then can be directly correlated to the concentration of the specific
molecules being measured.
A variation on the antibody/antigenfiber sensor is described in Sec.7.4.2. The
detection method is based on using a sandwiched material structure consisting of an
antibody layer, an antigen layer, and a detecting antibody layer. Following this
discussion, next Sects.7.4.3and7.4.4illustrate how relative movements between
twofibers or variations in the bend radius of afiber, respectively, can measure
changes in parameters such as pressure, stress, or temperature.
Other technology platforms that have been examined for waveguide biosensors
include optical interferometers, photonic crystalfiber-based devices, surface plas-
mon resonance devices, an individual or an array offiber Bragg gratings, and
nanoparticle based configurations. For these technologies, Sect.7.5 describes
fiber-based Mach-Zehnder, Michelson, and Sagnac interferometers.
Some representative photonic crystalfiber (PCF) devices that are being used in
different biosensor categories are described in Sect.7.6. One reason for using a PCF
in a biosensor system is that the air holes in a PCF serve as natural miniature
chambers for a liquid sample to enter [ 3 ]. This feature greatly reduces the volume of
a sample that is typically required in other categories of sensors. Another bio-
photonics fiber sensor configuration exploits intermodal interference between

Treat

interstitial

tumors

Tissue

pathology

Biosensor and

probe

applications

Detect specific

molecules

Measure O 2

levels in blood

Measure physical

changes: temperature,

pressure, stress, strain

Find molecular

concentrations

Measure features of

cells, proteins, DNA,

and other molecules

Endoscopy

Fig. 7.2 Diverse biosensing and biomedical measurement applications

7.1 Overview of Biosensors and Probes 199