Biophotonics_Concepts_to_Applications

pathogens such as bacteria and viruses. The biorecognition material is referred to as
areceptor, which has the characteristic to absorb or capture a specific analyte. In
immunology this analyte is called anantigen. An antigen is any substance that
causes the immune system of the human body to produce antibodies in order to
eliminate the antigen. Examples of antigens include chemicals that the immune
system views as toxins and microorganisms such as bacteria, fungi, parasites, and
viruses. The absorption process of the antigen by the receptor coating on the optical
fiber or waveguide results in a physical-chemical alteration that can change the
layer thickness, effective refractive index, degree of light absorption, or electrical
charge at the coating surface. An optoelectronic sensor, such as a spectrum analyzer
or a photodetector, then can measure variations in the optical signal parameters that
result from these changes in the physical characteristics.
For example, as noted in Fig.7.13(also see Fig.3.4), part of the optical power
of a propagating mode is contained in an evanescentfield that travels in the
cladding or coating [ 3 ]. Thus a change in the index of thefiber coating will induce a
slight perturbation in the mode near thefiber-to-coating interface. This modal
perturbation results a change in the optical power level in thefiber core. The light
power variation seen by the photodetector at that specific wavelength then can be
related to the concentration of the absorbed analyte. Applications of this method-
ology have been used for sensing glucose levels, pH levels, oxygen levels, and the
presence of antibodies.

7.4.2 ELISA

A technique calledenzyme-linked immunosorbent assay(ELISA) is a variation on
the class of antibody/antigenfiber sensor described above [ 23 , 25 , 26 ]. This pro-
cedure is a common serological test to check for the presence of certain antibodies
or antigens. The termserologyrefers to the scientific and clinical study of plasma
serum and other bodyfluids. The ELISA concept, which is illustrated in Fig.7.14,

Target molecules

(analyte)

Biorecognition molecule

layer (receptors)

Buffer solution

Optical fiber core

or planar waveguide

Guided mode Optical sensor signal

Evanescent

field tail

Captured

molecules

(analyte)

Fig. 7.13 Analyte sensing with an opticalfiber or planar waveguide via an evanescentfield
perturbation (J. Biomed. Opt. 19(8), 080902 (Aug 28, 2014). doi:10.1117/1.JBO.19.8.080902)

210 7 Optical Probes and Biosensors