Biosensors for the Detection of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 192

causes a change in the optical properties of the surface, which can therefore
by utilized for detection. One example of this is surface plasmon resonance
(SPR), where binding of the target changes the refractive index at the bio-
sensor surface. Other examples include the use of waveguides, fiber optics,
or ellipsometry. 1
SPR measures changes in the refractive index at the interface between
a planar metal surface and a dielectric material. Analyte-binding events are
detected by coupling photons from a light source to surface plasmons and
then measuring a change in the properties of the reflected light ( Fig. 7.5 ) .
Detectors that measure intensity, incident angle, wavelength, or phase of the
reflected light have been designed. SPR sensors have been used to detect a
range of analytes such as antibiotics, 23 vitamins, 24 hormones, 25 pesticides, 26
as well as bacteria and protozoa. A comprehensive review of the use of
SPR has been published by Homola. 27 Alternative techniques operating
on a similar principle of detecting changes in refractive index or resonant
reflection are the use of silicon microring resonators and photonic crystal
biosensors.
Another method of optical biosensing is those techniques facilitated by
waveguides. 1 Waveguides are typically made of glass, quartz, or polymer
films with a high refractive index embedded within a lower refractive index
material. Incident linear laser beams are therefore constrained within the
waveguide by total internal reflection (TIR), resulting in an evanescent
wave. A proportion of the light penetrates into the biolayer and is reflected
back into the waveguide after undergoing a phase shift that interferes with
the transmitted light. Changes in this interference pattern can thus be inter-
preted as changes in the biolayer allowing for detection of analyte-binding
events.
Interferometry is based on changes in the refractive index profile with
the evanescent field volume of a waveguide due to analyte binding. Optical
waveguide lightmode spectroscopy utilizes the change in resonance angle of
polarized light diffracted by a grating and incoupled into a thin waveguide
layer upon binding. TIR fluorescence is a method where the incident light
from TIR excites fluorescent molecules in the evanescent field, which pro-
duces a measurable fluorescent evanescent wave.
7.3.2. Electrochemical
Electrochemical techniques study interfacial phenomena by looking at the
relation between current and potential. A perturbation in either the current