the receptor and displaces water near
the waveguide surface, causing a change
in the velocity of the propagating light.
To measure this change, an adjacent but
unperturbed reference beam is optically
combined with the testing beam, creating an interference pattern of alternating
dark and light fringes. When refractive
index changes occur in the testing arm,
the interference pattern shifts, producing a resultant change in the relative
phase that is measured using a Fourier
transform algorithm and indicating
the presence and concentration of the
target antigen. No secondary or reporter
antibody is required. With detection
sensitivities on the order of 0.01 radian,
refractive index changes of less than 10-6
can be measured.
Current levels of detection are in
the femtomolar range for proteins and
less than 1,000 CFU/mL for whole
organisms. The interferometric sensor
provides a direct, near real-time measurement without the need for additional washing or incubation steps. No
labeling is required and no consumables
other than the buffer solution used
for collecting the sample are required.
The detection of Campylobacter jejuni
(103–107 cells/mL) in PBS using commercially available polyclonal antibody
immobilized waveguide is shown in
Figure 5. Quantitative reproducibility of
the assay was measured using independently prepared sensor chips and found
to be within 10 percent over several
orders of magnitude in concentration.
We have witnessed increasing efforts
in biosensor development for its great
potential for rapid and sensitive pathogen
detection. Advances have led to the development of a variety of nanomaterials-based sensor platforms, including metal
nanostructure-based surface plasmon
resonance, surface-enhanced Raman scattering, carbon nanotube, and graphene-based electrochemical sensors. However,
detecting low numbers of viable pathogens in complex matrices is still a hurdle
for biosensor application in food safety.n
Jie Xu, Ph.D., is a principal research scientist at the
Georgia Tech Research Institute.
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Figure 4. The electric field associated with the light beam propagating in the waveguide
layer extends above the waveguide surface into the cover layer. The waveguide material and
its thickness ( W) were chosen to support a single optical mode (m = 0) to optimize sensitivity.
Binding of a target antigen to the receptor (antibody) layer causes a change in the effective
Concentration (log10 CFU/m L)
Figure 5. (A) Typical time-response curves of the biosensor to heat-killed samples of 103–107
CFU/mL Campylobacter in PBS+. Samples of 1 mL were flowed at 1 mL/min in a closed loop.
A newly prepared waveguide was used for each sample. (B) After triplicate experiments, the
response (mean ± SE) at 10 () and 30 () minutes is shown as a function of concentration
on a log-log plot. The inset depicts the same data on a semi-log plot.