Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 9.7
Antibody-based biosensor. (a) The structure of an antibody. (b) The basic structure of a ClearBlue
Pregnancy test consists of two compartments in which antibodies are dispersed. (c) The introduction of a
sample containing the antigen will bind to the antibody on the latex sphere and move due to fl uid motion.
They encounter the antibody in the second compartment to form a complex that is retained in the test unit
to produce a blue line. (d) The formed antigen-antibody complex. The absence of the antigen in the sample
will not result in this complex.
chorionic gonadotrophin (hCG) protein (particularly, its free beta-subunit, hCG
)
in urine [Figure 9.7(c)]. hCG is an enzyme, the concentration of which increases
geometrically postconception during fetal growth. Hence, monitoring hCG
β
pro-
vides information concerning the progress of a pregnancy and the health of a fe-
tus. Monoclonal antibodies which bind specifically to hCG have been developed:
anti-
β
domain on hCG
and both are used in home pregnancy test devices. The urine is collected and led
through an absorbent sampler protruding from one end. A biosensor is held for
few seconds in the urine stream. If hCG is present in the urine, it is carried towards
hCG antibodies labeled with a dye. An hCG-hCG antibody/dye complex is formed
and moves towards a window in the case where there is another set of hCG an-
tibodies. At the window, these combine to form an hCG antibody/dye-hCG-hCG
antibody complex. The accumulation of complexes shows up as a line of the dye at
the window, indicating a positive pregnancy test. Commercial pregnancy test kits
can detect nano molar (or 10
−9
M) concentrations of hCG. If the line does not ap-
pear, this could be due to the absence of the hormone or insufficient urine on the
absorbent tip. To account for insufficient sample size variations, positive controls
are built within the device.
α
-hCG binds to the
α
domain and anti-
β
-hCG binds to the
β
9.3.3 Nucleic Acid-Based Biosensors
Nucleic acid biosensors utilize the complementary nature of the nucleic acids for
selective recognition capability. They are designed for the detection of DNA or
RNA sequences usually associated with certain bacteria, viruses, or given medi-
cal conditions. The DNA probes could be used to diagnose genetic susceptibility,
diseases, and paternity tests. Nucleic acid biosensors generally immobilize single-
strands (called probes) from a DNA double helix (or RNA) onto a surface as the
