Biology Reference
In-Depth Information
of the products of molecular methods e.g. amplification products from
polymerase chain reaction.
17
As this chapter focuses on biosensors for the
detection of whole pathogens, further discussion of this type of recognition
element is left until Chapter 8, which covers molecular methods.
Aptamers are small (2-25kDa) artificial nucleic acid sequences that
exhibit high affinity and specificity to target probes. This type of recogni-
tion element is produced through a series of selection and amplification
steps known as
S
ystematic
E
volution of
L
igands by
EX
ponential enrich-
ment (SELEX) to select a high-affinity probe. The advantages of aptamers
are that following the SELEX identification they can be synthesized easily
in large quantities and they can be produced against any target, even those
that do not elicit an immunogenic response. However, one main drawback
is that they are sensitive to nuclease attack, although this can potentially be
mitigated via chemical modifications.
Carbohydrates are a large class of biomolecules that are often involved in
pathogen interactions e.g. many pathogens recognize specific carbohydrate
sequences in the human gut in the process of initiating infection. These
interactions have inspired the use of carbohydrates as recognition elements
for pathogen detection. Advantages compared to antibodies are that car-
bohydrates do not denature on exposure to temperature or pH alterations
and hence are more stable, as well as the fact that carbohydrates are smaller,
thus allowing for higher surface density on the biosensor surface, which
can improve sensitivity. However, disadvantages are the low affinity of the
carbohydrate-protein interactions (10
2
-10
3
lower
K
d
values than antibody
recognition) and the obtainable specificity.
The specificity of carbohydrates is relatively broad, which can be a chal-
lenge for accurate detection of the intended target pathogen. However, in
some cases, such broad specificity may prove advantageous. For example,
some antibodies may be highly specific to the extent that mutants that
differ slightly from the original target are not detected; carbohydrates do
not suffer this limitation. One route to improve carbohydrate specificity
has been to combine carbohydrates with lectins. Lectins are a group of
proteins that strongly bind to specific carbohydrate moieties. Concanavalin
A, a widely used mannose- and glucose-binding lectin, has been employed
in biosensors for pathogens.
Whole-cell biosensors employ living cells as the biological recognition
element. The main advantage of this approach is that toxicity can be assessed;
this approach is being considered for the detection of low-concentration
micropollutants in water. However, widespread adoption of these sensors
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