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detection of 70 cells mL
−1
in 30 min based on evanescent field absorption,
55
and in the same year Zhu et al. reported 10
2
cells mL
−1
in 2 h using a fluores-
cent sandwich immunoassay.
56
In 2011, U-bent optical fibers were adopted
in an attempt to improve the LOD. However, during the 20 min allowed
for detection, detection of concentrations lower than 10
2
cfu mL
−1
was
not possible due to diffusion-limited transport to the sensor surface
57
(
Fig.
7.12
).Very recent work has applied bacteriophages on optical fibers, with an
LOD similar to the previous reports of 10
3
cfu mL
−1
.
58
The final optical technology used for
E. coli
is SPR. In 2005, Su and Li
compared direct detection with SPR and QCM using the same immuno-
capture strategy and 1 mL samples, achieving LODs of 10
5
and 10
6
, respec-
tively.
59
Also in 2005, Taylor et al. attempted to improve the SPR detection
by pretreatment of the bacteria as well as use of a secondary antibody
for signal amplification.
60
Again 1 mL samples were used, here at a flow
rate of 50 µL min
−1
. Bacterial modifications were either heat and ethanol
treated to render cells nonviable or lysis with detergent. Compared to Su
and Li, the SPR LOD for untreated
E. coli
was higher at 10
7
. The best
LOD was obtained for the detergent-lysed cells (10
4
cfu mL
−1
), explained
by better capture of small cell fragments and improved delivery of these
fragments to the sensor surface since smaller fragments diffuse faster. In
2006, Subramanian et al. trialed a different immobilization strategy using
SAM to immobilize the capture antibody closer to the SPR surface and a
Figure 7.12
Enhancement of penetration depth
D
P
in bent fiber probe, enabling eva-
nescent field overlap for detection of
E. coli
(
D
P
and biomolecules not drawn to scale).
Source:
Figure 1 from Ref.
57
. Reproduced with permission.
(For color version of this figure,
the reader is referred to the online version of this topic.)
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