Biomedical Engineering Reference
In-Depth Information
PD
CCD
camera
LSP
analyte
j
glass
Fig. 2.37
The LSP sensor
where
D
R.2
m
!/
1=2
=c,
2
D
.Lk
0
=2/
2
Œln.L=D/
C
iLk
0
=2=ln.1
C
L=D/;
f./
D
Œ.1
i/=
J
1
Œ.1
C
i/=J
0
Œ.1
C
i/,and
m
is the metal conductivity.
The applications of label-free sensors based on SPP in nanoparticles are reviewed
in (
Stewart et al. 2008
). For example, changes which appear in the extinction
of a nanoparticle film can be used to detect a target DNA or a protein. Simple
methods, such as separate immobilization of two nucleotide fragments, A
0
and
B
0
, on 13-nm gold nanoparticles to form two probes, which are able to hybridize
and aggregate in the presence of a target complementary nucleotide, denoted by
AB, are used. The sensor is similar to those discussed above and is able to detect
DNA targets at picomolar concentrations, up to femtomolar concentrations [see
(
Stewart et al. 2008
) and the references therein]. The SPP is used also to improve
the sensitivity of microarrays, which consist of immobilized biomolecules such as
DNA or proteins deposited as drops of 10-500m in size on a substrate, and further
used to bind targets such as DNA, through hybridization, or proteins via ligand-
receptor bindings for multiple detection processes. Using gold nanoparticles to label
the captured DNA targets, a target signal is enhanced by silver amplification via
electroless reduction of Ag ions on the surface of the Au nanoparticles. Femtomolar
concentrations of target sequence in human genomic DNA can thus be detected
without previous PCR amplification. Similar arrangements are used to detect the
multiplexed genotyping in human genome without target amplification or real-time
intracellular imaging of biomarkers in living cells. The reader must recognize how
sensitive is a label-free sensor based on SPP in nanoparticles. There are many other
label-free biosensors based on optical principles, such as optical resonators based on
whispery gallery modes or miniaturized optical interferometers; for recent reviews,
see (
Fan et al. 2008
;
Hunt and Armani 2010
).
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