Biomedical Engineering Reference
In-Depth Information
the bridging units which cross-link the NPs can be altered upon the application of an
external potential on the composite-modified electrode. Specifically, by the applica-
tion of a reducing potential, E
0.1 V vs. Ag QRE, the bridging units undergo an
electrochemical transition to a bis-aniline reduced state which exhibits
<
p
-donor
characteristics, whereas at potentials higher than E
>
0.1 V, the entire population of
p
the bridges exist in an oxidized quinoid state of a
-acceptor nature. Accordingly, the
introduction of an acceptor substrate into the electropolymerization solution results in
its association with the
-donor thioaniline residues that cap the Au NPs, by
p -donor-acceptor interactions, and upon electropolymerization, the acceptor substrate
also binds to the newly formed p -donor bis-aniline bridging units. To exemplify this
process, a high concentration, 1 mg mL 1 , of picric acid, (3), was introduced into the
electropolymerization solution [ 50 ]. Picric acid exhibits a molecular structure that is
analogous to TNT (4)(Fig. 3 ) with only a minor size difference between the hydroxyl
and methyl substituents associated with the molecules. Furthermore, both compounds
demonstrate comparable electron acceptor characteristics, suggesting that the picric
acid might be a good analog for the imprinting of TNT recognition sites. The use of
picric acid as imprint analog is favored over the direct implementation of TNT due to
its enhanced solubility in the aqueous electropolymerization medium, suggesting the
generation of a matrix with a higher density of imprinted sites for the association of
TNT. The subsequent removal of the picric acid from the matrix, by a prolonged
rinsing of the surface with a buffer, renders the surface with imprinted sites exhibiting
high affinity toward TNT. Similarly, a non-imprinted matrix was prepared by the
exclusion of the imprint analog (3) from the electropolymerization process. It should
be noted that both the imprinting and extraction stages were monitored in situ, by
following the changes in the SPR spectra corresponding to these processes. Figure 4A
shows the SPR curves obtained for the (3)-imprinted bis-aniline-cross-linked Au NPs
composite before, curve (a), and after, curve (b), the interaction of the matrix
with (4), 1 pM. A clear shift of the resonance angle is observed, consistent with
the association of (4) to the imprinted sites results in dielectric changes at the Au NPs
matrix. While the change observed in the minimum refractive angle,
p
y min ,issignifi-
cant, and may be used to provide a quantitative measure for (4), an alternative, more
convenient sensing mode is facilitated by fixing the incident light angle, y , while
monitoring the time-dependent changes of the reflectance,
R ,toyieldasensogram.
Figure 4B depicts two sensograms corresponding to the analysis of (4) by the non-
imprinted, left Fig. 4B-I ,andthe(3)-imprinted, right Fig. 4B -II, Au NPs composites.
In these figures, the injections of the TNT samples, with increasing concentrations, are
marked with arrows. Evidently, whereas the imprinted composite enables the detec-
tion of TNT with an ultra-sensitivity of 10 fM, the non-imprinted matrix shows a
1
D
10 3 -fold higher detection limit (10 pM). The enhanced performance by the
imprinted Au NPs matrix is attributed to the formation of the high affinity imprinted
sites for (4), allowing its efficient binding to the matrix. Using the resulting calibration
curves (Fig. 4C ) and employing a Langmuir-type fitting, the association constants of
(4) to the imprinted and non-imprinted matrices are estimated to be K a ¼
10 12
6
:
4
M 1 and K NI
a
10 9 M 1 , respectively. Interestingly, the two step reflectance
¼
3
:
9
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