Biomedical Engineering Reference
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structurally related molecule cyclohexane tricarboxylic acid, (8). The steric flexi-
bility of (8) yields an equilibrium mixture between the axial-equatorial
conformations of the carboxylic acid functionalities (Fig.
5B
) resulting in the
generation of imperfect imprinted recognition sites, exhibiting lower affinity for
the binding of RDX. Evidently, a decreased sensing performance for the detection
of RDX by the (8)-imprinted matrix was observed, as reflected by more than a
40-fold higher detection limit compared to the (7)-imprinted matrix.
The successful implementation of a carboxylic acid-functionalized molecule as
an analog for imprinting recognition sites for RDX was further expanded for the
sensing of other important explosives, such as pentaerythritol tetranitrate, PETN,
(9), nitroglycerin, NG, (10), and ethylene glycol dinitrate, EGDN, (11) (Fig.
6
)
[
52
]. Figure
6A
depicts the calibration curve obtained upon the sensing of PETN
on a citric acid, (12)-imprinted bis-aniline-cross-linked Au NPs composite. The
ionic and/or H-bonds between the carboxylate and the anilinium residues provide
strong affinity binding sites for (9), leading to a detection limit as low as 200 fM.
Under these conditions, a high association constant between the matrix and the
explosive,
K
a
¼
10
11
M
1
, was evaluated. Citric acid, (12), was, similarly,
implemented as imprinting substrate to generate imprinted sites in the Au NPs
matrix for nitroglycerine, (10) (Fig.
6B
). Evidently, significant reflectance
changes are observed upon the interaction of pM concentrations of (10) with
the (12)-imprinted matrix, curve (a). Similar SPR responses are, as expected,
absent in the analysis of (10) by the non-imprinted matrix, curve (b), which
reconfirms the important role of the imprinting in the detection paradigm. The
lower detection limit observed for the association of PETN with the
(12)-imprinted sites, as compared to the NG, may be attributed to the presence
of the OH group in (12) (Fig.
3
). The hydroxyl group, in addition to the three
carboxylic acid residues, mimic the four -ONO
2
functionalities of PETN,
whereas in the NG explosive (that includes only three -ONO
2
functionalities),
no contribution is made by the additional OH group associated with (12). It is,
thus, clear that (12) is structurally optimized to accommodate the PETN substrate
containing the four -ONO
2
functionalities.
Another important explosive, yet lacking significant scientific attention, is
ethylene glycol dinitrate, EGDN, (11). The chemical structure of (11) (Fig.
3
)
suggested that the imprinting analogs that may be adapted to yield imprinted sites
for (11) in the Au NPs matrices via electrostatic/ionic interactions are: maleic acid
(13), fumaric acid (14), and succinic acid (15). The three calibration curves
presented in Fig.
6C
show the reflectance changes obtained by the interaction of
variable concentrations of EGDN with the (13)-, (14)-, or (15)-imprinted Au NPs
composites. The results reveal a comparable sensing performance for all matrices
(Fig.
6C
). The superior performance by the (13)-imprinted matrix may be, however,
explained by the favorable existence of (11) in a state where the nitro substituents
adopt a “gauche” conformation [
76
]. Thus, the
cis
configuration associated
with the carboxylic acid functionalities of (13) allows the molecule to act as an
effective analog for EGDN, resulting in an enhanced sensing performance by the
(13)-imprinted matrix.
9
:
5
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