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excitation
emission
relaxation
S
0
-h
υ
1
h
υ
O
O
O
O
O
O
S r
S r
S r
S
S
S
n
n
n
S
0
S
1
S
1
′
bithiophene planarizes
Note:
υ
1
<
υ
2
emission
excitation
h
υ
O
O
S
0
O
O
-h
υ
2
S r
S
S r
S
n
n
S
0
S
1
bithiophene cannot
planarize
no relaxation
= Na
+
Scheme 2 A calixarene-based ion receptor with bithiophene backbone. The
top
scheme shows
excitation of the receptor in the absence of coordinated ion with normal planarization and
relaxation. The
bottom
scheme shows how ion binding restricts planarization and relaxation
resulting in higher energy emission [
13
]
Fig. 6 Polydiacetylene with
alternating double, single, and
triple bonds along the
polymer backbone
polydiacetylene
n
3.2 Biosensors
In addition to simple ions, CPs can be used to detect larger, potentially bio-active
materials. An early example of such a system employed the polymer polydiacety-
lene (PDA) (Fig.
6
) for viral detection [
14
].
A colorimetric detector was generated from a bilayer of ocadecyltrichlorosilane
(OTS) and polydiacetylene (PDA) with the potential for viral molecular recogni-
tion. In a living system, the infectious particles of the influenza virus are
encapsulated within a lipid bilayer to which the hemagglutinin (HA) lectin is
anchored. When cells come into contact with the virus capsules, the HA lectin
binds to
-glycosides of sialic acid on the cell's surface, resulting in endocytosis of
the virus and subsequent infection. In the biosensor, lipid chains functionalized
with sialic acid, an established carbohydrate for viral influenza recognition, were
prepared as Langmuir-Blodgett films, polymerized by UV radiation for PDA
cross-linking, and then lifted onto glass slides prepared with a monolayer of self-
assembled OTS (Fig.
7
).
a
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