Chemistry Reference
In-Depth Information
5.2 Holographic Glucose Sensors
Optical sensors consisting of polyacrylamide (pAAm) matrices have been
functionalised with boronic acid derivatives. 3-(Acrylamido)phenylboronic acid
(3-APB)
forms a reversible covalent bond with cis diol
[
8
]. Boronic acid
(pK
a
=
guration
(1); while at higher pH values (pH > pK
a
), the trigonal form reacts with OH
−
to
form the more stable negatively charged tetrahedral state (2), which can bind to cis
diol groups more readily (3
8.8) at low pH values is in an uncharged and trigonal planar con
*
4) (Fig.
5.1
a) [
9
,
10
]. The binding of cis diols forms
boronate anions, and subsequently swells the hydrogel through a Donnan osmotic
pressure increase [
11
]. This swelling increases the lattice spacing, which shifts the
Bragg peak to longer wavelengths (Fig.
5.1
b
-
d).
Holographic sensors comprising of 3-APB have been fabricated and their sensi-
tivity was optimised for glucose detection [
12
]. Maximal sensitivity was measured at
a 3-APB concentration of
-
20 mol% [
8
,
13
]. The hologram displayed a red Bragg
*
Fig. 5.1 Principle of operation of a Bragg grating-based glucose sensor. a Simpli
ed illustration
of the complexation equilibrium between the boronic acid derivatives and glucose. Glu = Glucose,
b Reversible swelling of the sensor by glucose modulates both the Ag
0
NP distribution spacing
and the refractive-index contrast, and systematically shifts the Bragg peak from c shorter to
d longer wavelengths as the hydrogel expands in the direction normal to the underlying substrate
