Chemistry Reference
In-Depth Information
Consequently, this resulted in water uptake, swelled the hydrogel and increased the
spacing of the Ag
0
NPs, primarily in the vertical (and to a much lesser extent in the
lateral) direction, which shifted the Bragg peak to longer wavelengths. Holographic
sensors (0.5
2.5 cm) were submerged in a plastic cuvette (reservoir) for pH
measurements. After a reading was taken, the reservoir solution was removed and
the cuvette was
×
flushed consecutively three times for each reading point. A spec-
trophotometer in re
fl
ection mode was used with a bifurcated cable to capture the
corresponding images. Figure
3.13
a shows the measurements of Bragg peaks of the
holographic pH sensor fabricated using silver halide chemistry. As the pH was
increased, the Bragg peak shifted to longer wavelengths by
fl
280 nm (Fig.
3.13
b).
An increase in lattice spacing consequently reduced the contrast of effective index
of refraction, hence the ef
*
ed
Henderson-Hasselbalch equation was adapted to determine the apparent pK
a
values
[
35
]:
ciency of the multilayer structure decreased. A modi
Dk
10
pK
a
pH
k
shift
¼
ð
Þ
3
:
1
ð
ð
Þ
þ
Þ
1
Fig. 3.13 Diffraction spectra of holographic pH sensors (6 % MAA) fabricated through silver
halide chemistry. a Visible-near-infrared diffraction spectra of a holographic sensor swollen by
different pH solutions using phosphate buffers at 24
C, b The Bragg peak shifts over three trials,
c Colorimetric readouts of the holographic sensor at various pH values. Na
2
HPO
4
-citric acid
(pH 3.00
°
8.00), Na
2
HPO
4
-HCl (pH 9.00), Na
2
HPO
4
-NaOH (pH 10.00) were mixed to obtain
buffers (150 mM) at desired pH values. Standard errors were calculated from the three subsequent
trials for each data point. Reproduced from Ref. [
17
] with permission from The Royal Society of
Chemistry
-
