Chemistry Reference
In-Depth Information
Fig. 3.23 Colorimetric readouts of holographic sensor
fl
flakes and their diffraction spectra as a
function of pH values. Images of a free-
fl
oating
fl
flakes, b paper strips at different pH values in
phosphate buffers (150 mM) at 24
C. The images were taken under white light illumination.
c Readouts for Bragg peak shifts as a function of pH change for
°
oating, paper- and
nitrocellulose-backed forms. d The reduction in the Bragg peak shift in holographic flakes. The
original holographic matrix showing the positions of two individual Ag
0
NP layers (red lines),
e The expansion on a plastic substrate, f in the flake form
fl
flakes in free-
fl
Table 3.2 Apparent pK
a
values of holographic pH sensors
Substrate
MAA
Glass
(SH)
Glass
(SR)
PMMA-
backed
(SR)
Flake
(SR)
Paper-
backed
(SR)
Nitrocellulose-
backed (SR)
pK
a
4.66
6.08
5.97
5.98
5.88
5.85
5.82
SH Silver halide chemistry
SR In situ size reduction of Ag
0
NPs (laser ablation)
(x direction) to its underlying substrate (shown in green) (Fig.
3.23
e). On the other
hand, holographic
x directions (Fig.
3.23
f). For
simplicity purposes, it was assumed that the gel does not expand in the y direction.
The red lines show the position of the fringes. The expansion of the holographic
matrix on a substrate is 2d, however, the expansion in the
fl
flakes can expand in both x and
-
flake form is d. Table
3.2
summarises the apparent pK
a
values measured from the holographic pH sensors
produced by different methods.
fl
3.5 Discussion
This chapter demonstrated the fabrication of holographic pH sensors via silver
halide chemistry and in situ size reduction of Ag
0
NPs in Denisyuk re
ection mode.
The use of a single laser pulse (6 ns, 350 mJ) to organise Ag
0
NPs within a
hydrophilic hydrogel matrix was reported. The holographic sensor was modulated
fl
