Chemistry Reference
In-Depth Information
spectrophotometer with
0.5 nm resolution, which is comparable to the accuracy of
electrochemical sensors. However, the sensitivity of the present sensors can be
improved by reducing the concentration of the crosslinker or increasing the
carboxylic acid groups in the polymer matrix [
20
].
In addition to colorimetric dyes and electrochemical sensing, optical sensors
have been developed. Sensing platforms in this area include absorption- and
luminescence-based optical systems,
±
fibre-optic and planar waveguide-based
sensors, and hydrogel-based systems that utilise diffraction and localised surface
plasmon resonance [
54
,
57
,
58
]. Other sensors include optical devices coupled with
pH indicators, which are based on weak organic dyes or changes on their optical
properties when they are protonated or deprotonated. For example, their absorption
or
fluorescence properties change in the presence of acidic or basic solutions, which
can be correlated with the concentration of H
3
O
+
ions. Optical sensors do not
require a separate reference sensor, and they can be easily miniaturised down to
1
fl
m. Additionally, they do not suffer from electromagnetic interferences, and
they can be used for remote and continuous measurements. However, the draw-
backs of the listed optical sensors include limited long term stability caused by
photobleaching or leaching of the sensing materials while also being affected from
temperature changes [
54
]. Notably, a change in ionic strength can alter the activity
coef
-
10
µ
cients and shift of the calibration plot [
59
]. Hence, corrections are required to
compensate for the ionic strength. Moreover, their operating pH range can be
extended; indicators with multiple pK
a
values or a group of receptors at different
pK
a
values have been used to improve their working range [
60
cial neural
networks (ANN) were developed to improve their dynamic range from pH 2.0 to
12.0 [
63
]. Such developments included optical pH sensor arrays [
64
,
65
]. Holo-
graphic sensors have advantages over other optical sensors since the characteristics
of the diffraction grating such as the angle of Bragg angle can be controlled using
arrangement of optical equipment and the nature of the laser light. Additionally, the
laser writing method described in this chapter can allow patterning nanostructures
such as carbon nanotubes, graphene and nanopillars [
66
,
67
]. Laser manufacturing
stands out as an ef
62
]. Arti
-
cient approach to mass produce optical sensors.
The silver-halide chemistry-based fabrication of holographic sensors requires
about 10 steps, and its reduction to 2
4 steps by alternative approaches such as
in situ size reduction of metal NPs can enable faster fabrication [
18
]. Another
critical step that will accelerate the interpretation of these sensors includes response
time, which should be achieved within a few seconds. The development of theo-
retical approaches to shorten the turnaround time will be helpful in the analysis of
any hydrogel-based sensor. Holographic pH sensors developed in this chapter can
be multiplexed with other holographic sensors on paper- or PDMS-based micro-
fl
-
fluidic devices or on contact lenses [
68
-
74
]. Since the readouts are colorimetric,
they can be quanti
ed by smartphones and wearable devices [
75
,
76
]. Additionally,
trials with clinical samples and comparison of the performance with commercial
sensors can prove the feasibility of holographic sensing for applications in medical
