Chemistry Reference
In-Depth Information
Chapter 3
Holographic pH Sensors
Dynamic photonic structures can be modulated by changing the periodic structure
and/or the index of refraction [
1
3
]. These dynamic photonic structures allow
responsive capability for sensing external stimuli to control the properties of light
and act as optical transducers [
4
,
5
]. Dynamic optical systems operating in the
visible and near-infrared region offer promise for designing adaptive materials and
sensors. Such devices have been prepared by microfabrication, self-assembly or a
combination of both [
6
-
16
]. However, achieving the attributes of a narrow-band
response with a wide operating range to construct optical sensors in a few steps in
hydrophilic polymers still remains a challenge. This chapter describes the con-
struction of holographic pH sensors by silver chemistry and laser ablation induced
in situ size reduction of Ag
0
NPs in hydrogel matrices using Denisyuk re
-
fl
ection
holography [
17
19
]. The holographic sensor consists of chemical-stimuli respon-
sive hydrogels with reversible narrow-band tuneability using Ag
0
NPs that are
organised in density-concentrated 3D regions. The optical characteristics of these
sensors were investigated by analysing the distribution of the mean diameter of Ag
0
NPs, effective refractive indices of patterned polymer-NP regions, and angular-
resolved measurements. The clinical utility of the sensor for the quanti
-
cation of
pH in arti
cial urine was demonstrated. The chapter also shows strategies for
fabricating holographic
fl
flakes and paper-based holographic sensors.
3.1 Holographic pH Sensors via Silver-Halide Chemistry
The first step in the fabrication of holographic sensors is the preparation of a
substrate, where the hydrogel matrix is deposited. 3-(Trimethoxysilyl)propyl
methacrylate in acetone (1:50, v/v) was poured onto glass microscope slides in an
aluminium tray [
17
,
20
]. After thorough coating, the excessive acetone/silane mix
was poured off whilst slides remained in situ due to surface tension. The slides
stored in the tray overnight in the dark, before removal and dark storage at room
temperature (Fig.
3.1
a). A monomer solution consisting of HEMA (91.5 mol%),
EDMA (2.5 mol%) and MAA (6 mol%) was prepared [
17
,
18
,
20
]. The solution
