Chemistry Reference
In-Depth Information
Chapter 4
Holographic Metal Ion Sensors
The quanti
cation of metal ions has applications in medical diagnostics, veterinary
screening and environmental monitoring. This chapter describes the development of
a holographic metal ion sensor through photopolymerisation. In contrast to the
nanoparticles (NPs) in silver halide chemistry, porphyrin molecules were chosen for
the construction of metal NP-free holographic sensors. A porphyrin derivative with
acrylate groups was synthesised to crosslink 2-hydroxyethyl methacrylate mono-
mers [
1
]. The porphyrin derivative also served as the light-absorbing material and
cation chelating agent. A single pulse of a Nd:YAG laser (
λ
= 532 nm, 6 ns,
350 mJ) in Denisyuk re
ection holography mode allowed formation of Bragg
diffraction gratings within the porphyrin cross-linked polymer matrix. Holographic
sensors had a reversible narrow-band tuneability within the visible spectrum to
report on organic solvents in water as a proof of concept, and concentrations of
metal cations such as Cu
2+
and Fe
2+
in aqueous media. The quanti
fl
cation of Cu
2+
ions has a potential application in the diagnosis of Wilson
s disease, a genetic
disorder in which copper accumulates in the tissues [
2
]. Similarly, the measurement
of Fe
2+
ions may help the diagnosis of hemochromatosis, hemolytic anemia,
paroxysmal nocturnal hemoglobinemia, and impaired biliary clearance [
3
].
The feasibility of incorporating chelating agents has been demonstrated in
crystalline colloidal arrays [
4
'
6
]. In holographic sensors fabricated through the
silver halide chemistry, incorporation of chelating agents into recording media was
investigated to develop ion-selective hydrogel matrices [
7
]. To incorporate crown
ethers in holographic sensors, functional groups allowing co-polymerisation of
crown ethers into the polymer matrices were needed [
7
]. These studies involved the
synthesis of methacrylate esters of homologous series of hydroxyether crown ethers
and their copolymerisation with hydroxyethyl methacrylate in the presence of a
crosslinker (i.e. ethylene dimethacrylate) to form a chelating hydrogel matrix. The
crown ethers tests included 12-crown-4, 15-crown-5, and 18-crown-6 pendant
functionalities, and they responded alkali and alkaline earth ions with varying
speci
-
city [
7
]. For example, holograms comprising of 18-crown-6 linearly
responded to K
+
ions over the physiological range while the readouts were not
affected by physiological variations in background Na
+
ion concentrations (
130
*
−
150 mM). The optimised hologram containing 18-crown-6 (50 mol%) showed
