Chemistry Reference
In-Depth Information
approach is to synthesise monomer mixtures that incorporate metal NPs in organic
solvents (0.1
1 mg/mL) produced by pulsed laser ablation [
15
,
16
].
Another fabrication approach that has been explored in this thesis was to
synthesise porphyrin derivatives that function as the crosslinker, the light absorbing
material, and the component of a diffraction grating. The use of this multifunctional
porphyrin permitted rapid fabrication of a photonic structure, which diffracted
narrow-band light. The sensor was fabricated in Denisyuk re
-
ection mode, how-
ever, no NP formation was required. As opposed to physical size reduction of Ag
0
NPs; in photopolymerisation, the porphyrin molecule absorbs the laser light and
further polymerises the hydrogel matrix at the antinode regions. This creates a
refractive index contrast of alternating polymerised and highly polymerised layers.
Since photopolymerisation is NP-free, it substantially reduces the requirements for
fabrication and eliminates the bleaching issues [
17
fl
20
]. Photopolymerisation will
play greater roles in the optimisation of the performance of holographic sensors and
reduce batch to batch variability.
Printing techniques or the use of photomasks during fabrication can introduce
user-friendly fool-proof text/quantity-reporting capabilities. Printing of recording
media is an untapped way of depositing the monomer solution and photosensitive
materials on substrates. This approach has practicality, miniaturisation capacity and
scalability in the deposition of holographic materials and construction of holo-
graphic arrays. For example, various polymers can be loaded on a cartridge that can
be
-
fit to noncontact, contact or airjet dispensers. As the monomers immobilise on
the surface of the substrate at room temperature or through UV-initiated free-radical
polymerisation, holographic arrays consisting of different analyte-sensitive mate-
rials can be constructed with controlled size. Since this approach minimises the
required volume of the monomer solutions, it has the potential to enable mass
production. Printing can also allow depositing holographic sensors on implantable
chips and contact lens sensors [
21
]. Furthermore, holographic sensor fabrication is
based on the use of a planar mirror, which produces an angular intolerant hologram
that limits the angle of view to
. Diffusers or lenses can be used to improve the
angular tolerance; however, holograms produced by this method have low-dif-
fraction ef
±
10
°
ciency (<5 %) [
22
]. Additionally, digital holography (no real object
requirement) may increase the diffraction ef
ciency, and allow multiplexing
through superpositioning of images [
23
,
24
].
7.2 Ligand Chemistry
This thesis described the ligands for holographic pH [
1
,
2
], metal cation [
3
] and
glucose sensors [
4
]. In sensing pH, the pHEMA matrix incorporated carboxylic acid
groups. When ionisable groups are copolymerised with other monomers, their
individual functions dictate the degree of pH-sensitivity of the pHEMA matrix. The
holographic pH sensors were tested in phosphate buffers and arti
cial urine to show
their putative clinical application. The Bragg grating was
finely modulated to
