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such as 2-(acrylamido)phenylboronate
uor-
ophenylboronic acid (5-F-2-MAPBA) [
38
], 4-vinylphenylboronic acid (4-VPBA)
[
39
], and their combinations with other comonomers [
40
,
41
] can be used to
improve speci
(2-APB)
[
37
], 2-acrylamido-5-
fl
city to glucose.
Like other hydrogel-based optical sensors, holographic sensors suffer from low
selectivity, which stands out as an issue for the development of products that can
compete with molecular dyes, electrochemical sensors and lateral-
ow assays.
Therefore, the emphasis on the functionalisation and the optimisation of holo-
graphic recording media needs to be increased. This requires further investigations
in the design of new receptors that can selectively bind to target analytes. Under-
standing the fundamentals of binding kinetics and reversibility will allow
constructing assays with improved control for real-time continuous monitoring
applications. Studies on hydrogel dynamics and characteristics including expan-
sion, shrinkage, diffraction ef
fl
ciency, control over NP size distribution, hydrogel-
analyte interactions, surface energy, release characteristics, reversibility, control of
hydrogel pore size, polymer decay, effect of porous and solid nanodopants, and
effect of temperature and moisture will be explored in the realisation of holographic
sensors. The quality and shelf life of the holographic sensor after long-term storage
also require investigations.
7.3 Multiplexing Holographic Sensors with Microfluidic
Devices
This thesis also described the initial steps towards using holographic sensors in the
fl
flake form and their integration with strip substrates. Paper matrices and nitrocel-
lulose membranes were utilised as wicking substrates to produce test strips. Three
techniques were developed to reduce the background noise due to re
ection from
the substrate: (i) Impregnating paper with Fe
3
O
4
, (ii) dyeing chromatography and
fl
filter papers to black with Procion Black MX-K, and (iii) dyeing nitrocellulose
membranes with DEKA-L fabric dyes. The resulting strip membranes were used to
wick and deliver the sample to holographic pH sensor
flakes to produce visual
colour changes. The test strips can be patterned by printing wax and be assembled
with lamination sheets
fl
to form multiplex paper-based micro
fl
uidic assays
(Fig.
7.1
a
d). In constructing high-throughput devices, any colorimetric sensor can
be multiplexed; however,
-
they require physical separation, multiple dyes and
fl
fluorophores, which are (i) not directional, (ii) work in different wavelength ranges,
(iii) may have cross talk either chemically or optically. Hydrogel-based sensors in
the present work can be multiplexed such that these complications are avoided.
Furthermore, in sensing glucose, the pH of the urine was corrected to 7.40 using an
alkaline solution, which might not be feasible at point-of-care settings. Addition-
ally, the holographic sensors can be affected from the variation in ionic strength;
therefore, pH and ionic strength sensors may be multiplexed with other sensors
