Chemistry Reference
In-Depth Information
diffract narrow-band light based on the volumetric changes of the matrix, Ag
0
NP
spacing, and index of refraction. When the pendant functional groups were ionised
(e.g. deprotonating carboxylic acid groups), the polymer swelled due to the elec-
trostatic and Donnan osmotic pressure forces that drew in or expelled counterions
along with water molecules, which altered the periodicity (lattice spacing) of the
diffraction grating. An increase or decrease in the lattice spacing shifted the
diffracted light to longer or shorter wavelengths. This systematic lattice modulation
consequently allowed a quantitative readout through wavelength changes of the
diffracted light, enabling spectroscopic measurement of colour changes as a func-
tion of pH. These sensors produced visually perceptible and reversible colour
changes at either side of the apparent dissociation constant (pK
a
) as a function of the
pH. The holographic sensors diffracted narrow-band light from the visible region to
the near-infrared region (
815 nm). The clinical application of the
diffraction gratings was demonstrated by pH sensing of arti
495
λ
peak
≈
-
cial urine over the
physiological range (4.50
9.00), with a sensitivity of 48 nm/pH unit between pH
5.0 and 6.0. As the pH was increased, the Bragg peak shifted to longer wavelengths
while the intensity decreased. This was attributed to a decrease in the effective
refractive index contrast since the expanding structure lowered the density of Ag
0
NPs present within a given volume. This was predicted by the model; both simu-
lations and experimental readouts showed agreement about the Bragg peak shift of
the diffracted light based on the lattice expansion and contraction.
In addition to holographic sensors, several advances in pH sensing have been
demonstrated. Electrochemical and
-
field-effect transistor based pH sensors have
utilised carbon
bre [
25
] and carbon/metallic [
26
] nanostructures, respectively.
Recent
rster resonance energy transfer using syn-
thetic DNA [
27
], genetically encoded red protein [
28
] and an antibody-conjugated
pH dependent dye [
29
] have been employed for intracellular monitoring. These
sensing mechanisms have selectivities down to 0.01 pH units from pH 2.0 to 12.0
[
30
]. Holographic sensors have a comparable accuracy to these sensors in the range
from pH 5.0 to 6.0; however, the dynamic range of holographic sensors needs to be
improved by incorporating different functional groups to induce a Bragg shift in the
desired dynamic range. The range of the pH sensitivity can be tuned by selecting
the desired acidic or basic monomers to cover the pH range of the application of
interest. For example, other functional groups can be chosen from tri
fl
uorescent sensors based on F
ö
uoromethyl
propenoic acid (TFMPA), dimethylaminoethyl methacrylate (DMAEM), and vinyl
imidazole to achieve a pH range from 2.0 to 9.0. In comparison to other colori-
metric sensors, for example, pH dependent dye-based sensors can only be used
once, while holographic sensors have the capability of being used for multiple
non-consumptive analyses. As opposed to electrochemical sensing, holographic
sensors do not require external power to operate for visual readouts. Additionally,
since the laser light forms the image of a planar mirror, the resulting photonic
structure produces an unidirectional diffraction, allowing readouts up to a meter
away. Dyes and
fl
fluorescence risk losing distinct signal, and remote wireless
readings are not feasible.
fl
