Chemistry Reference
In-Depth Information
4.2 Optical Readouts
4.2.1 Organic Solvents in Water
To demonstrate the tuneability of the porphyrin-based sensor, its response to
ethanol, methanol, propan-2-ol and dimethyl sulphoxide (DMSO) in DI water
(0.0
−
10.0 %, v/v) was investigated. An increase in the solvent concentrations
systematically swelled the hydrogel matrix. Figure
4.3
a shows a typical Bragg peak
shift from shorter (
620 nm) wavelengths as a function of
solvent concentration (e.g. ethanol). As the lattice spacing increased, the diffraction
ef
520 nm) to longer (
*
*
ciency of the periodic structure decreased. This may be attributed to the decrease
in the effective refractive index contrast in the expanding hydrogel matrix. The
expanding photonic structure displayed a colorimetric response in the visible
spectrum throughout the measurement (e.g. ethanol, Fig.
4.3
b). The sensor response
to solvents was fully reversible and rapid with 90 % of the equilibrium response
measured within 1 min. Figure
4.3
c illustrates the Bragg peak shifts as a function of
ethanol, methanol, propan-2-ol and DMSO concentrations over three trials, which
exhibited
3 % variation. The Bragg peak shifts for 10.0 % (v/v) ethanol, methanol,
propan-2-ol and DMSO were 100, 48, 162 and 56 nm, respectively. Consecutive
swelling/shrinking processes were reproducible over 30 successive changes without
hysteresis. Response time and recovery times for 10.0 % (v/v) ethanol, methanol,
propan-2-ol and DMSO were
±
50,
60 s;
30,
40 s;
50,
50 s; and
10,
*
*
*
*
*
*
*
20 s, respectively (Fig.
4.3
d). The water-alcohol solutions penetrated into the
hydrogel matrix and caused swelling, which increased with the alkyl chain length of
the alcohol [
18
]. In this respect, for example, water-ethanol mixture was more
effective than water-methanol mixture of the same concentration. This may be
attributed to an increase in hydrophilicity due to the increase in alkyl chain length.
The sensor response to pure organic solvents was also investigated. Bragg peak
shifts were recorded at
*
−
−
38,
24 and 37 nm from a starting peak wavelength of
*
520 nm for pure DCM, chloroform and THF, respectively. DCM and chloroform
caused a contraction. This may be attributed to the capacity of DCM and chloro-
form to expel the water molecules (RH 60 %) from the pHEMA matrix.
4.2.2 Quantification of Cu
2+
and Fe
2+
Ions in Aqueous
Solutions
The pK
a
values of mesoporphyrins are <6.5 [
19
]. Upon changing the pH of the
pHEMA-co-TACPP matrix from 7.0 to 3.0, the absorption peak shifted from
420 nm (pH 7.0) to 425 nm (pH 3.0) (Fig.
4.4
a). The apparent pK
a
values of the
pHEMA-co-TACPP matrix were
6.59 determined using the
Henderson-Hasselbalch equation (Fig.
4.4
b). A typical porphyrin molecule has four
pyrrole rings linked via methine bridges, resulting in a stable structure and aromatic
3.14 and
*
*
