Chemistry Reference
In-Depth Information
thickness of 10
2 mol%) of TACPP 2 may be
preferred for designing more rigid polymers. Fabrication of diffraction grating
consisted of passing a laser beam through a pHEMA-co-TACPP matrix, which was
aligned with an object (i.e. planar mirror) behind. In
±
2
µ
m. Higher concentrations (1
−
“
Denisyuk
”
re
fl
ection mode,
incidence and re
ected beams formed constructive wave interferences [
16
,
17
]. The
pHEMA-co-TACPP matrix was selectively patterned at the antinodes of the
standing wave, where the energy of the laser light was maximum, to form a
photonic structure, which acted as a narrow-band Bragg re
fl
ector. The laser energy
absorbed by the pHEMA-co-TACPP matrix depends on the laser wavelength,
power, pulse duration, dye
fl
s thermal (absorption), physical and chemical charac-
teristics. The photonic structure consisted of a periodic grating organised roughly
half of the wavelength of the laser light that was used to produce the grating. When
the system was exposed to the laser light, the pHEMA matrix was further cross-
linked with the help of the TACPP molecules at the antinodes of the standing wave.
UV-Vis absorption spectrum shows TACPP 2 molecules in THF with peaks at
422 nm (Soret band) and 453 nm (Fig.
4.2
a). UV-Vis absorption spectra showed
that TACPP 2 molecules covalently bound to pHEMA matrix before and after laser-
induced photochemical patterning (Fig.
4.2
a). The lattice spacing and the refractive-
index contrast of the sensor was modulated by external stimuli, which systemati-
cally shifted the Bragg peak from shorter to longer wavelengths when the hydrogel
expanded in the direction normal to underlying substrate (Fig.
4.2
b).
'
Fig. 4.2 Characterisation and principle of operation of a porphyrin-functionalised holographic
sensor. a TACPP 2 in UV-Vis spectroscopy showing peaks at 422 nm (Soret band) and 453 nm.
TACPP 2 molecules covalently bound to pHEMA matrix before and after laser-light exposure.
b The sensor comprised pHEMA (1) crosslinked with TACPP (2) was immediately ready to sense
the target (3). Swelling of the sensor in response to external stimuli shifted the Bragg peak from
shorter to longer wavelengths as the pHEMA-co-TACPP matrix expanded normal to its underlying
substrate. Reproduced from Ref. [
1
] with permission from The Royal Society of Chemistry
