B.Hydrogel volume changes [4]. J. Shin et

B.Hydrogel sensors for
detecting of pH
One
of the most used applications of hydrogels is the detection of pH. In
response to the change of pH value, pH responsive hydrogels change
their Volume, elasticity and mass reversibly . In the past years many
pH responsive hydrogel sensing technologies are evolved 2. All
Technologies dramatically change their degree of ionization at a
specific pH. The change of the net charge generates a electrostatic
repulsive force between the ionized groups. By creating this osmotic
swelling force, the volume changes 4. J. Shin et al. 8 utilized
the inverse opal hydrogel structures to produce a fast responding pH
sensor. This inverse opal hydrogel sensor technology is fabricated
through colloidal templating photopolymerization of 2-hydroxyethyl
methacrylate and ethylene glycol dimethacrylate 2,8,9. Inverse opal
hydrogels with tunable diffraction wavelength have a wide variety of
analytes. Due to their response to pH they show response to metal
ions, protons, glucose, and volatile organic compounds 8,9. For the
fabrication of a mechanically robust and fast response photonic
crystal pH sensor, a modifying of the flow cell method is used. The
bottom substrate is a standard microscope slide glass with a 25 x
38mm2
size.
The top substrate got a 3mm hole at the center of the slide glass,
for the use of high speed mini bench drill. With this method a
colloidal crystal template structure is made by three different
polystyrene microspheres 8. After the photo-polymerization under UV
light the polystyrene crystal structure is removed by chloroform.
Now a UV-vis spectrometer coupled with a reflected light microscope
is used to perform a pH-dependent reflectance measurement 2,8.The
responsive behaviours to pH of this inverse opal hydrogel were
investigated by the measuring of diffraction wavelength in the
aqueous hydrogel solution with different pH 9.
This
type of sensor shows a response time of about 12s for pH variations
between 5 and 6. The sensor has a long lifetime of over 6 months
without loss of response time or reproducibility in pH-driven color
change 2,8.