These changes are completely
reversible with variation in the pH. The spectral characteristic
of this sensor showed two maxima at 437 and 621 nm in
basic media related to DPA and VB, respectively. Thus,
these wavelengths were selected for further studies. It is
important to note that the absorption spectra of the immobilized
dyes are red shifted in comparison to those of their
soluble forms (almost 10 nm at two maximum wavelengths).
This can suggest that the structured conformation of the
immobilized dyes is more planar than that of its soluble
analogue [7]. The optical response of the sensor at two
maximum wavelengths is shown in Fig. 3. Fig. 3a and b
shows that the optode can be used for very low and high pH
values within 3 pH units in each extreme pH values.