Fig. 8 shows the color changes of TPTZ solutions after reacting
with Fe2+, Cu2+, Ni2+ and Co2+ ions. It is worth noting that these ions
showed interfering response to TPTZ at certain concentrations in
the solution state whilst with immobilization of TPTZ in the sol–
gel matrices and control of the variables of the preparation procedure
to tailor the network structure and pore size, tolerance limits
increased and a good selectivity can be observed for iron.