Fluoride-selective sensors based on polyurethane membranes doped with Zr(IV)-porphyrins

The feasibility of using Tecoflex polyurethane as a polymeric matrix for fluoride-selective membranes doped with Zr(IV)-octaethyl-(OEP) or Zr(IV)-tetraphenylporphyrins (TPP) is examined. Membranes containing cationic or anionic additives were prepared, with ionophore working according to neutral or charged carrier mechanism, respectively. Results are compared to those found previously using conventional poly(vinyl chloride) (PVC) as the membrane matrix. It was found that this polymer does not affect significantly the properties of these porphyrins, compared to poly(vinyl chloride) matrix. A dimer-monomer equilibrium determined recently to occur for Zr(IV)-porphyrins in PVC/o-NPOE membranes containing lipophilic anionic additives is also observed to occur (via UV-vis spectrophotometry) in the PU matrix. However, the equilibrium constants for dimer-monomer reactions appear to be lower in PU membranes compared to PVC films, as determined from the degree of super-Nernstian responses towards fluoride as well as the anion concentration ranges required to break the dimer as determined spectroscopically. Due to reduced dimerization of Zr(IV)[OEP]Cl₂ it was possible to obtain electrodes with PU/o-NPOE/KTFPB membranes exhibiting only slightly super-Nernstian (−64.6 mV/dec) response towards fluoride and response time (t₉₅ < 120 s) faster than observed for PVC-based membranes. Good working parameters were also obtained for this metalloporphyrin in PU membrane that forces neutral carrier mechanism (PU/DOS/TDMACl): F⁻ calibration slope −58.3 mV/dec and response time t₉₅ < 12 s. Tested membranes were subsequently applied for construction of miniaturized silicon-based sensors. Better fluoride selectivity was observed for sensors with Zr(IV)[OEP]Cl₂/PU/o-NPOE/KTFPB membranes (: ClO₄⁻ 0.7; Br⁻ −1.9; NO₃⁻ −1.9; Cl⁻ −3.1), compared to Zr(IV)[OEP]Cl₂/PU/DOS/TDMACl matrix (: ClO₄⁻ −0.8; Br⁻ −1.3; NO₃⁻ −1.5; Cl⁻ −2.1). However, latter composition was chosen to be better for flow measurement mode, as dimer formation can be totally prevented within this membrane. Sensors with Zr(IV)[OEP]Cl₂/PU/DOS/TDMACl maintained their characteristics at least for 2 months.