Expanded porphyrin incorporated solvent polymeric membrane electrodes: protonation and interaction with an analyte anion at organic/water interface as studied by optical second harmonic generation and Fourier transform infrared attenuated total reflectanc

Optical second harmonic generation (SHG) at the surfaces of poly(vinyl chloride) (PVC) supported liquid membranes based on expanded porphyrins was measured to better understand the relationship between protonation of ionophores at the membrane surface and selective EMF responses by the membranes toward anionic analytes in low pH ranges. Sapphyrin and rubyrin were chosen as representative examples of the ionophores and 3,5-dinitrobenzoate as their primary analyte anion. The square root of SHG intensity I2ω, of the sapphyrin based membranes increased in sigmoidal two steps according to the decrease in pH in the adjacent aqueous phase in the absence of 3,5-dinitrobenzoate. This result was attributed to the change of protonation form of sapphyrin, i.e. unprotonated-sapphyrin, monoprotonated-sapphyrin and diprotonated-sapphyrin, respectively, apparent surface protonation constants (apparent surface pKa1, pKa2) for sapphyrin at the water/membrane interface were determined therefrom. Apparent surface pKa1 thus estimated for sapphyrin was found to be close to the pHs at which the corresponding membrane begun to exhibit EMF and SHG responses toward 3,5-dinitrobenzoate. Fourier transform infrared attenuated total reflectance (FTIR-ATR) measurements of the sapphyrin incorporated membrane showed that 3,5-dinitrobenzoate was transported into the membrane phase only in the pH range where sapphyrin is highly protonated at the water/membrane interface and formed a complex with protonated sapphyrin. From these SHG, FTIR-ATR and EMF results, it is concluded that the high degree of protonation of sapphyrin at the membrane surface is requisite for the generation of anionic EMF responses toward the primary analyte anion. Apparent surface pKa1 values estimated for sapphyrin in the sapphyrin and KTpClPB based membrane and for rubyrin in the rubyrin based membrane were also found to match the pHs below which the corresponding membranes exhibit EMF responses toward 3,5-dinitrobenzoate.