Detrimental changes in the composition of hydrogen ion-selective electrode and optode membranes

Derivatives of phenoxazine dyes serve as chromoionophores in the field of chemical sensors. By studying the changes of the distribution of membrane components in ETH 5294 based, pH sensitive, ion-selective membranes using spectroelectrochemical microscopy (SpECM), the decay in the concentration of the protonated chromoionophore was generally much larger than expected upon previous studies. These recorded changes could not be interpreted with the leaching of the protonated or unprotonated forms of ETH 5294 or the tetraphenyl borate derivatives embedded in the membrane. Similarly, the changes could not be explained by the photochemical decomposition of the tetraphenyl borate derivatives (tetraphenyl borate, TPB− or tetrakis(4-chlorophenyl) borate, TpClPB−). The decrease in the absorbance value measured at 660 nm, at the absorbance maximum of the protonated chromoionophore has been linked to the photochemically initiated, singlet oxygen mediated decomposition of ETH 5294. The decomposition occurs only if the wavelength of the illuminating light is larger than ∼580 nm, and the rate of decomposition is facilitated by the presence of certain anions in the membrane (TPB−, and TpClPB−) and in the sample solution (Br−, I−, and Cl−) as they were penetrating the membrane by acid coextraction. On the other hand, the protonated chromoionophore was found to be stable in membranes cast with tetrakis[3,5-bis(trifluoromethyl) phenyl] borate anion (TFPB) or in acidic solutions of NO3−, ClO4−, SO4−, TFPB− and citrate anions. In comparison to ETH 5294 (chromoionophore I), ETH 2439 (chromoionophore II) was found completely stable, and the decomposition of ETH 5350 (chromoionophore III) was also hardly detectable. In our view, in the light of the experimental results summarized in this study some of the widely accepted views and practices in chromoionophore based sensors research should be changed and some of the results should be reevaluated.