Lipophilic tetramethylpiperidine N-oxyl (TEMPO) as a phase-transfer redox mediator in thin films for anion and cation sensing

Abstract

This study successfully demonstrates the use of a lipophilic derivative of TEMPO, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate, as a redox mediator for ion transfer processes at the interface between two immiscible electrolyte solutions. The results prove that TEMPO exhibits outstanding performance for mediating anion and cation transfer across polymeric ion-selective membranes doped with lipophilic ion-exchangers. The redox reaction at the inner interface of the electrode was controlled by cyclic voltammetry and allowed to modulate the flux of different ions according to the Hofmeister selectivity series. The behavior of this system was shown to agree well with the theory combining redox and phase-boundary potentials by comparing experimental and calculated cyclic voltammograms. Model ions were used to show the ideal Nernstian shift of the peak position while changing the phase-boundary potential at the outer interface when ion activity is varied in the aqueous phase. We also showcase that limitations from electrochemical instabilities in anion transfer observed using other redox probes are now overcome.