Stability Enhancement of Galvanic Redox Potentiometry by Optimizing the Redox Couple in Counterpart Poles

Potentiometry based on the galvanic cell mechanism, i.e., galvanic redox potentiometry (GRP), has recently emerged as a new tool for in vivo neurochemical sensing with high neuronal compatibility and good sensing property. However, the stability of open circuit voltage (E_OC) outputting remains to be further improved for in vivo sensing application. In this study, we find that the E_OC stability could be enhanced by adjusting the sort and the concentration ratio of the redox couple in the counterpart pole (i.e., indicating electrode) of GRP. With dopamine (DA) as the sensing target, we construct a spontaneously powered single-electrode-based GRP sensor (GRP_2.0) and investigate the correlation between the stability and the redox couple used in the counterpart pole. Theoretical consideration suggests that the E_OC drift is minimum when the concentration ratio of the oxidized form (O₁) to the reduced form (R₁) of the redox species in the backfilled solution is 1:1. The experimental results demonstrate that, compared with other redox species (i.e., dissolved O₂ at 3 M KCl, potassium ferricyanide (K₃Fe(CN)₆), and hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)) used as the counterpart pole, potassium hexachloroiridate(IV) (K₂IrCl₆) exhibits better chemical stability and outputs more stable E_OC. As a result, when IrCl₆^2-/3- with the concentration ratio of 1:1 is used as the counterpart, GRP_2.0 displays not only an excellent E_OC stability (i.e., 3.8 mV drifting during 2200 s for in vivo recording) but also small electrode-to-electrode variation (i.e., the maximum E_OC variation between four electrodes is 2.7 mV). Upon integration with the electrophysiology, GRP_2.0 records a robust DA release, accompanied by a burst of neural firing, during the optical stimulation. This study paves a new avenue to stable neurochemical sensing in vivo.