A dual electrochemical microsensor for simultaneous imaging of oxygen and pH over the rat kidney surface

Abstract

In this study, a dual microsensing electrochemical probe for measuring oxygen (O₂) and pH levels was developed based on a dual recessed Pt disk electrode (each disk diameter, 10 μm) with the use of two Ag/AgCl reference electrodes (one for each disk of the dual electrode). One of the recessed Pt disks of the dual electrode was electrodeposited with a porous Pt layer and then coated with a hydrophobic photocured polymer (partially fluorinated epoxy diacrylate, abbreviated as FED). The Pt-FED covered disk was used as an amperometric O₂ sensor and exhibited a linear current increase that was proportional to the P_O2 level (partial O₂ pressure) with high sensitivity (168.4 ± 33.8 pA mmHg⁻¹) and fast response time (t_90% = 0.17 ± 0.05 s). The other recessed Pt disk was electrodeposited with an IrO₂ layer. The potential between the IrO₂ deposited electrode and the Ag/AgCl reference electrode produced a reliable Nernstian response to pH changes (58.3 ± 1.5 mV pH⁻¹) with a t_90% of 0.43 ± 0.09 s. The sensor displayed high stability in the in vitro organ tissue measurements for at least 2.5 h. By using the developed dual O₂/pH microsensor as a probe tip for scanning electrochemical microscopy, the two-dimensional images of the location-dependent P_O2 and pH levels were simultaneously acquired and could be used to assess the surface of a rat kidney tissue slice. When compared to the corresponding medullary levels, both P_O2 and pH were observed to be higher in the cortex area, while the modest level gradient was observed near the cortex–medulla border. This finding suggests that there is a direct relationship between the tissue O₂ supply/consumption and pH, which is mainly determined by metabolite, such as CO₂, production.