A Carbon Dioxide Microelectrode for in Situ ρCO2 Measurement

Abstract

A microelectrode is described which permits a nondestructive method for measuring ρCO2 in living cells or small samples. Our objective was to develop an easily fabricated ρCO2 microelectrode that would permit analysis of small samples and withstand insertion into the soil. We developed a ρCO2 microelectrode by making use of a newly developed micro-pH electrode (MEPH4, WPI) and a semiliquid chlorotrifluoroethylene oil-wax gas permeable membrane. The fabricated microelectrode has a tip diameter of ≤120 μm (o.d.) and a response time of ≤3 min with a linear (nearly Nernstian) slope (58.0 ± 2 mV/log10 ρCO2). The ρCO2 microelectrode response is independent of the test solution pH and electrolytes. Measurements of ρCO2 with the microelectrode compared favorably with those from a standard ρCO2 macroelectrode (model 95-02, Orion). and the regression line had a slope of 1.2 and r = 0.98. This microelectrode is simple to fabricate, and the tip diameter is sufficiently small (± 120 μm) to permit measurement of CO2 in small sample volumes. The first direct potentiometric determinations of rhizosphere (soil-root interface) ρCO2 were carried out to demonstrate the performance of the microelectrode. Rhizoplane (<1 mm from the root) ρCO2 values at the five leaf stage were higher for Sordan-757 (9.14 ± 0.39 and 11.33 ± 0.28 kPa) than for Sordan-333 (7.15 ± 0.57 and 9.55 ± 0.30 kPa) on low and high CaCO3 soils, respectively. The ρCO2 microelectrode is an improved technique for examining the root-soil interface and other biological microenvironments.