An improved Severinghaus-type carbon dioxide microelectrode for use in biofilms

Abstract

Severinghaus-type carbon dioxide microelectrodes with a tip diameter of less than 20 m were constructed using anodically grown iridium oxide film (AIROF) as the internal pH sensor. The AIROF, which was formed at the tip of the iridium wire by cyclic voltammetry in diluted sulfuric acid, showed a super-Nernstian response to pH changes, with a slope between 65 and 80 mV/pH. Therefore, our microelectrodes were more sensitive to changes in carbon dioxide concentration than the previously described microelectrodes, which used liquid ion exchanger (LIX) pH membranes and had a Nernstian response of 59 mV/pH. When calibrated, our microelectrodes showed 65–76 mV per decade of µatm pCO2, and were more sensitive than the microelectrodes using LIX membranes, which showed 57 mV per decade of µatm pCO2. AIROF electrodes were serviceable after 1 month of storage, showing only a modest potential drift of 0.03 ± 0.01 mV/h. Their utility was demonstrated by measuring CO2 concentration profiles in Staphylococcus aureus (MN8) biofilms.