Abstract
A simple method to measure dissolved CO2 at a microscale would be beneficial for many scientific and medical applications. A simple amperometic microsensor for CO2 with a 20–100 μm wide tip was developed by placing a layer of acidic O2 trap solution containing Cr2+ in front of a Clark-type electrochemical sensor. The Clark-type sensor contains a Ag cathode in an ionic liquid, and also a Ag guard cathode behind the sensing cathode to prevent interference from reducible contaminants in the ionic liquid. The constructed sensors exhibited linear response over relatively large intervals of CO2 partial pressure, but for CO2 partial pressures of < 20 Pa the response was only about 60% of that observed at 20–1000 Pa. The slope of the calibration curve at 2–6 kPa was about 80% of the slope from 0 to 1 kPa. A high baseline signal caused by water diffusing through the membrane into the ionic liquid was avoided by making very conical sensors so that the water concentration around the cathode was kept low by diffusional transport into the bulk reservoir of ionic liquid, but elevated zero currents by a factor of about two was often observed for sensors more than 1 month old. The lifetime of the sensor by continuous operation can be more than 4 months, but with a slow decrease in sensitivity that may be caused by lower membrane permeability. Use of the sensor was demonstrated by measuring CO2 dynamics in the thallus of a red algae along with variations in O2 and pH.
Highlights
All living organisms convert carbon dioxide, by autotrophic consumption or heterotrophic production, and measurement of CO2 concentration changes are essential in many scientific and medical studies
The simplest CO2 measuring equipment based on gas permeable membranes are sensors where CO2 diffuses through the membrane and into a stagnant layer of buffer where the change in pH is measured by an electrochemical sensor (Severinghaus sensors) [3], or the change in pH results in protonation or deprotonation of an organic buffer that can be determined by optical means (CO2 optodes [4,5])
Microscale sensors are especially suited for measurement inside stagnant matrices such as plant or animal tissue, and we demonstrated use of the sensor by measuring CO2 dynamics inside the thallus of the red algae Gracilaria vermiculophylla
Summary
All living organisms convert carbon dioxide, by autotrophic consumption or heterotrophic production, and measurement of CO2 concentration changes are essential in many scientific and medical studies. Determination of dissolved CO2 in liquid samples is more difficult, but several methods have been developed that are based on diffusion of dissolved CO2 through a gas permeable membrane and resulting change in pH or conductivity in a medium behind this membrane [2]. The simplest CO2 measuring equipment based on gas permeable membranes are sensors where CO2 diffuses through the membrane and into a stagnant layer of buffer where the change in pH is measured by an electrochemical sensor (Severinghaus sensors) [3], or the change in pH results in protonation or deprotonation of an organic buffer that can be determined by optical means (CO2 optodes [4,5]). Addition of an degradable protein as carbonic anhydrase may, lead to a short lifetime of the sensor
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