Surface and electrochemical studies of a carbon dioxide probe based on conducting polypyrrole

This paper reports on the development of a Severinghaus carbon dioxide probe using polypyrrole (PPy) as the pH-indicator electrode. The electrochemically synthesized bicarbonate-doped PPy gave satisfactory potentiometric properties as pH-indicator electrode: sub-Nernstian response with a slope, m, of -39.06 mV pH(-1) at buffer solutions of pH 3-10, excellent linearity, r, of -0.999, and relatively fast response time, average t(r) = 3.6 min for three replicates (n = 3). For the CO2 probe design, the PPy/HCO3--coated Pt electrode was coupled with a miniaturized Ag/AgCl reference electrode. A thin layer (similar to0.5 mm) of 300 mul of 0.001 M NaHCO3 internal electrolyte is sandwiched between the two electrodes and the Teflon gas permeable membrane. The optimized CO2 probe exhibited good potentiometric properties (linearity = 8.78 x 10(-5) to 2.70 x 10(-3) M CO2 with r = -0.993; m = -47.42 mV/decade [CO2] with RSD = 17.04% at n = 3; LOD = 3.93 x 10(-5) M CO2 and t(r) = 4.71 min) when used only in one calibration measurement. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques were used to study the surfaces of the CO2 probe components: PPy/HCO3- film and Teflon gas permeable membrane. This study highlights the use of these surface-sensitive techniques to understand the electrochemical behavior of the fabricated CO2 probe. A correlation was made between the surface characteristics of the sensor components with the low reproducibility and repeatability of the sensor responses. (C) 2003 Elsevier Science B.V. All rights reserved.