Surface and electrochemical studies of a carbon dioxide probe based on conducting polypyrrole Bernard John V. Tongol a,1 , Christina A. Binag a,b,* , Fortunato B. Sevilla III b,c a Graduate School, The University of Santo Tomas, Espan ˜a, 1008 Manila, Philippines b Research Center for the Natural Sciences, The University of Santo Tomas, Espan ˜a, 1008 Manila, Philippines c College of Science, The University of Santo Tomas, Espan ˜a, 1008 Manila, Philippines Abstract 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 ¼ 3:6 min for three replicates (n ¼ 3). For the CO 2 probe design, the PPy/HCO 3  -coated Pt electrode was coupled with a miniaturized Ag/AgCl reference electrode. A thin layer (0.5 mm) of 300 ml of 0.001 M NaHCO 3 internal electrolyte is sandwiched between the two electrodes and the Teflon gas permeable membrane. The optimized CO 2 probe exhibited good potentiometric properties (linearity ¼ 8:78  10 5 to 2:70  10 3 M CO 2 with r ¼0:993; m ¼47:42 mV/decade [CO 2 ] with RSD ¼ 17:04% at n ¼ 3; LOD ¼ 3:93  10 5 M CO 2 and t r ¼ 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 CO 2 probe components: PPy/HCO 3  film and Teflon gas permeable membrane. This study highlights the use of these surface-sensitive techniques to understand the electrochemical behavior of the fabricated CO 2 probe. A correlation was made between the surface characteristics of the sensor components with the low reproducibility and repeatability of the sensor responses. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Electrochemical polymerization; Carbon dioxide sensor; SEM; XPS 1. Introduction The simplest way for detecting CO 2 is with the use of a potentiometric gas-sensing electrode. The Severinghaus elec- trode consists of a pH-sensitive glass electrode, a Ag/AgCl reference electrode, and an intermediate NaHCO 3 internal electrolyte layer, which is separated from the sample solution by a gas permeable membrane. Carbon dioxide diffusing through the membrane forms carbonic acid which dissociates and alters the pH of the internal electrolyte. Hydrolysis inside the electrode proceeds according to Eq. (1). The changes in pH are detected by the glass electrode [1] CO 2ðaqÞ þ H 2 O Ð H þ þ HCO 3  (1) The classical Severinghaus electrode has met with little success since the sensor is too expensive and cumbersome for miniaturization and has a relatively slow response. Particularly, the internal glass pH electrode employed suffer distinct limitations due to its fragility, lack of required detection for certain assays (e.g. ammonia in blood) and major interferences from other volatile and nonvolatile acids or bases that may be present in the sample [2]. The discovery of conductive electroactive polymers such as polypyrrole (PPy) heralded a new era in electrochemical research [3]. Thus, the application of conducting polymers in pH sensor devices has gained considerable interest in the recent years. Several papers have reported on the develop- ment and use of conducting polymers as pH-indicator electrodes: polyaniline (PAn) [4], poly(o-phenylenedia- mine) (PoPD) [5], poly(3-methylthiophene) (P3MTp) [6], and PPy [4,7]. Among the conducting polymers, PPy has been extensively studied because of its electroactivity, ease of preparation, and high porosity [8]. The pH-sensitivity of PPy has been attributed to the reversible protonation and Sensors and Actuators B 93 (2003) 187–196 * Corresponding author. Present address: Graduate School, The University of Santo Tomas, Espan ˜a, 1008 Manila, Philippines. Fax: þ632-731-4031. E-mail address: chrabina@ustcc.ust.edu.ph (C.A. Binag). 1 Present address: Faculty of Pharmacy, The University of Santo Tomas, Manila, Philippines. 0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00180-1