Emeraldine Base Thin Film Carbon Dioxide Sensor
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Emeraldine Base Thin Film Carbon Dioxide Sensor Mihai Irimia-Vladu1 and Jeffrey W. Fergus1 1 Auburn University, Materials Research and Education Center, 275 Wilmore Laboratories, Auburn, AL 36849, U.S.A. ABSTRACT A thin film of emeraldine base polyaniline in NMP was cast on an interdigitated electrode and its conductivity was investigated by impedance spectroscopy. The thin film responded to carbonic acid solutions of various pHs lower than 5. In general, an emeraldine base (EB) to emeraldine salt (ES) transformation occurs by protonation when the pH is less than 4. In the present case, the stages that occur prior to the onset of changes in total conductivity are detected by impedance spectroscopy. The sensor output was stable and reversible over a period of 3 months of testing.
INTRODUCTION Respiration, or carbon dioxide evolution, is a universal indicator for many biological processes, so the determination of carbon dioxide evolution can be used to monitor biological activity, such as bacterial growth in contaminated food. The sensor described herein consists of a thin emeraldine base-polyaniline (EB-PAni) film. The sensing mechanism is based on intermediate stages of transformation of emeraldine base polyaniline (EB-PAni) to a conductive salt type (ES-PAni) following exposure to a protonic acid. Carbonic acid, being a very weak acid, is unable to induce a conductivity change, but the intermediate steps that take place before the onset of this transformation are detected by impedance spectroscopy even when the total conductivity of the film is unchanged.
EXPERIMENTAL DETAILS Emeraldine base polyaniline with an average molecular weight of 5000 was purchased from Aldrich and treated in a 0.1 M NH4OH aqueous solution for 4 hours. The mixture was filtered through filter paper No1 (Watman Int.), then washed with 4 portions of 100 ml of water and the resulting cake was dried for 24 hours at 40 ºC in a vacuum oven. The 3% EB-NMP stock solution was produced by slowly adding 0.3 grams of EB powder in 10 mL of NMP (Aldrich, 99.9%) in a glove box under nitrogen with constant stirring. 1 µL of stock solution was cast on an interdigitated electrode (25 finger pairs of 15 µm interdigitated spacing, 3 mm long, 120 nm high) and the solvent was removed under flowing argon gas (Air Gas Co., 99.99% purity) at room temperature. The conductivity of the thin emeraldine base film was measured with impedance spectroscopy (Solartron SI 1260) using a frequency range of 3.2 × 107 to 1 Hz. The conductivity of the film was calculated using the measured resistance (R) in equation (1):
σ=
d [S/cm] 49 × h × f × R
(1)
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where f, h and d represent the finger length, height and interdigitated spacing respectively, as shown in figure 1, and 49 represents the number of conductive paths between 50 fingers. The contribution to conductivity given by the material above the finger height and between the finger ends and the side of interdigitated electrode was neglected.
TOP VIEW
f = 3 mm h = 120 nm d = 15 µm
Electrode
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