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Contribution of polymeric swelling to the overall response of capacitive gas sensors

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A new method for investigation of the swelling of polymers on exposure to gas or vapour has been devised and tested. It uses an optical profilometer (based on the chromatic aberration of a lens system) which is integrated into a computer-controlled gas-dosing and mixing setup. Gas and/or vapour concentration-dependent measurements have been carried out for thick layers of the polymers commonly used in gravimetric and capacitive gas sensors: poly(acrylic acid) (PAA), poly(vinyl pyrrolidone) (PVP), poly(ether urethane) (PEUT), and polydimethylsiloxane (PDMS). The thickness of PAA, PVP, and PEUT films changed significantly on exposure to humidity. These data have been used to derive the sorption isotherms of the respective polymers, which were found to be Henry or Flory–Huggins isotherms. Comparison of the geometrical (swelling) responses with capacitive responses revealed a strong correlation. The correlation, which occurs because both types of response are proportional to the water content of the polymer, is also valid for polymers with nonlinear gas responses. Finally the geometrical and electrical characteristics of the capacitive samples were used to explain the dependence of the capacitive response of different polymers on the concentration of the target gas or vapour. In this way was deduced that PDMS, which does not swell on exposure to humidity, swells in the presence of 2,3-dimethylpentane, for which no profilometer evaluations are yet available.

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The authors gratefully acknowledge support from EPFL-SAMLAB, Neuchatel, the group of Danick Briand, who provided the capacitive transducers used in the capacitance experiments.

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Correspondence to U. Altenberend.

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This paper is dedicated to Professor Franz Dickert on the occasion of his 70th birthday.

Appendix 1

Appendix 1

Effect of the geometry of an interdigitated structure on its capacitance

Explanation of why the capacitive sensitivity of an interdigitated structure depends on the interdigital ratio, r, is related to the geometry of the capacitive transducer and to field distribution. Dissimilar to a plane parallel capacitor, for which, in practice, all the electrical field lines are confined between the electrodes, for interdigitated capacitive transducers the field lines are, more or less, perpendicular to the transducer surface, filling the space above the electrodes up to a height no larger than the interdigital semi-period [16, 33]. The geometrical capacitance of the transducer is the capacitance measured with no additional dielectric layer on top (and has the value associated with an air column of height 0.5 λ and a dielectric constant of 1). Deposition of a polymer layer divides the sample capacitance into two components, that of the polymer (of height L and polymer dielectric constant >1) and that of the air (of height 0.5 λ − L and dielectric constant 1) which combine together (parallel and partially serial distributed connection) to give the actual sample capacitance. The higher (thicker) the polymer layer the larger is its contribution to the total capacitance of the sample and, as a consequence, its capacitive sensitivity to gases. Additionally, the polymer swelling, equivalent to an increase in L, will always result in a positive capacitance change if L is no larger than 0.5 λ and will have almost no effect for L > 0.5 λ.

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Altenberend, U., Oprea, A., Barsan, N. et al. Contribution of polymeric swelling to the overall response of capacitive gas sensors. Anal Bioanal Chem 405, 6445–6452 (2013).

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