Chemical sensors based upon polysiloxanes: comparison between optical, quartz microbalance, calorimetric, and capacitance sensors

Abstract

Optical, quartz microbalance, calorimetric, and capacitance transducers to monitor changes of thicknesses Δ d, masses Δ m, temperatures Δ T, and capacitances Δ C are coated with layers of chemically sensitive polysiloxanes. They are tested comparatively as prototype sensors to monitor organic aliphatic, aromatic, and halogenated gas components in air at different temperatures and partial pressures. The results of this comparative study using the different transducer principles are explained by reversible interactions between the organic component in the gas phase and the polymeric polysiloxanes. Characteristic diffusion constants, and hence response times, occur to adjust bulk concentrations of the organic components in the polymer matrices. For a specific polysiloxane with a certain dielectric constants ∈ 2, the distribution coefficient p g describing the thermodynamic equilibrium is related to the boiling temperature T b of the detected organic component and the operation temperature T of the sensor. For different polysiloxanes with different dielectric coefficients ∈ 2, the dipole moment μ of the organic component is related to the distribution coefficient p g with a specific dependence of the sensor signals Δ d, Δ m, Δ T, and Δ C on p g . Finally, the advantages and disadvantages of these transducer principles for specific practical applications are discussed.