Study of Analyte Diffusion into a Silicone-Clad Fiber-Optic Chemical Sensor by Evanescent Wave Spectroscopy

Abstract

The diffusion rates of various polar and nonpolar analytes in dimethylsiloxane were examined with the use of a commercially available 200-μm silica-core/300-μm silicone-clad fiber as the optical element for evanescent wave spectroscopy in the near-infrared spectral region. An analytical solution to Fick's second law was used to model the time-dependent analyte concentration at the core/cladding interface. Successful fit of the analytical solutions to infrared data verifies the assumption of constant diffusion coefficients that is necessary to solve the equation. Transport rates of polar analytes in silicone can be estimated with the use of a single-parameter model that results in diffusion coefficients of 3.2 × 10⁻¹, 1.6 × 10⁻¹, 8.1 × 10⁻⁷, and 3.9 × 10⁻⁷ cm²/s for methanol, ethanol, 2-propanol, and n-butanol, respectively. Estimating the transport of larger nonpolar analytes in the silicone cladding requires a two-parameter model that includes a diffusion coefficient and an interfacial conductance term. For pentane, hexane, heptane, and cyclohexane the resultant diffusion coefficients and interfacial conductance parameters are 6.9 × 10⁻⁷, 4.6 × 10⁻⁷, 4.4 × 10⁻⁷, and 2.3 × 10⁻⁷ cm²/s and 2500, 2000, 2000, and 600 μm⁻¹, respectively.