Several groups have shown that alkanethiol-modified pyrroles can be tethered to a gold surface, but there is often little evidence that, once oxidized, the resulting monolayer film is an organic conducting polymer. Using surface plasma resonance (SPR) spectroscopy, we demonstrate for the first time that, upon electrochemical oxidation, self-assembled alkanethiol-pyrrole films on gold show behavior characteristic of organic conducting polymers: we observe reversible changes in the optical constants of the organic film upon doping/dedoping. Since the optical constants are related to film conductivity, we show that the effective isotropic dielectric constant of the film obtained in the standard SPR data analysis can be interpreted in terms of in-plane and out-of-plane contributions to film conductivity. We find that the in-plane conductivity of oxidized 3-(ω-mercaptoundecyl)pyrrole is smaller, but of the same order of magnitude, than that found for thick films of polypyrrole. Most importantly, we observe reversible changes in the optical constants of the polymerized film, which are consistent with electrochemical switching of an organic conducting polymer whose conductivity is largest for the doped state and decreases for the dedoped state.
Temperature-dependent measurements of surface coverage and interfacial kinetics remain relatively unexploited in thin-film sensing applications that rely on optical surface-sensitive techniques such as surface plasmon resonance spectroscopy (SPR). These techniques are inherently sensitive to the optical properties of the bulk solution in contact with the thin film; therefore, quantitative thin-film sensing requires accurate refractive index data for bulk solutions at the conditions of interest. The refractive index for bulk solutions depends strongly on temperature, solution composition, and optical excitation wavelength. In this paper, we demonstrate the use of critical angle measurements for accurate, independent determination of the refractive index of bulk solutions and present results for different experimental conditions of solution temperature, solution concentration, and excitation wavelength. We also examine the implications of incorrect accounting of the bulk solution for the case of two-color SPR sensing of ultrathin organic films. This sensing technique, which depends inherently on the contrast in the dispersion of the refractive index of the film and the bulk solution, can be over 1 order of magnitude more sensitive than single-color SPR measurements. Critical angle measurements can be implemented in conjunction with SPR measurements and will be invaluable for thin film sensing application in which the bulk refractive index varies during the experiment, for example, in temperature-dependent SPR measurements, or for applications in which the solution refractive index is not known.
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