Temperature-Dependent Refractive Index Determination from Critical Angle Measurements: Implications for Quantitative SPR Sensing

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Abstract
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.