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

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.

Gold Organosol as a Real-Time Optical Sensor for Monitoring Solvent Refractive Index and Chain Length

“…It has now been seen that evanescent wave techniques such as surface plasmon resonance (SPR) spectroscopy can detect very low levels of chemical and bi-ological materials in real time without requiring analyte labeling and are sensitive to the refractive index changes at the solid/solution interface in the environment immediately adjacent to the sensor surface. 8,9 Recently, highly sensitive and selective chemical sensors based on surface plasmon resonance phenomenon have been developed for the detection of organic compounds such as morphine and IgE. [10][11][12] The characterization of the mass or thickness of the ultrathin organic films via SPR sensing 13,14 is also a versatile analytical approach, which is finding broad applications in biomedical, biomaterial, environmental, and agricultural research.…”

Evaluation of biosensor surfaces for the detection of microtubule perturbation

“…Due to fogging of the optical system, the lowest temperature achievable on the system was 15 °C, which is still an acceptable starting point for soluble (unpolymerized) tubulin, since normal polymerization does not take place at temperatures below 15 °C (Olmsted and Borisy, 1973). Since refractive index is also temperature-dependent (Grassi and Georgiadis, 1999), the response from a separate channel (reference channel) containing only sample buffer was subtracted from the sample channel to obtain the absolute change in response upon temperature change. The binding of tubulin to the surface at 15 °C was a slow process (Fig.…”

Surface plasmon resonance microscopy: Achieving a quantitative optical response

“…The initial calibration for the angle of incidence was performed by using the published values for the refractive index of water, n = 1.333. 22 Each critical angle reflectance data set was fit to a two-layer Fresnel model, 26 using literature values for the optical constants of the prism, 21 to calculate the refractive index of the bulk material (Supplementary Material, Fig. S2).…”