Precision measurement of the refractive index and thickness of GOx thin film at different pH levels

Glucose oxidase (GOx) is widely used in optical biosensors for glucose concentration measurement. There are many studies on the electrochemical properties of GOx, there is relatively little information about the refractive index (RI) and thickness of GOx thin film at different pH levels. This study develops a precision circular heterodyne interferometer (PCHI) to measure RI and thickness of GOx immobilized on the glass substrate with covalent bonding method at different pH levels. The results of this study reveal the non-linear dependence of RI on the pH levels of GOx, but the thickness is independent on pH levels of GOx. In addition, the comparison results are provided by commercial ellipsometer for measuring both RI and thickness, multimode fiber interferometer for measuring relative difference of RI, and field emission scanning electron microscope (FE-SEM) for the inspection of thickness. Based on these results, there is no significant difference between the proposed method and comparison methods. We believe that the valuable information of GOx can be helpful for the glucose sensor development.

► Glucose oxidase (GOx) is widely used in glucose sensor but there is less information about the optical parameters.
► First time to determine the refractive index and thickness of GOx thin film at different pH values.
► We applied precision circular heterodyne interferometer to determine the optical parameters of GOx.
► Our results validated with comparison methods and showed no significant difference.
► Refractive index of GOx is non-linear dependent on the pH values, and the thicknesses of GOx are independent of the pH values.