Evaluating the refractive index, thickness and porosity of ultrathin cellulose nanocrystal films with different polymorphs by SPR technique.

Abstract It is of crucial importance to know the quality of ultrathin films deposited on surface plasmon resonance (SPR) sensors prior to adsorption experiments. In this study, the optical properties of ultrathin cellulose nanocrystal films with various polymorphs (cellulose I, cellulose II and the hybrid of cellulose I/II), which deposited on gold surface of SPR sensors, were determined by a two-medium SPR technique and the influences of the second medium were assessed as well. The measured refractive index for ultrathin cellulose nanocrystal films with polymorphs of cellulose I, I/II and II was 1.453, 1.462 and 1.464, respectively, with a low margin of error about 0.2%. The porosity of according CNC films on SPR sensors was assessed to be 20.8%, 19.0%, and 18.5%. The measured film thickness for all deposited cellulose nanocrystal films was in the range of 25–35 nm, with a margin of error about 5%, accorded well with that examined by quartz crystal microbalance. The results showed that SPR surveys combined with Winspall analysis allow for simultaneous determination of the thickness, refractive index and the derived porosity, and provide a facile in situ quality control for the modified SPR sensors prior to adsorption experiments.