Circular Dichroism of Chiral Nematic Films of Cellulose Nanocrystals Loaded with Plasmonic Nanoparticles

In the search for induced chiral plasmonic activity, cholesteric films formed by cellulose nanocrystals have attracted great interest as potential hosts for plasmonic nanoparticles. Circular dichroism (CD) spectra of the composite films exhibit two peaks, one of which is ascribed to the cholesteric host and the other one to plasmonic chiroptical activity of the plasmonic nanoparticles. Here we report the results of comprehensive studies of extinction and CD properties of composite films formed by different types of cellulose nanocrystals and different types of plasmonic nanoparticles. We show that the second peak in the CD spectra acquired using CD spectrometers appears as the result of the local reduction of the CD signal of the host material, due to excessive absorption by the nanoparticles, and thus it cannot be interpreted as induced plasmonic chiroptical activity. Instead, we propose an alternative way to measure CD spectra of plasmonic cholesteric films by using Mueller matrix transmission ellipsometry. The results of this study are important for ongoing research in the field of chiral plasmonics and for the optical characterization of a broad range of chiral nematic nanostructured materials.