Speaker
Description
Cellulose nanocrystals (CNCs) are rod-like, highly crystalline nanoparticles. They are derived from cellulose, the most abundant biopolymer on Earth. Despite extensive work on CNCs, their refractive index (RI) is still not conclusively established. Reported values vary depending on the sample state and measurement method [1,2]. For sulfated CNCs, counterions are likely responsible for some of these variations. Counterions are bound to the sulfate half-ester groups and may modify the hydration environment at the CNC surface [6]. Understanding these interactions is relevant not only for RI measurements but also for fundamental CNC studies. Alkali ions can serve as labels in ultrastructural investigations, as we previously demonstrated [5].
This study examines how different alkali metal ions (from Li⁺ to Cs⁺, including H⁺) as counterions on the CNC surface affect the apparent refractive index. To assess whether sample preparation contributes to the measured RI, several exchange procedures are compared. These include a dialysis-based route and two different cation-exchange resins [4]. For all procedures, pH is adjusted to 7, and ultrasonication is applied to reduce agglomeration, as this step highly influences the measured RI.
Two optical methods are used. In the first, an established dispersion-based approach measures CNC suspensions by refractometry at different concentrations. The data are then extrapolated to obtain an effective RI of the CNC in water [1,2]. In the second method, spin-coated CNC thin films are analyzed by surface plasmon resonance (SPR) in air and isopropanol. Unlike refractometry, SPR probes a substantially less hydrated state. A cellulosic layer was also probed as a binding layer between the gold substrate and the CNCs. This additional RI data on pure cellulose shows that CNCs have a lower RI than pure cellulose, as previously reported [3]. Our results further show that heavier alkalis, such as Cs-CNCs, exhibit a lower RI than H-CNCs and lighter alkali forms. Although the absolute RI values differ between refractometry and SPR, both methods show a comparable decrease of about 0.03 in the RI from H-CNCs to Cs-CNCs. In SPR, this relative difference appears at all three investigated wavelengths. Overall, these findings indicate that the absolute RI depends on the measurement environment, while the relative counterion effect appears robust in the dataset. The different patches and exchange procedures only show minor variations in both measurement methods.
References
[1] Landry et al., For. Prod. J. 2011, 61, 104–112.
[2] Saveyn et al., Part. Part. Syst. Charact. 2002, 19, 426–432.
[3] Reid et al., Langmuir 2017, 33, 7403–7411.
[4] Petschacher et al., Nanomaterials 2022, 12, 3131.
[5] Knez et al., Small 2025, 21, 2500351.
[6] Ikegami, J. Polym. Sci. A-2 1964, 2, 907–921.