Gels were prepared by mixing various ratios of OCNF and starch (Soluble S9765, Sigma Aldrich, UK) solution (1:0.5 and 1:1 wt%) followed by vortex mixing until hydrogels were obtained. The required amount of cationic surfactants (Dodecyltrimethylammonium bromide (DTAB, purity≥98%, MW 308.34 g mol-1, Sigma-Aldrich, UK), cetyltrimethylammonium bromide (CTAB, purity 99%, MW 364.45 g mol-1, ACROS Organic, Fisher-Scientific, UK) were added to the starch solution prior to mixing with OCNF during the gel formation.

Rheological tests were performed using a stress-controlled rheometer (Discovery HR-3, TA Instruments, USA) equipped with a sandblasted plate-plate stainless steel geometry (40 mm) at 25oC. Approximately 1 mL of the gel was placed between the plates (with a plate-plate gap of 0.5 mm) and frequency, amplitude and flow sweeps were measured to determine the viscoelastic properties of the gels. Frequency sweeps were conducted, within the linear viscoelastic range, in strain control mode at 0.5% strain with an angular frequency range from 0.1 to 100 rad s-1. Amplitude sweeps were measured at an angular frequency of 1 Hz (6.28 rad s 1) covering the strain ranging from 0.01 to 50%. Finally, flow curves were measured to study the viscosity response of the sample to shearing, with a shear rate ranging from 0.01 to 100 s-1. 

ζ-potential measurements were conducted using a Zeta-sizer (Malvern Zeta-sizer Nano ZSP®, UK). Dilute solution (0.1 wt%) of samples in DI water were placed in the capillary electrode cell and the ζ-potentials measured as an average of 5 measurements from 100 scans each.

Iodine stock solution (50% of Lugol’s solution) was prepared by dissolving 0.25 g of iodine (Fisher Scientific, UK) and 0.5 g of potassium iodide (Sigma-Aldrich, UK) in 150 mL of DI water under magnetic stirring. Then 30 µl of the prepared iodine solution was added to each 5mL of diluted (25 times) gel sample before measuring the absorbance using a UV/visible spectrometer (Varian Cary 50 Probe) by scanning over the wavelength range of 290 to 800 nm.