DOI: 10.7924/r41v5qx4m
title: Data from: Synthesis of metal-modified nanocellulose as a biofilm analogue

description: In this study, the synthesis of nanocellulose, metal ion testing, imaging, mechanical characterization, and statistical analysis were conducted to investigate the structural and mechanical properties of nanocellulose hydrogels.
1. Nanocellulose Synthesis
The synthesis process began with raw cellulose powder (1-100 μm in size), which was dissolved in an ionic liquid solution composed of sodium hydroxide, urea, and water at a 7:12:81 mass ratio. This solution was stirred at 0 ºC in an acetone ice bath until it became clear, which took 30-45 minutes. After the cellulose dissolved, the solution was centrifuged at 10,000 RPM (13751 rcf) for 15 minutes at 0 ± 0.5 ºC. This separation yielded a solid gel and a viscous liquid containing suspended nanocellulose. The liquid component was then decanted into petri dishes and evaporated in a chemical fume hood over 24 hours to form nanocellulose hydrogels.
2. Metal Ion Testing and Absorption
For metal ion testing, nanocellulose hydrogels were exposed to solutions of various divalent metal ions to assess absorption effects on mechanical properties. Metal solutions were prepared at a concentration of 0.1 M for magnesium, calcium, nickel, strontium, cobalt, copper, and zinc. The nanocellulose hydrogels, cut into 10 mm diameter disks, were immersed in these metal solutions for 24 hours. Attempts to facilitate absorption before evaporation led to poor gelation and cross-linking, so this approach was refined.
3. Confocal Imaging
Confocal imaging was utilized to analyze the hydrated porous structure of nanocellulose films. Calcofluor white dye, which binds to cellulose, was used to stain the samples. After a 30-minute incubation, the samples were washed and imaged with a Zeiss LSM 900 confocal microscope, using a 400 nm excitation laser and a 10X objective lens. Images captured at detection wavelengths of 410-546 nm were then processed in MATLAB to estimate the void volume, providing a view of the hydrated porous volume in the nanocellulose structure.
4. Scanning Electron Microscopy (SEM)
To complement confocal imaging, scanning electron microscopy (SEM) was used as a label-free method to determine nanocrystal and pore sizes. Samples were prepared for SEM by critical point drying to preserve structural integrity and sputter coating with gold to reduce charging. Imaging was conducted using an Apreo S SEM at 5 kV. This process was performed on both unmodified nanocellulose and those modified with zinc and calcium ions, as these samples exhibited mechanical properties close to those of bacterial biofilms. Nanocrystal sizes were measured by hand using ImageJ software, and average sizes were calculated from measurements of 20 crystals across 3 separate samples. Error bars were added to represent the variability in nanocrystal size.
5. Mechanical Characterization
Mechanical characterization was performed using dynamic mechanical analysis (DMA) to assess the storage and loss moduli of both unmodified and metal ion-modified nanocellulose films. A TA Instruments RSA-G2 Solids Analyzer was used to measure these properties, with values taken from the zero-slope region of the strain-frequency curve. The resulting moduli were compared against internal standards and literature values for biofilms to determine how the nanocellulose films mimic biofilm-like mechanical characteristics.
6. Statistical Methods
Statistical analysis was applied to quantify the effect of each divalent metal ion on the nanocellulose hydrogels’ mechanical properties. Estimation statistics were used to provide a robust comparison of the experimental data, including mechanical values of unmodified nanocellulose hydrogels and alginate, against biofilm data from the literature. Data were presented using a Cummings estimation plot generated with EstimationStats, showing raw values with vertical error bars indicating a 95% confidence interval. The dataset included 30 replicate measurements from the same hydrogel and 3 different hydrogel batches, ensuring the statistical robustness of the findings.
Summary
Altogether, these methods and data provide a detailed assessment of the structural and mechanical characteristics of nanocellulose hydrogels, focusing on their response to metal ion exposure. This approach supports the study’s objective of understanding nanocellulose as a potential mimic for bacterial biofilms, with potential applications in areas such as wound dressing, metal remediation, and studies of bacterial growth environments.