M. Baladi, E. McDowell, J. Alston
Medical College of Georgia,
United States
Keywords: carboxylated cellulose, cellulose, biopolymer, citric acid, cCNCs, CNCs, artificial tissue
Summary:
Cellulose nanocrystals (CNCs) are increasingly explored for biomedical and sustainable material applications, and carboxylated CNCs (cCNCs) in particular show increased crosslinking potential. Current existing carboxylation methods rely on harsh reagents, complex reaction schemes, or low-yield processes, limiting scalability and yielding environmentally unfriendly products. This work investigates whether a simplified and low-cost organic mixed-acid hydrolysis method can reliably produce suitable cCNCs. The question behind this study: Can powdered cellulose be efficiently carboxylated using a streamlined, recyclable citric–hydrochloric acid hydrolysis process to produce cCNCs with sufficient carboxyl substitution for further use? There is a significant need for accessible, reproducible, and green synthesis routes that support sustainable nanocellulose production for research, biomedical, and industrial use. Cellulose hydrolysis and carboxylation were performed using microcrystalline or cotton-based cellulose suspended in varying amounts of citric and hydrochloric acids from 80–100°C. Six reaction conditions based on previous literature on the topic were tested to optimize yield and reduce reagent use. Following neutralization (to ensure protonation of carboxyl groups), washing, and freeze-drying, the resulting powders were characterized using FT-IR spectroscopy (to confirm successful carboxylation) and conductometric titration (to quantify carboxyl group concentration per gram and per cellobiose unit). Results confirm successful carboxylation across multiple reaction conditions, with the optimized process achieving the highest degree of substitution (0.637 mmol/g and 0.218 carboxyl units per cellobiose unit) while reducing acid volume by half. FT-IR spectra consistently exhibited the ester C=O peak at 1735 cm⁻¹, validating attachment of carboxyl groups to the cellulose anomeric carbon. Process adjustment demonstrated that acid use can be significantly reduced and that the acid mixture may be partially recycled without eliminating reactivity—supporting environmentally responsible scale-up. The reaction is consistent with an acid-catalyzed Fischer esterification mechanism, producing cCNCs with surface functional groups able to be further modified or crosslinked. These findings indicate that this mixed-organic acid method offers a practical and green alternative to more hazardous and environmentally unfriendly carboxylation routes such as TEMPO oxidation or strong inorganic acid treatments. The resulting cCNCs are suitable for chemical conjugation and incorporation into regenerated biopolymer, potentially yielding stronger and more versatile nanocellulose-based materials. Future work will focus on firstly expanding other chemistry of the cCNCs, and secondly evaluating crosslinked material performance in ionic-liquid regenerated cellulose systems as a precursor to studying its effect in biological systems.