Over the past few years, nanocellulose (NC), cellulose in the form of nanostructures, has been proved to be one of the most prominent green materials of modern times. NC materials have gained growing interests owing to their attractive and excellent characteristics such as abundance, high aspect ratio, better mechanical properties, renewability, and biocompatibility. The abundant hydroxyl functional groups allow a wide range of functionalizations via chemical reactions, leading to developing various materials with tunable features. In this review, recent advances in the preparation, modification, and emerging application of nanocellulose, especially cellulose nanocrystals (CNCs), are described and discussed based on the analysis of the latest investigations (particularly for the reports of the past 3 years). We start with a concise background of cellulose, its structural organization as well as the nomenclature of cellulose nanomaterials for beginners in this field. Then, different experimental procedures for the production of nanocelluloses, their properties, and functionalization approaches were elaborated. Furthermore, a number of recent and emerging uses of nanocellulose in nanocomposites, Pickering emulsifiers, wood adhesives, wastewater treatment, as well as in new evolving biomedical applications are presented. Finally, the challenges and opportunities of NC-based emerging materials are discussed.

Cellulose microfibrils (CMFs) and cellulose nanofibrils (CNFs) were isolated from hardwood pulp by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation, which produced negatively-charged carboxylate groups on their surface. First, these CMFs and CNFs were used to prepare microgels and nanogels, respectively, at different concentrations of cellulose and trivalent Al3+ cation by inducing ionic interactions between the negatively charged carboxylates and the metal cation. Then, two other cations (i.e., divalent Ca2+ and monovalent H+ were employed to understand the structure–property relationship these hydrogels. We characterized their morphology, chemical groups, mechanical properties, surface area, and pore size, and evaluated their drug-release behaviors using theophylline. Compared to the hydrogels prepared from divalent or monovalent cations, both microgel and nanogel prepared from trivalent Al3+ showed the highest stiffness and compressive strength, which indicated that they possessed the strongest ionic cross-linking via intra- and inter-fibrillar interactions. With a decrease in the valency of the cation used, the surface area of both hydrogels decreased, while their pore radius and calculated fibril diameter increased, indicating that a higher valency cation produced a hydrogel with higher porosity and a tighter network structure. The nanogel prepared from Al3+ also showed the highest swelling ratio and the lowest release of theophylline, while that of microgel was, in contrast, consistent. The low total drug-release behavior in nanogels was attributed to their compact and highly porous structure. The Higuchi model was the best-fit model of drug release kinetics. These results indicate that the characteristics and internal structure of hydrogel has a great impact on its properties and drug-release profile, and that it may be possible to finely tune hydrogel properties and drug release profile by altering the internal structure of hydrogels during its preparation

This study aimed to evaluate the effect of ammonium persulfate’s (APS) oxidation time on the characteristics of the cellulose nanocrystals (CNCs) of balsa and kapok fibers after delignification pretreatment with sodium chlorite/acetic acid. This two-step method is important for increasing the zeta potential value and achieving higher thermal stability. The fibers were partially delignified using acidified sodium chlorite for four cycles, followed by APS oxidation at 60 °C for 8, 12, and 16 h. The isolated CNCs with a rod-like structure showed an average diameter in the range of 5.5–12.6 nm and an aspect ratio of 14.7–28.2. Increasing the reaction time resulted in a gradual reduction in the CNC dimensions. The higher surface charge of the balsa and kapok CNCs was observed at a longer oxidation time. The CNCs prepared from kapok had the highest colloid stability after oxidation for 16 h (−62.27 mV). The CNCs with higher crystallinity had longer oxidation times. Thermogravimetric analysis revealed that the CNCs with a higher thermal stability had longer oxidation times. All of the parameters were influenced by the oxidation time. This study indicates that APS oxidation for 8–16 h can produce CNCs from delignified balsa and kapok with satisfactory zeta potential values and thermal stabilities.

Cellulose Nanofibrils (CNF) was successfully obtained by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation with the addition of different oxidant namely NaClO (Sodium Hypochlorite) i.e. 3, 4, 7, 10 and 15 mL followed by ultra-sonication treatment. Size distribution of nanocellulose was observed using particle size analyzer, while surface charged was measured using zeta potentiometer. At different level of oxidant, CNF obtained different charged with different size distribution. As the amount of oxidant increased, the size distribution of CNF increased which correlated to the higher CNF yield, however it decreased at maximum oxidant addition. Although, in general the yield for nanocellulose was very low. With zeta potential value about -48 mV, it showed very stable suspension in water for more than 8 months observation. An optimum oxidant level promoted thinner and longer CNF which further beneficial for better entanglement in the hydrogel formation application.

Using oil palm trunk (OPT) layered with empty fruit bunch (EFB), so-called hybrid plywood enhanced with palm oil ash nanoparticles, with phenol-formaldehyde (PF) resin as a binder, was produced in this study. The phenol-formaldehyde (PF) resins filled with different loading of oil palm ash (OPA) nanoparticles were prepared and used as glue for layers of the oil palm trunk (OPT) veneer and empty fruit bunch fibre mat. The resulting hybrid plywood produced was characterised. The physical, mechanical, thermal, and morphological properties of the hybrid plywood panels were investigated. The results obtained showed that the presence of OPA nanoparticles significantly affected the physical, mechanical, and thermal properties of the plywood panels. Significant improvements in dimension from water absorption and thickness swelling experiments were obtained for the plywood panels with the highest OPA nanoparticles loading in PF resin. The mechanical properties indicated that plywood composites showed improvement in flexural, shear, and impact properties until a certain loading of OPA nanoparticles in PF resin. Fracture surface morphology also showed the effectiveness of OPA nanoparticles in the reduction of layer breakage due to force and stress distribution. The thermal stability performance showed that PF filled OPA nanoparticles contributed to the thermal stability of the plywood panels. Therefore, the results obtained in this study showed that OPA nanoparticles certainly improved the characteristic of the hybrid plywood.