This paper provides an overview of recent progress made in the area of cellulose nanofibre-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibres (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterisation of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibres covered are those extracted from plants by acid hydrolysis (nanowhiskers), mechanical treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibrilbased composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymerisation from the surface of cellulose nanowhiskers by atom transfer radical polymerisation and ring opening polymerisation,

Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Above a critical concentration, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. We have examined the effect of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp. Longer hydrolysis times produced shorter, less polydisperse black spruce cellulose nanocrystals and slightly increased the critical concentration for anisotropic phase formation. Increased acid-to-pulp ratio reduced the dimensions of the nanocrystals thus produced; the critical concentration was increased and the biphasic range became narrower. A suspension made from a bleached kraft eucalyptus pulp gave very similar properties to the softwood nanocrystal suspension when prepared under similar hydrolysis conditions.

When used as fillers in polymer composites, the thermostability of cellulose crystals is important. Sulfate groups, introduced during hydrolysis with sulfuric acid, are suspected to diminish the thermostability. To elucidate the relationship between the hydrolysis conditions, the number of sulfate groups introduced, and the thermal degradation behavior of cellulose crystals, bacterial cellulose was hydrolyzed with sulfuric acid under different hydrolysis conditions. The number of sulfate groups in the crystals was determined by potentiometric titration. The thermal degradation behavior was investigated by thermogravimetric analysis. The sulfate group content increased with acid concentration, acid-to-cellulose ratio, and hydrolysis time. Even at low levels, the sulfate groups caused a significant decrease in degradation temperatures and an increase in char fraction confirming that the sulfate groups act as flame retardants. Profile analysis of the derivative thermogravimetric curves indicated thermal separation of the degradation reactions by the sulfate groups into low- and high-temperature processes. The Broido method was used to determine activation energies for the degradation processes. The activation energies were lower at larger amounts of sulfate groups suggesting a catalytic effect on the degradation reactions. For high thermostability in the crystals, low acid concentrations, small acid-to-cellulose ratios, and short hydrolysis times should be used.

Cellulose nanocrystals are promising candidates for applications in nanomedicine. To enable the use of fluorescence techniques in in vitro and in vivo studies, cellulose nanocrystals were labeled with fluorescein-5‘-isothiocyanate (FITC) via a three-step reaction, involving epoxy activation of the nanocrystal surface, opening of the epoxy rings with ammonium hydroxide, and coupling of FITC molecules to the primary amino groups. The FITC content of the labeled cellulose nanocrystals was determined by UV/vis spectroscopy. A FITC content of 0.03 mmol/g of cellulose, equivalent to 5 FITC moieties per 1000 anhydroglucose units, was obtained using the described method.

Cellulose nanocrystals (CNCs) prepared by H(2)SO(4) hydrolysis have sulfate groups on their surface, which have negative implications for some CNC applications. In this study, two desulfation methods were evaluated, and the properties of desulfated CNCs were compared to those of unsulfated CNCs, prepared by HCl hydrolysis. H(2)SO(4)-hydrolyzed CNCs from softwood sulfite pulp were subjected to either a mild acid hydrolytic desulfation or a solvolytic desulfation in dimethyl sulfoxide via the pyridinium salt. Removal of the sulfate groups was confirmed by conductometric titration and X-ray photoelectron spectroscopy. The effect of the desulfation procedure on the lateral crystallite dimensions was analyzed by X-ray diffraction. The extent of particle aggregation in the samples was assessed by atomic force microscopy and dynamic light scattering. The acid hydrolytic method achieved only partial desulfation and produced gradually decreasing sulfate contents upon successive repetition. The solvolytic method achieved nearly complete desulfation in a single step. The desulfated CNCs showed similar particle aggregation as the HCl-hydrolyzed CNCs, but the extent of aggregation was slightly less.

Elongated nanoparticles have recently been shown to have distinct advantages over spherical ones in targeted drug delivery applications. In addition to their oblong geometry, their lack of cytotoxicity and numerous surface hydroxyl groups make cellulose nanocrystals (CNCs) promising drug delivery vectors. Herein we report the synthesis of folic acid-conjugated CNCs for the targeted delivery of chemotherapeutic agents to folate receptor-positive cancer cells. Folate receptor-mediated cellular binding/uptake of the conjugate was demonstrated on human (DBTRG-05MG, H4) and rat (C6) brain tumor cells. Folate receptor expression of the cells was verified by immunofluorescence staining. Cellular binding/uptake of the conjugate by DBTRG-05MG, H4, and C6 cells was 1452, 975, and 46 times higher, respectively, than that of nontargeted CNCs. The uptake mechanism was determined by preincubation of the cells with the uptake inhibitors chlorpromazine or genistein. DBTRG-05MG and C6 cells internalized the conjugate primarily via caveolae-mediated endocytosis, whereas H4 cells internalized the conjugate primarily via clathrin-mediated endocytosis.

Model cellulose surfaces have attracted increasing attention for studying interactions with cell wall matrix polymers and as substrates for enzymatic degradation studies. Quartz crystal microbalance with dissipation monitoring (QCM-D) solvent exchange studies showed that the water content of regenerated cellulose (RC) films was proportional to the film thickness (d) and was consistent with about five water molecules per anhydroglucose unit. Sulfated nanocrystalline cellulose (SNC) and desulfated nanocrystalline cellulose (DNC) films had comparable water contents and contained about five times more water than RC films. A cellulase mixture served as a probe for studies of substrate accessibility and degradation. Cellulase adsorption onto RC films was independent of d, whereas degradation times increased with d. However, adsorption onto SNC and DNC films increased with d, whereas cellulase degradation times for DNC films were independent of studied d. Enhanced access to guest molecules for SNC and DNC films revealed they are more porous than RC films.

Cellulose nanocrystals (CNCs) are a biobased nanomaterial attracting increasing interest for a range of potential applications. This article reviews the current literature on the pulmonary, oral, dermal, and cytotoxicity of CNCs. Current studies of the oral and dermal toxicity of CNCs have shown a lack of adverse health effects, whereas studies of the pulmonary and cytotoxicity have yielded discordant results. Additional studies are needed to support the general conclusion that CNCs are nontoxic on ingestion or contact with the skin and to determine whether CNCs have adverse health effects on inhalation or elicit inflammatory or oxidative stress responses at the cellular level. This review underscores the importance of careful sample characterization and exclusion of interfering factors, such as the presence of endotoxins or toxic chemical impurities, for a detailed understanding of the potential adverse health effects of CNCs by various exposure routes.