Current Opinion in Colloid & Interface Science 19 (2014) 383–396

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Current Opinion in Colloid & Interface Science

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Nanocellulose properties and applications in colloids and interfaces☆

Carlos Salas a, Tiina Nypelö a,1, Carlos Rodriguez-Abreu c, Carlos Carrillo a, Orlando J. Rojas a,b,d,⁎a Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695, United Statesb Department of Chemical and Biomolecular Engineering, North Carolina State University, Campus Box 7905, Raleigh, NC 27695, United Statesc INL-International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, 4715-330 Braga, Portugald School of Chemical Technology, Department of Forest Products Technology, Aalto University, 00076 Aalto, Finland

☆ Special Volume for Current Opinion in Colloid and“Synthesis and Novel Applications of Biopolymer andKrassimir P. Velikov, Orlin D. Velev (Eds.).⁎ Corresponding author at: School of Chemical Techn

Aalto, Finland and Department of Forest Biomaterials, NCampus Box 8005, Raleigh, NC 27695-8005, United States

E-mail address: orlando.rojas@aalto.fi (O.J. Rojas).1 Current address: Department of Chemistry, Universit

Sciences Vienna, Konrad Lorenz Straße 24, A-3430 Tulln.

http://dx.doi.org/10.1016/j.cocis.2014.10.0031359-0294/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 July 2014Received in revised form 16 October 2014Accepted 17 October 2014Available online 30 October 2014

Keywords:NanocelluloseCellulose nanofibrilsCellulose nanocrystalsNanoparticlesSelf-assemblyAnisotropic particles

In this reviewwe introduce recent advances in the development of cellulose nanomaterials and the constructionof high order structures by applying some principles of colloid and interface science. These efforts take advantageof natural assemblies in the formoffibers that nature constructs by a biogenetic bottom-upprocess that results inhierarchical systems encompassing a wide range of characteristic sizes. Following the reverse process, a top-down deconstruction, cellulosematerials can be cleaved from fiber cellwalls. The resulting nanocelluloses,main-ly cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC, i.e., defect-free, rod-like crystalline residues afteracid hydrolysis of fibers), have been the subject of recent interest. This originates from the appealing intrinsicproperties of nanocelluloses: nanoscale dimensions, high surface area, morphology, low density, chirality andthermo-mechanical performance. Directing their assembly into multiphase structures is a quest that can yielduseful outcomes in many revolutionary applications. As such, we discuss the use of non-specific forces to createthin films of nanocellulose at the air–solid interface for applications in nano-coatings, sensors, etc. Assemblies atthe liquid–liquid and air–liquid interfaces will be highlighted as means to produce Pickering emulsions, foamsand aerogels. Finally, the prospects of a wide range of hybrid materials and other systems that can bemanufactured via self and directed assembly will be introduced in light of the unique properties ofnanocelluloses.

© 2014 Elsevier Ltd. All rights reserved.

1. Cellulose nanofibrils (CNF) and nanocrystals (CNC)

Cellulose constitutes the most abundant renewable polymerresource available. As a raw material, it is generally well known for itsuse in the form of fibers or derivatives in a wide spectrum of productsand materials. Cellulose fibrils are structural entities formed through acellular manufacturing process, cellulose biogenesis, stabilized byhydrogen bonds and van der Waal forces [85,135]. The fibrils containcrystalline and amorphous regions that can be generally separatedfrom the given cellulose source into amorphous and/or crystalline com-ponents, by mechanical, chemical or a combination of mechanical,chemical or enzymatic processes [135], yielding cellulose nanofibrils(CNF) or cellulose nanocrystals (CNC). Both, CNF and CNC have beenused recently to engineer newmaterials. The current regulatory efforts

Interface Science Volume onBiologically Derived Particles,”

ology, Aalto University, 00076orth Carolina State University,.

y of Natural Resources and Life

and name standardization for these nanocelluloses is a testament ofrapidly evolving activity in this field (for instance, CNF has been alsoreferred to as nanofibrillar cellulose NFC, while CNC has been cited asnanocrystalline cellulose NCC, cellulose nanowhiskers, etc.)

The unique properties of nanocellulose (CNF and CNC) such as lowdensity, biodegradability, high aspect ratio, high strength and stiffnesshave been discussed in several reviews ([45,49,50,67,98,110,135,168],among others). The abundance of OH groups on the surface of cellulosefavors the formation of hydrogen bonding, causing the cellulose chainsto assemble in highly ordered structures [85]. Hydrogen bonding plays acritical role in the nature of adhesion between nanocellulose with otherpolymeric materials present in the cell wall of fibers [61].

CNF are fibrils with lengths andwidths in themicrometer and nano-meter scales, respectively and they readily form networks and struc-tures when dispersed in, for example, aqueous media. CNF is usuallyprepared by liberation from the constituent fiber matrix and microfiberbundles [12,55,178,198] (Fig. 1). These nanofibrils contain both amor-phous and crystalline cellulosic regions [10] and can be differentiatedfrom CNC, which has a smaller aspect ratio and are prepared from fibersor fibrils via acid hydrolysis that degrades the amorphous regions, yield-ing highly crystalline cellulose nanoparticles [11] (Fig. 1). CNCs havegarnered a tremendous level of attention in the materials community.Biopolymeric assemblies, based on CNF and CNCwarrant such attention

Fiber deconstruction

5x5 m

1x1 m

Cellulose Nanofibrils (CNF)

Cellulose nanocrystals (CNC)

Pretreatment and shear

Fibers in wood Acid Hydrolysis

Non-crystalline

crystalline

Fig. 1. Schematic illustration of CNF and CNC production from fiber cell walls by mechanical and chemical treatments, respectively (Adapted from [77] with permission from The RoyalSociety of Chemistry and from Ref. [202] Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

384 C. Salas et al. / Current Opinion in Colloid & Interface Science 19 (2014) 383–396

not only because of their unique physical and chemical properties, butalso because of their inherent abundance, renewability and sustainabil-ity. They have been the subject of a wide spectrum of research efforts inthe development of rheology modifiers, coating components, opticaldevices, stabilizers of multiphase systems and as reinforcing agents in(nano)composites, all exploiting CNF and CNC intrinsic nanoscaledimensions, morphology, light weight and assembly behavior. Herewe highlight advances in nanocellulose utilization taking advantage oftheir colloid and interfacial behavior and their assembly at interfacesin solid films, emulsions, foams and aerogels.

As in typical colloidal systems, the size, shape andhigh surface area ofnanocelluloses influence the properties in aqueous media, for example,the optical characteristics, stability and rheology of their suspensions.At relatively high concentrations the cellulose nanocrystals self-assemble into a chiral nematic liquid crystalline phase, a property thathas been exploited in the preparation of functional films, as discussedin the next section.

2. Nanocellulose structuring in solid films

Orientation and alignment of nanocellulose have received increasedattention due to their suitability to form ordered structures, for exam-ple, in sensors and in optical devices. CNC is of special interest in suchapplications due to their aspect ratio, cylindrical shape, rigidity andchiral ordering, all leading to optical effects in aqueous media, in filmsor in solids templated from such structures. There is still a debate onthe origins of the chirality of CNC. The nematic ordering in colloidaldispersions is typically ascribed to the shape-anisotropy of CNC leadingto ordered arrangements and thus the ordering has been suggested tooriginate from chirality of the cellulose chains, the interaction betweenthe chiral surfaces and the twisted morphology of CNC [9,130].Khandelwal and Windle [91] have presented a thorough investigationon the role of CNC twisting concluding that it affects chirality and chiralalignment of cellulose in several length scales.

Observations regarding the alignment and structuring of CNC dis-persions have been extensively reported. In concentrated dispersionsCNC forms cholesteric ordering and arrange as a chiral nematic liquidcrystal. In such structures the nanoparticles align their long edges paral-lel to the horizontal plane of the aqueous dispersion and the crystal

layers forming a circular structure in the normal direction [154,155].The direction of the crystal axis is slightly rotated towards the planesbelow or above causing a helical alignment, with the pitch distance ofthe system defined as the spacing for the crystal layers on a plane tocomplete a full rotation. Intrinsic properties such as CNC dimensionsand surface charge affect ordering in the dispersed state. For example,short CNC order with a shorter pitch (shorter wavelengths) comparedto long CNC. The CNC surface charge originating from sulfate half-estergroups introduced during preparation with sulfuric acid hydrolysisincreases the electrostatic repulsion between the particles [54] whileincreased particle concentration increases the pitch [146]. The chiralstructure and assemblies formed by dispersed CNC are also affected bythe ionic strength as well as the nature of counter ions [43,44]; forexample, the pitch decreases with the increase of the ionic strength[146]. The reader is referred to the review by Abitol and Cranston [2]summarizing related effects.

When CNC dispersions are casted on a solid support to prepare solidfilms the particles canmaintain their alignment to form iridescent struc-tures with chiral nematic ordering [125,126] (see Fig. 2). The filmsreflect polarized light on a wavelength determined by the pitch of theliquid crystals [46]. Hence, the optical characteristic of the films andthe alignment of CNC in the films, as was the case for the aqueousdispersion, can be adjusted by changing the pitch as described above[146]. Reflected light and hence the color of dry films of CNC can alsobe varied by the thickness of the films, for example, as defined by con-structing a multilayer system consisting of alternating deposition ofCNC and a cationic polymer [30].

A recent approach reports on ultrasound treatment to change thechiral nematic pitch in CNC suspensions and the resultant dry films[15]. Sonication not only helped to reduce aggregation but was a suit-ablemean to control the film iridescent color with no need for additives(Fig. 3a). Sonication energy increases the wavelength of the lightreflected indicating a change in CNC alignment. External stimuli, electricormagneticfields can also affect CNCorientation in aqueous dispersions[3,31,33,34,71]. A few efforts have been made to align CNCs into a dryfilm by shear casting, convective assembly, and under electric or mag-netic fields (Fig. 3b and c). An early work by Yoshiharu et al. reportedon the preparation of oriented film using a rotating glass vial [194].The highly ordered film was formed using the CNC dispersion below

Fig. 2. Optical microscope image (a, crossed polars) of a CNC film (scale bar 40 μm). SEMimages of a fracture surface across the film a reshown ion b, c and d. In c and d the filmis oriented horizontally. The white arrows in c and d indicate examples of nanocrystalbundles pulled above fracture surface (Adapted from Ref. [130] with kind permissionfrom Springer Science and Business Media).

385C. Salas et al. / Current Opinion in Colloid & Interface Science 19 (2014) 383–396

the critical concentration for isotropic ordering and by drying the filmunder rotation (Fig. 3d). CNCs were applied on a solid support byusing convective and shear assembly by using a blade moving thethree-phase zone at the solid–liquid–air interface [71]. Similar workhas reported alignment of CNC by shear casting using a polymer system[37,153]. The parameters affecting the quality of alignment include thesubstrate–CNC interaction, dispersion concentration and speed of themoving blade. Application of CNC on a cationically charged substratefavors orientation in the direction of shear while keeping the dispersionconcentration below the concentration at which the CNC form thecholesteric phase [71].

CNCs tend to align perpendicular to the applied magnetic field [43]while maintaining the chiral structure. This effect can be used to tunethe pitch of CNC films by evaporating the solvent under a magneticfield [146] and the chirality can be twisted by applying a rotating mag-netic field [96]. Besides fundamental investigations on electric- ormagnetic-induced alignment, related effects have been utilized in thesynthesis of composites containing aligned CNC for improved tensilestrength [120] or in themanufacture of piezoelectricfilms [33,34] takingadvantage of the fact that CNC have a giant permanent electric-dipolemoment [56].

Unlike CNC, alignment of CNF is limited by their physical dimen-sions. Themicro-scale length of CNF prevents alignment to the same ex-tent observed for CNC. However, cellulose nanofibrils have been shownto self-align when loaded in composites for example, in the synthesis of

transparent films [161] in which CNF tend to align uniaxially parallel tothe surface and thus forms layered structures [190].

Nanopapers [52,53] or nanocellulose films have been reported fortheir unique properties such as low density, low porosity, transparency,low coefficient of thermal expansion, low air permeability and highstrength [135]. The porosity of nanopapers preparedwith CNF is typical-ly very low, this enables their good gas barrier properties [174,175].However, depending on the intended application porous materialsmay be preferred, for example in membranes carrying exposed adsorp-tion sites for target molecules as is the case of systems enclosing cellu-lose nanocrystals that have been developed for dye removal fromwater [88].

3. Nanocellulose in composites

Because of the potential improvement in strength, CNF and CNChave been extensively studied as strengtheners for composites. Severalreviews discuss related subjects (see for example [1,49,76,98]). Hereweonly highlight a few aspects from the view-point of interfacial phenom-ena. Incorporation of (hydrophilic) cellulose in non-polar polymermatrices is challenging due to usually poor adhesion between thecomponents, resulting in weak interfacial bonding. Klemm et al. [98]discussed the different methods for nanocomposite synthesis. Themain alternatives for incorporation of nanocellulose into a polymerma-trix include (a) dispersing dried cellulose into a hydrophobic matrix[150], (b) dispersing cellulose and the polymermatrix in a common sol-vent [58,167] or (c) using aqueous nanocellulose dispersion [127]. Thelatter is not applicable in the case of non-polar polymer matrices dueto the expected poor mixing and adhesion, as noted before. Because ofthese challenges solvent-exchange or drying are frequently applied ifaqueous nanocellulose dispersions are to be used. However, so-calledhornification of the cellulose component and the need for surface mod-ification of the cellulose to enable good dispersion remain limitingfactors [113].

The cellulosefiber–polymer interactions in compositeswith cellulos-ic fibers have been studied already since decades ago. The enhancementof such interactions is typically addressed by surface modification of thefibers [51,133] or by incorporation of compatibilizers [103,139]. Thesame principles apply to nanocelluloses, yet with additional challengesoriginating from their high surface area as well as their tendency toform gels with vast amounts of bound water. Surfactants have beenused as compatibilizer to incorporate CNC into hydrophobic matrices[17,93]. Surface grafting of CNC can also be used. For example, CNCwas grafted with poly(D-lactide) for incorporation in poly(L-lactide)[66] or with caprolactone oligomers in the case of poly(caprolactone)matrices [203]. Composites of Cellulose nanocrystals in a styrene-butadiene matrix can be formed by mixing in aqueous latex solution,followed by compression molding or using a common solvent forforming an organogel as a composite precursor [8].

Incorporation of nanocellulose in polystyrene, polyethylene andpolypropylene, some of the most widely used commercial hydrophobicsynthetic polymers, either as a reinforcement or to make the systemrich in renewable material composition, is a topical subject. Fujisawaet al. [59] prepared polystyrene composites reinforced with CNF thatwas oxidized by reaction in the presence of TEMPO (2,2,6,6-Tetramethylpiperidinyloxy) catalyst (TEMPO-CNF or TOCNF) utilizingdimethylformamide as a common solvent for the polymer and the filler.This approach enabled homogenous dispersion of the fibrils into thepolymer matrix leading to improvement in the tensile strength andelastic modulus. Composites polystyrene solutions loaded with CNCusing a common solvent and nonionic surfactants as compatibilizershave been reported in the form of electrospun fibers [158] or films [93]while telechelic poly(ethylene-co-butylene) was end-functionalizedwith a pyridine-based ligands to produce light-healable nanocompositesbased on CNC [29].

Fig. 4. Self-standing hydrogel produced by addition of HCl to carboxylated cellulosenanofibrils observed by naked eye (a) and under cross polarized lenses (b) (adaptedfrom Ref. [161] with permission from The Royal Society of Chemistry).

a

b

c

d

1 cm

Fig. 3. The effect of sonication energy on reflection of light of dried CNC films can be observed in the (a) panel with increased energy from left to right (reprintedwith permission from Ref.[15]. Copyright 2010 American Chemical Society). The images in (b) and (c) include self-organized and shear oriented films of CNC, respectively (adapted with permission from Ref. [37]Copyright 2013 American Chemical Society)while panel (d) illustrates alignment of CNCs by rotational shear (reprintedwith permission fromRef. [194]. Copyright 1997 American Chem-ical Society).

386 C. Salas et al. / Current Opinion in Colloid & Interface Science 19 (2014) 383–396

Contrary to the case of non-polar polymer media, incorporation ofnanocelluloses into a hydrophilic polymers such as PVA is greatly facil-itated by the similarity in surface energies and therefore simple mixingin aqueous media is a starting point for composite manufacture [4,116,127,147–149,200].

4. Colloidal behavior in hydrogels and liquid crystals

Nanocelluloses easily form hydrogels structures, i.e., materials com-posedmainly by water that is contained in a hydrophilic polymeric ma-trix [72,80]. Rheological characterization of CNC indicates the formationof liquid crystals [187]. As discussed in the previous section, they consistof self-organized structures with an order that also have the propertiesoffluids, for instance the ability toflow [18,89]. The viscosity andmoduliof CNC dispersions increases steeplywith concentration and two criticalvalues can be observed, namely, the overlap and the gelation concentra-tion,which depend strongly on the aspect ratio of the crystals [106]. Thethickening effect provided by the presence of nanocellulose in aqueousdispersions is attributed to the entanglement of the cellulosic chainswhich creates a network [109,144]. It is not surprising that comparedto CNC, CNF forms hydrogels more readily. Such hydrogels have beenproposed in a number of applications including scaffolds to storehuman cells [99,115].

Nanocellulose has been incorporated in the manufacture of both,hydrogels and liquid crystals, producing materials with unique proper-ties. The colloidal behaviors of these structures have been studied bydifferent research groups in the past years. Self-standing hydrogelsobtained solely by cellulose nanofibrils can be produced by differentmethods [40,41,161]. For example, addition of dilute hydrochloric acidto dispersions of the carboxylated nanofibrils at concentrations as lowas 0.4% w/v at pH = 2 promoted the formation of a hydrogel. Thesehydrogels were self-standing and anisotropic alignment of thenanofibrils was confirmed using cross polarized lenses (Fig. 4). Thereduction of the pH to low values induced a viscoelastic behavior thatfollowed a power law relation with the concentration of the nanofibersin water [161].

Addition of divalent or trivalent cations to a dispersion of carboxyl-ated cellulose nanofibrils in water triggers strong gelation of thesuspension [40,41]. Gelation occurs due to the formation of metal-

carboxylated bonds between cellulosic chains. These hydrogels arehighly viscoelastic, with the storage moduli related to the valence ofthe metal cation (which affect binding strength) [40,41].

Nanocellulose has been used also as reinforcement in the manufac-ture of composite hydrogels [68,192,193]. For instance, CNF and CNCwere used to reinforce polyvinyl alcohol-borax hydrogels and the effectof nanocellulose particle size and aspect ratio on the rheological proper-ties was studied. It was found that the addition of both, cellulose nano-fibers and nanocrystals, in amounts as low as 1% by weight, increasedconsiderably the storagemoduli (G′) of the hydrogel, by almost two or-ders of magnitude if compared to a hydrogel made only with polyvinylalcohol-borax. As expected, cellulose nanofibers aremore effective thancellulose nanocrystals in increasing the viscoelasticity of the hydrogeldue to the entanglement provided by the nanofibers [68]. Hydrogelswith carboxymethyl cellulose and CNC were proposed for medical ap-plications. As was the case in other systems, addition of CNC increasedthe storage modulus of the reinforced hydrogel [193]. Aldehyde-functionalization of the CNC was performed to facilitate cross-linking,resulting in an elastic hydrogel with a relatively high capacity for nano-particle loading [193].

387C. Salas et al. / Current Opinion in Colloid & Interface Science 19 (2014) 383–396

5. Structuringofnanocellulose at liquid–liquid interfaces: Emulsions

The self-assembly of nanoscale particles at interfaces is a well-known phenomenon that has been exploited in the stabilization ofPickering emulsions and foams. Cellulose nanoparticles also showsuch ability to self-assemble at liquid interfaces, with particular influ-ence of nanocellulose surface properties, shape and inter-particle inter-actions as well as nanocellulose–solvent interactions. Amajor driver forconsideration of nanocellulose in emulsion stabilization includes theirlow toxicity and biocompatibility [177].

The first studies on emulsification with cellulose materials involvedmicrofribillated cellulose (MFC). The hydrophilicity of cellulose fibrils,for example, in bacterial cellulose (BC) nanofibers having widthsbelow 100 nm, was found to favor the formation of oil-in-water (O/W) emulsions without the need for a significant reduction in interfacialtension [143]. Remarkably, the emulsions containing BC were quiteinsensitive to changes in ionic strength, pH and temperature, contraryto conventional emulsifiers such as thickening agents and surfactants.

Cellulosic surfaces can be hydrophobized by using agents such assilanes or alkylamines [124,189] so that stabilization of water-in-oil(W/O) emulsions can be made possible [7,114]. The degree of surfacemodification affects the wettability and therefore has an impact ondrop size and emulsion stability. It has been found that CNF acting asemulsion stabilizers are present as single or dispersed fibrils or forminglarge, network-like aggregates at the O/W interface. Such networkshave been proposed as main factor preventing coalescence of emulsiondroplets [188]. For fibrils produced with a high pressure homogenizer,emulsions with small oil droplet size and stable to creaming wereobtained when the number of homogenization passes during pro-duction was increased [185]. The collapse of emulsions stabilizedby cellulose fibrils can be reduced at low pH or high salt concentrationdue to electrostatic screening of the negatively charged particles. Insunflower oil-in-water emulsions stabilized solely by microcrystallinecellulose (MCC) it was found that MCC particles were able to reducethe lipid oxidation rate effectively, which was attributed to their abilityto scavenge free radicals and form thick interfacial layers around oildroplets [87].

Capron et al. reported on bacterial CNCs as stabilizer of O/W emul-sions [83] (Fig. 5). Above 60% droplet coverage by CNCs (average length~855 nm; width ~17 nm), a dense 2D interfacial network was formedand very stable, deformable droplets were produced. It appears thatfor CNCs to be effective as emulsifiers, their surface charge densityshould be lower than ca. 0.03 e/nm2 [84]. Providing this condition isfulfilled, CNCs with different sizes (length ranging from 100 to 900 nmand width of 13–17 nm) were able to stabilize emulsions. The CNCtendency to migrate to interfaces was attributed to the [200] β/(220)αhydrophobic crystalline edge (glucopyranose) that orients towardsthe oil phase. High-internal-phase O/W emulsions (HIPE) with gel-like behavior and 90% dispersed phase were produced in two steps(primary Pickering emulsion formation followed by droplet swelling)

Fig. 5. (a) Confocal laser scanningmicrograph of hexadecane droplets (BODIPY 564/570 stainedpoly(styrene) particles produced from styrene emulsions stabilizedwith bacterial CNC (b). Thepermission from Ref. [83]. Copyright 1997 American Chemical Society).

with CNCs (length ~89 and width ~13 nm) at concentrations as lowas 0.1 wt.% [20].

Assembly of liquid-core capsules with highmechanical stability wascarried out by Svagan et al. [171] and by using CNC as a host for thesynthesis of magnetic nanoparticles, Nypelö et al. [137] introducedPickering emulsion asmedia to producemicrobeads andmicrocapsules.They produced hybrid CNC magneto-responsive materials that can beused for sorption and separation. Svagan et al. [171] demonstrated thefabrication of capsules with composite shells formed by short cellulosenanofibers (1 μm) and CNC covalently cross-linked with aromaticdiisocyanate at the outer capsule wall and an inner layer dominatedby aromatic polyurea. The capsules showed exceptional mechanicalproperties. The Capron's group has also very recently reported on theformulation of new surfactant-free oil-in-water-in-oil (O/W/O) doubleemulsions, by using both native and hydrophobized NFC and CNC. Thedouble emulsions were very stable and comprised globules with sizesbetween 43 and 76 μm containing inner droplets of ca. 3 μm [35].

Varjonen et al. [181] functionalized CNFwith a fusion protein havingboth the ability to bind to cellulose and to be surface active. Protein-functionalized CNF assembled into tightly packed thin films at the air/water and at the oil/water interfaces, resulting in a synergistic improve-ment in the formation and stability of O/W emulsions. Temperatureresponsive emulsions were produced by Zoppe et al. [205] via graftingCNCs (diameter ~3–15 nm; length ~50–250 nm) with poly(NIPAM).The viscosity of the emulsions was found to increase in the vicinity ofthe LCST (lower critical solution temperature) of the polymer, but emul-sion de-stabilization occurred above the LCST, i.e., above 30–35 °C. Sèbeet al. [162] used esterified CNCs (length ~194 nm; thickness ~8 nm) forstabilization of O/WPickering emulsionswith different oils as dispersedphase. The emulsion stabilitywas dependent on the esterification agent,vinyl acetate being more effective than vinyl cinnamate as surfacemodifier for emulsification. Finally, Lee et al. [114] reported on W/Oemulsions stabilized solely by bacterial nanofibrils with a diameter of~50 nm and several micrometers in length, which were hydrophobizedby esterification with organic acids of various chain lengths. In order toobtain emulsions with water volume fraction higher than 60%, freeze-dried nanofibrils should be disentangled into individual units. The inter-nal phase content (ϕw) of the emulsions increasedwith the chain lengthof the esterification agent. These emulsions exhibited catastrophicphase separation when ϕw reached a certain value, contrary to cata-strophic phase inversion observed for other Pickering emulsions.

The case of oil-continuous emulsions is less reported, for obviousreasons. W/O emulsions with ca. 50% of acrylated soybean oil as contin-uous phase were stabilized solely by bacterial cellulose nanofibrilshydrophobized by silylation or esterification. Upon polymerizationand water removal, polymer foams were obtained [16]. Likewise, CNC-stabilizedW/O emulsionshavingpoly(lactic acid) (PLA) solution as con-tinuous phase was found to be an effective dispersion and alignmentmethod to control assembly of CNC into continuous composite ultrafinefibers produced by electrospinning [122]. Under a set of specific

) stabilizedwith bacterial CNCs (calcofluor stained) (a). Scanning electronmicrographs ofCNCs are clearly observed on the surface of the poly(styrene) particles (c). (reprintedwith

388 C. Salas et al. / Current Opinion in Colloid & Interface Science 19 (2014) 383–396

conditions, the as-spun composite ultrafine fibers assumed core–shellor hollow structures. In these structures, CNCs (diameter ~5 − 20 nmand length ~100− 500 nm) were aligned along the core (in the core−shellfibers), or on thewall (in the hollowfibers). CNC acted as nucleatingagent influencing PLA crystallinity, and improved the strength and stiff-ness of the electrospun composite fibers. Finally, multiple emulsions,specifically, oil-in-water-in-oil (o/w/o) emulsion types have been alsoreported by using nanocelluloses that were chemically modified withlauroyl chloride [35].

6. Structuring of nanocellulose at liquid-gas interfaces: Foamsand aerogels

CNF and CNC by themselves do not have strong interfacial activity toproduce stable foams, however amphiphilic behavior can be impartedby either polymer adsorption or by chemical surface modification, forexample, by attachment of hydrophobic moieties. For instance, aqueousfoams stabilizedwith up to 1wt.% CNFwere prepared by adsorbing pos-itively charged octylamine on the surface of the fibrils followedby intro-duction of air bubbles in the system through mixing and final freezedrying. The process resulted in highly porous, lightweight cellulosefoams (pore size ~300–500 μm) [23].

Similar to the stabilization of emulsions, the ability of nanocellulosesto self-assemble at the air/water interface has been exploited to producelightweight foams and aerogels suitable for applications such as insulat-ing, food packaging and adsorbent materials. Some recent approachesinclude the control of the morphology of cellulose foams by preparingprecursor aqueous foams [23,24] or Pickering emulsions [176].

Pickering emulsions stabilized by CNC (O/W emulsions, 20% contentof internal phase, cyclohexane) [24,176] were used in a two-step emul-sification process to increase the size of the initial emulsion droplets.The excess, continuous phase of the emulsions was removed by centri-fugation, and the resultant concentrated emulsionswere freeze dried toproduce a foam. The foams exhibited cell sizes similar to the droplets ofthe starting emulsion. Thiswas ascribed to the robustness of the startingemulsion drops to withstand centrifugation and freeze drying withoutcollapsing. The assembly of CNC at the interface was different depend-ing on the concentration of nanocrystals in the initial emulsions. ACNC concentration of 5 g/L produced foams with cell diameters around12 μm, with well-defined (polyhedral) pore distribution, whereas athigher CNC concentrations (10 g/L) the foam cells were similar in sizebut their walls were sheet-like structures. Functionalization of the

Fig. 6. SEM images of foams made from Pickering emulsions after freeze drying. The emulsio(images d–f). Note the structuring of the CNCs in the foam lamella (c and f). Reproduced with

foamswas achieved by addition of water soluble polysaccharides. Addi-tion of negatively charged alginate promoted coalescence leading to anetworked porous structure with no defined pore shape. In contrast,the addition of positively charged chitosan produced a cellular foamwith cell sizes similar to that of the initial emulsion droplets [176](see Fig. 6).

Aerogels, typically produced from gel precursors, are highly porousmaterials with low apparent density, large specific surface area and in-teresting properties such as low thermal conductivity. They are of inter-est in a variety of materials including thermal and acoustic insulators,super adsorbents, dielectrics, batteries and desalination media. Thepreparation of aerogels was first reported by Kistler [97], since thenthey have been synthesized from inorganic or organic gels as well astheir hybrids. Typically, aerogels have been fabricated from silica andmetal oxides, polymers and pyrolyzed polymers (carbon aerogels)[151]. The main drawback of traditional aerogels is their poor mechan-ical properties, which can be overcomeby the addition of nanomaterialssuch as graphene and carbon nanotubes [94,95]. The good mechanicalproperties of nanocellulose and the reported elastic modulus of143 GPa for CNC [170] combined with its high surface area makes it agood candidate for aerogel production.

The first report on the use of CNF for aerogels preparationwasmadeby Pääkkö et al. [145]who evaluated the influence of the freezingmeth-od on the preparation of aerogels from 2wt.% suspensions of CNF. Sincethen, different preparation methods have been reported, each of themaddressing the influence of different variables on the process of aerogelformation: Concentration of nanocellulose, method for preparation ofwell dispersed suspensions (hydrogel), cooling and freezing conditionsof the suspensions and process for solvent removal (typically achievedby freeze or supercritical drying).

Themorphology of the aerogels depends on the initial concentrationof the CNF dispersion. Low CNF concentrations (≤0.2 wt.%) produceaerogels with 3D porous network structure whereas at concentrationshigher than 0.5 wt.% physical crosslinking between the fibrils occur,resulting in 2D sheet-like structures. Suspensions of ultralow CNFconcentration (0.02 wt.%) from the supernatant after centrifugationproduce highly porous fibrous 3D networks with the fibrils assembledin bundles along their longitudinal direction [26].

The high amount of water in nanocellulose suspensions, and partic-ularly the strong hydrogen bonding between cellulose and water arechallenging aspects affecting water removal. The freezing of the sampleneeds to be carefully adjusted to gain control over the morphology of

ns were prepared with CNCs as stabilizer at concentrations of 5 (images a–c) and 10 g/Lpermission from Ref. [176] (Copyright© The Royal Society of Chemistry 2014).

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the final aerogels and to avoid their shrinkage and collapse and thustheir porosity. Since the presence of water induces strong capillaryforces that promote shrinkage of the aerogel, solvent exchange toreplace water prior to freezing/drying of the sample is a suitablealternative.

As implied in the preceding discussion, a variable to consider in thepreparation of cellulose foams and aerogels is the concentration of theprecursor dispersion and the high amount of water bound to the fibrils.An increased concentration of nanocellulose enhances the gel proper-ties, however there will be also aggregation of the nanoparticles. Thisaggregation influences the final morphology of the foam/aerogelformed. A high concentration of cellulose will make the removal ofwater from the structure more difficult, mainly due to strong capillaryforces.

High porosity foams were prepared from cellulose I nanofiber sus-pensions of different concentrations (0.7–10 wt.%). The results indicatethat CNF assembles as foams with sheet-like cell wall. Aerogels pro-duced from high concentration of CNF have lower porosity, higher den-sity, high degree of aggregation and improved mechanical propertiescompared to those prepared with low content of CNF [163].

A recent study evaluated the effect of concentration, particle size,surface charge and crystal structure on the self-assembling behavior ofCNC and CNF (at concentrations below 1wt.% during freeze drying pro-cess) [69]. The higher concentration range (0.5–1 wt.%) producedfoams with thick sheet-like lamellar structures whereas at lower con-centrations (0.05 wt.%) fibrous network structures, with orientation inthe direction of the freezing, were observed. The crystalline structureof the cellulose was also changed bymercerization to produce celluloseII crystals and nanofibrils. The results suggest that the crystallinity didnot affect the self-assembly behavior at higher concentrations (0.5 and1 wt.%). It was also suggested that the different self-assembly behaviorwas induced by hydrogen bonding (intramolecular and inter-chain)within celluloses I and II [69].

Studies on aerogels prepared from 2 wt.% CNF suspensions indicatethat quick freezing at cryogenic conditions (propane,−190 °C) allowedthe self-assembly of the CNFs into large structures forming a 3-D porousnetwork with uniform pores at different length scales. In contrast, slowfreezing induces extensive aggregation of the nanofibers producingaerogels with sheet-like pore walls [145].

Three different freezing–drying procedures in the production of CNFaerogels (freezing in liquid nitrogen, supercritical CO2 drying and fastfreezing in liquid propane followed by vacuum sublimation)were com-pared. The best results in terms of morphology and porosity wereobtained with supercritical CO2 drying and fast freezing with liquidpropane. The nanofibers assembled forming a porous network with nopreferred orientation whereas the freezing on liquid N2 induced forma-tion of sheet-like structures [102].

The CNF surface charge strongly influences aerogel formation andproperties. Nanofibers with a higher degree of oxidation produceaerogels with a high porosity and pore size, indicating the contributionof the negative surface charges on aerogel morphology [166]. The CNFcharge can be adjusted by the choice of nanocellulose source and alsoby the preparation procedure, mainly by surface modification. Surfacemodification of nanocellulose to adjust the electrostatic surface chargeshas been applied, typically by chemical modification of the precursorcellulosic fibers or by physical adsorption of polymers, proteins, poly-electrolytes and polysaccharides. For instance, cellulose aerogels ofvery low density were prepared from suspensions of carboxylated CNFby freezing in liquid nitrogen followed by freeze drying [13]. Aerogelsprepared from cellulose I nanofiber exhibited an improved mechanicalstrength after addition of xyloglucan to the CNF suspensions, reportedlydue to the increase of negative charges by xyloglucan. [163]. TEMPO-oxidized cellulose nanofibers (TOCNF) from wood and tunicate wereused to produce aerogels with low density (~10 mg/cm3) and high sur-face area (ca. 338 m2/g). For instance, 0.4 wt.% suspensions of TOCNFwere adjusted to pH 2 to produce self-standing hydrogels that were

freeze dried into aerogels. It was suggested that the reduction of pH in-duced the alignment of the nanofibrils in the precursor nanocellulosehydrogel due to the low ionization of the carboxylic groups [161]. Theself-assembly behavior of CNCs and TOCNF from rice straw were com-pared in the preparation of porous aerogels by fast freezing in liquid ni-trogen followed by freeze drying. The results indicate aggregation uponfast freezing, with the morphology depending on the size, the surfacecharge and the size distribution of the cellulose nanoparticles [81].Functionalized aerogels were prepared by using TOCNF to bind silvernanoparticles, forming a hydrogel that was cooled using a mixture ofethanol/dry ice and freeze dried into aerogel. The silver ions promotedaggregation of the nanofibers producing a sheet-like porous struc-ture [41].

CNF from different sources to prepare aerogels were compared bySehaqui et al. [164]: CNF from the enzymatic pretreatment of woodand TEMPO-oxidized CNF. Two solvent exchange procedures wereused prior to freeze drying: a one-step solvent exchange process ofthe CNF suspensions with tert-butanol and a six-step process with a96% ethanol solution, pure ethanol and tert-butanol followed by freezedrying. The aerogels produced from the six-step procedure showed ahigh fibrillar network structure, and displayed the highest surface areacompared with the ones obtained by the one step method [164].

The influence on aerogel morphology as a function of the source ofCNF (wood and cotton) and different methods of preparation (highintensity sonication, acid hydrolysis with either hydrochloric acid orsulfuric acid and TEMPO oxidation) was evaluated recently [25]. Theaerogels were prepared by cooling CNF suspensions of low concentra-tion (0.2–0.5 wt.%) at −20 ° C followed by freeze drying. The resultsindicated that the morphology of the aerogels depends on the typeand concentration of CNF, for instance CNF produced from sonicationand TEMPO oxidation produced aerogels with a 3D fibrous networkstructure at low concentration (0.1–0.2 wt.%)whereas a higher concen-tration (≥0.5 wt.%) the same CNF produced 2D sheet-like lamellarstructures with small pores sizes within it. On the other hand theaerogels from CNF produced by acid hydrolysis exhibited a sheet-likeporous structure with pore sizes of different length scales [25].

Further control on the morphology of cellulose aerogels wasachieved by using unidirectional freezing. The results indicate that fibrilalignment follows the direction of ice crystal growth during freezing.Low concentrations of CNF (1–2.5 wt.%) led to open porous structureswhile higher concentrations (3–8 wt.%) produced channel-like pores(lamellar channels) with some fibrils connecting (bridging) the foamcell walls, an effect that was ascribed to the aspect ratio of thenanofibrils [112].

Ultralow density aerogels from 0.6 wt.% suspensions of TEMPO-oxidized CNF (TOCNF) were obtained from rice straw fibers followingtwo freezing approaches prior to freeze drying: one step freezingusing either liquid nitrogen (−196 °C), cooling at −20 °C and cyclicfreezing (−20 °C, 15 h) and thawing (9 h) of the CNF suspensions.The aerogels prepared by cooling at −20 °C exhibited larger poreswith thin smooth walls (sheet-like) whereas those cooled at −196 °Cexhibited smaller poreswith sheet-like structures (Fig. 7). The structur-al differences observed were explained in terms of the rate of cooling,with the fastest cooling inducing small pores due to formation ofsmall ice crystals and the fastest favoring ice nucleation. These resultswere further confirmed by thewater absorbing capacity of the aerogels,with the ones prepared by one step freezing having the highest waterabsorbing capacity [82].

Bacterial cellulose (BC) has been used to prepare aerogels. Forexample, the approach used by Liebner et al. [123] included solventexchange with ethanol to remove water from the precursor hydrogelsfollowed by supercritical CO2 drying. The solvent exchange step wasuseful to avoid shrinkage/collapse of the aerogel structure duringdrying. Lightweight aerogels (8.25 ± 0.7 mg cm−3) with well-definedporous structure (10 nm) and high surface area (200 m2/g) were ob-tained. Hydrophobization of aerogels prepared from bacterial cellulose

Fig. 7. Aerogels prepared from freeze drying of 0.6% CNF suspensions frozen at−20 °C (a–c) and−196 °C (d–f). Radial cross-sections (a, d) and longitudinal sections are shown (c, f). Thelower left insets are photographs of aerogels. Inset scale bar = 500 μm (a, b, c and e) and 200 μm (d and f). The arrow in (f) points to one microfibril on the surface. Reproduced withpermission from Ref. [82] (Copyright© The Royal Society of Chemistry 2014).

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was achieved by reaction with alkyl ketene dimer through differentmethodologies followed by supercritical drying in CO2 [159].

The presence of BC helps to increase the stability of monomer foamsproduced by mechanical frothing of epoxydized soybean oil. The me-chanical properties were improved with addition of 0.5 wt.% BC whencompared to foams without BC but the Young's modulus of the foamscontaining 1 wt.% BC decreased. In addition, BC was noted not to inter-ferewith further polymerization of the foams and to reduce the porosityof the polymerized foams [117].

Recent advances on nanocellulose-based aerogels include starchfoams reinforced with microfibrilar cellulose [172], hybrid cellulose/montmorrillonite [62], silica-cellulose aerogels [160], functionalizedaerogels with oil absorbing properties and photo-switchable hydro-phobicity [90], aerogels for drug delivery [180], CNF as templates forceramic materials [134], hollow ceramic nanotubes [102], acrylic foams[191] as well as aerogels that can withstand stress under wet condi-tions [159,197].

In summary, aerogels from nanocelluloses offer an alternative toconventional aerogels in terms of renewability, lightweight andmechanical properties. Understanding the influence of the different var-iables involved in the preparation of the aerogels and the self-assemblyof the nanocellulose will provide better control onmorphology andwillallow tailoring the properties of the final materials. A tradeoff betweenthe effects of different variables, the freezingmethod (fast vs. slow), thesource and surface charge of nanocellulose, and the concentration of thestarting suspensions, altogether play important roles on the finalaerogel properties (including density, morphology, mechanical perfor-mance, and porosity).

7. Hybrid materials

Hybrid materials that combine organic or inorganic components areideally suited to fulfill specific application requirements. Typical exam-ples include composites formed by the incorporation of nanocelluloseinto a main component or matrix [92] and therefore the ability ofnanocellulose to formhybrid systems has been explored in combinationwith a variety of particles and polymers. One of the challenges in thisarea includes the possible lack of adhesion of nanocellulose with theother components. This has been approached by covalent surface mod-ification of CNF, adsorption of different biopolymers on the surface of

cellulose, etc. For example, DNA oligomers were grafted onto TEMPO-oxidized CNC produced from cotton. The DNA-grafted CNC self-assembled into larger aggregates compared to the unmodified CNC[131]. Likewise, the layer by layer (LbL) assembly technique has beenused to adsorb polymers onto the surface of CNF. Some examplesinclude the preparation of CNC-collagen hybrid films by adsorbingcollagen from acidic solutions onto CNF using LbL assembly, resultingin very thin layers (1.4–6 nm) of collagen. It was suggested that theassembly was driven by hydrogen bonding [36]. The LbL techniquewas also used to prepare hybrid luminescent single-walled carbonnanotubes-CNC films. Interestingly, CNC facilitated the dispersion of asignificant amount of the carbon nanotubes (up to 24%) in aqueousmedia and SEM imaging indicated a self-alignment of CNC along thenanotube axis, which was ascribed to the hydrophobicity of some ofthe crystalline faces of cellulose. These dispersions were used toproduce films that exhibited near infrared luminescence [140].

CNC has been used to prepare hard, transparent hybrid CNC-calciumcarbonatematerials [63] and CNC-inorganic hybridmaterials have beentested in the preparation of catalytic nanoparticles. For instance palladi-um nanoparticles (~3.6 nm diameter) were deposited onto the surfaceof CNCs, these hybrids served as catalyst for the hydrogenation ofphenol in aqueous media [28]. Similarly, carbonate-stabilized goldnanoparticles were deposited onto CNCs to produce a catalytic hybridwith gold loading of up to 1.55 wt.%, the hybridswere effective catalystsin the reduction of 4-nitrophenol [107]. Likewise, titanium dioxide-CNChybrids were prepared and their catalytic activity was evaluated for thephoto-degradation of a model dye [128].

CNC from bacterial cellulosewere used to reinforce poly(vinyl) alco-hol composites containing silver nanoparticles for improved thermaland mechanical properties [64] and the self-assembly of CNC by shearalignment into highly ordered crystalline structures have been used toproduce hybrid filmswith gold nano-rods, which exhibit strong surfaceplasmon resonance [19]. Bacterial CNF has also been used as templatesfor growing cobalt-ferrite nanoparticles to produce magnetic hybridaerogels with adjustable response depending on the concentration ofprecursor salts (FeSO4/CoCl2) [141].

Hybrids of nanocellulose with different clays have been explored forpackaging applications. Some examples include CNF-montmorillonitehybrid nanopaper [125,126], TOCNF-montmorillonite cast films [186](see Fig. 8), CNF-montmorillonite/PVA films with polyacrylic acid as

Fig. 8. SEM images of the cross sections of films made off TEMPO oxidized CNF (a), neat montmorillonite (b) and a mixture of 50:50 by weight of TEMPO-CNF and montmorillonite(adapted with permission from Ref. [186]. Copyright 2012 American Chemical Society).

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crosslinker [169], vermiculite-CNF hybrid films obtained by solventcasting [13], etc. The cationization of nanocellulose has been appliedand evaluated in preparation of hybrids with different layered silicates(smectite, talc, kaolin, vermiculite and mica). The results indicate adependence of the barrier properties with the amount of silicate in thecomposites [70].

In an attempt to mimic the properties and performance of naturalmaterials such as nacre, CNF was used in the preparation of biomimeticcomposite materials. A genetically modified class II hydrophobinproteinwas used to bind graphene flakes and CNF. Themechanical prop-erties of nanopapers improved after addition of graphene, furthermore,for a constant amount of graphene the Young's modulus increasedlinearly with the amount of engineered protein [104]. Similarly, hybridgraphene-cellulose-polyvinyl alcohol organic aerogels were preparedusing TOCNF and the results indicated materials with ultralow density,good mechanical and thermal properties [79]. The ability ofnanocellulose to interact with proteins and other biopolymers in natureopens opportunities for new applications and preparation of biomimeticmaterials [184].

8. Current prospects for nanocellulose: Oil and gas, composites,coatings and paints

The versatility of nanocellulose as a naturalmaterial makes room forconsideration in a variety of possible applications. As an example, theuse of nanocellulose in oilfield applications has captured the attentionas can be judged by patents in the subject area filed during the lastyears. In these patents different forms of nanocellulose have beenused; for instance bacterial cellulose was added to fracturing fluids toimprove its rheological properties and to reduce the friction lossthrough well casings [182,183]. Also bacterial cellulose has been pro-posed for substitution of conventional gellants or in combination withgellants to improve the rheological properties of drilling muds [119].More recently, CNC was added to well fluids to increase the strengthof cement or to increase the viscosity of water-based well fluids suchas fracturing and gravel packing fluids [156]. Cellulose nanocrystalshave been also proposed to prepare well treatment fluids to substituteconventional polymers in such formulations [105].

In a recent work, the reinforcement of polylactic acid using TEMPOoxidized cellulose nanofibers graftedwith amine-terminated polyethyl-ene glycol was proven to be more effective than reinforcing the samematrix with single wall carbon nanotubes. This was explained by theimproved dispersion of the nanofibers in the matrix. The compositesreinforced with cellulose nanofibers were stronger, tougher and moreoptically transparent compared to the composites based on carbonnanotubes [59]. Likewise, the incorporation of graphite nanoplateletsinto BCmembranes produced electrically conductive cellulosic compos-ites. These nanoplatelets were fixed to the BC network by different

methods and the resultant composites showed an increased electricalconductivity, with a conductivity peak for systemswith 9% nanoplateletloading in the composite [201]. The media used for growing bacterialcellulose were modified to incorporate polyvinyl alcohol and to createa composite assembled in situ that resulted in dramatically improvedmechanical, thermal and dimensional properties [22].

An application for nanocellulose in constant evolution is surfacecoating [6,38,39,65,121,132,152]. Starch-based coating formulationshave been filled with CNF and zinc oxide in order to prepare coatingsin antibacterial paper. The coated papers showed bactericidal activityagainst Gram positive andGramnegative bacteria [132]. Edible coatingsof gelatin and BC were prepared to coat banana and eggplant epicarpsfor preservation purposes. The wettability of such coatings was testedfor their adhesive properties and it was found to be highly influencedby the concentration of the cellulose nanofibers [6]. Cellulosenanocrystals have been also used to coat plastic films for flexible pack-aging applications. The cellulose nanocrystals reduced friction whilekeeping the optical properties of the coating [121,122].

9. Emerging nanocellulose applications

Initially, CNF research concentrated in Europe whereas CNCs weremainly investigated in North America. At the same time production ofdifferent nanocellulose grades is developing towards full industrial pro-cesses and the promise of emerging markets is gaining momentum.Rheology modification [39], bio-packaging materials [157] and rein-forced composites [93] are only a few of the applications discussed inthis review. In this sectionwe discuss recent prospects for nanocellulosein medical, cosmetic, electronic and sensing applications. Success insuch application requires a full understanding of nanocellulose interac-tions in colloid systems.

CNF is often pressed into thin transparent films referred to asnanopapers [174] and has been identified as potential packaging mate-rial, especially if it is furthermodified for barrier properties. Demand forenvironmentally friendly packaging materials is driving efforts in thisarea and will evidently lead to high volume products. In addition topassive packaging, CNF films have been shown to be suitable for loadingactive substances and for controlled release [111].

The highly crystalline CNC provide a rigid surface with tunable func-tional groups available formodification and grafting. Typical CNC gradesshow no cytotoxicity. For biomedical applications such properties arevery appealing, and hence, several research groups are suggesting CNCfor various medical uses [108]. Modified CNCs have been proposed intargeted delivery of chemotherapeutical drugs [42]. It was shown thatthe shape of CNC is beneficial for delivery of folic acid to targetmamma-lian brain cancer tumors. Recently Zoppe et al. [204] demonstrated CNC-based systems as viral inhibitors (alphavirus infectivity) and suggestedthat CNC can be applied for other viruses as well, for example for

Fig. 9.Modification of CNF (denoted as NFC) films with amine reactive groups for detection of biological species (reprinted with permission from Ref. [142]. Copyright 2012, AmericanVacuum Society.).

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inhibition of HIV. In addition to delivery, CNC offers potential for detec-tion and biosensing. For example, Edwards et al. have reported biosen-sors based on CNC by peptide conjugation for detection of humanneutrophil elastase [47,48].

As presented above the benefit of CNCs in medical applications anddrug delivery mainly rely on usage in liquid media. For biomedicalpurposes, where solid nanocellulosic materials are needed, CNF is thematerial of choice. Due to their benign nature, high available surfacearea, smoothness and reduced porosity, CNF films have been reportedas potential substrates for biosensors. These biosensors are typicallyprepared by binding peptides or proteins to the support matrix. CNFsubstrates activated by EDS/NHS chemistries have been shown to bindto bovine serum albumin (BSA), providing non-porous cellulosic filmsfor diagnostics [142]. TOCNF was used to prepare the support filmswith carboxyl groups, which were then converted to amine-reactivespecies. These substrates were then further used to bind BSA and poly-clonal antihuman IgG (Fig. 9). Another route to prepare biosensors forBSA and IgG detection has been reported by activating the CNF surfacevia co-polymer grafting. A peptide with specific affinity to human IgGwas conjugated to the grafted polymer for a highly selective bindingsystem [198,199]. The several beneficial properties already stated aswell as its potential as supporting material are expected to make CNFthe platform of choice in the development of bioactive interfaces.

Bacterial cellulose, with its a high crystallinity and purity [21] hasattracted interest especially in applications requiring contactwith tissueor otherwise demanding non-toxicity [196]. BC has been suggested forskin care applications such as forwound dressing [57] and their pelliclesare known to be able to bind a vast amount of water, which is beneficialin skin burns and can potentially absorb exudates. Commercial BC prod-ucts for such applications already exist. More opportunities are likely todevelop by combining topical delivery of a medicine on burned/woundareaswith other already established benefits. In addition to external usethere are excellent prospects for using BC derivatives in vivo. BC hasbeen studied as an interesting scaffold [173] or when combined withother materials [136]. For bone generation the required porosity of theBC scaffold can be easily attained by incorporating sacrificial substancesthat can be later removed and by creating spaces in the structure [195].

In addition to (bio)medical uses, nanocelluloses have characteristicsthat make them ideal for use in electronics. Recently CNF was shown towork as precursor of carbon nanofibers, for use as anode material insodium-ion batteries ([129], see also [74,27]). Moreover it was shownthat carbon fibers derived from CNF have a superior reversibility, therate capability required for fast charging and excellent cycling capacity.Preparation of CNF-based electroactive composites by coating cellulose

fibrils with polypyrrole has been demonstrated by Nyström et al. [138].The cellulose compositewas preparedwith a straightforwardmethod ofcoating the fibrils in aqueous solution followed by filtration and drying.The resulting composite was conductive, electro-active and suitable forenergy storage and electrochemically controlled separation.

Nanopaper has been used as a template to produce electronic mate-rials [75]. Taking advantage of the dense packing of nanocellulose in theform of a solid film and its minimal porosity, compared to traditionalpaper treated with conductive material, the obtained nanocellulosefilm exhibited improved conductivity [73]. Such cellulosic conductivematerial is an excellent candidate for developing flexible electronicswith the benefit of lightweight and good thermal and chemical stability.In fact, progress on the preparation of flexible electronics and transpar-ent nanocellulose films has been reported recently. When mixed withconductive materials nanocellulose enables the formation of a conduc-tive composite that can be further used as a supercapacitor; for examplethose obtained by BC films coated with carbon nanotubes enclosing anionic liquid polymer gel [86]. Similarly CNF-based supercapacitorshave been prepared in combination with graphene [60]. CNF has beenalso used in organic light emitting diodes by coating it with indium tinoxide film [118]. When applied as support for conductive film CNF al-lows use of the OLED in applicationswhere biocompatibility is required.CNCs have also been reported in composite electronics with tin oxidelayers for flexible organic field effect transistor [179]. The abovedescribed advances in flexible electronics highlights nanocellulose as asupporting or active material as well as a coating for flexible packaging[121].

Interest towards green applications has advanced steadily in thefield of catalysis. TOCNF has been demonstrated as ideal support ofcopper ions to prepare catalytic material via the Huisgen click reac-tion [100]. Similarly, gold particles have been incorporated in CNF sup-ports to manufacture composite structures with excellent catalyticactivity [14,101].

Due to their chiral characteristics and structuring ability, CNCs canbe used to promote alignment of inorganic particles. CNCs have beenused to template chiral structures of silica by adding a silica precursorto CNC dispersions followed by recovery of the inorganic structure bypyrolysis of the organic component [165]. This enabled preparation ofchiral ordered mesoporous materials with photonic properties thathave been proposed for applications such as chiral separation, sensingand catalysis. Likewise, CNCs have been used for templating poroustitania films by incorporating titania precursor to CNC suspensions[78]. In addition to the organization of the CNC the morphology of thefilms was reported to be dependent on the concentration of the titania

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precursor. The titania films can be utilized in photovoltaic devices andphotocatalysis.

10. Final remarks

In this reviewwe highlighted some uses of nanocelluloses (cellulosenanofibrils, CNF and cellulose nanocrystals, CNC) obtained, for example,by plant fiber deconstruction. Together with bacterial cellulose thesematerials are readily available, renewable, and sustainable and thusthey represent a natural resource of incredible importance in today'snanotechnologies. Nanocelluloses properties and attributes can be suit-ed to different applications and to create new materials. Some ofthe current research activities concentrate on the utilization ofnanocellulose in low-volume, high performance applications, as thosehighlighted in this review. However, solutions for traditional highvolume, low cost products and additives in papermaking [5], packaging[157], paints [32], composites, food, oil and gas and cement are emerg-ing rapidly.

Common to these activities is the fact that nanocelluloses have aninherent ability to self-assemble at interfaces. The development ofmethods for surface functionalization of nanocellulose can be expectedto allow further control to attain supra-structures and highly hierarchi-cal assemblies, much resembling the original, precursor cell wall offibers. Bringing together the knowledge of colloids and interfaces andnanocellulose will be critical in furthering their potential, taking advan-tage of structural, physical, chemical, thermal, piezoelectric, and otherproperties.

Acknowledgments

OJR is grateful for funding support by the Academy of Finlandthrough its Centres of Excellence Programme (2014-2019) and underProject “Molecular Engineering of Biosynthetic Hybrid MaterialsResearch” (HYBER). CR-A is grateful for funding from the EuropeanUnion's Seventh Framework Program (FP7/2007-2013) underCOOPERATION program NMP-theme, grant agreement no. 314212 andthe EU Operational Programme for Cross-border Cooperation: Spain-Portugal (POCTEP 2007-2013); European Regional Development Fund(ERDF).

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