Functionalization of cellulose nanocrystals for ...

Accepted Manuscript Functionalization of cellulose nanocrystals for advanced applications Juntao Tang, Jared Sisler, Nathan Grishkewich, Kam Chiu Tam PII: DOI: Reference:

S0021-9797(17)30100-5 http://dx.doi.org/10.1016/j.jcis.2017.01.077 YJCIS 21983

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

22 November 2016 15 January 2017 22 January 2017

Please cite this article as: J. Tang, J. Sisler, N. Grishkewich, K.C. Tam, Functionalization of cellulose nanocrystals for advanced applications, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.01.077

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Functionalization of cellulose nanocrystals for advanced applications Juntao Tang, Jared Sisler, Nathan Grishkewich, Kam Chiu Tam

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue, Waterloo, ON, N2L 3G1, Canada

Abstract Replacing the widespread use of petroleum-derived non-biodegradable materials with green and sustainable materials is a pressing challenge that is gaining increasing attention by the scientific community. One such system is cellulose nanocrystals (CNCs) derived from acid hydrolysis of cellulosic materials, such as plants, tunicates and agriculture biomass. The utilization of colloidal CNCs can aid in the reduction of carbon dioxide that is responsible for global warming and climate change. CNCs are excellent candidates for the design and development of functional nanomaterials in many applications due to several attractive features, such as high surface area, hydroxyl groups for functionalization, colloidal stability, low toxicity, chirality and mechanical strength. Several large scale manufacturing facilities have been commissioned to produce CNCs of up to 1 000 kg/day, and this has generated increasing interests in both academic and industrial laboratories. In this feature article, we will describe the recent development of functionalized cellulose nanocrystals for several important applications in ours and other laboratories. We will highlight some challenges and offer perspectives on the potentials of this sustainable nanomaterial.

List of abbreviations Cellulose nanocrystals (CNCs) Nanofibrillated cellulose (NFC) Small angle neutron scattering (SANS) High internal phase emulsion (HIPE) Didecyldimethylammonium bromide (DMAB) Cetyltrimethylammonium bromide (CTAB) Poly(dimetheylamino ethylmethacrylate) (PDMAEMA) Poly(oligoethylene glycol) methacrylate (POEGMA) Poly(methacrylic acid) (PMAA) Poly(N-isopropylacrylamine) (PNIPAM) Hydroxyethyl cellulose (HEC) Polyacrylamide (PAAM) Polarized optical microscopy (POM) Tetraethyl orthosilicate (TEOS) Tetramethyl orthosilicate (TMOS) Titanium(IV) ethoxide (TEOT) Urea-formaldehyde (UF) Poly(vinyl alcohol) (PVA) Poly(amidoamine) (PAMAM) Melamine-formaldehyde resin (MF) Polyrhodanine (PR) Poly(N-vinylpyrrolidone) (PVP) Chitosan oligosaccharide (CSos) Imipramine hydrochloride (IMI) Procaine hydrochloride (PrHy) Doxorubicin (DOX) Poly(ethyl ethylene phosphate) (PEEP) Vitamin C (VC) Fluorescein-5’-isothiocyanate (FITC) Rhodamine B isothiocyanate (RBITC)

1. Introduction In nanoscience and nanotechnology, the synthesis and modification of nanomaterials with well-defined structure and functionalities have attracted growing interest due to their many potential applications [1,2]. Recent advances in nanomaterials have led to the development of functionalized nanoparticles that hold promise in various industrial sectors, such as medicine, electronics, biomaterials and energy production [3,4]. However, a large proportion of chemicals used to produce nanomaterials are derived from petroleum-based resources, and they involve the use of toxic reagents that are harmful to the environment. Due to concerns on global warming and sustainable development, there is an urgent need to replace traditional raw material supply with those derived from renewable resources [5]. Furthermore, the ability to transform cheap and abundant material to yield high value products will offer significant advantage. It is well-known that cellulose is the most abundant naturally occurring polymer found in this planet [6]. It represents about 1.5x1012 tons (metric tonne) of total annual biomass production and is considered an inexhaustible source of raw material capable of meeting the increasing demand for environmentally friendly and biocompatible products [6,7]. Economical and environmentally friendly methods have been developed to process cellulosic materials by dissolving them in NaOH/urea solution or ionic liquids, as reported by Zhang’s [8–10] and Rogers’ laboratories [11,12], respectively. Unfortunately, the approach of disassembling cellulosic materials to their molecular entities sacrifices the attractive physical properties of the crystalline domains formed by the inter- and intra-molecular hydrogen bonds. By careful control of the disassociation of amorphous regions while retaining the crystalline domains, a new form of crystalline cellulose commonly referred to as nanocellulose is produced [7]. These nanocelluloses have size in the nanometer regime, and they possess many attractive characteristics, such as versatile fiber morphology, hydrophilicity, easy surface modification, large surface area and high aspect ratios [7]. Depending on their dimensions, origins and processing conditions, nanocellulose can be divided into two main categories, namely cellulose nanocrystals and cellulose nanofibrils. In this feature article, we will focus mainly on cellulose nanocrystals, however reviews on cellulose nanofibrils can be found in several recent publications [7,13–16].

2. Cellulose nanocrystals Cellulose nanocrystals (CNCs) are the crystalline domains extracted from wood fiber through acid hydrolysis. They are rigid, rod-like particles with a width of several nanometers and lengths of up to hundreds of nanometers [17,18]. The microscopic properties (physical and surface chemistry) of CNCs have an important bearing on their macroscopic properties (rheology, colloidal stability, etc), and these are summarized in Figure 1.

Figure 1 A summary of the physical and chemical properties of cellulose nanocrystals. 2.1 Physical properties The main physical dimensions for cellulose nanocrystals include the length (L), diameter (D) and aspect ratio (L/D), which are dependent on the source of cellulose or hydrolysis conditions (acid type, reaction time and temperature). CNCs derived from wood and cotton is usually shorter than that obtained from tunicate and bacterial cellulose because the latter possess a higher degree of crystallinity [17,18]. Lower fractions of amorphous regions make them more resistant to degradation from acid hydrolysis resulting in larger rod structures. Typically, the aspect ratio ranges from 10-30 for CNCs derived from cotton and up to approximately 70 for tunicate. Sulfuric and hydrochloric acids are the most commonly used acids in the hydrolysis process, but other strong acids, such as phosphoric and hydrobromic acid have also been reported [14,17]. Different acids may lead to significant differences in the dispersity and colloidal

stability of CNCs. For instance, CNCs derived from sulfuric acid hydrolysis disperses readily in water due to the abundance of negatively charged sulfate ester groups on their surface, while aqueous solutions of CNCs produced from hydrochloric acid hydrolysis display poor colloidal stability [18]. In addition to the properties discussed above, cellulose nanocrystals also possess other attractive features: large surface area (250~500 m2/g), and improved mechanical strength (tensile strength 7 500 MPa and Young’s modulus of 100~140 GPa)[19]. The extremely high Young’s modulus is an attractive characteristic for application in nanocomposites. Liquid crystalline behavior has also been observed for non-flocculating cellulose nanocrystal suspensions. In the dilute solution regime, CNCs are isotropic, and at higher concentrations the nanoparticles align to form an anisotropic nematic phase [17]. Beyond this critical concentration, CNCs dispersions display shear birefringence, and they can spontaneously phase separate into an upper isotropic and a lower anisotropic phase. The chiral nematic or cholesteric structure in the anisotropic phase possesses a helical twist along the main axis, with the orientation of each stack planes being rotated about the perpendicular axis. The parallel alignment of the CNCs is attributed to the well-known entropically driven self-orientation phenomenon, and the helix of cellulose nanocrystals is left-handed, reflecting the intrinsic chirality of crystalline cellulose. However, the pitch distance between the different planes can vary significantly, ranging from less than 1 to more than 50 nm, and it is a function of temperature, sonication time and ionic strength, but it is independent of the concentration of CNCs [20]. More interestingly, the chiral nematic structure of the suspension can be preserved via slow and complete evaporation of the water phase, yielding an iridescent film. The spectacular iridescent coloring originates from the reflection of light by the chiral nematic phases in a Bragg-type manner. The reflected color of the films can be manipulated by varying the pitch of the helical structure, and these iridescent materials are of great interest in coatings, security features and sensors.

2.2 Surface chemistry properties As the main chemical component of cellulose nanocrystals consists of cellulose chains, all classical chemistry on cellulose or polysaccharides is applicable to cellulose nanocrystals. However, CNCs is thought to be less reactive when compared to amorphous cellulose chains since most of the polymer chains are buried within the inaccessible crystalline regions. The monomeric glucose units of the cellulose chain possess three hydroxyl groups, which provide

reactive platforms for chemical modifications. Aside from abundant hydroxyl groups, the surface of CNCs may contain other types of functional groups that are directly related to its preparation and processing conditions. The common functional groups are sulfate groups (

) and

-

carboxyl groups (-COO ). With additional mild post-hydrolysis reactions, aldehyde groups (CHO), amino groups (-NH2) or thiol groups (-SH) may also be introduced to the CNCs surface. Depending on the specific functional groups on the surface, CNCs nanoparticles exhibit different charge properties. CNCs bearing sulfate or carboxylate groups on the surface are negatively charged over a wide range of pH conditions (above its pKa), while the amino groups are positively charged below the pKa values of the weak base. In addition, modifying the cellulose nanocrystals with quaternary ammonium groups will render their surface with permanent cationic charges.

Figure 2 Evolution of the number of research publications and citations on cellulose nanocrystals during the past ten years (2006-2016) according to ISI Web of Knowledge system.

3. Functional cellulose nanocrystals for advanced applications In the past 20 years, the investigation and utilization of cellulose nanocrystals in functional materials has become an active field of research activity and many researchers have dedicated their efforts to the study of this remarkable material, which is reflected by the growing number of publications and citations as summarized in Figure 2. The prospect of modifying and functionalizing cellulose nanocrystals is attractive as it enables the creation of advanced materials with new or improved properties [21]. By introducing the functional components (materials or chemical groups) to the system, synergistic effects can be achieved, which can

impart electronic, magnetic, catalytic, fluorescence and optical properties. Thus, their functionalities will be improved and potential applications in specific fields can be expanded. In the current article, we will highlight recent developments in the functionalization of cellulose nanocrystals and offer prospect and potentials of this component in novel sustainable nanomaterials for the future.

3.1 Emulsion stabilizer Currently, there is a growing market trend toward the formulation of products that can maintain the consumer perception of being natural and “green”. This has motivated the production of biobased nanoparticles for the formulation of Pickering emulsions for the food and cosmetic industries [22,23]. Cellulose nanocrystals are ideal for this application, and they have been shown to be effective Pickering emulsifiers [24,25]. They can stabilize monodispersed oil (hexadecane) droplets of ~4 μm in water phase against coalescence for 4 months (Figure 3A). This is due to the partitioning of stable CNCs nanoparticles at the oil-water interface that significantly enhance the stability of oil droplets [24]. Further research suggested that cellulose nanocrystals with a charge density greater than 0.03 e/nm2 could not efficiently stabilize oil droplets due to the strong electrostatic repulsions between the nanoparticles located at the oilwater interface [25]. In addition, neutral CNCs extracted by HCl hydrolysis performed better than sulfated CNCs at the oil/water interface. Kalashnikova and Carpon showed that the aspect ratio has a direct impact on the interfacial coverage, where a low aspect ratio resulted in a dense organization of short nanocrystals at the oil-water interface [26] (Figure 3B). Recently, the packing characteristics of CNCs (195 nm long, 23 nm width and 6 nm thick) with different surface charges at the interface was examined by Carpon and coworkers [27] using small angle neutron scattering (SANS). They reported that the average thickness of the layer around the oil droplets was determined to be 7 and 18 nm for charged and uncharged CNCs, respectively. This result supported the postulate that the (2 0 0) crystalline plane of the nanoparticles directly interacts with the interface. Due to the colloidal network structure that forms at the interface, oilin-water high internal phase emulsion (HIPE) systems [28] as well as water-in-water emulsions [29] could be stabilized. The HIPE system displayed a gel-like behavior, which can only be produced via a two-step method consisting of first the formation of the primary Pickering emulsion and second a subsequent swelling.

Figure 3 (A) Scanning electron micrographs of polymerized styrene Pickering emulsion stabilized by bacterial cellulose nanocrystals; Schematic representation of the stabilization of the Iβ cotton cellulose nanocrystals at the oil/water interface

[25]; (B) Scanning electron

microscopy (SEM) images of polymerized styrene–water emulsions stabilized by CNCs with different aspect ratios [26]; (C) Water-dodecane emulsions (1:1 by volume) stabilized by 0.25 wt.% CNCs and surfactant (a) CTAB and (b) DMAB with concentrations from 0 to 16 mM [30]; (D) Responsive behavior of emulsions (PDMAEMA-g-CNCs, 0.5 wt %) by adjusting the pH values of the aqueous phase [37]; (E) Schematic illustrating the pH-responsive behavior of Pickering emulsions stabilized by CNCs-POEGMA-PMAA and the demonstration for oil harvesting application [38]; (F) Illustration of a redispersable Pickering emulsion using tannic acid and HEC modified cellulose nanocrystals as stabilizer [40].

Aside from the investigation on emulsion systems stabilized by pristine CNCs, various modification strategies have been used to manipulate the surface functionalities of CNCs in controlling the physical properties of Pickering emulsions. This was achieved through the incorporation of surfactants [30], functional groups or surface active polymers, for example TEMPO oxidation and adsorption [31], periodate oxidation and amination [32], esterification [33], as well as long alkyl chain grafting [34]. Two types of cationic charged surfactants didecyldimethylammonium bromide (DMAB) and cetyltrimethylammonium bromide (CTAB) were adsorbed on the CNCs surface to tailor the hydrophobicity of the nanoparticles, and their

capability to stabilize emulsions was investigated by Cranston and coworkers [30]. They observed a double transitional phase inversion (from oil-in-water to water-in-oil and then back to oil-in-water) for emulsions stabilized by CNCs with increasing amounts of DMAB (a more hydrophobic molecule). However, no phase inversion could be induced for CNCs modified by CTAB (Figure 3C). Similarly, Capron and coworkers reported a simple method to prepare hydrophobic CNCs by adsorbing quaternary ammonium salts onto the TEMPO-oxidized CNCs, and the modified nanoparticles were capable of stabilizing inverse water-in-oil emulsions [31]. Pelton and coworkers studied the effects of both surfactant and water-soluble polymers (hydroxyethyl cellulose or methyl cellulose) on the properties of Pickering emulsions stabilized by cellulose nanocrystals [35]. The polymer coated CNCs nanoparticles produced emulsions with smaller droplet sizes, and the emulsions could resist coalescence when subjected to multiple cycles of heating and cooling. Compared to systems modified by physical adsorption, covalent chemical modifications can provide a more robust and versatile platform. Capron and coworkers tailored the hydrophobicity of CNCs and nanofibrillated cellulose (NFC) by chemical modification with lauroyl chloride (C12) [34]. They observed that the Pickering emulsions stabilized by the combination of two types of modified nanocellulose could be formulated into oil-in-water-in-oil (o/w/o) emulsions depending on the degree of substitutions. Sebe and coworkers modified the CNCs surface with vinyl acetate (VAc) and vinyl cinnamate (VCIn) and observed that the esterification treatment may significantly impact their utilization as Pickering emulsifiers [33]. VCIn-treated particles could only stabilize the cyclohexane-in-water emulsions, while the acetyl modified CNCs could be used to prepare stable ethyl acetate-in-water, toluene-in-water, and cyclohexane-in-water emulsions. Furthermore, in order to tailor the control of Pickering emulsions for specific applications, much research has been devoted to the development of emulsifiers that activate and deactivate in response to external stimuli. Zoppe et al. grafted thermo-responsive Poly(N-isopropylacrylamine) (PNIPAM) onto CNCs surface and compared the different nanoparticles in stabilizing emulsion systems [36]. They found that modified cellulose nanocrystals could stabilize the emulsions for a period of 4 months compared to unmodified CNCs nanoparticles. Tang and coworkers reported a dual-responsive (pH and thermo) system based on poly(dimetheylamino ethylmethacrylate) (PDMAEMA) grafted cellulose nanocrystals [37]. They demonstrated the feasibility of stabilizing both toluene- and heptane-in-

water emulsions under basic conditions. Decreasing the pH values lead to the protonation of tertiary amines on PDMAEMA chains, that promotes the electrostatic interaction between CNCs particles and polymer chains, resulting in a reversible particle aggregation and emulsion instability (Figure 3D). Following this, they grafted binary polymer brushes consisting of poly(oligoethylene glycol) methacrylate (POEGMA) and poly(methacrylic acid) (PMAA) on the surface of CNCs nanoparticles [38]. This would permit the control of the stability of Pickering emulsions using two types of triggers, i.e., POEGMA for temperature and PMAA for the pH. They demonstrated a reversible emulsification-demulsification process controlled by pH using the binary brush grafted nanoparticles, where the emulsification and oil-water separation could be repeated 5 times without any loss in efficiency (Figure 3E). Cellulose nanocrystal based Pickering emulsions could be formulated for use in encapsulation systems. Marquis and coworkers described a two-step approach to encapsulate oil microdroplets within alginate microgels [39]. The microdroplets were oil-in-water Pickering emulsions that were stabilized by cellulose nanocrystals and calcium carbonate. They further demonstrated that the CNCs layer could provide an ideal shell to prevent the coalescence of oil droplets. The Ca2+ released from the CaCO3 particles could be used for the gelation of alginate to form the microgel. Nile Red was used as a model compound to trace the release profile after encapsulation, and the double encapsulation protocol could provide better protection as well as sustained release when compared to the traditional method of encapsulation. Hu et al. has also reported another encapsulation system, which was based on emulsions stabilized by hydroxyethyl cellulose (HEC) modified CNCs nanoparticles [40]. They further coated emulsified corn oil in water emulsions with tannic acid, which can be transformed into solid dry emulsions (powders) via freeze-drying (Figure 3F). This work extended the use of surfactant free emulsions for food, cosmetic and pharmaceutical applications.

3.2 Templates for functional materials Hard templating using preformed mesoporous materials (also termed “nanocasting”) has emerged as a versatile technique to prepare materials that cannot be accessed through conventional lyotropic template synthesis, e.g. due to hydrolytic instability of precursors. Cellulose nanocrystal dispersions can exhibit lyotropic chiral nematic behavior at relatively low concentrations (e.g., 1-7 wt%), have lower viscosities and they form over shorter time scales

when compared to several other cellulose derivatives, i.e. ethyl cellulose or hydropropyl ethylcellulose. This has generated a strong interest in using evaporation-induced self-assembly protocols to prepare functional mesoporous materials with chiral nematic order [20,41]. In the templating approach, successive loading of precursors permeate through a stable mesoporous support, often followed by calcination to construct an interconnected network to produce the desired product. The remaining active components can either be templating materials (CNCs) or functional materials introduced in the synthesizing steps (Figure 4A and 4B). Many kinds of ordered mesoporous materials (e.g., carbon, metal oxides, and polymers) can be prepared through hard templating approaches. A summary on the different functional materials is documented in Table 1. MacLachlan and coworkers have published detailed reviews on the use of CNCs as a templating material [42,43], hence we will only discuss research reports published in the last 2 years. Nguyen et al. have demonstrated a representative method of fabricating new porous semiconducting material with chiral nematic structures [44]. This was achieved by first casting SiO2/C composite films by cocondensing SiO2 with cellulose nanocrystals and subsequently pyrolyzing the material. Then, magnesiothermic reduction was applied to finally covert the SiO2/C composite into silicon carbide. The chiral nematic hierarchical structure originating from the evaporation induced self assembly of CNCs was retained. Aside from templating chiral nematic structures in 2D films, cellulose nanocrystals have also been used to prepare three dimensional structural materials. Wang et al. used an inverse emulsion polymerization method to capture the chiral nematic structure of CNCs within microspheres [45] (Figure 4C). They demonstrated that the CNCs tactoids first formed within the water droplets, and subsequent polymerization would solidify the microspheres to retain their structure. By incorporating the hydrolysis reaction of silica within the microsphere formation process, organic-silica microspheres were also fabricated. Upon further removal of the organic matrix, the mesoporous silica microspheres with chiral nematic structures were obtained, and may have potential applications in optical devices and chiral separations.

3.3 Functional cellulose nanocrystal-Inorganic hybrids Inorganic materials are attractive components to be incorporated into cellulose nanocrystal systems due to their size-dependent magnetic, catalytic and optical properties. For

example, graphene or graphene oxide, noble metal nanoparticles, quantum dots as well as metal oxide nanoparticles have generated increasing interest among both academic and industrial laboratories. By taking advantage of cellulose nanocrystals, the aggregation behavior of thermodynamically unstable inorganic nanoparticles can be minimized or eliminated. CNCs are sustainable and ideal for use as supporting materials, and they are considered to be non-toxic and environmentally friendly. The ability of CNCs to form stable colloidal dispersions allows postprocessing into versatile products such as 1D dimensional inks and dispersions, 2D films or 3D composites. In addition, their structures and properties can be readily adjusted during the pretreatment and chemical modification process.

Figure 4 A representative illustration of synthesizing chiral nematic mesoporous materials using cellulose nanocrystals as templates [62]; (B) Optical characterization of CNCs/silica composite films and the corresponding mesoporous silica films [41]; (C) Schematicillustration on the preparation of PAAM/CNCs/Silica composites spheres and the characterization of composite spheres using polarized optical microscopy (POM), scanning electron microscopy and 3D reconstructed confocal fluorescence microscopy [45].

Table 1 Summary regarding functional materials using CNCs as templates Ref.

Precursors

Method

Properties or application

[41]

TEOS or TMOS

Remove CNCs

Mesoporous silica film

[46]

TEOS or TMOS

Remove CNCs +Ag NPs filled

Ag assembled in chiral nematic Silica film

[47]

TEOS or TMOS

Pyrolysis-remove SiO2

Mesoporous Carbon

[48]

TMOS and TiCl4

Mesoporous silica+TiCl4 infiltration + silica etching

Mesoporous TiO2

[49]

Ethylene-bridged organosilica precursors

Remove CNCs

Improving mechanical properties and flexibility compared to pure silica

[50]

TMOS

Remove CNCs

Ionic strength for color changing

[51]

PAAm hydrogel precusors

EISA (evaporation induced selfassembly)

Hydrogel sensor

[52]

Phenol-formaldehyde

Remove CNCs

Chiral mesoporous photonic resin

[53]

TMOS

Remove CNCs

[54]

TMOS

Remove CNCs

[55]

Phenol-formaldehyde

Remove CNCs

Chiral nematic Structures and Actuator Properties

[56]

TMOS PVA to reduce the crack

Remove CNCs+ CdS QDs

Mesoporous, chiral nematic order and luminescence

[57]

Urea formaldehyde

Alkaline treatment to remove UF resin

[58]

TMOS

Remove CNCs

Mesoporous chiral cellulose material displaying dynamic photonic properties Chiral nematic SiO2 for Gas Chromatographic separation

[59]

TEOT (Ti)

Remove CNCs

[60]

Precusor for prussian blue, organosilica precursors, TMOS

Remove CNCs and silica

[44]

TMOS

[61]

Phenol-formaldehyde

[45]

PAAM, TMOS Urea formaldehyde, cobalt ferrite precusor

Magnesiothermic reduction Remove PF, Photonic Patterns Printed (dry or wet) Inverse emulsion polymerization and romove CNCs

[62]

Remove UF and CNCs

Adding Polyols such as glucose to eliminate the crack Detailed investigation on the conditions for film (pH and ratio)

Mesoporous TiO2 film for solar cell Mesoporous coordination polymers Porous semiconducting material with chiral nematic structures Anticounterfeiting materials Noval mesoporous silica microsphere for optical device and chiral separation Electromagnetic interference (EMI) shielding

The deposition of nanoparticles onto the surface of cellulose nanocrystals can generate new hybrid materials that are suitable for use as heterogeneous catalysts in engineering applications, especially for wastewater treatment. By using rod-like pristine cellulose nanocrystals as substrate, various types of catalysts have been prepared including nickel nanocrystals [63], palladium nanoparticles [64,65], TiO2 nanocubes [66], and alloy nanoparticles [67]. They have been widely used in chemical reduction reactions (e.g. 4-nitrophenol to 4aminophenol; oxygen reduction reaction; hydrogenation of C-C and C-O multiple bonds), oxidations (benzyl alcohol to benzaldehyde), coupling reactions (Mizoroki-Heck coupling reaction) and photo degradations (methylene blue or methyl orange). Two excellent review articles on this topic have recently been published [68,69].

Figure 5 Schematic illustration on application of nano-catalysts (reducing 4-nitrophenol) using modified cellulose nanocrystals as carriers via different protocols, (A) Mussel-inspired polydopamine coating [70]; (B) PAMAM dendrimer grafting [71] and (C) Porous melamineformaldehyde resin coating [72]. Aside from the most developed approach on using pristine CNCs nanoparticles, other modification methods have been developed that introduce metal affinity groups or polymers on the surface of CNCs for the purpose of loading inorganic nanoparticles. Tang et al. reported a simple and facile approach using mussel-inspired polydopamine to deposit silver nanoparticles onto cellulose nanocrystals [70]. The catechol-rich polydopamine can be utilized as chelating groups as well as reducing agents, which has the added benefit of not requiring harsh conditions or toxic reducing agents to synthesize the nanoparticles (Figure 5A). By using the model reaction of 4-nitrophenol to 4-aminophenol, they found that the catalytic rate constants were 6 times faster than pristine silver nanoparticles reduced by dopamine. Chen et al. covalently grafted poly(amidoamine) (PAMAM) dendrimers onto the surface of oxidized CNCs using peptidic coupling [71]. The incorporated PAMAM dendrimers provided a well-defined hyper-branched structure, from which the cavities could be used as nano-reactors to control the size of gold

nanoparticles (Figure 5B). By optimizing the synthesis conditions, the best performance of the gold nanocatalyst to reduce 4-nitrophenol was reflected by a turnover frequency (TOF) of 5 400 h-1, which was exceptionally greater than other systems that used modified CNCs as supports. Very recently, Wu et al. extended these ideas to prepare highly porous cellulose nanocrystal support systems [72]. Mesoporous structures were achieved by coating a layer of melamineformaldehyde resin (MF) using polycondensation reactions. The nitrogen enriched resin provided chelating sites that can improve metal ion binding and the porous structure can confine the growth of nanoparticles with a uniform size distribution (Figure 5C). After depositing the Pt or Pd nanoparticles onto MF-CNCs, the hybrid material displayed superior catalytic properties towards the reduction of 4-nitrophenol with a TOF of up to 3168 h-1. In an effort to combat the problem of water pollution, rapid and recyclable treatment materials based on cellulose nanocrystals were prepared by Chen et al. [73]. They deposited superparamagnetic Fe3O4 nanoparticles onto the surface of cellulose nanocrystals, resulting in nanorods that could respond to external magnetic triggers. In order to improve the stability of CNCs against oxidation, a uniform silica layer was coated onto the hybrids. This coating layer could increase the onset decomposition temperature by 60 oC when compared to pristine CNCs. Through further grafting of -cyclodextrins (β-CD), the resulting CNCs@Fe3O4@SiO2 @β-CD hybrids exhibited good adsorption properties towards model pharmaceutical residues in water, such as procaine hydrochloride and imipramine hydrochloride, with an adsorption capacity of 13 mg/g and 14.8 mg/g, respectively. Another example of inorganic-CNCs hybrid application is the use of such system as antimicrobial agent. Silver or zinc oxide nanoparticles (Ag or ZnO) have been widely investigated for this purpose. Drogat et al. reported an approach to deposit silver nanoparticles onto aldehyde functionalized cellulose nanocrystals, where the aldehyde groups were used to reduce Ag+ to Ag0 in mild conditions [74]. The silver nanoparticles were in the size range of 20 to 45 nm and the composite exhibited excellent antimicrobial properties. Shi et al. developed a method to load silver nanoparticles using polydopamine coated cellulose nanocrystals [75]. The hybrid material exhibited improved antimicrobial properties towards gram negative Escherichia coli, and gram positive Bacillus subtilis bacteria with a minimal inhibition concentration (MIC) of 4 µg/ml and 8 µg/ml, respectively. TEM studies were conducted to understand the improved antimicrobial

properties of the hybrid materials. They attributed this result to the high concentrations of silver ions released from the silver nanoparticles near the surface of the organisms, which kill the bacteria. This process was assisted by the adhesive properties of polydopamine towards the cell membranes. Yao and coworkers prepared ZnO/cellulose nanocrystal hybrids through a one-pot green synthesis method, in which the modified carboxyl groups could function as stabilizing and supporting agents for the deposition of ZnO nanoparticles [76]. Carboxyl groups were introduced during the hydrolysis process using citric and hydrochloric mixed acids (C6H8O7/HCl). The obtained ZnO nanoparticles with a hexagonal wurtzite structure and the smallest average diameter of 42.6 nm displayed promising antimicrobial properties against the model bacteria; Escherichia coli and Staphylococcus aureus. Other applications could also be found in bioimaging/biosensing, such as quantum dots (QDs) or carbon dots being the most well-known. Chen et al. demonstrated a one-pot synthesis to prepare well-dispersed QDs in aqueous solution using oxidized cellulose nanocrystals [77]. A co-precipitation method was introduced to synthesize the CdS@ZnS core-shell quantum dots. The carboxylate groups provided the sites for coordinating Cd2+ ions that allow for the in-situ nucleation and growth of QDs on the CNCs surface. The coating of a ZnS shell was introduced to reduce the toxicity of hybrid materials as well as enhance the photo-emission intensity. They further demonstrated that HeLa cells can uptake the composite nanoparticles effectively, with intense red photoluminescence mostly observed in cytoplasmic regions (Figure 6A). In addition, they reported a quantum dot-cellulose nanocrystal system that can be used for anti-counterfeiting applications [78]. Structural colorful films were fabricated via layer-by-layer self-assembly of oppositely charged CdS quantum dot modified cellulose nanocrystals on a flexible poly(ethylene terephthalate) substrate. CNCs-COOH@CdS and CNCs-PEI@CdS were utilized in the assembly process, with the emission peaks around 650 nm (size 4 nm) and 480 nm (size 2.1 nm), respectively. The fabricated films displayed tunable structural colors from film interference and adjustable emission colors from the quantum dots, which demonstrates their promising applications in anti-counterfeiting devices (Figure 6B).

Figure 6 (A) Confocal fluorescence micrographs of HeLa cells showing the uptake of CNCs/CdS@ZnS quantum dots with red emissions. The cytoskeleton actin was stained green [77]. (B) Schematic illustration showing the quantum dot-cellulose nanocrystal films for the purpose of anti-counterfeiting application. The film was fabricated via a layer-by-layer selfassembly process and the thickness and roughness were characterized by scanning electron microscopy [78].

3.4 Functional Cellulose nanocrystal-Organic hybrids In this section, various types of functional groups, small organic molecules or organic polymers will be discussed. Combining cellulose nanocrystals with both natural and synthetic polymers expands their use in wastewater treatment, energy storage and biomedical applications. Sodium alginate, chitosan, 4-vinylpyridine, pyrrole, rodanine and poly (ethyl ethylene phosphate) are among several organic polymers that have been used to enhance the utility of CNCs. Many recent reviews exist that discuss the use of CNCs in wastewater treatment, energy and biomedical applications, and readers can refer to these for a deeper insight into these topics [5, 7, 16]. We have briefly summarized the application of cellulose nanocrystal-organic hybrids in

respect to three major challenges facing the world, such as environmental, energy and biomedical sectors. We will first discuss the application of CNCs in wastewater treatment. Environmental pollution in developing countries is becoming more severe. Water is one of the basic necessities needed to sustain and support life. CNCs is nontoxic and possesses high surface area, which is a key characteristic for its application in wastewater treatment, especially as an adsorbent. Luong and coworkers were the first to report the use of pristine CNCs to adsorb a cationic dye, methylene blue, with the maximum capacity determined to be 101 mg/g determined from the Langmuir isotherm [79]. Following that, Batmaz et al. modified the CNCs surface through TEMPO oxidation to increase the surface charge density, which increased the maximum adsorption capacity of methylene blue to 769 mg/g [80]. However, it should be noted that using nanoparticles as adsorbents could be problematic for downstream separation processes (i.e. coagulation or high speed centrifugation), which limit their large scale applications. Mohammed et al. incorporated high surface area CNCs into a negatively charged biopolymer (alginate) matrix [81,82] (Figure 7). A further ionic crosslinking process was introduced to produce macrosize hydrogel beads. With this composite material, the CNCs adsorption capability was retained, and the use of hydrogel beads facilitated the easy separation in batch adsorption or in a continuous flow packed bed system [82]. Also, by impregnating CNCs into the hydrogel matrix, it can contribute to a higher surface area for adsorption as well as enhanced mechanical properties. Cranston and coworkers designed crosslinked cellulose nanocrystal aerogels ultilizing the Schiff-base chemistry. The ultralightweight (5.6 mg/cm3) and porous (99.6%) aerogels exhibited enhanced mechanical properties and shape recovery capability in water [83]. More interestingly, they further demonstrated that the aerogels could be used as superabsorbents for oil-water separations as they have strong affinity towards different solvents. Aside from the decontamination techniques, such as adsorption or absorption, another promising large scale process to explore and exploit is flocculation. Cranston and colleagues functionalized the surface of CNCs with poly(4-vinylpyridine), a pH-responsive polymer that can tune the hydrophilic/hydrophobic properties of the nanoparticle [84]. At pH values lower than the pka, the pyridine motifs were protonated, resulting in a positive charge on the nanoparticle surface that ensured a stable colloidal dispersion. While increasing the pH values, the deprotonation of the

pyridyl groups led to more exposed hydrophobic groups, causing the nanoparticles to flocculate and precipitate from aqueous dispersions.

Figure 7 (A) Schematic showing the preparation of CNCs-alginate hydrogel beads [81]; (B) Comparison of the adsorption capability of pure alginate and CNCs-alginate hydrogel beads towards methylene blue [81]; (C) Schematic diagram of a fixed bed adsorption column process for removing methylene blue from contaminated water [82]. There is immense value in using nanotechnology to rejuvenate the forest industry by producing value added wood-derived products, especially in resolving issues, such as energy storage [5]. Cellulose nanocrystals are ideal candidates for fabricating nanostructures with enhanced properties, which has already been reflected by assembling them into supercapacitors or batteries. Wu et al. modified the CNCs surface with carboxylic groups through a TEMPO mediated oxidization process for such purposes [85]. These carboxylic groups formed strong hydrogen bonds with pyrrole monomers, which were subsequently polymerized in-situ onto the CNCs surface with the introduction of an oxidant. The optimized coating condition was achieved through tuning the mass ratio of pyrrole to CNCs. By using the three-electrode system, they further demonstrated that the optimized samples exhibited a supercapacitor behavior with a

capacitance of 248 F g-1 at the scan rate of 0.01 V s-1. However, they also encountered cycling instability as revealed by the rapid capacitance decay over extended cycles due to the inhomogeneous coating of conductive polymers as well as structural breakdown upon cycling. A solution was advanced by introducing an amphiphilic polymer (poly(N-vinylpyrrolidone) (PVP)) that acted as a buffer layer for the favorable growth of polypyrrole [86]. This second generation of conductive CNCs displayed a smooth and uniform coating, which increased the conductivity from 4.5 to 36.9 S cm-1 (Figure 8A). They further demonstrated that the uniform coating improved the cycling stabilities of the capacitative systems (Figure 8B). The material exhibited a capacitance of 338.6 F g-1 at 2 A g-1 with a good capacity retention of 87.3% over 2 000 cycles under the current density of 10 A g-1. As cellulose nanocrystals are a comparatively (in this application field) low-cost biomaterial from nature, they could be considered as an abundant carbon source for pyrolysis processes, and the carbon materials could be designed with versatile nanostructures for fabricating electrode materials. Pang et al. described a method to prepare nitrogen and sulfur dual-doped mesoporous carbon materials [87]. They first coated the CNC surface with a conjugated polymer, polyrhodanine (PR), which provided a heteroatom source for doping. Then, evaporation induced self-assembly, and the hydrolysis of tetraethyl orthosilicate (TEOS) was used to fabricate PR-CNCs/silica composite films. Subsequent pyrolysis and removal of the silica template generated a N, S co-doped mesoporous carbon that could be used as a cathode material for lithium sulfur batteries (Figure 8C). Through doping N and S into the carbon lattice, an enhanced chemisorption of lithium polysulfide was achieved, which could deliver a high capacity of 1 370 mA h g-1 at C/20 and discharge/charge for 1 100 cycles at 2C rate with a very low capacity fading of 0.052% per cycle. Liu et al. also reported a porous nitrogen doped carbon derived from cellulose nanocrystals and urea [88]. The product displayed superior catalytic performance toward oxygen reduction reactions in alkaline media when compared to a commercial Pt/C catalyst. This is reflected by the comparable electrocatalytic activity, a better tolerance to the methanol crossover effect and an improved long-term durability.

Figure 8 (A) Morphologies of polypyrrole coated onto CNCs surface are important for their supercapacitor behavior, TEM images showing the morphology difference by using two different coating protocols (TEMPO-CNCs and PVP-CNCs) [86]; (B) A comparison of capacitance (Cs) loss between two generations of polypyrrole coated CNCs; also the cartoon to propose the mechanism of better performance using PVP layer as a binder [86]; (C) Schematic illustration of the synthesis of nitrogen/sulfur-doped mesoporous carbon for the utilization of lithium-sulfur battery electrode [87].

The breadth of applications for nanoparticles in the biomedical sector is enormous, ranging from drug delivery, antioxidant, antimicrobial, florescence biomarker and tissue engineering [13,16,89]. For instance, Akhlaghi et al. developed a novel drug delivery system by grafting the surface of oxidized CNCs with chitosan oligosaccharide (CSos) [90]. Using procaine hydrochloride as a model drug compound, they determined that the drug loading can approach up to 14% w/w and the modified system revealed a fast release over around 1 hour at pH 8. Later, they used isothermal titration calorimetry (ITC) to further understand the interactions between model drugs (imipramine hydrochloride (IMI) and procaine hydrochloride (PrHy)) and CS os [91]. They found that IMI exhibited a more dominant binding enthalpy compared to PrHy, indicating

that IMI had a higher binding ability toward CNCs carriers. Then, drug selective electrodes were used to monitor the drug release profiles. They demonstrated that both PrHy and IMI displayed fast response by tuning the pH values of the system, and the amount of IMI released from CNCsCSos was higher than PrHy. Wang et al. used a “grafting to” method to modify CNCs with biodegradable polymers for drug delivery [92]. Propargyl-terminated poly(ethyl ethylene phosphate) (propargyl-PEEP) was synthesized by ring-opening polymerization and subsequently grafted onto azide-modified CNCs nanoparticles through Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) “click” chemistry (Figure 9). The highly negative charged modified nanoparticles could be used to load doxorubicin (DOX) via electrostatic interactions. When the complex was internalized by tumor cells, the acidic environment disrupted the interactions between the nanoparticle and DOX, leading to a pH-triggered release. They also demonstrated

Figure 9 (A) Schematic illustration of the synthesis pathway of CNCs-g-PEEP via (CuAAC) “click” reaction and the formation of DOX-loaded cellulose nanocrystals; (B) In vitro drug release profile of DOX-loaded CNCs-g-PEEP at pH 5.0 and 7.4 [92].

that the CNCs-based carrier showed good biocompatibility towards both HeLa cells and L929 cells, with average cell viabilities above 90%, even at high concentrations. Aside from specific drugs for therapy or drug delivery, an antioxidant Vitamin C was formulated with modified CNCs. CNCs-CSos was used to complex Vitamin C (VC) via a further ionic gelation through

triphosphate crosslinking [93]. The encapsulation efficiency of VC into a complex approached up to 71.6% and 91% at pH 3 and 5, respectively. The system displayed a sustained release of active components (Vitamin C) up to 20 days and showed promising radical scavenging and antioxidant activity. Apart from drug delivery systems, Tang et al. reported on a stable antimicrobial system that is based on a polyrhodanine coated cellulose nanocrystal [93]. The coating procedure was easy and feasible using an in-situ oxidation polymerization procedure. The optimized sample exhibited promising antimicrobial properties towards both E. coli (Gram negative) and B. subtilis (Gram positive). Also, their toxicity towards HeLa cells were demonstrated to be low within the concentrations that displayed favourable antimicrobial properties. In addition to antimicrobial properties, they found that the modified nanoparticle displayed a pH-dependent optical characteristic [94]. The color of the dispersion gradually changed from pale red to blue violet when the pH was increased from 2.04 to 12.04. The redox reversibility of the hybrid nanomaterial in response to pH was retained when transformed into different geometries, such as 1D printable inks, 2D flat films and membranes, and 3D hydrogel beads (Figure 10). They anticipated that the modified material could be processed into vacuum packaging materials, with the effective antimicrobial properties providing long-term protection for fresh meat or vegetables. The pH-dependent optical property may also be used to assess the quality of food as the metabolism of microorganisms may lead to pH changes.

Figure 10 (A) Summary of the UV-Vis spectra of a 0.01 wt% CNCs@PR dispersion at different pH values; (B) Sigmoidal plots of maximum absorption wavelength and absorbance@peak versus pH. (C) Schematic illustrating the redox reversibility of the hybrid nanomaterial in response to pH was retained when transformed into different geometries [94].

The purpose of CNCs as potential carriers for cell uptake originates from its size, hydrophilic nature and chemical composition. Modifying the surface with fluorophores can generate nanoscale biomarkers that could be used for the localization and quantification of nanoparticles within the cell. Roman and coworkers were the first report the fluorescence characteristic using modified CNCs by conjugating the surface with fluorescein-5’isothiocyanate (FITC) [95]. They used spectrofluorometry, fluorescence microscopy and flow cytometry to study the in-vivo interaction of modified CNCs within cells. Luong and co-workers compared the application of negatively charged FITC and positively charged rhodamine B isothiocyanate (RBITC) modified cellulose nanocrystals in cell internalization [96]. They found that the negatively charged FITC-CNCs could not be internalized by the cells due to the strong electrostatic repulsions between the anionic CNCs and anionic cellular membrane. However, RBITC-CNCs exhibited low cytotoxicity and excellent membrane permeability in various cell lines, which suggests that the surface charge and conjugated elements are essential factors when functionalizing the nanoparticle as carriers to cells for bioimaging and drug delivery.

4. Conclusion and perspectives The consumption of fossil fuels to produce energy and petrochemicals over the last 50 years has resulted in the rapid accumulation of green-house gases that is impacting the livelihood of many communities around the globe. There is thus a compelling motivation to seek alternative sources of energy and raw materials not derived from fossil fuels, but from renewable resources. Research on the functionalization of CNCs is at its infancy, and the development of scalable synthetic protocols for the chemical modification of CNCs, and fundamental understanding of this new class of nanomaterial will create new opportunities and markets. We believe sustainable nanomaterials will be extremely important in addressing two critical issues confronting our world, namely energy and the environment. The utilization of cellulose nanocrystals will reduce our dependence on conventional carbon sources (e.g. crude oil) and cellulosic materials are excellent carbon sinks for capturing carbon dioxide. This featured article outlines research activities that explore and exploit sustainable nanomaterials for a variety of applications. We summarize the most recent developments on the modification and utilization of functionalized cellulose nanocrystals. The modifications have expanded the use of this remarkable nanomaterial to applications, such as emulsion stabilizers,

anti-microbial agents, controlled delivery systems, sustainable catalysts, templating agent for mesoporous nanostructures, and various energy and electronic systems. Studies on the modification and functionalization of cellulose nanocrystals have generated important fundamental understanding necessary for designing novel functional systems for potential applications. Around the globe, several large scale production facilities to manufacture pristine cellulose nanocrystals were commissioned. Some of these facilities include the following: (a) Celluforce-1 000 kg/day (Canada), (b) American Process-500 kg/day (USA), (c) Holmen-100 kg/day (Sweden), (d) Alberta Innovates-20 kg/day (Canada), (e) US Forest Products Lab-10 kg/day (USA), (f) Blue Goose Biorefineries-10 kg/day (Canada), and (g) India Council for Agricuture Research-10 kg/day (India). In addition, there are other smaller facilities in countries, such as China, Brazil etc. that are not listed here. Consumers can now purchase significant quantities of CNCs for field trials and large scale product formulations and evaluation. We are at the stage where we will see the adoption of CNCs in product formulations in applications, such as coatings, pulp and paper, consumer and personal care systems, wastewater treatment and biomedical engineering. The future for CNCs is bright, and the research activities in academic and industrial laboratories will generate many new discoveries and applications. This feature article provides a glimpse amongst other of one such activity in the Laboratory of Functional Colloids and Sustainable Nanomaterials at the University of Waterloo, Canada.

Acknowledgements The research funding from CelluForce and AboraNano facilitated the research on CNCs. K. C. Tam wishes to acknowledge funding from CFI and NSERC.

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Graphical Abstract