From Basic Research to Real Applications - ACS Publications

Invited Feature Article pubs.acs.org/Langmuir

Cellulose as a Scaffold for Self-Assembly: From Basic Research to Real Applications Hua Tian and Junhui He* Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Cellulose has received a tremendous amount of attention both in academia and industry owing to its unique structural features, impressive physical−chemical properties, and wide applications. This natural polymer is originally used for packaging, paper, lightweight composites, and so forth and is now being developed for various new areas, such as antibacterial treatment, catalysis, water purification and separation, and biological and environmental analysis. In the current article, we summarize the recent developments in the self-assembly of cellulose with various species including metal ions and metal and metal oxide nanoparticles. Then we highlight several key application areas of cellulose-based composites by reviewing the recent representative literature in each area. A significant part of this review demonstrates some exciting innovations for a wide range of practical applications of cellulose-based composites. Some challenges are also discussed with a view toward future developments.

1. INTRODUCTION Ever since the successful preparation of nanocrystalline materials by Gleiter in the 1980s,1 nanomaterials and nanotechnology have been developing rapidly, with broad applications in physics, chemistry, material science, catalysis, and biomedical science.2 The nanoscale endows materials with marked changes in properties and results in improved performance and even new functionalities. There are abundant papers in the literature showing that the performance of nanomaterials depends on their particle size and uniformity.3−5 To avoid the agglomeration of nanomaterials during application, facilitate their recovery and reuse, and meet specific requirements in engineering applications, the assembly of nanomaterials onto various scaffolds with large surface areas has been proven to be an effective strategy.6 A number of materials have been selected as scaffolds or supports for fabricating functional composites, including carbon materials (activated carbon,7 carbon nanotubes,8 graphite,9 graphene,10 etc.), silica-based materials (molecular sieves and mesoporous silicon oxide),11−13 ceramics,14,15 and celluloses.16,17 Using these scaffolds, a large variety of functional and well-designed architectures have been successfully developed through employing specific approaches. These assembled composites present some particular physical, chemical, electronical, or biological properties compared to the free nanomaterial counterparts and hold great promise for various applications. Cellulose, one of the bioderived materials, holds an important position in abundant organic raw materials. Since Payen first recognized cellulose as a definitive substance in 1838,18 this biodegradable, biocompatible, and renewable material has received a great deal of attention in academic research and industrial commercialization. Cellulose can be © XXXX American Chemical Society

derived from a variety of sources, from woody plants and agricultural byproducts to fungi, bacteria, and invertebrates. Owing to its excellent physical and chemical properties, cellulose has served in numerous applications during the past decades19−21 and continuously shows potential in many new applications.22−25 One of the most intriguing applications for cellulose is as the scaffold material of composites, mainly in the form of fiber, cotton, textile, paper, membrane, or film. The functionalization of cellulose materials greatly increases their functionality and expands their application scope. In this article, we review recent developments in cellulosebased composites. First, we will discuss three general chemical routes for cellulose functionalization. Then their potential applications in antibacterial reaction, water treatment, catalysis, and biosensors will be described on the basis of the vast number of research results available. Finally, we will present some representative designs that enable cellulose-based composites to be more practical for real applications.

2. SELF-ASSEMBLY OF METAL IONS ON CELLULOSE FIBERS Cellulose is a polyacetal of β polyglucan, covalently linked with OH groups at C4 and C6 atoms.26 Unmodified cellulose has a low adsorption capacity as well as variable physical stability.27 To overcome such drawbacks and extend the application areas Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular Assembly Received: May 30, 2016 Revised: July 1, 2016

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DOI: 10.1021/acs.langmuir.6b02033 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. Illustration of the procedure for the preparation of (a) phosphorylated cellulose and (b) Ti4+-phosphate functionalized cellulose.35 Copyright 2012 Elsevier.

Figure 2. Detection principle for acquiring quantitative information on sample lactoferrin content based on the emission distance within a microfluidic paper channel.38 Copyright 2015 American Chemical Society.

filter paper.38 First, TbCl3 was deposited in a straight microfluidic channel on filter paper by means of an inkjet printer. Then, the printed paper was partially cut into a certain shape and treated with anionic polysaccharides, followed by complete drying at 37 °C. For the quantification of lactoferrin, the sample liquid traveled along the channel of distance-based μPAD, and lactoferrin molecules in liquid were continuously consumed by the formation of fluorescent Tb3+-lactoferrin complexes until the complete depletion of analyte occurred (Figure 2). Thus, the concentration of analyte in the sample liquid could be observed as a function of the length of the green fluorescent line under UV illumination. Metal ions such as Pt2+ could quantitatively functionalize cellulose, allowing the formation of sensitizer units. The modified celluloses exhibit efficient upconversion abilities, enabling the tumor-imaging utility. Nagai et al. designed a comodified cellulose templated triplet−triplet annihilation photon upconversion (TTA-UC) system, employing a platinum(II)-tetraphenylporphyrin (PtTPP) sensitizer and rhodamine B as an emitter (Figure 3).39 The comodified cellulose had a J-aggregated construction due to aggregationinduced self-assembly containing dimer-like edge-to-edge association from platinum(II) porphyrin arrays on a cellulose surface. The increase in the concentration of cellulose with rhodamine B and the PtTPP moiety could result in a higher aggregation state and an intensity enhancement of upconversion photoluminescence.

of cellulose, various chemical functionalizations have been conducted by means of halogenation, etherification, esterification, silylation, oxidation, and grafting based on the natural advantage of abundant OH groups at the surface of cellulose fibers.28−31 For instance, the OH group of cellulose can be easily esterified by H3PO4, P2O5, or POCl3, resulting in the formation of phosphorylated cellulose, which is readily chelated with metal ions. As early as the 1950s, a metal ion−cellulose complex was synthesized during wood treatment with various metal salt solutions for timber preservation.32 Over the past several decades, metal-ion-modified cellulose has been a subject of great interest in detection, photo upconversion, ion-exchange, catalysis, and so on.19,33,34 For example, the high sensitivity and specificity of Ti4+-phosphate-functionalized cellulose have been studied for phosphopeptides enrichment.35 The OH group on the C6 of cellulose was first esterified by phosphoric acid, and then the as-prepared phosphorylated cellulose with relatively high substitution was immersed in Ti(SO4)2 aqueous solution to chelate Ti4+ via strong interactions (Figure 1). During the phosphopeptide enrichment, empty sites of Ti4+ ions that were temporarily occupied by water molecules were replaced by phosphorate groups of phosphopeptides. The study indicated that after enrichment based on functionalized cellulose, 14 phosphopeptides could be detected with a high signal-to-noise ratio. To date, there has been an exciting convergence of microfluidic paper-based analytical devices (μPAD), which possess expanding application fields from medical diagnosis to environmental analysis and food quality monitoring.33,36,37 The detection motifs are often based on chromogenic substances placed on paper substrates. As a typical example, Yamada et al. reported a distance-based μPAD using the Tb3+ cation as the fluorescent detection reagent on surface-modified cellulosic

3. SELF-ASSEMBLY OF METAL NANOPARTICLES ON CELLULOSE FIBERS Noble metal nanoparticles are currently among the most studied nanomaterials in the fields of catalysts, antibacterial agents, and detectors.10,40−44 However, the agglomeration and instability of metal nanoparticles during their use may largely B


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Pt. On the basis of these results, our group recently optimized the in situ reduction method to produce Ag nanoparticles on natural cellulose fibers.43 Monodisperse Ag nanoparticles with an even smaller size (2.7 nm) were achieved by controlling the NaBH4 concentration. The minimum inhibitory concentration (MIC) tests proved that these Ag-loaded cellulose fibers possessed a high antimicrobial efficiency for prohibiting the propagation of Escherichia coil (E. coli). The synthesis of metal nanoparticles on cellulose was also realized using cellulose itself as the reducing agent. For example, Nadagouda et al. prepared nanocomposties of carboxymethyl cellulose (CMC) with transition metals such as Cu, Ag, In, and Fe.49 In this approach, CMC acted as the capping and reducing agent. The spontaneous reduction of metals could be accomplished by reacting CMC aqueous solution with metal salts under microwave irradiation, without using any other reductant or surfactant. The reaction between a metal ion and CMC was a metal displacement reaction, where the Na+ ion from CMC was replaced by the transition-metal ion. However, this approach is feasible only for transition metals; noble metals such as Au, Pd, and Pt could not easily undergo the metal displacement reaction with the Na+ ion. Cai et al. developed a simple one-pot approach to reducing some metal ions to metal nanoparticles by hydrothermal treatment with cellulose gel, where the cellulose gel acted as both the reducing and stabilizing agent.50 The metal nanoparticles could be well dispersed and stabilized in the cellulose aerogel. The size of the metal nanoparticles was tunable by the hydrothermal temperature. Very recently, Chen et al. fabricated an amidoxime surface-functionalized bacterial cellulose (AOBC) without affecting the morphology of bacteria cellulose.51 The AOBC could be used as a reducing agent and carrier for Au nanoparticles distributed on the bacterial cellulose surface. Amidoxime groups of AOBC enhanced the interaction between bacteria cellulose and Au nanoparticles, and the content of amidoxime groups in AOBC could control the Au particle size. Previous studies show that the nanoporous structure of cellulose fibers plays an important role in the implantation and growth of noble metal nanoparticles. Besides as a support material, the nanoporous structure and high oxygen (ether and hydroxyl) density of cellulose fiber could constitute an effective nanoreactor for in situ synthesis of metal nanoparticles. The nanoporous structure facilitated the introduction of metal ion and reductant into cellulose fibers and simultaneously promoted the removal of byproducts from fibers. The ether oxygen and hydroxyl group of cellulose could anchor metal ions tightly via ion−dipole interactions and stabilize the formed metal nanoparticles by a strong bonding interaction with their surface metal atoms.6 This case is similar to that on nanoporous titania.52 During the synthesis of metal−TiO2 composites,

Figure 3. (a) Structure of comodified upconverting cellulose. (b) Schematic illustration of the aggregation-process-induced emission enhancement. (c) Upconversion imaging of cellulose-Rho99.88% PtTPP0.12%.39 Copyright 2015 American Chemical Society.

reduce their efficiency. A commonly used strategy to address these issues is to disperse metal nanostructures on the surface of scaffold materials. For an ideal scaffold, a large surface area, strong interaction with metals, and low cost are necessary. Adopting abundant cellulose as a scaffold for metals could be a feasible means to maintaining their high activities and simultaneously reducing cost.45,46 Currently, the methods of hybridizing celluloses with metal nanomaterials are dominated by postimmobilization and the in situ growth of metal nanoparticles on cellulose substrates. To produce metal− cellulose composites using the postimmobilization method, attention should be paid to controlling the high dispersity and uniformity of metal nanoparticles. A surface pretreatment is usually employed to introduce bonds of the metal and the substrate.47 For the in situ growth approach to fabricating metal−cellulose composites, process parameters need to be controlled, including the type of solvent, type and concentration of metal precursor and reducing agent, reaction time and temperature, and so forth. To induce anchoring sites for the metal precursor nucleation and control the morphology of the produced metal nanoparticles, some chemical promoters or surfactants are usually used. In 2003, He et al. reported for the first time that a series of noble metal nanoparticles could grow on the surface of cellulose fibers using a facile in situ growth approach.6,43,48 Ag, Au, Pt, and Pd nanoparticles of less than 10 nm could be quickly synthesized on porous cellulose fibers just through the reduction of metal ions by NaBH4 at room temperature (Figure 4).6 The nanoparticle size increased and the size distribution became broadened with the increase in metal ion concentration in solution. Under the identical metal ion concentration, the smaller the metal/glucose ratio, the smaller the nanoparticle size and the narrower the size distribution, except for elemental

Figure 4. Cellulose specimens with (a) no particles and (b) Ag, (c) Au, (d) Pt, and (e) Pd metallic nanoparticles.6 Copyright 2003 American Chemical Society. C


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Similar to many other composites, the homogeneous dispersion of metal oxide nanoparticles over the cellulose fibers is a difficult challenge to face during the synthesis process because the aggregation of metal oxide nanomaterials has already been proven to reduce the performances of obtained composites. To overcome this issue, various methodologies have been developed to synthesize or immobilize metal oxide nanoparticles on cellulose fibers with high dispersity, such as hydrolysis, sol−gel, the hydrothermal process, and the sonochemistry method.74−76 Metal oxides were incorporated into cellulose scaffolds generally in two ways: one is a two-step process. To control the nanoparticle size and morphology in cellulose−metal oxide composites, it is better to form monodisperse nanoparticles first and then assemble these nanoparticles onto cellulose materials. The other way is a direct growth approach that can ensure strong cellulose−metal oxide interaction in cellulose−metal oxide composites. In this process, nanoparticle nucleation and growth on cellulose fibers usually requires a high-temperature and high-pressure environment. Manganese oxide has been considered to be a promising catalyst for formaldehyde oxidation.3,40,41,78 However, pure manganese oxide nanomaterials suffer from dust pollution and catalyst loss in real air purification. In 2011, our group synthesized a novel MnO2/cellulose composite by an in situ generation approach.77 MnO2 nanosheets were prepared on cellulose fibers by the reduction of KMnO4 using oleic acid as a reductant. TEM images confirmed that MnO2 nanosheets were arranged roughly perpendicular to the surface of the cellulose fiber (Figure 6). Because of this unusual nanostructure, MnO2/ cellulose composites displayed excellent catalytic activity for

metal ions were stabilized by oxygen atoms in the TiO2 network in a way similar to that for crown ethers. After reduction, such coordination is insufficient to maintain isolated metal atoms, and these metal atoms tend to coalescence, forming metal nanoparticles. The formed metal nanoparticles were then stabilized by a strong bonding interaction between the outermost orbitals of its surface atoms and the surrounding oxygen atoms of the TiO2 matrix.52−54

4. SELF-ASSEMBLY OF METAL OXIDE NANOPARTICLES ON CELLULOSE FIBERS Another application of cellulose fibers is as scaffolds for the assembly and immobilization of metal oxide nanoparticles. The fabrication of metal oxides on cellulose fibers can not only offer metal oxides better stability and easier recyclability but also improve their performance, thus making the application of metal oxides very popular in various fields. To fabricate the cellulose−metal oxide composites, it is essential that the surface of the cellulose substrate is faithfully lined with inorganic ions. As mentioned earlier, the cellulose fibers possess abundant surface OH groups and intrinsic porous nanostructure, which makes them an ideal scaffold. These cellulose−metal oxide composites combine the processability of cellulose fibers and the superior performances of metal oxide materials. Until now, this passionate endeavor has been dedicated to designing and synthesizing cellulose−metal oxide composites, including TiO2,55,56 ZnO,57,58 Fe2O3,59 Fe3O4,60,61 Co3O4,62 MnO2,63 Cu2O,64,65 CuO,66,67 Al2O3,68 SiO2,69 ZnO-SiO2,70 CuCoFeOx,71 and ZnO-CdS.72 The obtained composites significantly expand the application fields of metal oxides, ranging from water treatment, disinfection, catalysis, and chromatography to electrodes. Figure 5 shows a representative example of nanostructured cellulose composites containing metal oxide nanomaterials.73

Figure 5. Schematic illustration of the interaction between the hydroxyl groups of the cellulose chain and the TiO2 nanorods.73 Copyright 2015 Elsevier.

There were strong hydrogen bonding interactions between hydroxyl groups of the cellulose chain and TiO2 nanorods. Cellulose fibers served as scaffolds for the dispersion of photocatalytic nanoparticles. The presence of TiO2 nanorods limited the molecular motion of cellulose polymer chains and spontaneously increased the membrane porosity of cellulose chains. This composite membrane showed excellent photocatalytic performance in the phenol degradation under UV and visible -light irradiation.

Figure 6. Transmission electron microscopy (TEM) (a), magnified TEM (b) images, and digital photographs of MnO2/cellulose fibers (c). Reproduced from ref 77. Copyright 2011 American Chemical Society. D


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Figure 7. Digital photographs and scanning electron microscopy (SEM) images of Au0.5Pt0.5/MnO2/cotton obtained at varied Au0.5Pt0.5/MnO2 loadings: (a, b) 6%, (c, d) 10%, (e, f) 15%, (g, h) 20%, and (i, j) 25%. Adapted with permission from ref 48. Copyright 2013 Springer.

formaldehyde oxidation, about 9−17 times higher than that of pure MnO2 power. As natural and conventional cellulose, cotton fibers are one kind of the most commonly used cellulose materials. Cotton fibers are mainly used as supports because of their unique properties, including abundantly available space, nontoxicity, and good adsorption performance. One recent interest has been to load or grow catalysts on cotton-based scaffolds to fabricate purification or self-cleaning devices. Recently, our group developed a facile method to prepare Au0.5Pt0.5/MnO2/ cotton catalysts.48 Au0.5Pt0.5 alloy nanoparticles of 2−5 nm were first dispersed on the surface of nestlike MnO2, with little agglomeration. Subsequently, the obtained Au0.5Pt0.5/MnO2 powder was adhered to the surface of cotton fibers in an aqueous solution by an adsorption process (Figure 7). The catalytic oxidation results demonstrated that Au0.5Pt0.5/MnO2/ cotton catalysts exhibited excellent catalytic performance during the complete oxidation of formaldehyde. It was concluded that the improved performance of the Au0.5Pt0.5/MnO2/cotton composites results from the valence states of MnO2, MnO2 porous structures, the synergistic effect between Au0.5Pt0.5 nanoparticles and MnO2 and the synergistic effect between Au0.5Pt0.5/MnO2 and cotton fiber. Most importantly, the introduction of cotton overcomes the disadvantages of powder catalysts including a high pressure drop, dust pollution, and difficulties in practical engineering applications for indoor air purification. Similarly, Manna et al. successfully coated Ag@ZnO nanostructures on cotton fabrics by a bioinspired mineralization route to prepare visible-light-driven self-cleaning fabrics.79 They used poly(allylamine) for the formation of Ag@ZnO nanostructures from a water-soluble zinc salt under mild conditions. The entrapped polyamine in the ZnO matrix facilitated the reduction of Ag(I) to form Ag(0) without any use of an external reducing agent. The results demonstrated that the presence of Ag nanoparticles provided the ZnO-coated fabrics with not only an improved photocatalytic property but also visible-light-driven activity (Figure 8). Furthermore, these functional cotton fabrics displayed effective antimicrobial activity against both Gram-positive and Gram-negative bacteria. The surface functionalization of cellulose usually requires a delicate design to afford a well-defined property. For instance, in nature, cellulose is hydrophilic and not suitable for removing oils and organic solvents from water. Through the incorporation of functional moieties, however, the cellulose surface can be turned into a superhydrophobic surface. Peng et al. fabricated a superhydrophobic magnetic cellulose sponge for

Figure 8. Ag@ZnO on fabric and its photocatalytic and antibacterial activities.79 Copyright 2015 American Chemical Society.

the separation of free oil/water mixtures and surfactantstabilized W/O emulsions.60 A cellulose sponge was obtained by a sol−gel method and freeze-drying. A thin layer of Fe3O4 was then coated on the cellulose sponge surface, and a surface chemical modification was then performed via the self-assembly of the hexadecyltrimethoxysilane monolayer. The modification by hexadecyltrimethoxysilane endows the sponge with high hydrophobicity. The purpose of coating Fe3O4 was to increase the roughness of the surface and provide a magnetic separation property to the sponge. In another report, cellulose-based microspheres with both superhydrophobicity and superparamagnetism were prepared by Lin’s group.80 Upon modification with a catechol-bearing fluorinated polymer, the hierarchical magnetic Fe3O4 microspheres became superhydrophobic and were able to form various liquid marbles for liquid droplet transportation and manipulation. The magnetic property of Fe3O4 made the manipulation of liquid droplets feasible and easy. The use of poly(DOPAm-co-PFOEA) offered the liquid marbles remarkable stability. The magnetic property ensures easy separation of materials from reaction mixtures and therefore contributes to more sustainable and greener industrial processes. So far, combining supports with magnetic nanomaterials has attracted considerable attention. Cao et al. developed a simple coprecipitation− electrostatic-self-assembly technique to prepare a biocompatible magnetic cellulose nanocrystal.81 The electrostatic interaction between the cellulose nanocrystals and chitosan and that between chitosan and Fe3O4 were found to be the key driving forces for the formation of the composite. The obtained magnetic cellulose composites were demonstrated be to potential carriers for the immobilization of enzyme, papain, and pseudomonas cepacialipase.81,82 Serve et al. presented a simple method for the purification of influenza virus particles using a novel magnetic sulfated cellulose particle.83 Compared to the previously established centrifugation method, use of the E


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Figure 9. Schematic of the energy band structure and the charge-transfer process (left) and ultraviolet visible absorption spectra (right) of CCNFs/ ZnO/CdS composites. Adapted from ref 72. Copyright 2015 Springer.

Figure 10. (a) Untreated paper. (b) Silver nanoparticle (NP) paper. (c) CuNP paper.96 Copyright 2015 Royal Society of Chemistry. (d) Log reduction of E. coli and E. faecalis bacterial counts after permeation through the AgNP paper with different silver contents.95 Copyright 2011 American Chemical Society. (e) Filter unit with cross-sectional area (22.1 cm2) and head height (9 cm).96 Copyright 2015 Royal Society of Chemistry. (f) Log reduction values of the E. coli bacterial count after permeation through the CuNP paper with 65 mg Cu/g paper (black bars), CuNP paper with 10 mg Cu/g paper (gray bars), and untreated paper (white bars).93 Copyright 2014 Elsevier.

contact with pollutants. CdS, as a visible-light photosensitizer, extended the light response range of the CCNFs/ZnO composites to the visible-light region (Figure 9).

novel magnetic sulfated cellulose particles reduced the influenza A virus particle purification time from 3.5 h to 30 min before mass spectrometry analysis. In recent years, the immobilization of TiO2 or ZnO nanoparticles onto a cellulose substrate has attracted increasing interest in both academia and industry. These composites exhibit some interesting performance, such as antibacterial, adsorption, photocatalysis, and self-cleaning.84−89 El-Naggar et al. deposited TiO2 nanoparticles in situ onto cotton fabrics without seriously damaging the fabrics.90 Urea nitrate was used as a peptizing agent for the first time to convert titanium hydroxide to TiO2 nanoparticles. TiO2 nanoparticles loaded on cotton fabrics were composed of aggregated particles with an average size of less than 50 nm. These treated fabrics performed well in bacterial reduction and UV protection regardless of washing. The composites containing multiple metal oxides usually exhibit superior performance compared to those containing only one metal oxide.91,92 As an example, Li et al. successfully synthesized a binary ZnO/CdS catalyst on electrospun cotton cellulose nanofibers (CCNFs).72 It was found that the cellulose nanofibers could increase the absorption of visible light and the

5. APPLICATIONS OF CELLULOSE-BASED COMPOSITES The functionalization of cellulose materials with organic polymers, inorganic materials, metals, or metal oxides can change their physicochemical properties and impart them with some additional performance for specific applications. Actually, the resultant cellulose-based composites have been applied in various advanced applications in a wide range of fields including antibacterial, water treatment, catalysis, optoelectronic devices, biosensors, bioimaging, and medical diagnosis. In this section, we will give an overview of selected recent advanced applications of cellulose-based composites. 5.1. Antibacterial. To meet health, safety, and environmental demands, cellulose-based composites have been widely studied during the past few decades as antibacterial agents because of their unique properties including environmental friendliness, biocompatibility, high stability, easy availability, and nontoxicity. The common antibacterial agents used are F


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Figure 11. (a) Magnetic responsiveness of γ-Fe2O3 nanoparticles, γ-Fe2O3@cellulose hydrogels, and γ-Fe2O3@CA. (b) Adsorption kinetics of Cr(VI). (c) Adsorption isotherms of Cr(VI).118 Copyright 2015 American Chemical Society.

In addition to silver nanoparticles, cellulose materials modified with various metal oxide nanoparticles (e.g., TiO2, ZnO, and CuO) have also attracted many researchers to exploit them as antibacterial agents during the past few years. For example, Chauhan et al. immobilized ZnO nanowires on the surface of cellulose fibers of paper matrices by a single-step hydrothermal method.99 Paper matrixes without ZnO nanowires did not show any antibacterial performance, whereas the ZnO-immobilized paper matrixes displayed a complete inhibition of E. coli growth (i.e., 99.99%) under visible-light illumination for 6 to 9 h. Recently, the copper oxide-modified paper was also found to be effective for the inhibition of bacteria.64,67 The mechanism of the antibacterial action of copper oxide composites is that copper oxide provides a sustained release of copper ions. These copper ions may damage the bacterial cell membrane and enter cells to disrupt enzyme function, which leads to the death of bacteria.100,101 5.2. Water Treatment. With rapid industrialization and population expansion, water pollution has been become a serious problem for the environment and human society today. Large amounts of toxic organic compounds, dyes, and heavy metals are discharged from industrial, agricultural, and domestic wastewaters and eventually into water systems.102,103 A combination of biotechnology and nanotechnology may provide a new solution to the old problems. The rapid development of modified cellulose materials opens up new opportunities in water treatment and these materials have already been demonstrated to be very promising candidates for the treatment of high-volume and low-concentration polluted water.104 Through surface chemical modification with various functional groups or species such as carboxyl, cyclodextrin, metal ions, metals, metal oxides, cellulose materials can achieve excellent adsorption performance for water contaminants, including dyes, pesticides, Ag(I), Cu(II), Fe(III), Ni(II), Cr(VI), Cd(II), Hg(II), Pb(II), and so on.27,105−111

diverse, usually composed of cellulose materials (such as cellulose nanocrystals, cotton fabrics, textile materials, and paper) and a secondary component ranging from organic molecules and polymers to metals and metal oxides. Among them, silver-based materials are the most popular system being researched and explored because of their rapid and efficient antibacterial activity against a wide range of bacteria and viruses. Early in 2003, He et al. developed a very facile in situ method to synthesize silver nanoparticles anchored on cellulose.6 The preparative procedure is simple and fast. In a successive paper, we demonstrated that functionalized cellulose could prohibit the propagation of E. coli.43 The silver-loaded cellulose fibers displayed effective antibacterial activity with E. coli propagation effectively prohibited. It is interesting that the antibacterial action is sustainable and not contaminating. After the fibers were immersed in water for 72 h, no silver was found in the water. Later, Dankovich et al. improved this in situ approach to embed silver or copper nanoparticles in thick paper (Figure 10).93−96 Bacteria were killed just as testing water passed through the paper, resulting in log 8.8 and log 3 reductions of viable E. coli and Enterococci faecalis (E. faecalis) bacteria, respectively. Their experiments indicated that the silvernanoparticle-loaded paper remained unchanged and no bacteria were observable on the paper after bacteria filtration. The silver and copper levels released in the effluent water were all below the recommended limits in drinking water. Ramaraju et al. also developed an Ag nanoparticleimmobilized cellulose nanofibril film that showed high antibacterial performance against both Gram-positive and Gram-negative bacteria.97 Shi et al. successfully enhanced the dispersion stability of silver nanoparticles through in situ anchoring of them on the surface of modified cellulose nanocrystals with dopamine.98 This hybrid had high stability and showed antimicrobial activity that is approximately 4 times better than that for free silver nanoparticles. G


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For example, a cellulose/graphene composite has recently been prepared by Zhang et al. for the removal of triazine pesticides from water.112 This composite could effectively adsorb six different triazine pesticides with a higher adsorption capacity compared to that of graphite carbon, cellulose, and graphene. The improved performance is attributed to the rough and wrinkled surfaces with a lamellar structure in the cellulose/ graphene composite, which provides an advantageous condition for the adsorption process. Inorganic ion exchangers including Ti(IV)-, Na(I)-, and Sn(IV)-based materials are known to be one of the most efficient agents for the removal of heavy metals from water.113−117 Fe(III) has a strong affinity for arsenic anions. Zhao et al. designed and prepared a Fe(III)-loaded ligand exchange cotton cellulose adsorbent (Fe(III)LECCA).29 Through cross-linking, activation, and ammonification to improve the chelating ability of cellulose, Fe(III) ions were loaded onto cotton cellulose. Fe(III)LECCA possessed a surface area of 2.23 m2 g−1, a moisture content of 87%, and many large pores (5−30 μm) on the surface and within the inner part. The batch adsorption experiments indicated that this adsorbent displayed a good adsorption capacity, selectivity, and adsorption rate for arsenate removal from drinking water. As(V) 65% could be adsorbed by Fe(III)LECCA within 2 min, and the adsorption capacity could be up to 23.8 mg g−1 at a water flow rate of 26.0 BVh h−1. The treated water could meet the United States Environmental Protection Agency (USEPA) 10 ppb standards. More recently, Wan and co-workers exploited a template synthesis combined with chemical coprecipitation to achieve well-dispersed hybrid γ-Fe2O3@ cellulose aerogels (γ-Fe2O3/CA).118 The composites exhibited an excellent superparamagnetic property (Figure 11). Moreover, γ-Fe2 O 3/CA could rapidly remove Cr(VI) from contaminated water. Compared to other traditional nanoparticles for water purification, these composites display several advantages, such as environmental benefits, strong mechanical strength, easy recycling and separation by magnet attraction, and flexibility to design the desired shape and dimensions. 5.3. Catalysis. Catalysts of high surface area and low nanoparticle agglomeration are always much desired in heterogeneous catalysis. An enlargement of the surface area can be achieved by decreasing the particle size and creating a porous structure. However, these routes could not effectively solve the problems of particle agglomeration and loss during applications. The immobilization of catalytic nanoparticles onto the surface of cellulose materials offers an attractive avenue to circumvent these problems in various types of heterogeneous catalysis reactions. In a recent report, a direct method was developed to synthesize a cellulose composite with Ag nanoparticles.119 Ag nanoparticles were grown in a few days at room temperature, in the presence of light, without using any additives other than cellulose nanocrystals. This composite catalyst was very active for the hydrogenation of aldehydes, 4-nitrophenol, alkenes, and alkynes. In another interesting study, palladium was immobilized on cellulose paper by simply dipping the paper into a dispersion of oleylamine-stabilized Pd nanoparticles, followed by drying with a hair dryer.120 This functionalized paper could be employed as a highly efficient, robust, and recyclable catalyst for the oxidative homocoupling of arylboronic acids, the Suzuki cross-coupling reaction, and the nitro-to-amine reduction. Catalytic reactions were performed just by immersing the modified paper strips into the corresponding reaction medium. To develop an efficient and cost-effective catalyst for H2O2

decomposition that supplies sufficient propulsive thrust for small-scale, autonomous underwater vehicle propulsion systems, Claussen’s group deposited platinum nanourchins onto cellulose films toward a recyclable, durable, and efficient catalyst.121 This organic−inorganic nanohybrid provided a large surface area, a high proportion of the [111] face of platinum, and a porous structure. The Pt-based organic−inorganic catalysts could lower the activation energy for H 2 O 2 decomposition by 50−63% over that of conventional materials and by 13−19% for similar Pt nanoparticle-based structures. In addition to the aforementioned cellulose−metal composites, a new class of cellulose composites that consist of a hybrid nanostructure and cellulose as a secondary scaffold have been the subject of intensive studies more recently. It has also been indicated that unusual properties can be achieved using these hybrid nanostructures. For instance, the immobilization of metal−organic framework (MOF) materials onto polymeric substrates improves the accessibility to active sites during catalytic reactions. Neufeld and colleagues successfully deposited copper(II) benzene-1,3,5-tricarboxylate (CuBTC) crystals onto the surface of carboxyl-functionalized cotton fabrics.76 They discovered that this CuBTC-cotton composite showed excellent catalytic performance for generating therapeutic bioagent NO from S-nitrosocysteamine at therapeutic levels. The immobilization of CuBTC onto cotton fabrics greatly improved the long-term activity in NO release. The supported CuBTC-cotton catalyst demonstrated a NO release rate of 19 ± 3.0 nM s−1 with a release time of roughly 4 h, whereas the unsupported catalyst reached reaction completion within just 90 min. 5.4. Detection. Since the first report on the application of cellulose-based paper in microfluidic assays by Whiteside’s group in 2007,20 paper-based analytical devices (PADs) have attracted significant interest and are currently an active field in analysis and sensing research. PADs have several unique advantages resulting from the intrinsic properties of paper, including low-cost, portability, ease of use, and pumpless operation. Recent developments have expanded their application fields to biomedical analysis, environmental monitoring, foodborne contaminant detection, and fluid manipulation.122−126 PADs usually employ colorimetric, electrochemical, or fluorescent technology for analysis and quantification.127−130 Among them, colorimetric detection is one of the most widely used methods because colored metal−ligand complexes are easily discernible with the unaided eye. Hand spotting of reagents onto a patterned paper surface is the simplest and the most broadly used route for preparing PADs. However, this method has drawbacks, such as low efficiency and timeconsuming and tedious operations, hindering their widespread implementation. Recently, new technologies involving the inkjet printing of reagents have been developed to overcome these problems. The colorimetric reagents are inkjet printed onto the reaction zone or channel, followed by drying. When targets are present in the test zone, a color change occurs and the experimental result can be observed by the naked eye. For example, to develop a distance-based paper visual quantitative method for point-of-care tests, Wei et al. designed a microfluidic distance readout sweet hydrogel integrated PAD to quantitatively detect cocaine.131 This target-responsive enzyme-embedded hydrogel could release glucoamylase, which would produce glucose by a glucoamylase-catalyzed amylolysis reaction. This induced a cascade reaction on the H


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Figure 12. (a) Air-sampling cassette with the paper-based sensor. (b) Sensing device connected to the air-sampling pump by a rubber tube. (c) Digital images of the tests zones of the colorimetric devices show the colorimetric responses in the range of 1−500 μg g−1 bisphenol A (BPA) (three trials). (d) Comparison between the colorimetric BioPAS and GC methods for the detection of BPA in household dust specimens at the same sampling locations.132 Copyright 2015 American Chemical Society.

PAD to produce a distance signal as a stained stripe. Then quantitative detection could be achieved on the basis of the distance signal readout. In a recent publication, Alkasir and co-workers expanded the application of PAD to the measurement of dust specimens by designing a compact portable colorimetric paper-based biosensing device with integrated sampling/analysis units.132 Their device comprised an air-sampling cassette and a colorimetric paper-based enzyme sensor, which is used for the detection of BPA based on color change (Figure 12). BPA is always used in a variety of commercial and consumer plastics products and is harmful to human health. Once the paper-based sensor was in contact with the BPA-dust sample, BPA was oxidized enzymatically to its corresponding quinone, which then reacted with a chitosan layer of the sensor to form a charge-transfer quinine-imine complex with strong absorption in the visible range. The detection data could be gathered with the naked eye. They established a color database based on a (RGB) color space. This database could numerically quantify the color intensity, and the BPA concentration could thus be

established. The detection limit of this device was as low as 0.28 μg g−1. The traditional gas chromatography method was also used to validate the colorimetric biosensing method for household dust analysis (Figure 12d). It demonstrated that the ability of this colorimetric method compared well to the established method. Despite the advantages of low-cost, rapid detection, and portability, more efforts are needed for PADs to realize their real applications. First, the determination and quantification of targets are usually achieved with the help of optical or imaging instrumentation, which would limit their accuracy. Second, most known paper-based assays operate in aqueous solutions. The use of organic solvents with PADs has been very challenging because of the difficulty of creating oleophobic or lipophobic barriers.133 Thus, expanding the operating liquids to include organic solvents is necessary to broaden the application fields of PADs to the detection of water-insoluble targets, such as pesticides, drugs, and chemical warfare agents. I


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Figure 13. (a) The drinkable book. (b) The polluted water is passing through the paper filter. (c, d) Use in the developing world.134

sensitive and quantitative on-site detection of cocaine.124 Cocaine recognition was realized using an anticocaine aptamer, and the target concentration is obtained by the attenuation of the luminescence of the paper device. With the help of a smartphone, quantitative results could be achieved very quickly with high sensitivity. This device exhibited high applicability in both human saliva samples and blood samples.

6. REAL APPLICATIONS Since He et al. first reported the facile in situ synthesis of noble nanoparticles in cellulose fibers in 2003,6,43,48,94,95 the most exciting progress therein has doubtlessly been that made by Dankovich and co-workers. They developed the bactericidal materials for improving drinking water quality by eliminating bacteria through filtration. They applied the silver- or coppernanoparticle paper filter for point-of-use (POU) water treatment,93,96 which is a potential solution to poor-quality drinking water. The silver- or copper-nanoparticle paper filters displayed excellent bactericidal performance in laboratory tests. After microbial disinfection, the paper filters retained their high performance. Moreover, the levels of silver and copper released into the filtered effluent water remained below the World Health Organization drinking standard. The simple manufacturing process, light weight, no additional energy consumption, and low cost endow the technology with great promise as a new POU water treatment technology. Very recently, Dankovich and co-workers designed a practical and user-friendly device called a drinkable book (Figure 13a). The drinkable book contains 25 pages, and each page is embedded with silver nanoparticles. The book can not only kill deadly waterborne microorganisms to make dirty water safe for drinking but also provide hygiene education, telling users the dangers of drinking dirty water and how to use the book. Each page of the book is a water filter (Figure 13b). The packaging of the book is designed to be the filter holder. It is estimated that each page can treat up to 100 L of water. The drinkable book has been tested in Bangladesh, Ghana, and South Africa, and the results were satisfactory (Figure 13c,d). Another notable example that must be mentioned is the PADs for on-site analysis. Worldwide popularization of smartphones offers an opportunity for the real applications of on-site analysis using PADs as sensors. This technology, known as mobile sensing, is a very young and quickly developing research field. For example, Holomic developed a highperformance, economical, hand-held tool for testing alcohol and drugs.135 A rapid diagnostic test strip was inserted into a smartphone and used for lateral flow immunochromatographic assay. The test data could be entered on a smartphone interface and then easily saved or e-mailed. Recently, He et al. reported a portable upconversion nanoparticle-based paper device for the

7. CONCLUSIONS Cellulose represents a versatile scaffold for self-assembly toward advanced composites and devices. This review has attempted to introduce recent works in the rapidly growing research field of cellulose-based composites from perspectives of basic research to real applications. The functionalization of cellulose to expand its functions has been realized by the hybridization of cellulose with a variety of species including metal ions, metals, and metal oxides. Cellulose-based composites are believed to have great potential for a wide range of practical applications in antibacterial treatment, catalysis, water purification and separation, and biosensors. We believe that further efforts to create new inspiring composites with cellulose as the scaffold would enable appealing technologies and applications. However, challenges remain in both fundamental design and practical applications of cellulose-based materials. For example, current popular cellulose-based composites used as antibacterial agents and water treatment materials are confronted with the issue of stability, which hinders their long-term use. Low-cost and portable μPADs exhibit great promise for on-site analysis but suffer from high overall analysis costs, low selectivity, and poor stability, limiting their long-term and large-scale field use. Great efforts may be devoted to higher levels of sophistication and miniaturization with enhanced reliability and selectivity. The combination of μPADs with smartphones is one of the future directions for creating new and practical technologies, which can significantly reduce the analysis cost and realize remote in-field testing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: (+86) 10 82543535. Notes

The authors declare no competing financial interest. J


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Biographies

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Hua Tian is currently an assistant professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS). She earned her MEng degree from Ocean University of China and Ph.D. from Research Center for Eco-Environmental Sciences, CAS, under Prof. Z. Hao. Her research interests involve functional nanomaterials and their energy and environmental applications. Junhui He is a professor and Head of the Functional Nanomaterials Laboratory at the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS). He also serves as the Director of the Center for Micro/Nanomaterials and Technology, TIPC. He received his Ph.D. degree under Prof. E. Wang from the Institute of Photographic Chemistry, CAS. He then spent 2 years as a postdoctoral fellow with Prof. N. S. Allen at Manchester Polytechnic and over 4 years as an STA fellow and FRS researcher with Prof. T. Kunitake at Frontier Research System (FRS), RIKEN. He returned as part of the Hundred Talents Program. His research interests involve functional nanomaterials, biomimetic materials, thin films, smart surfaces, and sensors and their energy and environmental applications.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 21571182 and 21271177), the Science and Technology Commission of Beijing Municipality (Z151100003315018), and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. We are also grateful to Dr. Yue Zhang for her help with language improvement.

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