Ting Chen,Ce Sun,Xiuzhi Tian,Xue Jiang,Meiyun Zhang

College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology,Xi'an,Shaanxi Province,710021,China

Abstract: Natural polysaccharides are synthetic macromolecules located in the cell wall and intracellular, intercellular, and secretory cells. These are essential components of life activities. Cellulose and its derivatives, chitosan,alginic acid, starch, and other natural polymer materials that exist in nature are polysaccharides. This article briefly reviews the structure and sources of several natural polysaccharides, focusing on the modification design strategies of polysaccharides such as graft polymerization, oxidation,etherification, and esterification on cellulose (mainly cellulose nanocrystals,CNC),chitosan(CTS),and other polysaccharides and derivatives in the fields of biomass energy, composite materials, wastewater treatment, and biomedicine. Based on the theory of modification and application of polysaccharides at the present stage, future research goals and modification strategy design are prospected.

Keywords: cellulose; chitosan; composite materials; water treatment; drug release

1 lntroduction

At present time, people's quality of life has been greatly improved due to the development of innovation, technology, and products. Moreover, human society is also facing two major problems: energy shortage and environmental pollution. The development of green, environmentally friendly, renewable, and sustainable materials has become a major theme.Polysaccharides are polymerized carbohydrates composed of sugar chains combined with glycosidic bonds with at least ten or more monosaccharides which has high molecular weight[1]. Natural polysaccharides are biological macromolecules synthesized by organisms and are located in the cell wall and intracellular, intercellular, and secretory cells. It is an essential component of life activities[2-3]. To date,more than 300 types of natural polysaccharide compounds have been reported in the literature,such as fungal polysaccharides, plant polysaccharides, animal polysaccharides, seaweed polysaccharides, and bacterial polysaccharides. Cellulose and its derivatives,chitosan, alginic acid, starch, and other natural polysaccharides are widely present in nature.According to our definition, polysaccharides include substances composed of sugar residues connected by glycosidic bonds and molecules containing polymerized sugar structures connected to amino acids,peptides, proteins, lipids, and other structures by covalent bonds[4-5].

Natural polysaccharides have many excellent biological activities,including immune regulation,antitumor, anti-virus, anti-oxidation, and hypoglycemia.The biological activity, as well as physical and chemical properties of natural polysaccharides are closely related to their structure.To improve the effects of polysaccharides and make them more widely used in food, medicine, materials, and other fields, researchers often use scientific methods to modify polysaccharide molecules. The modification of natural polysaccharides includes the physical, chemical, and biological methods. Among them, the various functionalization brought by the chemical modification of polysaccharides has been the goal of researchers. The chemical modification of polysaccharides mainly relies on the hydroxyl (—OH), carboxyl (—COOH), or amino (—NH2) groups on the molecular structure of polysaccharides to undergo oxidation[6],esterification[7],etherification[8], graft copolymerization[9], and other reactions. This process is also called polysaccharide derivatization. Based on the application of natural polysaccharide derivatives in different fields such as biomass energy, composite materials, water treatment,and biomedicine, this article reviews the different modification strategies designed by researchers on cellulose, chitosan, starch, and other polysaccharides in recent years. In addition, innovations and challenges related to future polysaccharide modifications have been proposed.

2 Natural polysaccharides

2.1 Cellulose

Cellulose is the most abundant and widely distributed natural renewable organic polymer. It has the advantages of environmental protection, low cost,good biocompatibility, biodegradability, and biological, chemical, and thermal stability. Cellulose is a macromolecular polysaccharide composed of small glucose molecules linked throughβ-1, 4-glycosidic bonds. Its molecular formula is (C6H10O5)n,the structure diagram is shown in Table 1. Its molecular weight ranges from several hundreds to tens of thousands[2]. In its molecular chain, each glucose molecule contains three hydroxyl groups,which are located at positions 2, 3, and 6 of the glucose molecule. During cellulose formation,multiple cellulose macromolecular chains are stacked and entangled with each other, and more hydrogen bonds are formed between the macromolecular chains and within the molecules. Macromolecules have high crystallinity and compact structures, which make them insoluble in most solvents, such as water and common inorganic and organic solvents[10]. The strength and thermoplasticity of cellulose are poor,and they are resistant to chemical corrosion. All of these factors have a great influence on its dissolution,modification, molding processing, and application.Currently, cellulose and its derivatives are widely used in the fields of light industry, chemical industry,national defense, medicine, energy, biotechnology,and environmental protection[11-12]. This is due to various modification treatments of cellulose fibers,including physical modification, chemical modification, and biological modification. After the modification treatment, the heat and abrasion resistance, mechanical strength, water or oil repellency, and antibacterial activity of the cellulose were improved.

2.2 Chitosan

Chitin is the second-largest renewable resource in nature after cellulose in terms of output. It is also the most essential animal structural material, following the protein bone glue in biology and the largest nitrogencontaining natural organic matter on Earth, except for protein[13]. When the degree of deacetylation of chitin reaches 50%-100%, it becomes chitosan; the system name isβ-(1,4)-2-amino-2-deoxy-D-glucose, and its molecular formula is (C6H11NO4)n, the structure of the chitosan is shown in Table 1. Chitosan and cellulose have similar chemical structures. Cellulose has a hydroxyl group at the C2 position, and the group of chitosan is replaced by an amino group at the C2 position. Chitosan is the only alkaline polysaccharide among all the natural polysaccharides.It contains many properties such as biodegradability, biocompatibility,non-toxicity, antibacterial properties, biological activity, and physical and chemical properties, which are widely used in chemicals, textiles, agriculture,environmental protection, biomedicine, papermaking,and other fields.

The molecular chain of chitosan contains a large number of hydroxyl and amino groups, making it a compact crystalline structure.The main factor that limits the application of chitosan is due to its poor solubility.Therefore,it is necessary to selectively modify chitosan to provide special functionalization while using its biological activity.Based on previous studies,chitosan's chemical reaction usually involves only two hydroxyl groups and one amino group on glucosamine, mainly carboxylation,alkylation,esterification,and Schiff base formation,or cross-linking reactions[14-15].

2.3 Alginic acid

Alginic acid is a natural polysaccharide aldehyde acid present in the cell walls of brown algae such as kelp.A linear copolymer formed by 1,4-bonding betweenβ-Dmannuronic acid (M) andα-L-guluronic acid (G), the contents of G and M in alginic acid has a significant effect on the gelling properties of the fiber, and the chemical formula of alginic acid is (C6H8O6)n[16](Table 1). Alginic acid combines with various cations in seawater to form various alginates. Alginate can thicken, suspend, emulsify, stabilize, form a gel and film, and spin a fiber. It has a wide range of applications in the food,paper,and cosmetic industries.In recent years,it has been used extensively in the field of biomedical engineering.

2.4 Starch

Starch is a macromolecular carbohydrate, a natural polysaccharide macromolecular compound formed by the polymerization of glucose molecules. The basic unit and chemical formula of starch areα-Dglucopyranose and (C6H10O5)n, respectively. The structure diagram is shown in Table 1. There are abundant sources of starch, mainly in the roots, stems,leaves, fruits, and pollen organs of higher plants,particularly in potatoes and grains. Starch is a renewable material that is degradable and it does not pollute the environment. It is divided into amylose and amylopectin. Amylose is composed of D-glucose connected byα-1, 4-glycosidic bonds; the branch position of amylopectin isα-1,6-glycosidic bonds, and the rest areα-1,4-glycosidic bonds. Starch contains a large number of reactive hydroxyl groups, which provides a structural basis for its modification[17].

The modification of starch is based on the inherent characteristics of natural starch, using physical,chemical, or biological treatment, to improve the performance of starch and expand its application range.Cyclodextrin (CD) is a general term for a series of cyclic oligosaccharides produced by amylose under the action of cyclodextrin glucosyltransferase produced byBacillus, with a degree of polymerization of 6-12[18].Raw starch materials extracted from plants can be saccharified to obtain glucose, which is then fermented by enzyme and certain strains to produce high-purity lactic acid. Polylactic acid (PLA) is synthesized through chemical synthesis, which is a new type of biodegradable material[19]. These modified starches have good properties and are widely used in food,papermaking, water treatment, biomedicine, packaging,and other fields.

3 Pretreatment of cellulose for easier hydrolysis

With the gradual depletion of petroleum resources and the deteriorating environment, vigorously promoting the use of fuel ethanol technology has become an important part of energy development strategies in many countries, and the purpose of reducing dependence on fossil energy and greenhouse gas emissions[20]. At present, domestic fuel ethanol is mainly based on biomass (corn, wheat straw, and other foodstuffs) as raw materials. Sugars are extracted through enzymatic hydrolysis, which are further fermented to produce ethanol[21]. The whole process has the advantage of using biological enzyme technology to provide an easier hydrolyzation of sugars in foodstuff; however, disadvantages include the high cost of food and agricultural land pressure. In recent years, researchers have further proposed cellulosic ethanol technology,a high-end clean energy technology that can replace traditional food ethanol technology to produce clean ethanol fuel using cellulosic biomass materials that are widely available[22-23].

Cellulosic ethanol technology[24]is mainly based on cellulose, which has a wide range of sources in nature,as raw materials, and pretreatment is carried out to promote its hydrolysis into sugars and further fermentation into ethanol. This technology has the advantages of wide raw material sources and low cost,but it also has the disadvantage of low hydrolysis efficiency.

Cellulose has a large number of intramolecular and intermolecular hydrogen bonds, and relatively high crystallinity. Therefore, it has a high stability and resistance to chemical erosion. Cellulose must be pretreated to promote cellulose hydrolysis to become a fermentable carbohydrate. In the current report, the pretreatment methods for lignocellulosic materials include physical pretreatment, such as microwave radiation, liquid hot water, and other physical methods,chemical pretreatment using alkali or acid, different solvent systems, such as organic solvents[25], ionic liquids[26], and (super- or sub-) critical solvents[27].Compared with physical pretreatment, chemical pretreatment requires many chemicals, which increases the cost of treatment and causes serious environmental pollution.

According to the structural characteristics of cellulose, researchers performed chemical modifications to impair the stability of its structure to improve the hydrolysis of cellulose. According to previous reports, after certain chemical modifications,cellulose becomes unstable, making it easier to hydrolyze. A study from Borsa et al[28]found that hydrolysable property of carboxymethyl cellulose with a low degree of substitution was better than that of the original cellulose.In addition,the hydrolysis ofβ-(1,4)-glycosidic bonds between glucose molecules is increased due to the covalent bonding of reactive dyes to cellulose, which promotes a decrease in the degree of polymerization of cellulose molecular chains and a large loss of hydrogen bond strength.This phenomenon is called "reactive tendering" in the textile industry[29].The researchers found that substituting various groups on the position C6 of cellulose is beneficial for destroying the stability of cellulose and promoting hydrolysis. Therefore, the chemical modification of cellulose can increase the yield of sugars in cellulose hydrolysis, make cellulosic ethanol fuel a renewable green energy,and become economically competitive.

In general plant fibers, microcrystalline cellulose(MCC)accounts for approximately 73%,and the rest is amorphous cellulose. MCC is obtained on an industrial scale from wood and cotton cellulose; however,materials obtained from water hyacinth[30], coconut shells[31], sugar cane bagasse[32], and jute[33]have not been sufficiently studied. MCC is a free-flowing,extremely fine, short rod-like or powder-like porous particle that is hydrolyzed by dilute acid to the levelling off degree of polymerization (LODP) of 15-375. It is insoluble in water, dilute acids, organic solvents, and grease. It partially dissolves and swells in dilute alkali solutions. Owing to its special properties,such as a low degree of polymerization and large specific surface area, MCC can be used in medicine,food, cosmetics, and light chemical industries. Jiang et al[29]chose 2,4,6-trichloro-1,3,5-triazine (TCT) to modify MCC (Fig. 1(a)). The impermeable crystal structure of MCC was covalently attacked by cyanuric chloride, and X-ray photoelectron spectroscopy (XPS)was used to prove the successful introduction of a certain amount of cyanuric chloride, while using cross polarization/magic angle spinning (CP/MAS),13Cnuclear magnetic resonance (13C-NMR), and wideangle X-ray diffraction (WAXD) characterizations showed that the crystalline structure of MCC was destroyed, and the ratio of crystal planes changed dramatically. These changes increased the hydrolysis efficiency of MCC catalyzed by dilute acids.

The d-spacing of MCC should affect the acid hydrolysis of cellulose because the position of water molecules in the crystal lattice depends on the dspacing of the crystal plane[34].Each crystal plane had a different d-spacing and reactivity.The ratio of the three main crystal planes in cellulose is (101ˉ), (101), and(200), which affect the accessibility of cellulose to chemical substances. As shown in Fig. 1(c), the diameter of the hydronium ion is 0.9 nm[35], and the increase in d-spacing between crystal planes promotes the hydrolysis of cellulose. The ratio between the three main crystal planes affects the hydrolysis ability of cellulose.The number of crystal planes(101)and(101ˉ)relative to(200)(R)can be calculated using Eq.(1).

According to the results, it can be concluded that the increase in crystal planes promotes the hydrolysis of cellulose, as larger d-spacing makes cellulose easier to be accessed. However, the improvement is not as good as it is expected because the remaining TCT groups on cellulose are hydrophobic, which can be added to the molecular chain of cellulose. The hydrophilic groups may form water molecular channels in an aqueous environment, increasing the accessibility of cellulose(Fig. 1(d)). Jiang et al[29]introduced sulfanililic acid(SAA) with TCT to generate SAACT, which was then reacted with MCC(Fig.1(b)).Finally,it was found that the hydrophilicity of the sulfonic acid groups on SAA formed water molecular channels, which greatly increased the efficiency of cellulose hydrolysis. These graft modifications can cause the destruction of the crystal form of cellulose or increase the hydrophilicity, thereby significantly increase the yield of reducing sugars, resulting in lower costs, less resource consumption, and more eco-friendly aspects,which have great expectations for potential applications.

Fig.1 Design strategy and principle of cellulose modification to promote hydrolysis. (a) The hydrolysis of TCT modified MCC; (b)SAA introduced into TCT to form SAACT;(c)draft of different d-spacing values of the three main crystal planes that are(101ˉ),(110),and(200)in MCC,and comparison of their accessibilities to hydronium;(d)the accessibility of cellulose can be increased by building the water molecular channel in water environment [29]

4 Modification of nanocellulose for composite materials

Due to the increase in people's demand and the overexploitation of mineral resources such as petroleum,mankind faces the depletion of these mineral resources.According to a report byNikkeion August 6, 2018, the Organization for Economic Cooperation and Development (OECD) recently released a statement that the global production of plastic waste continues to increase,with more than 300 million tons of petroleumbased plastic flowing into the environment every year.The non-degradability of petroleum-based plastics occupies many land areas, causing long-term and deepseated ecological and environmental problems; air pollution due to the wind blowing of plastics in the air;water pollution due to the floating plastic bottles and non-biodegradable materials. Moreover, if animals digest garbage by mistake, it will harm their health,which leads to indigestion and starvation.As the global ecological environment continues to deteriorate,various countries have taken actions to research and develop alternatives of bio-based plastics[36-38].Therefore, the development of degradable green plastic materials is urgently needed worldwide.

PLA is one of the most widely studied bio-based and biodegradable material. Its advantages include high mechanical strength, high melting temperature, good biodegradability, good sustainability, and low cost.PLA is a degradable biopolymer material that is ideal,has the most development potential, and can replace petroleum resources[39]. It is widely used in packaging and biomedical materials.

Cellulose nanocrystal (CNC) has attracted considerable attention as nanofillers to enhance the crystallization of PLA and improve the mechanical properties of composites. However, two challenges were identified: the inferior thermostability of CNC and the compatibility of CNC and polymeric matrix[40].Surface modification has proven to be an efficient method to overcome the two problems mentioned above. Many surface modification processes have been studied, such as the adsorption of surfactants[41-42],TEMPO-mediated oxidation[43-44], cationization[45],esterification[46], and polymer grafting[47]. However, the weak combination of the modifier and cellulose in surface absorption results in low durability, and the weak compatibility of the modifier and cellulose in the organic reaction causes low modification efficiency.These disadvantages can be avoided by graft polymerization.

Fig.2 The synthesis of triazine derivatives onto CNC surface using BA,HA,and DA [48-49]

4.1 CNC modified with triazine derivates and blended with PLA

Numerous chemical and physical modifications have been attempted to improve the thermal stability and reduce the hydrophilicity of CNC.Yin et al[48-49]studied the synthesis of triazine derivatives using n-butylamine(BA), hexylamine (HA), and dodecylamine (DA) with different carbon chain lengths and grafted the synthesized triazine derivatives onto the CNC surface(Fig. 2). It was found that adding modified CNC could improve the mechanical properties of PLA composites.Triazine, a hexahydroxy N-containing heterocyclic compound, is an important intermediate in dye and pharmaceutical industries. The TCT has three reactive chlorine groups and can directly react with various functional groups, such as amino, hydroxyl, and epoxy groups[50]. It proved that TCT could be a candidate for activating hydroxyl groups on the surface of CNC.

Compared with BA and HA, DA with a longer hydrophobic aliphatic carbon chain was used to obtain an ideal effect. The hydrophobic aliphatic chain covered the surface of the modified CNC, as a result,the thermal stability and hydrophobicity of CNC-TCTDA were further improved, compared with CNC-TCTBA and CNC-TCT-HA. In addition, compared with pure CNC and CNC modified with TCT-HA, CNC modified with TCT-DA could be more uniformly dispersed in the PLA matrix, which indicates that the interface interaction between the nanofiller and the PLA matrix was further enhanced. The enhanced interaction led to improved thermal stability and the mechanical properties of the composite.The elongation at break of PLA composite increased by 80%, while maintaining other properties (such as the light transmittance of the composite).

Fig.3 Interaction modes between CNC,APS and PEG:(a)cross-linking between CNC and APS via hydrogen bonds;(b)grafting of APS onto CNC through covalent bonds;(c)APS-PEG grafting on CNC surface [54]

4.2 CNC modified with silicane and poly(ethylene glycol)/acrylate

Poly(ethylene glycol) (PEG), as an efficient plasticizer and compatibilizer for biopolymers and nanocomposites, is biocompatible, non-toxic, and ecofriendly[51-52]. Numerous studies on the application of using PEG in CNC surface modification have been conducted.PEG can be used as a carrier to increase the dispersion of CNC in a polymer matrix[53]. To the best of our knowledge, chemical grafting of PEG on the CNC surface is difficult to comprehend owing to the chemical inertness of the PEG chains. In another study by Yin et al[54],p-aminopropyltriethoxysilane was selected as an intermediate to graft PEG onto the CNC surface (Fig. 3). This is an environmentally friendly and sustainable strategy for modifying CNC, which improves the thermal stability and hydrophobicity without damaging the crystal structure of CNC. After analysis, the self-crosslinking of silane formed a threedimensional network structure, where the PEG layer as a compatibilizer and carrier was successfully grafted onto the CNC surface,and the TEM image showed that the morphology and size distribution of the CNC grafted with ammonium persulfate (APS)-PEG were not seriously disturbed. The nanoparticles prepared by this method can avoid thermal decomposition at high temperatures (approximately 300℃) and exhibit low hydrophilicity. In the process of compounding with PLA, the APS-PEG-grafted CNC was more uniformly dispersed in the PLA matrix than the original CNC. In addition, during the modification process, the CNC composites did not aggregate and carbonize significantly. Compared with pure PLA, the modified CNC-reinforced PLA composite showed higher crystallinity, higher nucleation density, and faster crystal ball growth rate, and the smaller spherulite size improved the strength and elongation at break of the composite material. The functionalization of CNC through amino silane and epoxy-polyethylene glycol tackled the two challenges of CNC/PLA composite,namely poor thermal stability and compatibility.

4.3 CNC modified with the controlled polystyrene(PSt)chains

The graft polymerization of CNC can avoid the weak combination and weak compatibility of the modifier and cellulose, resulting in high modification efficiency and better durability. However, traditional free radical polymerization still has the problem of poor control of the grafted molecular chain structure[55]. Therefore,controlled polymerization is the best modification strategy for CNC.

Yin et al[56]adopted the graft polymerization of styrene (St) via surface-initiated atom transfer radical polymerization (SI-ATRP) of CNC to modify its thermostability and compatibility. The modified CNC were dosed into polymethylmethacrylate (PMMA)nanocomposites by solution casting. In addition, the compatibility between the modified CNC and the PMMA matrix was improved owing to the organic polymer chains on the surface of the modified CNC.The breaking strength and elongation at break of the PMMA/CNC composite improved with the addition of CNC, which was attributed to the excellent mechanical properties of the CNC. Furthermore, the thermal stability of PMMA/CNC composite was improved by the introduction of CNC, and the composite exhibited excellent transparency at the same time.Further studies are needed to fully understand the interaction between CNC and the PMMA matrix, and to better control the grafting density of PSt and the length of the polymer chain grafted on the CNC surface.

4.4 Lipase-catalyzed laurate esterification of CNC

Cellulose can be transformed in various ways to achieve better compatibility with a hydrophobic polymer matrix, owing to the abundant —OH groups on the cellulose surface. In particular, environment friendly modification methods are needed[57]. The goal is to chemically modify the hydroxyl groups of CNC to decrease the hydrophilicity of CNC without compromising their structural integrity or thermal stability.

Yin et al[58]reported successful esterification of cellulose surfaces in a benign solvent system, whereby dodecanoic acid was attached to CNC, significantly increasing its hydrophobicity. To improve the compatibility between the CNCs and PLA matrix,CNCs were topo-chemically modified to laurate esters(CNC-LAA) using immobilized lipase as the esterification catalyst. The modified CNCs exhibited hydrophobic properties that were attributable to a homogeneously grafted hydrophobic layer on the CNC surface. The nanoparticles showed good thermal stability because of esterification, probably due to less amount of amorphous regions of the modified CNC.Crystallization of PLA was significantly enhanced by the inclusion of CNC-LAA particles due to the nanoparticles that acted as nucleating agents for the PLA.The mechanical properties of the CNC-LAA/PLA composite were improved, which was attributed to the enhanced interfacial interaction between the modified nanofillers and laurate ester groups.

5 Modification of polysaccharide for wastewater treatment

Wastewater from dyeing or printing processes makes a significant contribution to water pollution. Most dyes used for dyeing and printing have similar characteristics, such as specific colors, complex structures, low biodegradability, and potential toxicity.Therefore, dye wastewater should be decolorized during the first stage of treatment[59]. In recent years,many methods, such as flocculation, precipitation,extraction, evaporation, adsorption, ion exchange,oxidation, advanced oxidation, incineration,electrochemical treatment, biodegradation, and membrane filtration have been reported to remove color from wastewater[60].

5.1 The modified chitosan porous particles

Due to the rapid development of the textile industry,dyes have become one of the main sources of serious water pollution. Dyes, particularly water-soluble dyes,are difficult to separate from wastewater because of their inert properties, such as the degradability of light,oxidants,and biological reagents.

Recently, adsorption technology using chitosan(CTS) derivatives has been developed to replace conventional treatment processes to adsorb anionic dyes in aqueous solutions. CTS has renewability,biocompatibility, biodegradability, and non-toxicity and thus has advantages over other adsorbents. In addition,the primary amino groups of CTS can be protonated by hydrogen ions in the solution; hence, anionic dyes can be strongly adsorbed by electrostatic attraction[61-62].However, CTS cannot be directly used to adsorb dyes from its wastewater because it lacks good chemical and mechanical stability and high adsorption capacity and efficiency, which can be changed by crosslinking and grafting modified products. Jiang et al[63]synthesized glycidyl methacrylate (GMA)-modified CTS (CTSGMA) bearing double vinyl groups through the ringopening reaction of GMA with the C6—OH of CTS(Fig. 4(a)). Thereafter, CTS-GMA was radically copolymerized with methyl methacrylate (MMA) to obtain the graft copolymer (CTS-GMA)-g-PMMA.Finally, an inverse emulsion together with wet phase inversion was used to prepare (CTS-GMA)-g-PMMA with increased amorphous regions of porous particles.It was proved by the characterization that they had strong affinities for anionic dyes (e.g., reactive brilliant blue 19, reactive brilliant violet 5) dissolved in water.The adsorption process is pH-controlled,which ensures easy adsorption and desorption of the dyes.

5.2 β-cyclodextrin(β-CD)based polyelectrolyte

When treating dye wastewater by flocculation,flocculants are one of the key factors that determine the decolorization effect and efficiency. At present,compared with traditional metal-based and synthetic polymers, natural polysaccharide-based modified flocculants have been remarkable due to their biodegradability, renewability, low cost, and nontoxicity[64].β-CD is a derivative of starch and its related compounds after enzymatic or acid degradation. It has the potential to decolorize because it can form guestguest complexes with certain dyes or their groups[65].In addition, it has better water solubility and is easier to modify through homogeneous chemical reactions.

How in the world did you know that? asked Plumb. I packed your parachute, the man replied. Plumb gasped4 in surprise and gratitude5. The man pumped his hand and said, I guess it worked! Plumb assured him, It sure did. If your chutehadn t worked, I wouldn t be here today.

In a study by Jiang et al[66], without the use of surfactants, new hydrophobically modified polyelectrolytes containingβ-CD moieties were prepared by solution polymerization of St, maleic anhydride (MAH) esterifiedβ-CD (β-CD-MAH), and acryloyloxyethyl trimethyl ammonium chloride(AETAC). Introducing the St moieties as well as reducing the cationic AETAC moieties onto the backbone of the copolymer was effective in obtaining a polyelectrolyte with improved hydrophobicity (Fig. 4(b)). Its chemical composition was confirmed by Fourier transform infrared (FT-IR) and1H-NMR spectra. As a new flocculant, it exhibited enhanced properties in decolorizing the C.I. reactive violet 5 solutions, that is, faster decolorization speed, stronger resistance against inorganic salt, more flexibility of pH value, and wider flocculation window. For the given flocculant/dye solution system,apart from such roles as polymer bridging,electrostatic,and covalent bonds,the hydrophobic effect contributed to the flocculated decolorization mechanism. However, their role was dominated by the chemical structure of the dye and flocculation conditions, particularly the temperature and pH value.

5.3 The cationic dialdehyde nanocrystalline cellulose(CDAC)filled chitosan films

The final product of the complete oxidation of cellulose is carbon dioxide and water; however, partial oxidation can introduce new functional groups such as aldehyde groups, ketone groups, carboxyl groups, or enol groups into the cellulose macromolecules to form oxidized cellulose with different water-soluble or insoluble properties. Among them, periodate attacks position C2 or C3 to produce highly reduced dialdehyde cellulose(DAC), which has attracted the attention of researchers[67-68].

CNC is a promising nanofiller for reinforcing CTS films.The flocculation of the CNC suspension in acidic CTS solution is a key problem that results in many shortcomings such as poor tensile strength.A derivative of nanocellulose—CDAC, obtained from the Schiff base reaction between the (2-hydrazinyl-2-oxoethyl)-trimethylazanium chloride (GT) reagent and DAC, was synthesized in the recent study by Tian et al[69]to avoid flocculation (Fig. 4(c)). A CDAC suspension was prepared by successive oxidation-reductive amination of CNC. The oxidation of CNC led to smaller rod-like nanocrystals with reduced crystallinity. The final experimental results showed that CDAC acts as a crosslinker as well as filler, improving the mechanical strength of the composite films. CDAC and CTS in the composite film exhibited a synergistic dye removal effect, accounting for the relatively high adsorption capacities of the composite films. Flocculation occurs in the mixture of CNC/CTS but disappears in the mixture of CDAC/CTS.

Fig.4 The modified polysaccharide for wastewater treatment.(a)The modified chitosan porous particles [63];(b)the β-CD based polyelectrolyte [66];(c)the CDAC filled chitosan films [69];(d)the cationic polyelectrolyte grafted CNC [71]

5.4 The cationic polyelectrolyte grafted CNC

Cellulose-based flocculants have gradually become a model for the use of polysaccharide biomass resources in wastewater treatment, but the removal of dyes takes a long time owing to the flocculants' low degree of polymerization and high water solubility. CNC produced by the strong acid-assisted hydrolysis of natural cellulose may be preferred flocculants to achieve efficient and fast dye removal. CNC had a large specific surface area of approximately 480 m2/g.Many surface hydroxyl groups can be converted into active sites for the immobilization of dyes[70]. CNC tend to aggregate once they are combined with dyes.The high crystallinity of the CNC further facilitated the rapid sedimentation of the destabilized nanocrystals.

In the study by Jiang et al[71], CNC was surfacemodified by grafting polymerization of acryloyloxyethyl trimethyl ammonium chloride(AETMAC) with the initiator of ceric ammonium nitrate (CAN) (Fig. 4(d)). The PAETMAC grafting on the CNC surface loosen the hydrogen-bond network among the tremendous surface hydroxyl groups, offer three-dimensional spaces for adsorption of dyes, play the role of bridging,which promotes the growth of dyecontaining flocs and accelerates the precipitation of flocs.

According to other studies, polysaccharide-based adsorbents or flocculants have the advantages of rapid decolorization, wide flocculation window, flexibility of pH value, and strong tolerance of inorganic salts;however,two problems were identified:

(1) They only work with certain dyes. Negatively charged materials can only remove cationic dyes from wastewater. Positively charged materials can only remove anionic dyes from wastewater.

(2) High cost and second pollution. The preparation process of absorbents and/or flocculants is complicated and requires large amounts of chemical reagents. The treatment of formed flocs results in secondary pollution and adds to the cost. Absorbents could not be regenerated after dye adsorption.

With these current problems, further studies need to focus on the preparation of amphoteric polysaccharides through simple and environmentally friendly methods.For example, in ionic liquids, cellulose is grafted with amphoteric groups to reduce crystallinity, increase porosity, and contain amino and carboxyl groups at the same time, thereby improving the accessibility of dyes.The ultimate goal is to achieve adsorption and desorption by adjusting the pH value of the dye, which is effective for both anionic and cationic dyes.

6 Natural polysaccharides for bio-applications

Hydrogels are three-dimensional networks composed of hydrophilic substances with strong water absorption properties. They have excellent biocompatibility, soft texture, high permeability, and hydrophilicity to small molecules, and have a wide range of applications in biomedical applications, such as tissue engineering,wound healing, and drug delivery[72-73]. Polysaccharide carbohydrates are rich in sources, renewable, and biodegradable, therefore, they are a much more promising matrix for preparing biomedical hydrogels compared to polyvinyl alcohol, polyethylene glycol,and polyacrylamide.

In present research reports, polysaccharide-based hydrogels such as cellulose, hemicellulose, chitosan,and sodium alginate have been widely studied in the field of biomedicine. Lou et al[74]chose a derivative of chitosan, carboxymethyl chitosan (CMCS), to crosslink dialdehyde-β-cyclodextrin (DA-β-CD) to prepare the target hydrogel (Fig. 5).Among them, CMCS itself has good biocompatibility, antibacterial, anti-oxidation,biodegradability, and non-cytotoxicity, and is an ideal material for preparing biomedical hydrogels. DA-β-CD was produced by the oxidation ofβ-CD with sodium periodate. The DA-β-CD can be cross-linked with CMCS with amino groups to form a hydrogel for drug delivery.

DAC is used as a high-efficiency and low-toxicity cross-linking agent for the preparation of PVA/DAC hydrogel films, particularly for topical applications,such as drug-loaded patches, wound dressings, and cosmetics. Muchová et al[75]used DAC as a green crosslinking agent to synthesize a biocompatible DAC/PVA film for transdermal drug delivery applications and could control the amount of DAC and the weight average molecular weight of PVA; two factors were used to optimize the performance of the hydrogel[75].The membrane had high porosity and drug loading(high water content), mechanical properties were easy to handle, the best adhesion to the skin was extracted from all test samples,and the transdermal drug delivery was improved. The hydrogel membrane was biocompatible, it had no cytotoxicity and no negative impact on cell growth and morphology. In addition, the hydrogel did not support cell migration and attachment to its surface, ensuring that the hydrogel patch can be easily removed from injured or damaged skin even after use.

In addition to cellulose and chitosan,polysaccharides such as starch, dextrin, and sodium alginate are also widely used in the field of biomedicine. Polysaccharide derivatives have anti-tumor and immune-promoting activities, anti-viral activities,antibacterial properties, and other excellent biological properties, as a result, polysaccharide-based products have great potential in drug delivery systems,degradable biofilms, bio-enzyme immobilized carriers,tissue engineering,and other applications.

Fig.5 Natural polysaccharides for bio-applications,DA-β-CD crosslinked CTS hydrogel for drug release [74]

7 Conclusions

Natural polysaccharides have abundant reserves,excellent biodegradability, and biocompatibility, and there are a large number of active groups (—OH, —NH2, etc.) in the chemical structure that can be modified to achieve multiple functions. This article reviews the different modification strategies of cellulose, chitosan, and other polysaccharides in the fields of bioenergy, composite materials, dye wastewater treatment, and biomedicine. Combined with the current research status, many challenges in the modification of polysaccharides will be encountered in the future.

(1) The degree of polymerization of natural polysaccharides cannot be controlled, resulting in instability of chemical modification.

(2) The pretreatment of cellulose should be performed by a combination of biological, physical,and chemical technologies, and the use of solvents and biological enzymes should be given priority to reduce energy consumption and pollution as much as possible.

(3) There are many shortcomings in the chemical modification of polysaccharides, such as the high cost of modification, difficult operation, side reactions, and inability to accurately control the modification sites,which greatly restricts the large-scale application of polysaccharide modification technology.

(4) There are still gaps in the analysis of the modification mechanism of polysaccharides by researchers in many aspects, and the study of the modification mechanism is an important subject to be discussed in the future. Polysaccharides have many advantages that will soon be discovered and utilized by more researchers.