Abstract
With the improvement of living standards, the human demand for antibacterial materials has increased. Cellulose, as the most abundant polymer in the world, is natural, biodegradable, and renewable, which makes it a promising raw material for the production of antibacterial materials. Silver nanoparticles (AgNPs)-cellulose antibacterial composites exhibit good biocompatibility and antimicrobial properties. These materials are easily degraded chemically and are environmentally friendly. Therefore, the AgNPs-cellulose antibacterial composites exhibit broad utilization prospects in environmental protection, medicine, chemical catalysis, and other fields. Several methods are used to manufacture such materials. This paper reviews three common techniques: the physical method, the in situ chemical reduction method, and the covalent bonding method. The differences and relationships are identified, and the advantages and disadvantages are compared among these three methods. Lastly, the present situation and the development potential of the AgNPs-cellulose antibacterial composites are discussed in this review.
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Review of Silver Nanoparticles (AgNPs)-Cellulose Antibacterial Composites
Yongjian Xu,a,b,c Shasha Li,a,c Xiaopeng Yue,a,c,* and Wanli Lu a,c
With the improvement of living standards, the human demand for antibacterial materials has increased. Cellulose, as the most abundant polymer in the world, is natural, biodegradable, and renewable, which makes it a promising raw material for the production of antibacterial materials. Silver nanoparticles (AgNPs)-cellulose antibacterial composites exhibit good biocompatibility and antimicrobial properties. These materials are easily degraded chemically and are environmentally friendly. Therefore, the AgNPs-cellulose antibacterial composites exhibit broad utilization prospects in environmental protection, medicine, chemical catalysis, and other fields. Several methods are used to manufacture such materials. This paper reviews three common techniques: the physical method, the in situ chemical reduction method, and the covalent bonding method. The differences and relationships are identified, and the advantages and disadvantages are compared among these three methods. Lastly, the present situation and the development potential of the AgNPs-cellulose antibacterial composites are discussed in this review.
Keywords: Silver nanoparticles; Cellulose; Cellulosic antibacterial materials
Contact information: a: Shaanxi Key Laboratory on Paper Technology and Specialty Papers, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China; b: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China; c: National Demonstration Center for Experimental Light Chemistry Engineering Education (Shaanxi University of Science & Technology), Xi’an, Shaanxi 710021, China;
*Corresponding author: yuexiaopeng@sust.edu.cn
INTRODUCTION
With the improvement of environmental awareness, research for development of biodegradable materials from renewable sources is increasing. Biopolymers, which are less expensive and occur in abundance in nature, have become the focus. A representative of biopolymers presenting these advantages is cellulose (Cherian et al. 2011). However, similar to the great majority of biopolymers, the characteristics of cellulosic materials determine that they can undergo bacterial attachment (Pasmore et al. 2001). Since cellulosic materials are commonly used in biomedical fields, the spreading of infections within hosp21itals has motivated scientists to develop new and efficient antibacterial materials for fighting infections, more especially as components of wound dressings and antifouling coatings (Drogat et al. 2011; Tsai et al. 2017). Fortunately cellulose and its derivatives have been demonstrated to be good materials for functionalization, and it is a favorable base material to address the antibacterial problem (Shin et al. 2008; Díez et al. 2011).
Cellulosic antibacterial materials generally can be described as the combination of cellulose and antimicrobials. This combination endows new properties and expands the applications of cellulosic materials (Bakumov et al. 2007; Li et al. 2009). Generally, there are three types of common antimicrobials: inorganic (Xu et al. 2016), organic (Roy et al. 2008), and natural (Alonso et al. 2009). The main raw materials of organic antimicrobials are lipids, alcohols, and phenols, such as quaternary ammonium salts, ethanol, metformin, formaldehyde, organic halogen compounds, organic metal, pyridine, imidazole, haloalkane, and iodide. Although organic antimicrobials are widely used in all aspects of life, due to their bactericidal, antiseptic, and mildew-proof effects, their applications in the fields of textile, health, medicine, and some other areas are limited due to their intrinsic toxicity. Natural biological antimicrobials, which mainly come from plant extraction, include amino acids, natural peptides, and polysaccharides and are abundant and environmentally friendly. However, natural biological antimicrobials require a complex preparation process and have higher costs, which restricts the research and applications of these antimicrobials. Represented by silver, zinc, and copper, the inorganic antimicrobials, with high antimicrobial activity and low toxicity, have gained extensive attention (Jensen et al. 2000; Métraux and Mirkin 2005; Torres et al. 2007; Tao et al. 2008; Liu and Hurt 2010; Zhang et al. 2011; Henzie et al. 2012). Among these antimicrobials, silver nanoparticles (AgNPs), which has been demonstrated to possess excellent antibacterial activity through mechanisms involving the release of Ag+ ions that affect the replication of DNA (Marini et al. 2007) or the collapse of the proton-motive force across the cytoplasmic membrane (Holt and Bard 2005), have become a target of great interest for their relatively nontoxicity to human cells (Vimala et al. 2010).
Cellulose, the most abundant renewable resource on earth, mainly comes from plants and bacteria. Multifarious cellulosic materials, such as nanofibrillated cellulose (NFC) or regenerated cellulose (RC), can be obtained via physical, chemical, or other methods. Hence, there is an enormous potential for the development of AgNPs-cellulose antibacterial composites.
This paper reviews three methods for the combination of cellulose and AgNPs, i.e., physical methods, the in situ chemical reduction method, and covalent bonding methods. The differences and relationships between these methods, as well as their advantages and disadvantages, are compared. Furthermore, the specific forms and applications of AgNPs-cellulose antibacterial composites are discussed. This paper aims to promote the development of AgNPs-cellulose antibacterial composites.
PHYSICAL METHODS FOR THE PREPARATION OF AgNPs-CELLULOSE COMPOSITES
The physical methods can be classified as either wet or dry processes according to whether there is water medium during the production process. There is no morphology requirement for raw material in wet process; however, it is generally a block in the dry process.
Wet Process
The wet process includes the adsorption characteristics of natural cellulosic materials. The cellulosic materials and nano-silver colloid are prepared separately. The nano-silver colloid can be obtained by varied methods, such as chemical reduction (Chen et al. 2002; Hao et al. 2002; Anderson et al. 2014), photoinduction (Jin et al. 2001; Callegari et al. 2003; Jin et al. 2003; Basuny et al. 2015), or electrochemistry (Braun et al. 1998). Then AgNPs are blended with cellulosic materials, without chemical reactions. The cellulosic materials merely serve as a carrier or matrix. Figure 1 illustrates the basic principle of this method. AgNPs-cellulose antibacterial composites should be prepared according to the expected product types (such as membrane, fiber, or powder) and the categories of raw cellulosic material, as reviewed below.
Fig. 1. The wet process
Membrane type products
Cellulosic membranes are widely used in ultrafiltration, microfiltration, reverse osmosis, and forward osmosis processes (Shibata 2004). Due to the rapid development of novel cellulose solvents, regenerated cellulose (RC) membranes have captured extensive attention. Typically, there are two approaches to load AgNPs onto cellulose membrane materials.
RC membranes can be prepared before the AgNPs are loaded (Ahamed et al. 2015). Benavente et al. (2017) fabricated RC membranes and nano-silver colloids. The AgNPs-containing RC membrane products were obtained by dipping the RC membranes into the nano-silver colloid. However, the disadvantage of this approach is that the AgNPs cannot enter the interior of the RC membrane.
The second method solves the above problem. The AgNPs and the cellulose solution are mixed before the casting of the membrane. For instance, AgNPs can be prepared using the modified Tollens’ process and added into the cellulose-alkali/urea solution to obtain an approximately 0.5 mm-thick AgNPs-containing RC membrane (Chook et al. 2012). The AgNPs and cellulose are mixed uniformly in the homogenous cellulose solution. Thus, the AgNPs are incorporated into the interior of the RC membrane during the casting process. The final RC has antibacterial ability for a long period.
Due to alternative solvent systems, cellulose derivatives are more favorable for fabricating membrane materials, such as the AgNPs-containing hydroxypropyl methylcellulose membrane (Lloret et al. 2012). In addition, Caloca et al. (2017) fabricated AgNPs-containing polyethylene glycol/cellulose acetate ultrafiltration membranes by filtering nano-silver colloid through the polymer membrane. Faria et al. (2017) proposed the fabrication of antimicrobial membranes through the incorporation of graphene oxide-silver nanocomposites into a cellulose acetate polymeric matrix. The membranes presented strong antibacterial activity, being able to inactivate adhered bacteria at a rate of 90% compared to pristine cellulose acetate membranes.
Fiber type products
In the field of health care, fibers characteristics play an important role in the final product performance. Medical cloth items, such as surgical gowns, will have antibacterial ability if the fibers are loaded with AgNPs. Several reports have focused on the AgNPs-loading modification of cellulosic fibers. Raghavendra et al. (2013) fabricated a AgNPs-containing mixed solution with arabic gum and guar gum as reducing agents. The cellulose-Ag compound fibers were obtained by immersing cotton fibers in the mixed solution. Csóka et al. (2012) prepared a AgNPs colloid by a modified Tollens method using D-glucose as the reduction agent. The silver colloid solution was added to a water suspension of cellulose fibers to prepare fiber sheets.
Cellulose acetate (CA) is a thermoplastic resin derived from natural fibers. Based on the wide utilization of CA in varied areas such as the pharmaceutical and textile industries, it seems necessary to prepare AgNPs-containing CA fibers. Kendouli et al. (2014) prepared a AgNPs colloid via polyhydric alcohols reduction, and the AgNPs colloid was further blended with a CA solution to obtain AgNPs-containing CA fibers by electrospinning. They also proposed another approach where prepared CA fibers are immersed into a AgNPs colloid.
Powdery type products
Powdery material is usually employed to fill the other main component, i.e. a mobile matrix. Accordingly, the antibacterial properties of compound materials can be achieved by loading AgNPs into the powdery material. Martins et al. (2012) prepared a nano-silver colloid via glucose reduction. The surface modified NFC was mixed with the AgNPs colloid, and the obtained powdery compounds were used as fillers in starch-based coating formulations to produce antimicrobial paper products.
Dry Process
Wet treatments can cause environmental concerns and high water and energy consumption. Dry routes including sputtering are considered ecofriendly processes and offer the advantage of modifying only the material surface.
Recently, a novel technique for incorporating AgNPs into paper surface using a flame pyrolysis procedure has been proposed by Brobbey et al (2017). The technique is known as Liquid Flame Spray. This method demonstrates a dry synthesis approach for depositing AgNPs directly onto paper surface in a process which produces no effluents. The production technique is scalable for industrial production of antibacterial paper. Irfan et al. (2017) also employed the dry process but to prepare antimicrobial functionalized cotton fabric. They deposited antimicrobial silver nanoclusters/silica composite on cotton fabric by radio frequency co-sputtering method. The study is expected to be applied to surgeon gowns in the future.
Summary
In general, the advantages of the physical method, i.e., its convenient operation process and relatively high silver loading ratio, are remarkable. However, due to the relatively weak bonding of adsorption, the AgNPs adsorbed on the surface of cellulosic materials may be partially lost during the utilization process. In other words, the products prepared by this method have poor laundering durability (Ilić et al. 2009). Thus, the antibacterial potential may be restricted to a certain degree. Separately, in wet process an obvious disadvantage of this method is that the use of reducing agents can be toxic and undesirable. In addition, there is heavy water consumption and waste water production with hazardous chemicals. On the contrary, dry routes avoid these problems perfectly. It is an environmentally friendly method and conforms to the needs of industrial production.
IN SITU CHEMICAL REDUCTION METHOD FOR THE PREPARATION OF AgNPs-CELLULOSE COMPOSITES
In situ chemical reduction has been widely used to overcome the inefficiency of physical adsorption. Compared with physical adsorption, in situ chemical reduction involves the slow growth of AgNPs in cellulosic materials. In other words, having an ionic radius of about 0.1 nm, the silver ions can pass into the interior of cellulosic materials. After the AgNPs have increased in size, they were wrapped by the skeleton of cellulosic materials and could not drop out. Thus the AgNPs can grow in situ in the interior of materials.
Based on whether the cellulosic materials participate in a redox reaction, the in situ chemical reduction method can be divided into two branches. First, cellulosic materials can serve as a matrix for the growth of AgNPs. In this case, there is no chemical reaction between the cellulosic materials and the silver precursor. Second, chemical reactions occur between cellulose (or its derivatives) and a silver precursor. In that case, the cellulosic materials play a dual role of reducing agent and matrix.
Cellulosic Materials as a Matrix
Because the cellulosic materials do not participate in a chemical reaction in this approach, an additional reducing agent must be added to reduce the silver ions.
A one-step method can be used to prepare powder/fibers from cellulosic materials, such as microcrystalline cellulose (MCC) powder (Vivekanandhan et al. 2012). Nanocrystalline cellulose (NCC) can serve as the matrix, allowing glucose to reduce Tollen’s reagent to produce AgNPs at room temperature (Wang et al. 2016). Pinto et al. (2009) prepared cellulose nanocomposites by adding a solution of AgNO3 drop-wise into an ice-cold NaBH4 solution containing the cellulose and stirring vigorously over 2 h. Gaminian and Montazer (2017) decorated AgNPs on electrospun cellulose nanofibers (CNFs) through a facile method. The CNFs were treated with silver nitrate, ammonia, and sodium hydroxide and subsequently with dopamine as reducing and adhesive agent. Ag ions on the CNF surface were photo-reduced to AgNPs using UVA irradiation to produce a dense layer of AgNPs on the nanofibers. This approach has been employed widely due to its convenient operation (Son et al. 2006; Li et al. 2011a; Li et al. 2011b; Liu et al. 2011; Jang et al. 2014; Li et al. 2016; Prema et al. 2017). However, the silver ions and reducing agent are added in a system, which partially hinders the in situ synthesis of AgNPs in cellulosic materials. This potential hindrance is the main disadvantage of this method.
Some bulk form AgNPs-containing cellulosic materials, such as membrane, paper, or gel, and can be prepared by a relatively complex two-step method. Figure 2 shows the general operation, which includes the following two steps: i) cellulosic material is immersed in a silver precursor solution to adsorb adequate Ag ions, and the loose Ag ions on the surface of the cellulosic material are washed away by deionized water; and ii) the cellulosic material containing Ag ions is added into a reducing agent solution to form AgNPs. In most cases, silver nitrate is selected as a silver precursor, whereas the reducing agent can be chosen from varied substances, such as sodium borohydride (He et al. 2003; He et al. 2005; Luong et al. 2008; Maneerung et al. 2008; Zhu et al. 2009; Ahmad et al. 2016; Yan et al. 2016), sodium citrate (Tankhiwale and Bajpai 2009; Hebeish et al. 2013), triethanolamine (Barud et al. 2008; Barud et al. 2011), glucose (Chook et al. 2017), chitosan sulfate (Breitwieser et al. 2013), Cassia alata leaf extract (Sivaranjana et al. 2017), bioflocculant (Muthulakshmi et al. 2017), UV-irradiation (Rehan et al. 2017), hydrazine, hydroxylamine, or ascorbic acid (Maria et al. 2009). In this method, silver ions are adsorbed inside the cellulosic materials when the materials are soaked in silver nitrate solution. The silver ions are further in situ reduced to AgNPs, and the AgNPs grow up in the reductive solution. During in situ reduction, the particle size of the AgNPs is controlled by adjusting the soak time in the reducing agent solution.
Fig. 2. The two-step method
Cellulosic Materials Serve as Both a Matrix and a Reducing Agent
A remarkable characteristic of this method is that it has no reducing agents other than the cellulosic material. Thus, an advantage of this method is that it reduces the consumption of chemicals. This approach is illustrated in Fig. 3.
Fig. 3. In situ chemical reduction method, in which cellulosic materials serve as both a matrix and a reducing agent
Due to the intrinsic reducibility of cellulose, either natural cellulose, RC, or some of cellulose derivatives can be employed as a reducing agent without any processing. For example, the AgNPs embedded cellulose films or microspheres were fabricated by immersing the materials in an aqueous AgNO3 solution at 80 °C for 24 h (Wu et al. 2012). Wu et al. (2014a) developed a similar method to synthesize and impregnate AgNPs onto bacterial cellulose (BC) nanofibers via immersing the BC nanofibers into a silver ammonia solution at 80 °C for 10 min. Kolarova et al. (2017) prepared AgNPs-cellulose composite film by reduction of silver nitrate by methyl cellulose. Elayaraja et al. (2017) generated AgNPs by immersing TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-oxidized BC in AgNO3 solution and keeping it the dark over night at 40 °C. The prepared AgNP deposited BC was a promising alternative to control the shrimp pathogen. Because the reducibility of natural cellulose is usually very weak, additional energy, e.g., microwave heating (Oluwafemi et al. 2016; Xu et al. 2016) or water bath heating (Yang et al. 2012a; Emam et al. 2014; Wu et al. 2014b), may be necessary during the redox reaction between cellulose and silver ions.
Natural cellulose can be introduced in specific reductive groups for the in situ reduction of silver ions. Wu et al. (2008) fabricated dialdehyde cellulose via a periodate oxidation reaction, and reductive aldehyde groups were successfully introduced. Cheng et al. (2013) prepared AgNPs/cellulose compounds using aminocellulose as a combined reducing and capping reagent.
Summary
In physical methods, nano-silver colloid has to be prepared in advance. However, there is no need for such preparation before the in situ chemical reduction method. Compared with physical adsorption, in situ chemical reduction is simpler, and the silver loading ratio is improved to a certain extent. Furthermore, the antibacterial endurance of AgNPs-containing cellulosic materials is effectively enhanced due to the internal loading of AgNPs within cellulosic materials. Especially, when cellulose itself plays a role as reducing agent, the process is conducted without using ordinary reductants, which often are hazardous.
COVALENT BONDING METHOD FOR THE PREPARATION OF AgNPs-CELLULOSE COMPOSITES
Because the fixation ratio of AgNPs and the antibacterial endurance of AgNPs- containing cellulose fibers are influenced by the interaction between fibers and particles, a strong and effective bridge should be established to enhance the combination between cellulosic materials and AgNPs. Thus, the covalent bonding method has been examined in a preliminary study. The procedures of the covalent bonding method are as follows: i) AgNPs are immobilized by a suitable dendrimer; and ii) the covalent bonds between polymer-parceled AgNPs and modified cellulose are formed during the crosslinking reaction process.
Zhang et al. (2013) firstly prepared amino functional AgNPs by a one-step reaction between silver nitrate and amino-terminated hyperbranched polymer (HBP-NH2). And then the amino functional AgNPs were grafted on the NaIO4 oxidized cotton fabric. The preparation process is shown in Fig. 4. The AgNPs grafted oxidized cotton fabric showed excellent antibacterial property and laundering durability. After exposing to 50 consecutive home washing conditions, the Ag content of AgNPs grafted oxidized cotton fabric only decreased from 149.88 to 116.08 mg/kg, and the bacterial reduction was maintained over 96% against both S. aureus and E. coli.
Fig. 4. The preparation of AgNPs grafted oxidized cotton fabric (reprinted with permission from Zhang et al. 2013)
Fig. 5. The amidation reaction between NFC and DENAgNPs (reprinted with permission from Ramaraju et al. 2015)
Ramaraju et al. (2015) prepared AgNPs-containing cellulosic antibacterial material using this method (Manna et al. 2001; Bendi and Imae 2013). The preparation process included three procedures: i) a silver nitrate solution and NH2-terminated fourth generation poly (amido amine) dendrimer (PAMAM) solution were mixed together, and sodium borohydride solution was slowly added to form a PAMAM containing AgNPs (DENAgNPs); ii) NFC, suspended in water, was reacted with equimolar coupling reagents, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N–hydroxy-succinimide (NHS) to form a stable NHS-carboxylated NFC; and iii) an aqueous DENAgNPs solution was added into the NHS-carboxylated NFC suspension to form DENAgNPs-NFC via an amidation reaction. The amidation reaction is shown in Fig. 5.
Summary
There are some disadvantages with the covalent bonding method, such as the larger chemical consumption and the prolonged preparation period. Nevertheless, the greatest benefit of this method is the relative strength and permanence of the covalent bond between the cellulosic materials and the AgNPs-wrapped polymers. The AgNPs are tightly immobilized on the cellulosic materials, endowing permanent antibacterial activity. Although this preparation method is in the early stages, it may be the focus of future research.
CYTOTOXICITY ANALYSIS OF AgNPs
Since AgNPs are widely used as an antibacterial agent, which can kill microorganisms cells, their biological activities on human cells are worth considering. Some research has been focused on this subject and has been reported.
Travan et al. (2009) indicated that AgNPs did not show any cytotoxic effect toward three different eukaryotic cell lines, i.e., mouse fibroblast-like (NIH-3T3), human hepatocarcinoma (HepG2), and human osteosarcoma (MG63) cell lines. They thought that this was due to the fact that AgNPs could exert their antimicrobial activity by simple contact with the bacterial membrane, while they could not be taken up and internalized by eukaryotic cells. Panáček et al. (2009) found that AgNPs exhibited no cytotoxic effects on human fibroblasts at silver concentrations of 0.05 mg/L to 54 mg/L. However, the research of Greulich et al. (2009) showed that AgNPs exhibited cytotoxic effects on human mesenchymal stem cells at high concentrations but also induced cell activation at high but nontoxic concentrations of AgNPs. Recently Shaheen and Fouda (2017) investigated the cytotoxicity of AgNPs, and their research demonstrated that AgNPs were safe for eukaryotic human cell represented by HepG2, Mcf7 and BHK cellline.
Through the research we can know that AgNPs exhibit distinguishing toxicity to different human cells. Nevertheless we can conclude that AgNPs are relatively nontoxic to human cells. Consequently, AgNPs can be regarded as sufficiently safe to merit further investigations in the applications of medicine, wound dressing, food packaging, and some other fields.
Fig. 6. The photographs of AgNPs-containing (A) modal fibers (reprinted from Pivec et al. 2017) and (B) RC membranes (reprinted with permission from Cai et al. 2009. “Nanoporous cellulose as metal nanoparticles support,” Biomacromolecules 10(1), 87-94; Copyright (2018) American Chemical Society and with permission from Chook et al. 2014).
FORMS AND APPLICATIONS OF AgNPs-CELLULOSE COMPOSITES
For various applications, AgNPs-containing cellulosic antibacterial materials should take different forms, as shown in Figs. 6 through 8. The general map of applications of AgNPs-cellulose composites is shown in Fig. 9. Natural cellulose fiber materials loaded with AgNPs are usually made into antibacterial paper products (Xu et al. 2015) or mixed with fluff pulp to manufacture antibacterial disposable hygiene products. AgNPs-containing NFC materials, which are normally fabricated into membrane, aerogel, or hydrogel products, are widely used in food packaging, chemical catalysis, environmental protection, and other fields (Dong et al. 2013; Das et al. 2015). AgNPs-containing MCC maintains the original powder morphology of MCC, and it is utilized as functional filler (Silva and Unali 2011). AgNPs-containing RC can be spun into silks for textile materials (Pivec et al. 2017) or cast into membranes for effluent treatment (Weis et al. 2005; Kallioinen et al. 2010; Puro et al. 2010). Singla et al. (2017a) prepared AgNPs and cellulose nanocrystals nanobiocomposites (NCs) in film and ointment forms. NCs were found to significantly enhance in vivo skin tissue repair by decreasing production of inflammatory cytokines and increasing fibroblast proliferation, angiogenesis, and finally tissue neo-epithelization and regeneration in less than 14 days by favoring collagen deposition. NCs may serve as potential candidates as antibacterial wound dressings for accelerating tissue repair and regeneration, such as serving for diabetic patients (Singla et al. 2017b).
Fig. 7. AgNPs contained in various products: (A) paper (reprinted with permission from Xu et al. 2015), (B) NFC membranes (reprinted with permissions from Dong et al. 2013 and Ramaraju et al. 2015), and (C) NFC gels (reprinted with permissions from Dong et al. 2013 and Das et al. 2015)
Fig. 8. AgNPs-containing BC membranes (reprinted with permissions from Yang et al. 2012b and Li et al. 2015)
Fig. 9. The applications of AgNPs-cellulose composites (reprinted with permissions from Adepu and Khandelwal 2017; Tang et al. 2015; and from Singla et al. 2017b)
Because it is easy to shape, BC mostly has been formed into membranes. The AgNPs-containing BC membrane can be utilized as an antibacterial artificial skin to replace the defective skin, or as a medical antibacterial dressing to replace the traditional cotton wool, gauze, and bandages used to treat burns or chronic ulcerative disease (Czaja et al. 2006, 2007; Fu et al.2013). Adepu and Khandelwal (2017) fabricated BC-AgNPs antifouling materials. They found that food stuff was protected from microbial spoilage for 30 days when stored in BC-AgNPs nanocomposites having < 2% silver, whereas spoilage was noticed within 15 days for food stuff stored in regular polythene bag. Therefore, BC impregnated with AgNPs serves to be highly useful material for food packaging as well as healthcare systems. With its superior mechanical properties and excellent biocompatibility, the AgNPs-containing BC membrane has great potential in medicine and health care (Yang et al. 2012b; Li et al. 2015).
CONCLUSIONS
- This paper reviewed three common techniques used to prepare AgNPs-containing cellulosic antibacterial materials, i.e., the physical method, in-situ chemical reduction method, and covalent bonding method. The specific forms and application status of AgNPs-containing cellulosic materials prepared by the three methods are introduced.
- Commonly, the physical method is convenient to operate, and the loading ratio of AgNPs to cellulosic material is relatively high. However, the physical adsorption force between the AgNPs and the cellulosic material is correspondingly weak, and this is a drawback of this method. Compared with the physical method, the operation process of the in-situ chemical reduction method is obviously simplified, and the antibacterial endurance of AgNPs-containing cellulosic materials is effectively enhanced due to the internal loading of AgNPs in cellulosic materials. Considering the final product performances, the covalent bonding method is superior to the above two methods due to the covalent bond combination between cellulosic materials and AgNPs-wrapped polymers. However, some disadvantages now exist, such as the larger chemical consumption and the prolonged preparing period.
- The AgNPs-containing cellulosic antibacterial materials can be widely used in food packaging, chemical catalysis, environmental protection, functional materials, textiles, skin beauty products, medical items, health care, and other fields. To date, the exploration of the application of AgNPs-containing cellulose materials is still in progress. In general, the research on BC membranes and CA fibers is mature, and there have been commercial products in the market. Hence, the AgNPs-containing BC membranes and CA fibers are expected to be the earliest to achieve industrial production.
ACKNOWLEDGMENTS
This research was funded through a grant of the Natural Science Foundation of Shaanxi Province (Grant No.2015JQ3088). The authors gratefully acknowledge the Open Fund of the State Key Laboratory of Pulp and Paper Engineering, the South China University of Technology (201730), and the Shaanxi University of Science and Technology Academic Leader Training Program (2013XSD25).
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Article submitted: August 1, 2017; Peer review completed: October 21, 2017; Revised version received: November 14, 2017; Accepted: November 15, 2017; Published: December 4, 2017.
DOI: 10.15376/biores.13.1.Xu