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Article In Situ Synthesis of on Cellulose Fibers Using D-Glucuronic Acid and Its Antibacterial Application

Guangxue Chen, Linjuan Yan, Xiaofang Wan *, Qiankun Zhang and Qing Wang *

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China; [email protected] (G.C.); [email protected] (L.Y.); [email protected] (Q.Z.) * Correspondence: [email protected] (Q.W.); [email protected] (X.W.)

 Received: 25 August 2019; Accepted: 19 September 2019; Published: 23 September 2019 

Abstract: The development of ecofriendly procedures to avoid the use of toxic chemicals for the synthesis of stable silver nanoparticles (AgNPs) is highly desired. In the present study, we reported an eco-friendly and green technique for in situ fabrication of AgNPs on bleached hardwood pulp fibers (bhpFibers) using D-glucuronic acid as the only . Different amounts of D-glucuronic acid were introduced and its effect on the size and distribution of AgNPs on the bhpFibers was discussed. The morphology and structures of bhpFibers@AgNPs were proved by electron microscope-dispersive (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Then, a series of bhpFibers@AgNPs with different AgNPs loadings were also prepared by adjusting the concentration of the AgNO3 solution. After a papermaking process via vacuum filtration, the prepared papers displayed an outstanding antibacterial performance against (gram -negative) and Staphylococcus aureus (gram-positive). It is foreseeable that the bhpFibers@AgNPs have a promising application in the field of biomedical.

Keywords: green synthesis; silver nanoparticles; D-glucuronic acid; cellulose fibers; antibacterial properties

1. Introduction AgNPs have been widely accepted as the most efficacious metal with properties [1–5]. However, the dissociative AgNPs or silver may cause inevitable harm to human health and the environment, which greatly hinders the practical application [6]. Therefore, AgNPs have generally been in situ generated onto various polymeric matrices [7–10]. These silver-based possess excellent antimicrobial properties against a wide scope of pathogens, which play vital roles in medical antibacteria [11–13], wound treatment [14,15], food packaging and textiles [16,17]. Since the size, morphology and structure of AgNPs are strongly related to their antimicrobial properties, the synthesis process should be technologically controlled. Among all the matrices, cellulose has attracted much attention due to the advantages of abundant resources [18–21] and rich hydroxyls in its molecular structure [22,23]. Recently, methods have been developed for the fabrication of cellulose/AgNP nanocomposites. Zhu et al. [24] presented an in situ synthesis of AgNPs in pristine cellulose fibers using NaBH4 as the reduction agent. The mean size and size distribution of AgNPs were adjusted by changing the concentration of NaBH4. Maria et al. [25] developed bacterial cellulose/silver nanocomposites based on different reductants of hydrazine and hydroxylamine, with the help of such additional agents as gelatin or . Li et al. [26] fabricated cellulose–silver nanocomposites using (EG) as a reducing agent with a

Materials 2019, 12, 3101; doi:10.3390/ma12193101 www.mdpi.com/journal/materials Materials 2019, 12, 3101 2 of 12 microwave-assisted method. Clearly, the use of organic solvents or the use of toxic chemicals as reducing agents during the preparation are unfavorable for the environment, which largely limits their actual application in biomedical fields. Thus, non-toxic green reductants such as [27] and ascorbic acid are commonly chosen for the synthesis of AgNPs in matrices. However, due to the weak reduction of these reducing agents, larger particle sizes or clusters of AgNPs in cellulose have been observed, which lead to poor antibacterial effects [28]. For example, Li et al. [29] found that polyhedral silver particles with a size of 250 nm exhibited relatively poor dispersion in the cellulose matrix when using ascorbic acid as a reducing agent. In order to further improve the greenness of the in situ generation of AgNPs, using some biological substances, including plant and plant extracts [30,31], as reducing agents has become a new trend. Aladpoosh et al. [32] prepared AgNPs on cotton fabric using ashes of S. rosmarinus. The results showed that aggregation of particles in the matrix was unavoidable. Sri Ramkumar et al. [33] first prepared a seaweed extract that was obtained from seaweed by using a mixer grinder, then AgNPs were synthesized by bio-reduction of AgNO3 with the prepared seaweed extract. It is evident that the introduction of these plant reducing agents might cause unexpected aggregation of AgNPs and cumbersome processes. Therefore, it is still a challenge to prepare the AgNPs with small size and uniform distribution in the matrix with green and ecofriendly techniques. D-glucuronic acid (DLA) might be a most attractive candidate for the synthesis of silver nanoparticles among these chemical reducing agents because of its outstanding features. First, DLA is abundant in many biological systems and is non-toxic [34], which is crucial for ‘ ’. Besides, DLA has abundant hydroxyl groups and a terminal group in its units [35,36], which might be exploited as glucose. Most of all, DLA is water soluble and possesses negatively charged carboxyl groups, which might electrostatically interact with Ag+ and promote the distribution of Ag+ in the cellulose matrix [36,37]. It also has been reported that DLA was able to reduce Cr+5 to Cr+3 [38]. From this point, we can except that the DLA can also be used to reduce silver ions to AgNPs on cellulose fibers. Herein, a one-step green hydrothermal technique for in situ fabrication of AgNPs on bleached hardwood pulp fibers (bhpFibers) was proposed by only using DLA as the reducing agent. Different amounts of DLA were introduced and its effect on the size and distribution of AgNPs on the bhpFibers was discussed. Surface morphology and structures of bhpFibers before and after surface modification were characterized using SEM images, XRD patterns, FT-IR spectra and EDS mapping. Then, a series of bhpFibers@AgNPs with different AgNPs loadings were also prepared by adjusting the concentration of the AgNO3 solution. After the papermaking process, the antibacterial properties of bhpFibers@AgNPs against the Escherichia coli (gram-negative) and Staphylococcus aureus (gram- positive) were separately investigated.

2. Experimental

2.1. Materials A pulp board made from poplar was supplied by Yueyang Paper Co., Ltd., Yueyang, China. Silver nitrate (AgNO3, AR) was purchased from Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China). The solution (NH H O, 25 wt%) was provided by Guangzhou Chemical Reagents Factory 3· 2 (Guangzhou, China). Sodium hydroxide (NaOH, > 98 %) and D-glucuronic acid (DLA) were obtained from Sinopharm Chemical Reagent Company (Shanghai, China). All the reagents were analytical purity used without further purification. All the water used was deionized water.

2.2. Pretreatment of Bleached Hardwood Pulp The bleached hardwood pulp was first defibered (100 rpm/min) using a fluffer to separate the interwoven fibers in water without altering the original structure of the fibers. Then bleached hardwood Materials 2019, 11, x FOR PEER REVIEW 3 of 12

2.3. Synthesis of bhpFibers@AgNPs The bhpFibers@AgNPs were prepared through reduction of AgNPs in situ on bhpFibers (Scheme. 1). In a typical procedure, 0.2 g of dried bhpFibers were dispersed in 30 mL of deionized water. Then, 10 mL of Ag(NH3)2+ solutions with appropriate concentrations were added into the above suspension and transferred to the refrigerator overnight. Subsequently, 10 mL of different concentrations (0, 0.1, 0.2, 0.4 and 0.6 mg/mL) of DLA solutions were dropwise introduced in 20 minutes and conducted for 2 h at 50 °C . The reaction mixtures were then centrifuged (8000 rpm, 3 min) and rinsed with deionized water three times to obtain bhpFibers@AgNPs composites. The Ag(NH3) solutions were prepared by adding 25 wt% ammonia water dropwise into 20, 40, Materials602019 and, 1280, 3101mM silver nitrate solutions until the pH of the mixtures was about 11. A few drops of3 of 12 NaOH were added to the obtained Ag(NH3)2+ solutions to adjust the pH value to 11.5. AgNPs on bhpFibers were synthesized according to the following reactions: pulp fibers (bhpFibers) with a beating degree of 48 ◦SR could be obtained after the beat with a speed of AgNO3+2NH3·H 2O ⇌ Ag(NH3)2+ + NO3- + 2H2O (1) 25,000 rpm by the PFI pulping machine (Mark VI, 621, Hamjern Maskin, Oslo, Norway). 2[Ag(NH3)2]OH + C6H10O7 (DLA) ⇌ C5H9O6COONH4 + 2Ag↓+3NH3 + H2O (2) 2.3. Synthesis of bhpFibers@AgNPs The2.4. bhpFibers@AgNPsFabrication of bhpFibers@AgNP were prepared-based paper through reduction of AgNPs in situ on bhpFibers (Scheme1). In a typicalAs procedure, shown in Scheme 0.2 g 1, of the dried bhpFibers@AgNP bhpFibers were-based dispersed paper was in fabricated 30 mL of via deionized a vacuum water.filtration Then, + 10 mLprocess. of Ag(NH Typically,3)2 solutions the prepared with bhpFibers@AgNPs appropriate concentrations was dispersed were in 100 added mL deionized into the above water to suspension form and transferreda stable pulp to suspension the refrigerator under overnight.gentle stirring. Subsequently, Then, the suspension 10 mL of was diff pourederent concentrations into the filtration (0, 0.1, 0.2, 0.4funnel and 0.6with mg a Φ/mL) = 0.22 of μm DLA microporous solutions weremembrane. dropwise After introduced being vacuum in 20-dried minutes at 60 °C and for conducted 12 h, the for bhpFibers@AgNP-based paper was successfully prepared. A series of papers with different amounts 2 h at 50 ◦C. The reaction mixtures were then centrifuged (8000 rpm, 3 min) and rinsed with deionized of AgNPs were also prepared by adjusting the concentration of silver ions in the solution. water three times to obtain bhpFibers@AgNPs composites.

Scheme 1. Schematic of the fabrication of bhpFibers@AgNPs and bhpFibers@AgNP-based paper.

AgNPs:Sch Stableeme 1.silver Schematic nanoparticles; of the fabrication bhpFibers: of bhpFibers@AgNPs Bleached hardwood and bhpFibers@AgNP pulp fibers. -based paper. AgNPs: Stable silver nanoparticles; bhpFibers: Bleached hardwood pulp fibers. The Ag(NH3) solutions were prepared by adding 25 wt% ammonia water dropwise into 20, 40, 60 and2.5. 80 Characterization mM silver nitrate solutions until the pH of the mixtures was about 11. A few drops of NaOH + were added to the obtained Ag(NH3)2 solutions to adjust the pH value to 11.5. AgNPs on bhpFibers Morphological characteristics and energy dispersive spectroscopy (EDS) were captured with were synthesized according to the following reactions: emission scanning electron microscopy (SEM, Zeiss Merline, Oberkochen, Germany). Crystal structures were performed in the range of 4° to 90° on X-+ray diffraction (XRD, Philips PW3040/00 AgNO + 2NH H O Ag NH ) + NO − + 2H O (1) X’Pert MPD, Westborough,3 MA, USA3· ) 2 using a diffractometer3 2 with3 Cu Kα2 radiation at the 3 kW.

2[Ag(NH ) ]OH + C H O (DLA) C H O COONH + 2Ag +3NH + H O (2) 3 2 6 10 7 5 9 6 4 ↓ 3 2 2.4. Fabrication of bhpFibers@AgNP-based paper As shown in Scheme1, the bhpFibers@AgNP-based paper was fabricated via a vacuum filtration process. Typically, the prepared bhpFibers@AgNPs was dispersed in 100 mL deionized water to form a stable pulp suspension under gentle stirring. Then, the suspension was poured into the filtration funnel with a Φ = 0.22 µm microporous membrane. After being vacuum-dried at 60 ◦C for 12 h, the bhpFibers@AgNP-based paper was successfully prepared. A series of papers with different amounts of AgNPs were also prepared by adjusting the concentration of silver ions in the solution.

2.5. Characterization Morphological characteristics and energy dispersive spectroscopy (EDS) were captured with emission scanning electron microscopy (SEM, Zeiss Merline, Oberkochen, Germany). Crystal structures were performed in the range of 4◦ to 90◦ on X-ray diffraction (XRD, Philips PW3040/00 X’Pert MPD, Materials 2019, 12, 3101 4 of 12

Westborough, MA, USA) using a diffractometer with Cu Kα radiation at the 3 kW. Chemical structures were characterized by FT-IR spectra (FTIR-ATR, Bruker VerTex 70, Billerica, Massachusetts, Germany). Chemical compositions were performed on X-ray photoelectron spectroscopy (XPS, Escalab 250 spectrometer, Thermo Electron Corporation, Waltham, MA, America). The thermal gravimetric analyses were investigated by a TGA Instruments (TGA, TA Q500, New Castle county, Delaware, America) heating from 25 to 700 ◦C with a heating rate of 10 ◦C/min in a nitrogen atmosphere at a flow 1 rate of 40 mL min− . The silver content was measured by inductively coupled plasma atomic emission spectrometry (ICP-ms, Hitachi Z-2000, Tokyo, Japan).

2.6. Antimicrobial Activity Test Antibacterial detections of the solid plate medium (inhibition zone measurement) are as follows: 10 mL of the melted solid Luria-Bertani (LB) medium was poured into a Petri dish (90 mm in diameter) before waiting for solidification. Then, Escherichia coli (E. coli) DH5α or Staphylococcus aureus (S. aureus) was cultured in a liquid LB medium at 220 rpm and 37 ◦C until the optical density (OD) 600 was about 0.6, and the concentration of bacteria reached a 108 colony-forming unit (CFU)/mL. The obtained E. coli or S. aureus suspension was taken out to be spread on LB agar plates three times with sterile tweezers. Finally, the prepared bhpFiber-based paper and bhpFibers@AgNP-based paper were placed on the culture medium for full contact. The inoculated microorganisms were cultured at 37 ◦C for 24 hours. The antibacterial property of the composites paper was evaluated by the size of the inhibitory zone.

3. Results and Discussion

3.1. Effect of DLA on in-situ preparation of AgNPs on bhpFibers SEM was used as a powerful tool to vividly illustrate the morphologies of bhpFibers before and after surface modification. The morphological features and particle size distribution of AgNPs that varied with the concentration of DLA are exhibited in Figure1 and Figure S1; the pristine bhpFibers in Figure1a are interwoven by a three-dimensional network composed of interpenetrated cellulose fibers. The interconnected structure increased the fibers’ surface roughness, affording good support for the anchoring of AgNPs. After in situ reduction of AgNPs without the aid of DLA, the bhpFiber surfaces were distributed with obviously large particles or clusters of AgNPs of 158.59 nm, as shown in Figure1b and Figure S1b. When the concentration of DLA was 0.1 mg /mL, the AgNPs with a smaller size of average diameter 34.47 nm gradually appeared in Figure1c and Figure S1c, in which most of the particles were spheres, but there still existed some large aggregated silver particles. When the DLA amount increased to 0.2 mg/mL, the obtained nanoparticles of bhpFibers had a uniform size distribution and an average diameter of 36.27 nm, as shown in Figure1d and Figure S1d; they were all spheres and well separated from each other owing to the sufficient DLA molecules, suggesting that the addition of DLA was beneficial for the reduction of aggregated particles and the formation of nanoparticles with uniform and small particle size. With the amount of DLA increasing to 0.4 and 0.6 mg/mL, silver nanoparticles with a larger size, of average diameters 56.13 nm and 60.82 nm, were prepared on bhpFibers as shown in Figure1e,f and Figure S1e,f. This might be due to more reducing + agents causing faster reduction of Ag(NH3)2 and then forming large AgNPs [27]. Figure1g shows intuitively the particle size of AgNPs on bhpFibers under different amounts of DLA, indicating that the appropriate concentration of DLA can be exploited to control the different sizes of AgNPs. Meanwhile, the smaller AgNPs with larger specific surface areas are accessible for the interaction between AgNPs and bacteria, affording higher antimicrobial effects than larger AgNPs [28]. Based on the results of Figure1, 0.2 mg /mL of DLA was conducive to the generation of AgNPs with the smallest particle size and uniform distribution on bhpFibers. Compared to the silver particles (80–120 nm) reduced by glucose [27,39], DLA acts as a reducing agent to achieve smaller particle size (36.27 nm) and better dispersion of silver nanoparticles in the cellulose matrix. This may be because DLA molecules could bind to Ag ions through carboxyl group chelation, which facilitates the reduction of Ag ions [40]. Materials 2019, 12, 3101 5 of 12 Materials 2019, 11, x FOR PEER REVIEW 5 of 12

Figure 1. The SEM images of (a) bhpFibers and bhpFibers@AgNP-based paper prepared by 20 mM AgNO and different concentrations of DLA: (b) 0 mg/mL, (c) 0.1 mg/mL, (d) 0.2 mg/mL, (e) 0.4 mg/mL 3 and (f) 0.6 mg/mL; (g) silver particle sizes at different concentrations of DLA. Materials 2019, 11, x FOR PEER REVIEW 6 of 12

Figure 1. The SEM images of (a) bhpFibers and bhpFibers@AgNP-based paper prepared by 20 mM

AgNO3 and different concentrations of DLA: (b) 0 mg/mL, (c) 0.1 mg/mL, (d) 0.2 mg/mL, (e) 0.4 mg/mL and (f) 0.6 mg/mL; (g) silver particle sizes at different concentrations of DLA.

3.2. Characterization of the Pristine bhpFibers and bhpFibers@AgNPs Composites The crystalline phases of pristine bhpFibers and bhpFibers@AgNP-based paper were characterized by X-ray diffractometry (XRD), shown in Figure 2a. The XRD results show that the bhpFibers had three diffraction characteristic peaks at 2θ = 14.8, 16.5 and 22.5 corresponding to the — Materialstypical2019 (1,1120),, 3101 (110) and (200) planes of cellulose fibers (JCPDS50-0926), respectively [41]. Compared 6 of 12 with bhpFibers, the bhpFibers@AgNPs showed typical XRD pattern of Ag nanocrystals on different crystal planes. Moreover, there were five peaks at 2θ = 38.1°, 44.3°, 64.4°, 77.5° and 81.6° in the 3.2.patterns. Characterization These peaks of the Pristine were in bhpFibers accordance and with bhpFibers@AgNPs the JCPDS database Composites (No. 89-3722), which correspondedThe crystalline to thephases (111), of (200),pristine (220), bhpFibers (311) and and (222) bhpFibers@AgNP-based lattice planes of Ag [ paper42], suggesting were characterized the synthesis of silver in the crystalline state by using DLA as the reducing agent was successful. In by X-ray diffractometry (XRD), shown in Figure2a. The XRD results show that the bhpFibers had addition, no typical peaks of substance such as Ag2O were observed, demonstrating that the pure three diffraction characteristic peaks at 2θ = 14.8, 16.5 and 22.5 corresponding to the typical (110), AgNPs were synthesized on the surface of bhpFibers. (110) andThe (200) structures planes of of the cellulose pristine fibers bhpFibers (JCPDS50-0926), and bhpFibers@AgNP respectively-based [41 paper]. Compared were investigated with bhpFibers, theby bhpFibers@AgNPs FT-IR in Figure 2b. showed As for the typical pristine XRD bhpFibers, pattern the of Ag peak nanocrystals at 3330 cm−1 onwas di attributedfferent crystal to the planes. Moreover,characteristic there absorption were five peaksof O–H at stretching. 2θ = 38.1 ◦The, 44.3 vibration◦, 64.4◦ ,bands 77.5◦ atand 2901 81.6 cm◦ −in1 and the 1368 patterns. cm−1 can These be peaks wereassigned in accordance to the asymmetrical with the JCPDS and database symmetrical (No. stretching 89-3722), of whichthe C-H corresponded group, respectively. to the (111), The (200), (220),absorption (311) and band (222) at 1055 lattice cm−1 planescorresponded of Ag to [42 C],–O suggesting–C stretching the mode synthesis from the of glucosidic silver in units the [ crystalline26]. stateThe by peak using at 894 DLA cm as−1 was the related reducing to the agent C–H was rocking successful. vibration In of addition, cellulose. noThese typical bands peaks appeared of substance in both the pristine bhpFibers and the modified cellulose fibers, suggesting the chemical structure of the such as Ag2O were observed, demonstrating that the pure AgNPs were synthesized on the surface cellulose was barely changed. No new peaks appeared in the bhpFibers@AgNPs, indicating that the of bhpFibers. interaction between cellulose fibers and AgNPs was a physical .

Figure 2. XRD patterns (a) and FT-IR spectra (b) of bhpFibers and bhpFibers@AgNP-based paper. Figure 2. XRD patterns (a) and FT-IR spectra (b) of bhpFibers and bhpFibers@AgNP-based paper. The structures of the pristine bhpFibers and bhpFibers@AgNP-based paper were investigated The XPS technique of pristine bhpFibers and bhpFibers@AgNP-based paper1 could provide by FT-IR in Figure2b. As for the pristine bhpFibers, the peak at 3330 cm − was attributed to the further information about the structure and chemical state of AgNPs on cellulose fibers.1 As depicted1 characteristicin Figure 3a absorption, the pristine ofbhpFibers O–H stretching. showed the The binding vibration energy bands spectra at of 2901 C1 s cmpeaks− and (288 1368eV) and cm O1− can be assigneds peaks to (533 the eV), asymmetrical respectively, and which symmetrical can be inferred stretching to lignocellulosic of the C-H characteristic group, respectively. peaks of Thecellulose absorption 1 bandmaterials at 1055 [ cm30]−. Aftercorresponded surface modification, to C–O–C stretching Ag3d peaks mode were from observed the glucosidic in the bhpFibers@AgNPs, units [26]. The peak at 1 894demonstrating cm− was related the formation to the C–H of AgNPs. rocking A vibration high resolution of cellulose. XPS spectra These of bands Ag3d (appearedFigure 3b) in was both the pristineundertaken bhpFibers to analyze and the the modifiedvalence state cellulose of Ag on fibers, the surface suggesting of cellulose the chemical fibers, it can structure be seen of that the the cellulose waspeaks barely of changed.Ag3d3/2 at 368 No eV new and peaks Ag3d appeared5/2 at 374 eV in appeared, the bhpFibers@AgNPs, demonstrating that indicating the stable thatzero- thevalence interaction betweenAgNPs cellulose had been fiberssuccessfully and AgNPs loaded wason the a physicalbhpFibers adsorption. surface [43,44]. The XPS technique of pristine bhpFibers and bhpFibers@AgNP-based paper could provide further information about the structure and chemical state of AgNPs on cellulose fibers. As depicted in Figure3a, the pristine bhpFibers showed the binding energy spectra of C1 s peaks (288 eV) and O1 s peaks (533 eV), respectively, which can be inferred to lignocellulosic characteristic peaks of cellulose materials [30]. After surface modification, Ag3d peaks were observed in the bhpFibers@AgNPs, demonstrating the formation of AgNPs. A high resolution XPS spectra of Ag3d (Figure3b) was undertaken to analyze the valence state of Ag on the surface of cellulose fibers, it can be seen that the peaks of Ag3d3/2 at 368 eV and Ag3d5/2 at 374 eV appeared, demonstrating that the stable zero-valence AgNPs had been successfully loaded on the bhpFibers surface [43,44]. MaterialsMaterials 20192019,, 1112,, x 3101 FOR PEER REVIEW 77 of of 12 12 Materials 2019, 11, x FOR PEER REVIEW 7 of 12

FigureFigure 3. ((aa)) XPS XPS spectra spectra of of bhpFibers bhpFibers and and bhpFibers@AgNP-based bhpFibers@AgNP-based paper; paper; ( (bb)) Ag3d Ag3d XPS XPS spectrum spectrum of of bhpFibers@AgNP-basedFigurebhpFibers@AgNP-based 3. (a) XPS spectra paper.of paper. bhpFibers and bhpFibers@AgNP-based paper; (b) Ag3d XPS spectrum of bhpFibers@AgNP-based paper. Besides,Besides, X-ray X-ray energy energy dispersive dispersive spectroscopy spectroscopy ma mappingpping (EDS), described in Figure 44,, showsshows thatthat thethe elements elementsBesides, of of X-ray C, C, O O energy and and Ag dispersive were uniformly spectroscopy distri distributedbuted mapping on on the (EDS), surface described of fibers, fibers, inwhich Figure also 4, proved shows thatthat bhpFibersthe bhpFibers e were were successfully coated with AgNPs.

Figure 4. EDS elemental mapping images of the bhpFibers@AgNP-based paper. Figure 4. EDSEDS elemental elemental mapping mapping images images of of the the bhpFibers@AgNP bhpFibers@AgNP-based-based paper paper.. 3.3. The effect of AgNO3 on the deposition amounts of AgNPs on bhpFibers 3.3. The effect of AgNO3 on the deposition amounts of AgNPs on bhpFibers 3.3. The effect of AgNO3 on the deposition amounts of AgNPs on bhpFibers Subsequently, the effects of the concentration of AgNO3 were explored by mixing different Subsequently, the effects of the concentration of AgNO3 were explored by mixing different concentrationsSubsequently, of AgNO the effects3 (20–80 of mM) the concentration with 0.2 mg/mL of AgNO of DLA.3 were As shown explored in byFigure mixing 5, when different the concentrations of AgNO3 (20–80 mM) with 0.2 mg/mL of DLA. As shown in Figure5, when the concentrationconcentrations of of AgNO AgNO3 increased3 (20–80 mM) from with 20 to 0.2 80 mg/mLmM, the of size DLA. of the As prepared shown in AgNPs Figure on 5, bhpFibers when the concentration of AgNO3 increased from 20 to 80 mM, the size of the prepared AgNPs on bhpFibers increasedconcentration slightly of AgNO from 36.54,3 increased 43.86 andfrom 51.75 20 to to 80 45.05 mM, nm. the The size AgNPs of the contentprepared in AgNPsthe composites on bhpFibers could beincreased determined slightly by from the 36.54,amount 43.86 of andresidual 51.75 atto 45.05700 °C nm. in The TGA Ag curvesNPs content (Figure in theS2) composites and the weight could be determined by the amount of residual at 700 °C in TGA curves (Figure S2) and the weight

Materials 2019, 12, 3101 8 of 12 increased slightly from 36.54, 43.86 and 51.75 to 45.05 nm. The AgNPs content in the composites couldMaterials be 2019 determined, 11, x FOR PEER by the REVIEW amount of residual at 700 ◦C in TGA curves (Figure S2) and the weight8 of 12 percentagesMaterialsMaterialsMaterialsMaterialsMaterials of2019 2019 Ag2019, ,11 11 2019 ,2019,were ,x11 x FOR ,FOR, ,x11 11 FOR, calculated PEER, x PEERx FOR FOR PEER REVIEW REVIEWPEER PEER REVIEW REVIEW REVIEW to be 9.36%, 10.26%, 12.91% and 11.72%, demonstrating that88 ofof8 12of12 more8 8 12 ofof 1212 silverpercentage nanoparticless of Ag were were calculated incorporated to be into 9.36%, bhpFibers. 10.26%, This 12.91% result and supported 11.72%,the demonstrating results of the that scanning more electronsilverperc perc nanoparticlesperc microscopyentageentagepercpercentageentageentagess of ofs Ag ofAg ins swere Ag ofwere ofwere Figure AgwereAg calculated incorporatedwere calculatedwere 5calculated. Moreover,calculated calculated to to be tobe into be9.36%,to 9.36%,to the 9.36%,be be colourbhpFibers. 9.36%, 9.36%,10.26%, 10.26%, 10.26%, 10.26%, of10.26%, 12.91% the12.91% 12.91%This obtained 12.91% 12.91% and andresult and 11.72% 11.72% and and 11.72% papers supported 11.72% 11.72%, ,demonstrating demonstrating, demonstrating and,, demonstrating demonstrating ICP-ms the results that that analysis that more more that that more of moremore the of silver nanoparticles were incorporated into bhpFibers. This result supported the results of the thescanning AgNPssilversilver electronsilver content silver nanoparticles nanoparticles nanoparticles nanoparticles microscopy on bhpFibers were were were werein incorporated was incorporatedFigure incorporated incorporated also 5 measured,. Moreover, into into bhpFibers.into into bhpFibers. as bhpFibers. bhpFibers.the shown colourThis This in ThisresultThis Tableofresult the result result1supported . obtainedsupported It cansupportedsupported be the seenpaper the results the the thatresultss and results results the of ofICPthe paper ofthe of - ms the the scanningscanningscanningscanningscanning electron electron electron electronelectron microscopy microscopy microscopy microscopymicroscopy in in FigureinFigure Figure inin FigureFigure 5 5. . Moreover, 5Moreover,. Moreover, 55.. Moreover,Moreover, the the the colour colour thethecolour colour colourof of the ofthe the ofobtainedof obtained the theobtained obtainedobtained paper paper paper spaperpapers and ands and sICP s ICP andand ICP--msms ICPICP-ms -- msms colouranalysis darkened of the AgNP withs the content AgNPs on content bhpFibers when was the also concentration measured of, as AgNO shown3 was in Table within 1. 60It can mM. be When seen analysisanalysisanalysisanalysisanalysis of of the ofthe the ofAgNPof AgNP the theAgNP AgNPsAgNPs content contents contentss contentcontent on on onbhpFibers bhpFibers onbhpFiberson bhpFibersbhpFibers was was was also also waswas also measured measured alsoalso measured measuredmeasured, ,as as, shown asshown, , shown asas shownshown in in Table inTable Table inin Table1. Table1. It 1.It can Itcan 1.1. can Itbe Itbe can canbeseen seen seenbebe seenseen morethat the AgNO paper3 (higher colour darkened than 60 mM) with was the added,AgNPs thecontent impregated when the AgNPs concentration on the bhpFibers of AgNO3 decreased was within a thatthatthat the thethatthat the paper paper the thepaper paper colourpaper colour colour colour colourdarkened darkened darkened darkened darkened w with ithw iththe thew w iththe ithAgNP AgNP the AgNPthe AgNP sAgNPs content contents contentss content content when when when when the whenthe the concentration concentration the concentrationthe concentration concentration of of AgNO ofAgNO AgNO of of AgNO AgNO3 3was was3 was within 3within3 was was within within within little.60 mM. A When similar more phenomenon AgNO3 (higher was reported than 60 formM) the was cellulose added,/AgNPs the impregated membrane AgNPs [27]. on It isthe noteworthy bhpFibers 6060 mM.60 mM. mM.6060 When WhenmM. mM. When When moreWhen more more AgNO moreAgNOmore AgNO AgNO AgNO3 3(higher (higher3 (higher33 (higher (higher than than than 60 60than than mM)60 mM) mM)60 60 was mM) wasmM) was added, added, was was added, added, added, the the the impregated impregated the theimpregated impregated impregated AgNPs AgNPs AgNPs AgNPs AgNPs on on onthe the theon onbhpFibers bhpFibers the thebhpFibers bhpFibers bhpFibers thatdecrease alldede thecreasedecreased creaseadede AgNPslittle.creasedcreased a ad little. little. aAd dlittle.on asimilara A little.Alittle. bhpFibers similarAsimilar similar A Aphenomenon similarsimilar phenomenon phenomenon phenomenon in phenomenonphenomenon Figure was was 5was werewas reported reported reported waswas reported highly reportedreported for for for dispersed the thethe forthefor cellulose/AgNPs cellulose/AgNPs the thecellulose/AgNPs cellulose/AgNPscellulose/AgNPs and no visible membrane membrane membrane membranemembrane agglomeration [27] [27] [27] .[27] .It It [27] [27].is isIt. It.is. It It is isis occurred,noteworthynoteworthynoteworthynoteworthy whichnoteworthynoteworthy that was that thatall that the attributedall allthat that all the theAgNPs all theall AgNPs AgNPs the the AgNPs to on AgNPs AgNPs the on on bhpFibers on strong bhpFibers bhpFibers onon bhpFibers bhpFibers bhpFibers interaction in inin Figure inFigureFigure Figureinin Figure betweenFigure 555 werewere5 were 55 were highlywere highlyhydroxyl highly highly highly highly dispersed dispersed dispersed dispersed and dispersed dispersed oxygen-containing and and and and no no and and no visible no visible no novisible visible visible visible groupsagglomerationagglomerationagglomeration onagglomeration theagglomerationagglomeration cellulose occurred, occurred, occurred, occurred, with occurred, occurred, which the which which which AgNPs was which which was was was attributed [ attributed 40 attributedwas was ]. attributed According attributed attributed to to to theto the the tothe to strong strong the the strong abovestrong strong interaction interaction interaction interaction analysis, interaction interaction bet bet bet between 60ween ween bet bet mM ween ween hydroxyl hydroxyl of hydroxyl hydroxyl AgNO hydroxyl hydroxyl and and 3 and was and and and suoxygenfficientoxygenoxygen-containingoxygen foroxygenoxygen--containing achievingcontaining-containing--containingcontaining groups groups thegroups groups desiredon groupsgroups on theon onthe thecellulose silver onthe oncellulose cellulose thethecellulose content cellulosecellulose with with with with the onthe the withwith the bhpFibersAgNPsAgNPs AgNPs thetheAgNPs AgNPsAgNPs [ [[4040 [] surfaces.40]. ..According According ][According[.4040 According]].. AccordingAccording to to tothe tothe the the toabove toabove the theaboveabove aboveaboveanalysis, analysis, analysis, analysis, analysis,analysis, 60 60 60 60 6060 mM of AgNO3 3was3 sufficient for achieving the desired silver content on bhpFibers surfaces. mM ofmM AgNOmM ofmMmM ofAgNO3 AgNO ofwasof AgNOAgNO sufficientwas was3 3sufficient waswas sufficient sufficient sufficientfor forachieving for achieving forachievingfor achievingachieving the the desiredthe desired thethedesired desireddesired silver silver silver silver silvercontent content contentcontent on on on bhpFibers bhpFibers onbhpFiberson bhpFibersbhpFibers surfaces. surfaces. surfaces.surfaces.

FigureFigureFigureFigure Figure5. 5. The The5. The 5. SEM 5.SEM The TheSEM images imagesSEM SEM images images images(a (a– –d)(ad)– of d)(a of(a bhpFibers@A–of–bhpFibers@Ad)d) bhpFibers@A of of bhpFibers@A bhpFibers@AggNPNPgNP-based-basedgg-NPNPbased -paper-based basedpaper paper prepared paperpaperprepared prepared preparedprepared by by 0.2by 0.2 0.2mg/mLby bymg/mL 0.2 mg/mL0.2 mg/mL mg/mL DLA DLA DLA and andDLA DLA and and and differentdifferentdifferentdifferentdifferent concentrations concentrations concentrations concentrations concentrations of of AgNO ofAgNO AgNO of of AgNO 3AgNO:3 :( a(3a): )(20 a203)3: mM,: 20( mM,(aa) )mM, 20 20 ( b(mM, bmM,) )(40 b40) mM,( 40mM,(bb) )mM, 40 40 ( c (mM, c)mM, )60( 60c) mM, 60( mM,(cc) )mM, 60 60 ( d(mM, dmM,) )(80 d80) mM. (80mM.(dd) )mM. 80 80 mM. mM.

FigureFigure 5.5. The SEMSEM imagesimagesTableTable (a–d)(aTable– d)1.Table Table1. ofThe ofThe1. bhpFibers@AgNP-basedThebhpFibers@A 1.color 1.color The Thecolor and andcolor color and the the and and thegAgNPs AgNPsNP the AgNPsthe-based AgNPs AgNPs content content content paper content content on on theon the prepared theon samples. onsamples. the samples.the samples. samples. by 0.20.2 mgmg/mL /mL DLADLA andand

didifferentfferent concentrationsconcentrations ofof AgNOAgNO33:(: (a)) 20 mM,mM, ((b)) 4040 mM,mM, ((cc)) 6060 mM,mM, ((dd)) 8080 mM.mM. bhpFibebhpFibebhpFibebhpFibebhpFibebhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@AgbhpFibers@Ag SampleSampleSampleSampleSample Table 1. The color and the AgNPs content on the samples. rsrsTable rs rsrs 1. NPsTheNPsNPs -color-2020NPsNPs- mM20 mM and-mM-2020 mM mMthe AgNPs NPsNPsNPs--4040NPsNPs - contentmM40 mM -mM-4040 mMmM on the NPsNPs NPssamples.--6060NPsNPs- mM60 mM -mM-6060 mMmM NPs NPsNPs--8080NPsNPs- mM80 mM -mM-8080 mMmM bhpFibers@Ag bhpFibers@Ag bhpFibers@Ag bhpFibers@Ag AgNPsAgNPsSampleAgNPsAgNPsAgNPs bhpFibers 00 0 00 8.28.2 8.2mg/g mg/gNPs-20 8.2mg/g8.2 mg/gmg/g mM 22.422.4NPs-4022.4 mg/g mg/g22.422.4 mg/g mM mg/gmg/g NPs-603333 mg/g33 mg/g mg/g33 mM33 mg/gmg/g NPs-8031.831.831.8 mg/g mg/g31.8 mM31.8 mg/g mg/gmg/g contentcontentcontentcontentcontent bhpFibe bhpFibers@Ag bhpFibers@Ag bhpFibers@Ag bhpFibers@Ag SampleAgNPs content 0 8.2 mg/g 22.4 mg/g 33 mg/g 31.8 mg/g rs NPs-20 mM NPs-40 mM NPs-60 mM NPs-80 mM

AgNPs ColorColorColor Color Color 0 8.2 mg/g 22.4 mg/g 33 mg/g 31.8 mg/g content

3.4.3.4.3.4. Antibacterial Antibacterial3.4. 3.4.Antibacterial AntibacterialAntibacterial P Propertiesroperties Properties PPropertiesroperties of of bhpFibers@AgNP ofbhpFibers@AgNP bhpFibers@AgNP ofof bhpFibers@AgNPbhpFibers@AgNP--BBasedased-Based- -PB BPaperasedasedaper Paper P Paperaper Color

3.4. Antibacterial Properties of bhpFibers@AgNP-Based Paper

Materials 2019, 12, 3101 9 of 12

Materials 2019, 11, x FOR PEER REVIEW 9 of 12 3.4. Antibacterial Properties of bhpFibers@AgNP-Based Paper It has has been been reported reported that that the the antibacterial antibacterial properties properties of the of silver-based the silver-based nanocomposites nanocomposites are related are relatedto the mass to of the AgNPs mass in theof matrixAgNPs [ 24 in,26 the]. Herein, matrix the [24, antibacterial26]. Herein, activities the ofantibacterial bhpFibers@AgNP-based activities of bhpFibers@AgNP-based paper prepared by 20 mM and 60 mM AgNO3 were tested on Gram- paper prepared by 20 mM and 60 mM AgNO3 were tested on Gram- negative E. coli and Gram-positive S.negative aureus, E. respectively. coli and Gram As presented-positive inS. Figureaureus6, ,respectively no inhibition. As zones presented were observed in Figure nearby 6, no the inhibition pristine zonesfiber-based were papers,observed indicating nearby the that pristine the papers fiber- didbased not papers, show anyindicating antibacterial that the activity. papers The did inhibitionnot show any antibacterial activity. The inhibition zones of the bhpFibers@AgNP-based papers with 20 mM zones of the bhpFibers@AgNP-based papers with 20 mM AgNO3 for E. coli and S. aureus were 5 mm AgNOand 3.53 mm,for E. respectively, coli and S. aureus while were the inhibition 5 mm and zones 3.5 mm, of the respectively, bhpFibers@AgNP-based while the inhibition paper zones with 60 of mM the bhpFibers@silver nitrateAgNP for E.- colibasedand paperS. aureus withwere 60 6mM mm silver and 5 nitrate mm. These for E. results coli and demonstrated S. aureus were that the6 mm prepared and 5 papersmm. These showed results outstanding demonstrated antibacterial that the prepared activity against papers bothshowedE. coli outstanding(Gram-negative) antibacterial and S. activity aureus (Gram-positive).against both E. coli This (Gram is because-negative) AgNPs and can S. aureuspenetrate (Gram the- cytoplasmpositive). This of E. is coli becauseand S. AgNPs aureus, and can penetrateinteract with the cellcytoplasm components, of E. causingcoli and damageS. aureus to, and bacteria. interact Moreover, with cell with components the increasing, causing of AgNPs damage in papers,to bacteria. the inhibitionMoreover, zones with the against increasingE. coli and of AgNPsS. aureus in increasedpapers, the simultaneously. inhibition zones These against results E. coli clearly and S.indicated aureus increased that the antibacterialsimultaneously. ability These only results stems clearly from theindicated AgNPs that impregnating the antibacterial inside ability bhpFibers. only Astem betters from antibacterial the AgNPs effect impregnating was obtained inside accompanied bhpFibers. by the A increasebetter antibacterial of silver content effect in was the bhpFibers. obtained accompanied by the increase of silver content in the bhpFibers.

Figure 6. The antibacterial property of bhpFibers and bhpFibers@AgNP-based paper generated using Figure 6. The antibacterial property of bhpFibers and bhpFibers@AgNP-based paper generated using aq. AgNO3 solutions of concentrations 20 and 60 mM, respectively, against E. coli and S. aureus. aq. AgNO3 solutions of concentrations 20 and 60 mM, respectively, against E. coli and S. aureus.

4. Conclusion In summary, an environmentally benign synthesis method was provided for in situ fabrication of AgNPs on bhpFibers using DLA as the only reducing agent, which was then used for the preparation of antibacterial papers. SEM results indicated that the AgNPs were homogeneously dispersed in the matrix and AgNPs were efficiently synthesized by means of XRD and XPS

Materials 2019, 12, 3101 10 of 12

4. Conclusions In summary, an environmentally benign synthesis method was provided for in situ fabrication of AgNPs on bhpFibers using DLA as the only reducing agent, which was then used for the preparation of antibacterial papers. SEM results indicated that the AgNPs were homogeneously dispersed in the matrix and AgNPs were efficiently synthesized by means of XRD and XPS measurement. FT-IR indicates that the interaction between cellulose fibers and AgNPs was a physical adherence. Different amounts of DLA were introduced and its effect on the size and distribution of AgNPs on the bhpFibers was discussed. It was found that 0.2 mg/mL DLA was most conducive to the generation of AgNPs with the smallest particle size (36.27 nm) and uniform distribution. Based on this, a serial of bhpFibers@AgNPs with different amounts of AgNPs were prepared by adjusting the concentration of AgNO3. The results indicate that 60 mM of AgNO3 was sufficient for achieving the desired silver content on bhpFibers surfaces. Moreover, the bhpFibers@AgNP-based paper exhibited outstanding antimicrobial performance against E. coli (gram negative) and S. aureus (gram positive). Therefore, it is believed that this eco-friendly and green synthesis approach for fabrication of the bhpFibers@AgNPs has a promising application in the field of antibacterials.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/19/3101/s1, Figure S1: Size distribution of bhpFibers@AgNPs prepared by 20 mM AgNO3 and different concentrations of DLA: (b) 0 mg/mL, (c) 0.1 mg/mL, (d) 0.2 mg/mL, (e) 0.4 mg/mL and (f) 0.6 mg/mL, Figure S2: TG curves of pristine bhpFibers and bhpFibers@AgNPs prepared by 0.2 mg/mL DLA and different concentrations of AgNO3: 20 mM, 40 mM, 60 mM and 80 mM. Author Contributions: Conceptualization, G.C., Q.W. and X.W.; Methodology, L.Y.; Software, L.Y.; Validation, G.C., X.W. and Q.W.; Formal Analysis, L.Y. and Q.W.; Investigation, L.Y. and Q.Z; Resources, G.C.; Data Curation, L.Y.; Writing-Original Draft Preparation, L.Y. and Q.W.; Writing-Review & Editing, L.Y. and X.W.; Visualization, L.Y.; Supervision, G.C. and Q.W.; Project Administration, G.C.; Funding Acquisition, G.C. Funding: This work was supported by the Science and Technology Planning Project of Guangdong Province (No.2017B090901064), the Science and Technology Project of Guangzhou City (No.2016070220045) and the China Postdoctoral Science Foundation (2018M633054). Conflicts of Interest: The authors declare no conflict of interest.

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