Sheet-like Cellulose Nanocrystal-ZnO Nanohybrids as Multifunctional

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Sheet-like cellulose nanocrystal-ZnO nanohybrids as multifunctional reinforcing agents in biopolyester composite nanofibers with ultrahigh UV-Shielding and antibacterial performances Somia Yassin Hussain Abdalkarim, Hou-Yong Yu, Chuang Wang, Lili Yang, Ying Guan, Linxi Huang, and Juming Yao ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00188 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Bio Materials

Sheet-like cellulose nanocrystal-ZnO nanohybrids

as

multifunctional

reinforcing agents in biopolyester composite nanofibers with ultrahigh UV-Shielding and antibacterial performances Somia Yassin Hussain Abdalkarima, Hou-Yong Yua*, Chuang Wanga, Lili Yanga, Ying Guana, Linxi Huanga, Juming Yaoa* a

The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of

Ministry of Education and National Engineering Lab for Textile Fiber Materials & Processing Technology, College of Materials and Textile, Zhejiang Sci-Tech University, Xiasha Higher Education Park Avenue 2 No.928, Hangzhou 310018, China E-mail: [email protected]; [email protected]

ABSTRACT: The uses of inorganic metal oxide and ZnO nanohybrids as UV absorbers have potential to increase the production of UV-protective textile, which will also overcome the drawbacks of organic molecules and prevent negative impacts on human health and environment. In this work, sheet-like cellulose nanocrystal-ZnO (CNC-ZnO) nanohybrid was successfully developed by one-step hydrothermal method. The obtained CNC-ZnO nanohybrids as UV absorber and antibacterial agents were introduced into biopolyester (poly (3-hydroxybutyrate-co-3-hydroxy valerate, PHBV) by using electrospinning process. The addition of sheet-like CNC-ZnO can greatly enhance PHBV thermal stability and crystallization ability. In addition, excellent antimicrobial ratios of Escherichia coli and Staphylococcus aureus, and high absorbency of solution A (9.82 g/g) were obtained for the composite nanofibers with 5 wt % CNC-ZnO. Moreover, most of the UV irradiations were blocked out for both UVA (99.72%) and UVB (99. 95%) with high UPF value of 1674.9 in the resulting composite nanofibers with 9 wt % CNC-ZnO. This study provides a novel

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ACS Applied Bio Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

method to produce sheet-like CNC-ZnO with multifunctional properties and its nanocomposite for potential uses as wound dressings and other functional biomaterials. KEYWORDS: Cellulose nanocrystal, Zinc oxide, Sheet-like nanohybrids, Multifunctional properties, Poly(3-hydroxybutyrate-co-3-hydroxy valerate), Biomedical application.

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1. INTRODUCTION Generally, an ideal wound dressing materials should fulfill many requirements not only non-toxicity, biocompatibility, biodegradability, no surface residual colonies, wound desiccation, but also should protect the wound from micro-organisms, infections, contaminations, and slight performance loss after UV radiation to bacterial purification. Recently, extensive research efforts have been focused on the harmfulness of ultraviolet radiation (UVR), because the overexposure of UV radiation can attack human bodies such as skin burns or even skin cancer, and photodegradation to organic materials (polymer or plastic materials).1,2 Here, different approaches have been devoted to developing ultraviolet protective materials in order to reduce the collision of UV radiation from direct sun exposure or any other sources, which could greatly cause harmful effects on human health and environment.3,4 In this perspective, various studies of nanocomposites composed of polymers with inorganic/organic UV absorbers have shown a bright future for using as UV-shielding materials, but more attention is needed on their multifunctional application.5-7 The usage of organic molecules as UV absorber into polymer matrix showed significant enhancement in the UV-shielding performance of polymer matrix, unfortunately organic molecules could make the polymer suffer from photodegradation and migration after long-lasting exposure to UV light.5, 8-10 In contrast, the combined effects of inorganic metal oxide nanoparticles such as (ZnO, TiO2, GO, and SiO2) as hybrid materials in the polymer matrix provide new prospects to overcome the above limitation of organic molecules UV absorber, indeed the photocatalytic effects of metal oxide nanoparticles could degrade the polymer matrices.11-14 More significantly, ZnO nanoparticles based polymer nanocomposites have received considerable attentions, because of their high optical transparency, low refractive index, and non-toxicity. It is believed that ZnO nanoparticles are considered to be the most effective absorber of UV radiation among other inorganic nanoparticles, and mostly used as UV-protective nanofillers,

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antibacterial agents, and sensors.15-17 For instance, Sabira et al. reported that the addition of ZnO showed outstanding UV protective performances for poly(vinylidene fluoride) matrix to realize application as solid-state lighting devices.18 Lizundia et al. successfully prepared nanocomposites film containing both functional ZnO and poly(L-lactide) (PLLA), and the addition of ZnO into PLLA could absorb most of UV light and provide high transparency to the nanocomposites film.19 Also, RodrigoBalen et al. found that the incorporation of ZnO nanoparticles into poly methyl methacrylate( PMMA) composites in forms of nanofiber and film showed improvements in both UV-shielding and thermal performances of PMMA matrix.20 Chen et al. reported that the effectiveness of polyacrylonitrile (PAN) -ZnO/Ag composite nanofibers may possibly transform by using different microstructures of ZnO and could be used in smart textiles, flirtations, antibacterial and UV-protective textile.21 To date, ZnO nanoparticles are being mostly incorporated with petroleum-based polymers for potential uses as antimicrobial food packaging or UV shielding by using the traditional solvent casting and melt mixing methods.22,23 In this context, the use of ZnO nanoparticles in the biodegradable polymer matrix through electrospinning technique is still a matter for further research. It is well known that poly(3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) as one of the polyester family has replaced the use of petroleum-based plastic in various application areas such as food packaging, and biomedical materials, due to their biocompatibility, biodegradability, and good properties for cell adhesion and proliferation.24,25 Although the majority of previous studies have been performed to the crystallization behavior, antimicrobial, mechanical, and thermal properties of PHBV nanocomposites.26-28 It is well known that ZnO nanoparticles terned to aggregate due to their high surface energy, therefore the homogeneous dispersion of the ZnO nanoparticles within polymer matrix could be more difficult. Although many studies have reported on the uses of chemical grafting process or templates to overcome the aggregation of ZnO nanoparticles, most of these studies required

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multiples process, and long time for preparation, which was not efficient for large-scale production.29 Furthermore, a specified surfactant or protective colloid have already been used to improve the dispersion of ZnO nanoparticles in the polymer matrix. Nevertheless, most of these surfactants were toxic to greatly limit their uses in the biomedical application. Recently, the uses of cellulose nanocrystal (CNC) can act as templates to fabricate ZnO based nanohybrids or composites due to their unique features arising from high stiffness to nanometric size. Therefore, the uses of CNC based ZnO nanohybrids have attracted great interest owing to the electrostatic interaction between CNC and ZnO that can improved the dispersibility of nanohybrids within polymer matrix.31 Few publications have been reported on the fabrication of different morphology of CNC-ZnO nanohybrids using various preparation methods, the summery of these studies are summarized in Table 1.32-35 Up till now, there have been limited reports on evaluating of the crystallization behavior, antimicrobial, and UV-shielding performance of PHBV/CNC-ZnO composite nanofibers. Previously, we reported the application of PHBV/CNC-ZnO nanocomposites for both nanofiber and films. Also, the obtained nanocomposites presented a possible mechanism for using such CNC-ZnO on the degradation property of PHBV film, and showed improvements in the antibacterial activity, thermal and mechanical properties.36,37 By understanding the combined effects of inorganic metal oxide nanoparticles and biomass nanocellulose (From most abundant natural cellulose resources) into biodegradable polymer provide new prospects for the sustainable use of nanotechnology and nanocomposites for biomedical materials. The main purpose of this work is to fabricate and characterize a novel sheet-like CNC-ZnO nanohybrid by using CNC as carriers/templates to enhance the dispersion of ZnO nanoparticles into PHBV matrix. In accordance, the effect of this nanohybrid on the crystallization behavior, UV-shielding, antibacterial properties, thermal stability, and absorbency of solution A of PHBV nanofiber were studied. We believe that the obtained composite nanofibers could provide

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multi-functional properties for potential uses as optical and antibacterial wound dressing biomaterials. To date, no report on this unique UV absorber in the properties of PHBV nanofiber has been presented. Table 1. Summary of studies investigating various preparation methods of CNC-ZnO nanohybrids Method Precipitation

Shape Flower-like nanoroad

Size/Diameter 2.56 μm

Ref. 32

method

clusters

Precipitation method

Sphere-like structure

143.1 nm

Ref. 33

Precipitation method

A hexagonal wurtize structure

19.3 nm

Ref. 34

In-situ solution casting

Irregular disc-like structure

65 nm

Ref. 35

Sheet-like converted to

210 nm

This work

technique Hydrothermal method

flower-like structure 2. EXPERIMENTAL SECTION 2.1. Materials The (MCC) was purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial. Hydrochloric acid (HCl) sodium hydroxide was given by Hangzhou Shuanglin Chemical Reagent Company. Citric acid (C6H8O7) was bought from Hangzhou Gaojing Fine Chemical Industry Co., Ltd. Zinc chloride (ZnCl2), and sodium hydroxide (NaOH), were purchased from Tianjin Yongda Chemical Reagent Company. Ethanol (C2H5OH), Chloroform (CHCl3), and DMF were bought from Guoyao Group Chemical Reagent Co., Ltd. Commercial PHBV (Mn = 5.90×104, and the molar ratio of HV is 2.57%) was obtained from Tianan Biological Material Co., Ltd. (Ningbo, China). 2.2. Synthesis of sheet-like CNC-ZnO nanohybrids

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The CNC-ZnO nanohybrids were prepared by a one-step hydrothermal process as shown in Scheme 1. The CNC was prepared according to previous prparaion method in our group. Briefly; MCC as starting material (6g) was added into 300 mL mixed acid solution consisting of (3M C6H8O7, and 6M HCl) with volume fractions of (90% C6H8O7/10% HCl), and then the mixture solution was heated at 80 Ԩ for 6 h. After acid hydrolysis process, the residual acid in the CNC suspension was neutralized with 3 M ammonia solution at room temperature, and washed several times by centrifugations with deionized water, and then final CNC suspension with the solid (CNC) content of 4.5 g/L was obtained.37 Then (5.0 mmol, 0.6815g) of ZnCl2, NaOH with (15 mmol; 0.6g) and 25 mL of deionized water were added into bottle glass and homogenized at room temperature under strong stirring, afterward, 10 g of CNC suspension was added to the mixture and stored at 80 ºC for 24 h in an oven. Under these conditions, the CNC-ZnO nanohybrids were obtained and washed several times by centrifugations with deionized water followed by ethanol. Finally, the resultant sheet-like CNC-ZnO nanohybrids were dried at 60 ºC for 12 h.

Scheme 1. Schematic illustrating possible experimental preparation procedure of sheet-like CNC-ZnO nanohybrids and their electrospinning process 7



2.3. Preparation and fabrication of composite nanofibers The PHBV/CNC-ZnO composite nanofibers were successfully prepared through electrospinning process. In a typical procedure, sheet-like CNC-ZnO nanohybrid as a model of nanofillers at weight loadings of (3, 5, 7, and 9 wt %) was dissolved in mixed solvent of chloroform/DMF 90/10 (v/v) and sonicated under ice bath for 30 min. Furthermore, the (10 wt % PHBV) was also dissolved in a mixed solvent and stirred at 70 Ԩ for 30 min. Then the two solutions were blended together and mechanically stirred for one day until the clear solution was obtained. The resulting solution was filled in a commercial plastic syringe (10 mL) fitted with a needle of 0.7 mm inner diameter and the feeding rate was 1 mL/h. The polymer solution was electrospun at 18 kV power supply at room temperature, and the nanofibers were collected from aluminum foil which was placed on cylindrical roller with rotating of 220 rpm and distance between tipെto-collector was 15 cm. Moreover, the characterizations properties of sheet-like CNC-ZnO nanohybrids and their composite nanofibers including the morphology and microstructure, elemental analysis, optical properties, chemical structure, thermal stability, antibacterial test, simulated fresh blood test, FE-SEM image of CNC, and the results of conductivity and viscosity of composite nanofibers were provided in Supplementary Information. 3. RESULTS AND DISCUSSIONS 3.1. Surface morphologies and microstructures Figure 1 represents the surface morphologies of CNC-ZnO nanohybrids, pristine PHBV, and composite nanofibers reinforced with various CNC-ZnO contents of 3, 5, 7 and 9 wt %. Figure S1 shows FE-SEM image of CNC with rod-like structure and diameter of 20 ± 5 nm was obtained (Supporting Information). While with using mid concentration of Zn2+ ions (5 mmol of ZnCl2), CNC-ZnO nanohybrid with uniform nanosheet structure transformed to the 8


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flower-like structure on the surface of CNC, while the average diameter of 200 ± 10.4 nm with 440 ± 22 nm length as shown in Figure 2, as well as sheet thickness of (40-80 nm).This result indicates that the concentration of Zn2+ ions possibly will perform as the main feature to modify the microstructures of the CNC-ZnO nanohybrids. As shown in Figure 1, the obtained nanofibers with the regular surface for pristine PHBV and rough surface with high loading of CNC-ZnO content (7 and 9 wt %) and 50-60µm in thickness. Figure 2 illustrates the average fiber diameter distributions, as the content of CNC-ZnO nanohybrids increased, the average diameter was reduced from 540 ± 27 nm for pristine PHBV nanofiber to 250 ± 12.5, 230 ± 11.5, 220 ± 11, and 210 ± 10.5 nm for PHBV composite nanofibers with various CNC-ZnO contents of 3, 5, 7 and 9 wt %, respectively. The reduction in nanofiber diameter was greatly affected by the physical properties including viscosity, conductivity, and surface tension of the electrospinning PHBV solutions by adding CNC-ZnO nanohybrids. In Figure (Supporting Information), as the content of CNC-ZnO increased, the electrical conductivity was increased from 2.64 ± 0.05 µS/cm to 3.19 ± 0.07 µS/cm with 9 wt % CNC-ZnO. Actually, the increased electric conductivity can stretch the fibers during the electrospinning process to decrease nanofiber diameter. Additionally, the increment of CNC-ZnO nanohybrids can restrict the molecular interaction and chain entanglement of PHBV matrix, which lead to a reduction in the viscosity of electrospinning solution.38 It is well known that CNC-ZnO nanohybrids can be embedded inside fiber structure or placed on the fiber surface and this dispersion of CNC-ZnO had a major effect on the average fiber diameter. As observed in Figure 1, the 5 wt % CNC-ZnO nanohybrids were homogeneously dispersed inside the nanofiber structure, and this behavior was due to the formation of the strong intermolecular hydrogen bonding interactions between CNC-ZnO and PHBV matrix. Although the CNC-ZnO was efficiently dispersed within the PHBV matrix during preparation process of the electrospinning solution, an agglomeration might still happen within the nanofiber structure during the electrospinning

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process at high loadings of CNC-ZnO (7 and 9 wt %). To evaluate the existence of CNC-ZnO nanohybrids within PHBV nanofiber, the energy dispersive X-rays analysis (EDX) for composite nanofibers with various CNC-ZnO contents (3−9 wt %) was carried out to determine their distribution as shown in Figure 3. It can be observed that the existence of zinc, carbon, and oxygen elements in the EDX spectrum, indicating successfully loadings of CNC and ZnO nanoparticles in the composite nanofibers.

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Figure 1. FE-SEM images of neat PHBV and composite nanofibers reinforced with various sheet-like CNC-ZnO content

Figure 2. The average diameter distribution of CNC-ZnO and composite nanofibers reinforced with various sheet-like CNC-ZnO contents

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Figure 3. EDX spectra of composite nanofibers reinforced with various sheet-like CNC-ZnO contents. 3.2. Chemical structure The Chemical structure of pristine PHBV and composite nanofibers are illustrated in Figure 4. The composite nanofibers showed characteristic bands at 3436 cm-1, 1746 cm-1, and 1725 cm-1, attributing to O‒H stretching vibrations of hydroxyl groups, free C=O, and hydrogen-bonded C=O groups, respectively.28 Compared to pristine PHBV nanofiber, the peak of hydrogen-bonded C=O was increased from 1721 cm-1 to 1725 cm-1 for the composite nanofibers, which signified that the existence of strong hydrogen bonding interaction was formed in the composite nanofibers. In addition, new absorption peak at 452 cm-1 was related to Zn–O stretching vibration, suggesting the successful fabrication of composite nanofibers from PHBV matrix containing CNC-ZnO nanohybrids.39,40 For more details, the appearance

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of hydrogen bonding interactions in the obtained composite system was further studied in the range from 1800 and 1680 cm-1 as shown in Figure 4b, and curve-fitted by Gauss/Lorentz spectral function in order to estimate the hydrogen bond fraction (FH−CO) using the similar reported equation of the composites elsewhere.28 As shown in Table 2, significant differences on the hydrogen bond fraction could be found and FH−CO increased from 0.35 to 0.41 for composite nanofiber with 9 wt % of CNC-ZnO. This increment can correspond to the form of strong hydrogen bonding interaction in composite nanofiber system. It can conclude that the strong hydrogen bonding interaction has a significant influence on the boundary linkage between CNC-ZnO and PHBV macromolecules to enhance the performance of the composite nanofibers.

Figure 4. FT-IR spectra (a), (b) carbonyl stretching region (VC=O) in the infrared spectra for the composite nanofibers, and (c) peak de-convolution for the composite nanofiber with 9 wt % sheet-like CNC-ZnO.

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3.3. Thermal stability crystallization and melting behaviour The addition of sheet-like CNC-ZnO nanohybrids into PHBV matrix not only enhanced their biocompatibility and biological properties but also improved their thermal stability of the composite nanofibers. The TGA and DTG scans of CNC-ZnO nanohybrids and composite nanofibers with a variety of CNC-ZnO contents are presented in Figure 5(a and b). All the composite nanofibers including pristine PHBV illustrate single degradation temperature. According to the data of thermal parameter values in Table 2, pristine PHBV shows the lowest thermal stability (T0 = 216.8 Ԩ), whereas the composite nanofiber with 9 wt % CNC-ZnO content exhibited the highest thermal stability (T0 = 242.1 Ԩ) compared with other samples. Furthermore, initial degradation temperature (T0), maximum degradation temperature (Tmax), and complete degradation temperature (Tf) were shifted to higher values passing from 216.8, 249.4, and 271.3 Ԩ for pristine PHBV to 242.1, 267.8, and 283.8 Ԩ for the composite nanofiber with 9 wt % CNC-ZnO. Therefore, the enhancement on the thermal stability of the resultant composite nanofibers could be ascribed to the combined effects of hydrogen bonding interaction and heat resistance of CNC-ZnO nanohybrids, which could improve the degradation temperature of composite nanofibers, providing superior thermal stability than pristine PHBV.37,41 From above, it can be concluded that the obtained nanofiber composite showed improvement on the thermal parameter, The T0, Tmax, and Tf values were increased by 25.3, 18.5, 12.5 Ԩ, respectively. The melting and crystallization behavior of PHBV composite nanofibers are provided in Figure 5(c, and d). The melting crystallization temperature (Tmc) of composite nanofibers with 7 wt % CNC-ZnO showed a significant shift to a higher temperature from 40.7 to a maximum value of 57.4 Ԩ and then would be reduced with increasing CNC-ZnO content. Besides, the degree of crystallinity was slightly increased from 47.6% for pristine PHBV to 48.3% for the composite nanofiber with 9 wt % CNC-ZnO (Table 3). This result can be 14


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explained by the CNC-ZnO could decrease the energy barrier to accelerate PHBV nucleation and its melt configuration/crystallization.26 More than 7 wt % CNC-ZnO addition caused the agglomeration of CNC-ZnO in the composite nanofiber to weaken their nucleation efficiency (Supported FE-SEM result Figure 1). Moreover, all composites nanofibers including pristine PHBV exhibited cold crystallization peak (Tcc) during the second heating curve in Figure 5d. The cold crystallization temperature was increased from 36.2 to a maximum value of 50.3Ԩ for the composite nanofibers with 7 wt % CNC-ZnO (Table 3), the increase in Tcc values could be caused restricted PHBV chain mobility by hydrogen bonding interaction with increasing of CNC-ZnO contents (supported FT-IR result Figure 4). It can easily understand that the formation of hydrogen bonding interactions possibly will fix CNC-ZnO nanohybrids and PHBV chains to the motion of crystalline PHBV molecular chains.42,43 The addition of CNC-ZnO enhanced melting temperature of PHBV nanofiber (Tm). The pristine PHBV showed multiples melting temperature (Tm1, Tm2, Tm3) corresponded to fusion-meltingrecrystallization process of the PHBV matrix. Whereas the composite nanofibers illustrated two melting peaks (Tm1, Tm2), and the Tm2 was considered as the true melting temperature of the composite nanofibers. As CNC-ZnO content increased from 3 to 9 wt %, Tm2 of PHBV/CNC-ZnO composite nanofibers were gradually increased from 149.3 to 155.8 Ԩ, indicating improvement of PHBV crystal perfection.

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Figure 5. TGA (a), (b) DTG, and DSC curves (c) first cooling scan, (d) second heating scan for the composite nanofibers with various CNC-ZnO contents. Table 2 Hydrogen bond fractions (FH−CO), and TGA pentameters of pristine PHBV and the composite nanofibers with various CNC-ZnO contents Sample

a

FH−CO

T0

Tmax

Tf

(oC)b

(oC)b

(oC)b

PHBV

—

216.8

249.4

271.3

3%

0.35

237.8

262.9

281.2

5%

0.36

239.2

263.4

282.4

7%

0.38

241.4

265.6

282.7

9%

0.41

242.1

267. 9

283.8

CNC-ZnO

—

326.3

379.8

391.5

a

FH−CO was obtained from de-convoluted FT-IR spectra.

b

T0, Tmax, and Tf were obtained from TGA curves at the heating rate of 10 oC min-1.

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Table 3 Thermal analysis pentameters and crystallinity (Xc) of pristine PHBV and composite nanofibers with various CNC-ZnO contents Tmc

∆Hmc

TCC

Tm1

Tm2

Tm3

∆Hm

XC

(oC)

(g/J)

(oC)

(oC)

(oC)

(oC)

(g/J)

(%)a

PHBV

40.7

34.4

36.2

96.1

117.3

130.9

69.8

47.6

3%

55.1

30.7

49.4

133.3

149.3

155.3

68.9

48.4

5%

56.5

34.7

49.8

134.1

155.6

—

65.6

47.1

7%

57.4

34.0

50.3

134.5

155.8

—

65.7

48.1

9%

55.2

30.5

49.2

134.8

155.3

—

64.5

48.3

CNC-ZnO

—

—

—

—

—

—

—

87.9

Sample

a

Crystallinity (XC) was calculated from the XRD pattern of CNC-ZnO nanohybrids and for

composite nanofibers based on the equation of Xc=∆Hm/∆Hm100%PHBV×(1–¢CNC-ZnO), where ¢CNC-ZnO is weight fraction of the CNC-ZnO content and ∆H100 100% of PHBV= 146.6 J g-1 37 for composite nanofibers. 3.4. Spherulite Morphology The spherulite morphologies of PHBV and its composite nanofibers were further studied by using polarized optical microscopic (POM) and the POM images are presented in Figure 6. It could be seen that the pristine PHBV spherulites with a typical Maltese cross and there was the free distance for PHBV spherulite growing before contacting with each other this free space in the PHBV spherulite was restricted owing to increased number of PHBV spherulites, which acted as a barrier for neighbour growing spherulites.44 As the CNC-ZnO content increased, the spherulites size was decreased from 130 µm for pristine PHBV to 112 µm for the composite nanofiber with 9 wt % CNC-ZnO. Actually, the spherulite size and the nucleation density of composite nanofiber were affected by the hydrogen bonding interaction,

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free space in the PHBV spherulites, and especially the nucleation effect of CNC-ZnO. Firstly, the free space in the PHBV spherulitic was restricted owing to increase the number of PHBV spherulites which acted as barrier for neighbour growing spherulites. Secondly, when PHBV nanofiber incorporated with CNC-ZnO content blow 7 wt%, the nucleating effect of CNC-ZnO played main role in increase of nucleation density for composite nanofiber (with 5 wt% CNC-ZnO), leading to reduction of PHBV spherulite size. However, with increasing CNC-ZnO content up to 7 wt%, the nucleation effect of CNC-ZnO was weakened due to the agglomeration of CNC-ZnO in the PHBV nanofiber (Supported by FE-SEM Figure 1), as a result, nucleation density was slightly reduced to cause tiny increase of the spherulites size. The growth curves of spherulite radius have been illustrated with time during the isothermal crystallization process (Figure 6). The growth rates (G) of the spherulite was determined by the fitting slope of the spherulite radius vs time. It can be seen that the G value of pristine PHBV was 0.897 µm/s, while G value of composite nanofibers was decreased gradually to 0.66 µm/s, and 0.72 µm/s with increase of CNC-ZnO content from 5 to 9 wt %. This observation suggested that the increment of CNC-ZnO content can improve nucleation abilities of PHBV crystal nucleus, and restrict the diffusion limitation of the molecular chain of spherulites. However, compared with 5 wt % CNC-ZnO, the growth rate was slightly increased from 0.66 to 0.72 µm/s for composite nanofiber with 9 wt % CNC-ZnO, due to the tiny CNC-ZnO agglomeration to restrict the formation of intermolecular interactions between CNC-ZnO and PHBV matrix to improve distribution of molecular chain pieces and thus increase growth rate.26 It demonstrated that the CNC-ZnO agglomeration could possibly decrease the whole surface area of CNC-ZnO and allow the PHBV chains to move freely and easily. Indeed, more CNC-ZnO in the composite nanofibers can give more nucleation sites to induce chain folding of PHBV chains to form smaller PHBV spherulites with faster nucleation rate. The formation of hydrogen bonding interaction in the composite nanofibers

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could restrict chain movement of PHBV chains to limit growth of PHBV spherulite. Overall, the rate of induced nucleation crystallization was higher than the limitation rate of spherulite growth, and thus the nucleation density, overall crystallization rate and crystallinity of PHBVcomposite were improved.

Figure 6. Optical microscopic images of spherulite morphology for pristine PHBV (a), and composite nanofibers with 5 wt % (b) and 9 wt % CNC-ZnO (c). (Inserts are their corresponding radial growth rates of spherulites). 3.5. UV-Shielding Performance It is known that the ZnO is considered to be as most high UV-shielding materials with low refractive index and acts as a physical filter against the UV irradiation.17Hence, in this work, CNC-ZnO nanohybrid with UV-shielding effect has been added into PHBV nanofiber in order to reduce the environmental contamination of the PHBV matrix. The UV 19



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transmittance spectra of pristine PHBV and composite nanofibers containing different concentrations of CNC-ZnO are shown in Figure 7a. The pristine PHBV displayed transmittance of 85% at 800 nm wavelength, whereas the transparency of all the composite nanofibers was still high in the visible region around 78.9% even with high loading of 9 wt% CNC-ZnO, which was almost close to that of pristine PHBV. Addition of CNC-ZnO nanohybrids showed an increase in UV absorption regions of both UVB and UVA, along with the reduction of the reflectance region of both UVB and UVA of PHBV composite nanofibers, confirming that the adding of CNC-ZnO nanohybrid can increase absorption regions for both UV irradiations and induce an increase of UV-shielding quality. Moreover, the ultraviolet protection factor (UPF) and percentages of UVA and UVB radiation blocking were evaluated with UV-2000F Textiles UV Factor Tester (Labsphere Company, USA) according to AATCC 183:2000 standard by using the following equations. 45

UV protection factor (UPF) 

280



400

280

UVA blocking (%)  100 

400



100

E ( ) S    T    d

400

T ( )d

320 400



320

UVB blocking (%)  100 

100

320

d

T ( )d

280 320



E ( ) S    d

280

d

(1)

(%)

(2)

(%)

(3)

Where E (λ) is the relative erythema spectral effectiveness, S (λ) is the spectral irradiance (Wm−2 nm−1), dλ is bandwidth, and λ is wavelength. The UPF value was calculated for UV-A in the range of 320–400 nm, and for UVB between 280 and 320 nm. Table 4 illustrates the percentages of UV-A and UV-B radiation blocking and UPF values of pristine PHBV and composite nanofibers. When 9 wt % CNC-ZnO was added to PHBV nanofiber, most of the 20


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UV irradiations were blocked out for both UV-A (99.72%) and UV-B (99.95%) without any drop in the transparency (around 78.9%). As illustrated in Table 4, the UPF value of pristine PHBV was about 60.27, while with adding of 9wt %CNC-ZnO, the UBF value of for composite nanofibers increased to 1674.89, demonstrating the excellent UV shielding performance of composite nanofiber. The high quality of UVA and UVB shielding properties of composite nanofibers could make such nanofiber desirable for use as optical materials and some biomedical applications. In order to investigate the band gap of the composite nanofibers, this band gap was estimated through Tauc’s equation.14

 hv    hv  Eg 

2

(4)

Where α, h, v, β, and Eg are absorption coefficient, Planck’s constant, light frequency, constant, optical energy band gap, respectively. The optical energy band gap is determined using A Tauc plot (αh ν)1/2 against photon energy (hν), the band gap can be obtained by yield a straight line through the edge of the curves to the x-axis as presented in Figure 7b. It is clear that pristine PHBV had not any absorption of the band gap in the region, but with introducing 3‒9 wt % CNC-ZnO, the composite nanofibers showed band gap in the range of (3.26‒3.23 eV). This result could be attributed to the semiconductor effect of ZnO and formation of strong chemical interactions in the composite nanofibers system.2 The band gap of composite nanofibers was slightly decreased with the addition of 3‒9 wt % CNC-ZnO and this narrow bandgap could greatly improve UV shielding performance of composite nanofibers. The capability of absorbing the excess exudates is an important factor for wound dressing’s materials. The solution A for the composite nanofibers is presented in Figure 7c. The absorbent abilities were measured after immersing the nanofiber into solution A according to British standard test method of natural wound exudates.36 The absorbency values of pristine PHBV and composite nanofibers in the range of (2.27 ± 0.05‒9.82 ± 0.1 g/g) were 21



seen in Figure 7c. The highest absorbency was found for PHBV/CNC-ZnO composite with 5 wt % CNC-ZnO, and the high absorbency possibly was generally due to the chemical and physical structure of the composite nanofibers. It is easy to understand that the CNC-ZnO with more hydrophilic groups resulted in absorbing more water for composite nanofibers. Also, the strong interactions and high porosity of the composite nanofibers can generate capillary force to make the nanofiber effectively absorbing high levels of wound exudates. However, with the high CNC-ZnO loading of 7−9 wt %, the absorbency value was slightly decreased due to poor CNC-ZnO dispersion. More significantly, the gray arrow in Figure 7d shows an evaluation of the UV-shielding performance and transmittance of PHBV/CNC-ZnO nanofibers in this work to compare with other UV shielding performance for different nanocomposite.36 Although many UV absorbers were used for enhancing the optical properties of nanocomposites materials, however, most of these studies can only enhance UV shielding performance or transmittances of nanocomposites. For example, they can be seen in nanocomposites system of poly vinyl alcohol/hollow dopamine-melanin (PVA/Dpa-h) provide (UV shielding of 99.8%, transparency of 61%),2 poly vinyl alcohol/ graphene oxide (S-GO/PVC; UV-shielding of 95%, transparency of 25%)),14 poly(L-lactide)/ZnO (PLLA/ZnO; (UV-shielding of 61%, transparency of 95%)),19 polystyrene/ZnO (PS/ZnO; (UV-shielding of 57%, transparency of 70%)),46 cellulose Acetate/ graphene oxide ((CA/GO; (UV-shielding of 36%, transparency of 80%)),47 microeincapsulated phase change materials/titanium dioxide doped methyl methacrylate shell (MPCMs/TiO2; (UV-shielding of 50%, transparency of 98%)),48 and polyurethane-lignin/zinc oxide (PU-QAL/ZnO; (UV-shielding of 75%, transparency of 58%)).49 Compared to these UV-shielding materials, our PHBV/CNC-ZnO composite nanofibers showed excellent optical properties for both UV-shielding performance of 99.72%

22


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and transparency of 78.9%. Therefore, the result of this study had great significance for development in functional biomaterials fields.

Figure 7. (a) UV−vis diffuse reflectance spectra, (b) Tauc’s plots, (c) the absorbency of

solution A (simulated fresh blood) for composite nanofibers with various CNC-ZnO content and (d) overview of the UV-shielding and transparency of PHBV/CNC-ZnO composite nanofibers compared with other UV shielding materials including, PVA/Dpa-h,2 pPVC/S-GO,14 PLLA/ZnO,19 PS/ZnO,46 CA/GO,47 MPCMs/TiO2,48 and PU-QAL/ZnO.49

23



Page 24 of 37

Table 4 Percentage blocking of UVA and UVB, and UPF values of composite nanofiber with

various CNC-ZnO contents Percentage blocking samples

UPF UVA

UVB

PHBV

96.54

98.82

60.27

3%

98.78

99.95

933.15

5%

99.41

99.95

1347.22

7%

99.59

99.95

1521.50

9%

99.72

99.95

1674.89

3.6. Antimicrobial properties The antimicrobial activity of composite nanofibers with 5 wt % CNC-ZnO was evaluated on simulated field water on Gram-negative bacteria Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus (S. aureus) by using qualitative methods. In brief, two types of bacterial cells (S. aureus and E. coli) were mixed with a liquid medium initial concentration of (1 ×10−6 colony-forming units) . Then simulated field water was prepared by using 10 mL of liquid medium with full bacteria was added to 100 mL of water. After that, a 2×2 cm square sample of pristine PHBV and composite nanofiber with 5 wt % CNC-ZnO was added to 15mL of the mixture liquid and then incubated for 12 h at 37 °C. Finally, 100μL of from the treated mixture was placed on plates and the plates were stored in the incubator for another 12 h at 37 °C. As presented in Figure 8, the pristine PHBV did not show any bacterial reduction against the tested bacteria. In contrast, significant reductions in bacterial amounts were found in the composite nanofibers with 5 wt % CNC-ZnO aligned with Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus); this observation can be only due to antibacterial activity of ZnO. The antibacterial action of ZnO corresponded to

24


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different issues, for instance, the interface between ZnO and bacterial or the metal release of antimicrobial ions, surface area, polar surface, morphology.50-52 Furthermore, due the photocatalysis and metal release of ZnO under light irradiation could produce electron–hole pairs. The hole (h+) interacted (HO−) on the surface of CNC-ZnO nanohybrids, creating highly active free radicals of superoxide anion (O2−), hydroxyl radicals (HO−), and perhydroxyl radicals (HO2+). Therefore, these free radicals can damage the bacterial cell.37,50 The incorporation of composite nanofibers into simulated field water demonstrated a significant bacterial reduction and the total quantity of bacterial colonies in the solid medium were decreased approximately to the zero (Figure 8a and b), and the water became almost transparent in Figure 8c. It indicates that the obtained composite nanofibers showed the excellent antibacterial ratios of 100% for both E. coli and S. aureus (Figure 8d). Moreover, the inhibition zone of Gram-negative E. coli and Gram-positive S.aureus bacteria were obtained according to the disc diffusion method. Briefly, columnar gels were placed on the culture dishes coated with 100μL of bacteria was dispersed in the plates and pristine PHBV and nanofiber composites with addition of 5 wt % CNC-ZnO placed in the plates and the plates were incubated at 37 Ԩ for 24 h. Figure 5 shows the formation of inhibitory zones. Obvious that the composite nanofiber with addition of 5 wt % CNC-ZnO showed growth inhibition ring of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were 3 and 4.5 mm, respectively as illustrated in Figure 9. In contrast, no inhibition ring was formed with pristine PHBV as a control sample. Thus, such composite nanofibers may be used as a novel of biomedical materials with high antibacterial activity.

25



Figure 8. The digital images of the quantitative mean colony-forming units against two

bacteria (a) S. aureus and (b) E. coli bacteria, (c) digital images of the water disinfection effect with the addition of PHBV and composite nanofiber with 5 wt % CNC-ZnO after incubating for 12 h, and (d) antibacterial ratio.

Figure 9. The digital images of the formed inhibition ring of PHBV and composite nanofiber

with 5 wt % CNC-ZnO against two bacteria (a) S. aureus and (b) E. coli bacteria. 26


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4. CONCLUSIONS

This study reported the effects of functionalized sheet-like CNC-ZnO nanohybrids on the properties of PHBV nanofiber. It is found that sheet-like CNC-ZnO nanohybrids as good UV absorber were homogeneously incorporated with PHBV nanofiber, the nucleation density, overall crystallization rate and crystallinity of PHBV composite nanofibers were improved, meanwhile the obtained nanofiber composite showed an increments on the thermal degradation temperature, the T0, Tmax, and Tf values were greater than pristine PHBV by 25.3, 18.5, 12.5 Ԩ, respectively. Moreover, the UV irradiations of the resulting composite nanofibers were blocked out for both UVA and UVB without any drop in the transparency (around 78.9%) together with a band gap of 3.23 eV, in addition to excellent antibacterial ratios of 100% for both E. coli and S. aureus. Besides, the highest absorbency was found for composite nanofibers with loaded of 5 wt % CNC-ZnO. The high quality of UVA (99.95%), UVB (99.72%), and UPF value of (1674.89), and strong antibacterial activity of composite nanofibers

could

make

such

nanofiber

desirable

for

use

as

multifunctional

optical/antibacterial biomaterials. 

ASSOCIATED CONTENT * Supplementary Information

All characterizations of CNC, sheet-like CNC-ZnO nanohybrids and their composite nanofibers including the morphology and microstructure, elemental analysis, optical properties, crystalline structure, chemical structure, thermal stability, photocatalytic test, antibacterial test, Absorbency of solution A (simulated fresh blood test), FE-SEM image of CNC, and the results of conductivity and viscosity of composite nanofibers were provided in (Supplementary Information). Corresponding Author

* Hou-Yong Yu (H.Y. Yu), Juming Yao (J. Yao); Tel.: 86 571 86843618; E-mail addresses: 27



[email protected]; [email protected].

Notes

The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

The work is funded by Key Program for International S&T Innovation Cooperation Projects of China (2016YFE0131400), and Candidates of Young and Middle-Aged Academic Leader of Zhejiang Province (11110331271703). 1

28


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

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Graphical Abstract


Sheet-like Cellulose Nanocrystal-ZnO Nanohybrids as Multifunctional

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