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Flexible and monolithic zinc oxide bionanocomposite foams by a bacterial cellulose mediated approach for antibacterial applications† Peipei Wang,a Jun Zhao,a Ruifei Xuan,a,b Yun Wang,a Chen Zou,a Zhiquan Zhang,c Yizao Wand and Yan Xu*a The use of self-assembled biomacromolecules in the development of functional bionanocomposite foams is one of the best lessons learned from nature. Here, we show that monolithic, flexible and porous zinc oxide bionanocomposite foams with a hierarchical architecture can be assembled through the mediation of bacterial cellulose. The assembly is achieved by controlled hydrolysis and solvothermal crys-

Received 11th October 2013, Accepted 7th February 2014 DOI: 10.1039/c3dt52858h www.rsc.org/dalton

tallization using a bacterial cellulose aerogel as a template in a non-aqueous polar medium. The bionanocomposite foam with a maximum zinc oxide loading of 70 wt% is constructed of intimately packed spheres of aggregated zinc oxide nanocrystals exhibiting a BET surface area of 92 m2 g−1. The zinc oxide bionanocomposite foams show excellent antibacterial activity, which give them potential value as selfsupporting wound dressing and water sterilization materials.

Introduction Bionanocomposite foams assembled from inorganic nanoparticles through the mediation of biological molecules (biomolecules) at nanoscale represent an emerging and intriguing area of research await to be exploited. One technological advancement is the possibility to integrate flexibility, porosity and functionalities into one monolith that could lead to applications and nanodevices as diverse as biocatalysis, environmental protection, biomedicine and energy production.1 Compared with other methods of nanocomposite foam preparation like phase separation/emulsion freeze drying, gas foaming and solvent casting,2 a biomolecule mediated approach offers a way to design flexible, monolithic and lightweight bionanocomposite foams with hierarchical architectures and structural functions due to the rich surface functionalities and self-assembly properties of biomolecules. Even more intriguing is that it offers a solution to avoid aggregation of inorganic nanoparticles and allows the fabrication a State Key Lab of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China. E-mail: [email protected]; Tel: +86 431 85168607 b College of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China c College of Chemistry, Jilin University, Changchun 130012, China d School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China † Electronic supplementary information (ESI) available: Additional data. See DOI: 10.1039/c3dt52858h

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of bionanocomposite foams with high nanoparticle concentrations.3 Structural functions like the iridescence of nacre pearls and the wave guiding property of sea sponges are key attributes of biominerals that have inspired the development of functional bionanocomposite foams. The use of self-assembled biomolecules in functional nanocomposite design is one of the best lessons learnt from nature. In biominerals, inorganic nanoparticles are abosrbed into hierarchical architectures of self-assembled biomolecules such as proteins4a and polysaccharides.4b Among the biomolecules, bacterial cellulose (BC) has attracted considerable attention due to its superior mechanical and optical properties in addition to its biocompatibility, biodegradability and renewability.4c It is a linear polysaccharide, consisting of bundled molecular chains (termed nanofibers) based on β-1,4-linked anhydro-D-glucose units. The nanofibers of BC have hydroxyl-rich surfaces, driven by hydrogen bonds, they self-assemble forming a hierarchical architecture with interconnected porosity. BC has been used for developing flexible magnetic nanopapers,3 biosensors,5 organic light-emitting diodes,6 electrical conducting papers,7 and flexible luminescent materials.8 The potential of BC to mediate the mineralization of inorganic species was well manifested by the growth of non-agglomerated, porous and flexible ferromagnetic cobalt ferrite nanocomposites.3 The successful assembly of TiO2/BC nanocomposites showing enhanced photocatalytic properties9 and multipurpose advanced composites10 demonstrates the general applicability of BC as a template for

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the organization of functional nanocomposites with increased levels of spatial organization. Zinc oxide is a wide band gap n-type semiconductor, holding promise in wide-ranging applications such as in chemical11 and biological12 sensing, piezoelectric devices,13 dye-sensitized solar cells14 and antibacterial agents.15 Recent studies have shown that ZnO nanoparticles have effective antibacterial activity towards gram-positive and gram-negative bacteria. Among other factors, photocatalytic bacterial-disruption and reactive oxygen species (ROS) bacterial-attack16 play essential roles in the antibacterial activity of ZnO nanoparticles. The large specific surface area and interconnected porosity enhance the antibacterial activity of ZnO due to increased reactive sites and ease of mass transfer.17 Research has shown that organizing ZnO nanoparticles into a hierarchically porous architecture with interconnected pores and a high specific surface area optimizes the functional performance and even renders new functionalities.3,17b For example, a ZnO aerogel obtained by ALD displays enhanced light harvesting and excellent power efficiency,18 a ZnO aerogel obtained by temperature gradient chemical gas-phase synthesis exhibits excellent low-temperature ethanol sensing properties,19 and hierarchical ZnO/Cu corn-like materials with mesoporosity and high specific surface areas show high photocatalytic and antibacterial activity.17a The cellulose-templated synthesis of crystalline ZnO has been recently reported,7,20 it is thus worth exploring whether monolithic and flexible ZnO/BC nanocomposite foams with tunable ZnO contents and specific surface areas can be assembled from ZnO nanoparticles, and if such bionanocomposite foams exhibit enhanced antibacterial activity, hence offer a facile approach to produce selfsupporting antibacterial materials based on ZnO. Here, we describe the assembly of monolithic and flexible ZnO/BC bionanocomposite foams with tunable ZnO loadings, high specific surface areas and hierarchical porosity. The bionanocomposite foam with a maximum ZnO loading of 70 wt% exhibits a Brunauer–Emmett–Teller (BET) surface area of 92 m2 g−1. The assembly is achieved in less than 12 h by seeding under reflux conditions followed by gentle solvothermal crystallization in a non-aqueous polar medium. Replication of the hierarchical structure of bacterial cellulose is achieved by controlled hydrolysis of the zinc precursors.20b,21 To demonstrate the functionality, we show that the 70 wt% ZnO/BC foam exhibits excellent antibacterial activity to E. coli as characterized by a 98% reduction in viability of E. coli after 1 h of treatment under UV irradiation at 365 nm from the initial bacterial concentration of 107 CFU mL−1. The minimal inhibitory concentration of the 70 wt% ZnO/BC foam is 35 μg mL−1 in saline medium.

Experimental

Paper

nol (AR), acetic acid (AR) and monopotassium phosphate (AR) were purchased from Beijing Chemicals Company, China. Commercial zinc oxide powder (ZnO powder) was purchased from Tianjin Huadong Chemical Industry, China. Acetobacter xylinum HN001 was kindly provided by the School of Materials Science and Engineering, Tianjin University, China. D-Glucose anhydrate was purchased from AMRESCO LLC, yeast extract from OXOID and lacto-peptone from Beijing Aoboxing Biotechnology Company, China. The Mueller–Hinton (MH) medium was purchased from Qingdao Hope Biol-Technology Co. Ltd. Distilled water was used in all experiments. Preparation of BC and the BC aerogel BC was cultured by the bacterial strain acetobacter xylinum HN001 in a culture medium of 100 mL deionized water (DI) containing 2.5 g of D-glucose anhydrate, 0.75 g of yeast extract, 1 g of lacto-peptone and 1 g of monopotassium phosphate. The culturing medium was adjusted to pH = ∼5 using 30% acetic acid and sterilized at 121 °C for 20 min using HIRAYAMA HVE-50, Japan. Culturing was conducted at 30 °C for 7 d under static conditions. The as-cultured BC was purified using 1 wt% NaOH solution at 90 °C for 30 min, followed by repeated washing using distilled water until pH = 7. It was stored at 4 °C in a closed vessel containing distilled water. The BC aerogel was recovered from the as-cultured BC by solvent-exchange and lyophilization: the as-cultured BC was impregnated in ethanol under gentle agitation at 30 °C for 6–7 h, and the process was repeated three times using fresh ethanol. Next, the ethanol-exchanged BC was placed in a container with tertiary butanol under gentle agitation at 30 °C for 6–7 h and the process was repeated three times using fresh tertiary butanol. The solvent-exchanged BC was placed in a freeze-drier to liberate organic solvents and generate the BC aerogel. The BC aerogel has a calculated porosity of 99% based on the method reported elsewhere22 and a BET surface area of 166 m2 g−1. Assembly of ZnO/BC foams The ZnO/BC bionanocomposite foams were assembled in two steps as illustrated by a typical experiment. Step 1: BC aerogel was cut to specified dimensions and placed in a reflux vessel containing 0.5 g of zinc acetate dihydrate and 30 mL of ethanol. Refluxing took place at 78 °C for 5 h forming an intermediate product. Step 2: the intermediate product of Step 1 was sealed in a 30 mL PTFE-lined stainless steel autoclave containing 20 mL of ethanol and 0.2 g of hexamine. Solvothermal crystallization took place at 85 °C for 6 h forming ZnO/BC organogels, which were then isolated from the reaction mixture and solvent-exchanged using the procedures described above. The ZnO/BC foams were recovered from the organogels by freeze drying as white foams, having a porosity of 98.7% and density of 0.02 g cm−3 based on the reported methods.22

Chemicals

Characterization

All reagents were used as received. Zinc acetate dihydrate (AR), hexamine (AR), anhydrous ethanol (AR), anhydrous iso-propa-

Powder X-ray diffraction data (XRD) were acquired on a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (50 kV,

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200 mA). Scanning electron microscopy (SEM) images were taken on a JEOL JSM 6700F. Transmission electron microscopy (TEM) measurements were conducted on a FEI Tecnai G2S-Twin with a field emission gun operating at 200 kV. Nitrogen adsorption–desorption isotherms were measured at liquid nitrogen temperature on a Micromeritics ASAP 2020M system. The BET method was used to calculate the specific surface areas based on the nitrogen adsorption data. All samples were degassed at 200 °C for 10 h before measurements were taken. The mesopore size distribution was calculated using the Barrett–Joyner–Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) data were recorded on an ESCALAB 250 Xray photoelectron spectrometer using a monochromated X-ray source (Al Kα hν = 1486.6 eV). FTIR spectra were collected on a Bruker IFS 66v/S FTIR spectrometer. Thermogravimetric (TG) analysis was conducted on a NETZSCH STA 449C TG thermal analyzer. The concentration of Zn2+ in solutions was analyzed using an electrochemical system (CHI 660A, Shanghai Chenhua Scientific Instruments Co, China). Antibacterial tests The antibacterial activity was tested towards gram-negative E. coli (strain DH5α) provided by the Medical School of Jilin University. BC and ZnO powder were used as references throughout the tests. All items used were sterilized at 121 °C for 20 min using HIRAYAMA HVE-50, Japan. The antibacterial behaviour of all samples were tested under controlled irradiation at 365 nm (12 W) for 1, 3 and 5 h, respectively. Control experiments were conducted in the dark for the same duration. Tests were repeated three times to ensure data reproducibility. Viable bacteria were monitored using the standard protocol by counting the number of colony-forming units (CFU). The antibacterial activity of the ZnO/BC foams towards E. coli was tested under daylight by exposure to bacterial suspensions. A typical experiment was conducted as follows: E. coli was grown for 24 h with shaking at 220 rpm in a MH medium, then 10% of the bacterial suspension was taken out to grow at 37 °C for 3–4 h. It was centrifuged, washed with a 0.85% NaCl solution (aq.) and centrifuged again to give a bacterial concentration of approximately 108 CFU mL−1. 5 mL of the bacterial suspension was diluted tenfold in 0.85% NaCl solution (aq.) to yield a final bacterial suspension of around 107 CFU mL−1. The final bacterial suspension was tested with the ZnO/BC foams, specifically 0.01 g of 70 wt% ZnO/BC and 0.017 g of 40 wt% ZnO/BC (about 0.1 mg mL−1 of ZnO), while 0.01 g of the BC aerogel and 0.007 g of ZnO powder were used as controls. The viable bacteria were monitored by counting the number of CFU on the MH agar plates. The % reduction in viability, (N0 − N)/N0, was determined by calculating the CFU per mL of the culture, where N0 denotes the number of CFU at the beginning of the treatment and N stands for the number of CFU after the treatment for a specified period. The mean values were calculated from three independent experiments. For the determination of minimal inhibitory concentration (MIC), 0.0025 g, 0.005 g, 0.01 g, 0.05 g and 0.1 g of the 70 wt% ZnO/BC foam were each placed in 50 mL of the final bacterial

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suspension at 37 °C for 24 h with shaking at 220 rpm. 100 μL aliquots of serial dilutions (1 : 105) in 900 μL of MH media were plated on MH agar plates, and next incubated for 24 h at 37 °C to give an estimate of the viable bacterial counts as CFU.

Results and discussion Assembly of ZnO/BC foams The ZnO/BC foam containing the maximum 70 wt% of ZnO exhibits a unique hierarchical architecture constructed from the spheres of wurzite ZnO and BC nanofibers (Fig. 1A and 2F). The spheres are composed of aggregated ZnO nanocrystals threaded by crystalline BC nanofibers, giving rise to a unique string-beaded morphology (Fig. 1B). The ZnO nanocrystals are around 10–20 nm in size and the ZnO spheres have diameters in the range of 100–200 nm where ZnO is highly crystallized as revealed by high resolution TEM (Fig. 1C). The ZnO/BC foams were assembled in a non-aqueous polar medium from zinc acetate dihydrate in two steps. In Step 1, wurtzite ZnO of low crystallinity becomes visible after 5 h as indicated by the reflections at 31.7, 34.4 and 36.2° (JCPDS 361451). BC is present at the end of Step 1 as indicated by the (002) reflection at 22.45° (JCPDS 03-0289). A control experiment was conducted in the absence of BC. Neither amorphous nanoparticles nor nanocrystals were observed, suggesting the critical role of the BC aerogel in the hydrolysis of zinc acetate dihydrate, the polycondensation of the hydrolyzed zinc species and the nucleation of wurtzite ZnO.

Fig. 1 The ZnO/BC foam with 70 wt% of ZnO: (A) SEM image showing the string-beaded morphology. (B) TEM image showing aggregated ZnO nanocrystals of approximate dimensions of 10–20 nm. (C) High resolution TEM of the ZnO/BC foam showing well crystallized wurtzite ZnO.

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Fig. 2 Normalized XRD patterns based on the (002) reflection of BC: (A) time zero. (B) 1 h. (C) 3 h. (D) 5 h (Step 1). (E) 2 h. (F) 6 h (Step 2).

The intermediate product of Step 1 is flower-shaped containing largely amorphous species with a minor presence of the wurtzite ZnO phase (Fig. 2D and 3B). In Step 2, the intermediate product of Step 1, ethanol and hexamine of the specified stoichiometry (Experimental) are autoclaved at 85 °C. The formation of ZnO spheres surrounding the BC nanofibers is accompanied by the increasing crystallinity of the wurtzite ZnO phase (Fig. 2E and F). The ZnO/BC nanocomposite with the maximum 70 wt% of ZnO was obtained after 6 h (Fig. S1,† 2F and 3D). The packing density of the ZnO spheres increases as the reaction proceeds. A ZnO/BC foam with 40 wt% of ZnO is obtained after 3 h of solvothermal crystallization at 85 °C, in which the spheres of the aggregated ZnO nanocrystals are distant from each other (Fig. 3C, S1†). The foam with 70 wt% of ZnO is obtained after 6 h at 85 °C, where the spheres of the aggregated ZnO nanocrystals are intimately connected giving rise to fully wrapped BC nanofibers (Fig. 3D). Varying the assembly conditions including initial reaction compositions and crystallization times caused no further increase in ZnO loading. The ZnO/BC foams are flexible and lightweight with tunable ZnO contents and varying thicknesses (Fig. 3E and F). To understand the role of solvents in the assembly of the ZnO/BC foams, control experiments were conducted using the residual solvents (ethanol–hexamine) of Step 2 without any treatment according to the specified stoichiometry. Similar ZnO/BC foams are obtained where the spheres of the ZnO nanocrystals are formed around the BC nanofibers, however, the sizes appear to be less uniform (Fig. S2A†). The results suggest that ethanol–hexamine is not evidently consumed in the assembly, hence, it can be reused. In another attempt, a one-pot assembly approach was explored by autoclaving zinc acetate dihydrate, BC, ethanol and hexamine of the specified stoichiometry in the temperature range of 35–50 °C for 5 h, followed by 6 h at 85 °C. ZnO/BC foams constructed of large (∼500 nm) and loosely packed ZnO spheres were obtained (Fig. S2B†). Two additional control experiments

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Fig. 3 Assembly of ZnO/BC foams tracked by SEM: (A) BC aerogel containing hydrogen-bonded network of BC nanofibers. (B) The intermediate product of Step 1 with a flower-shaped morphology. (C) A ZnO/BC foam with 40 wt% of ZnO. (D) The ZnO/BC foam with 70 wt% of ZnO. (E) A photo of 70 wt% ZnO/BC foams of varying thicknesses. (F) A photo of the 70 wt% ZnO/BC foam showing its flexibility.

were conducted as follows: (1) reflux and solvothermal crystallization were carried out in iso-propanol of the same volume in the place of ethanol; (2) refluxing in ethanol and crystallization took place in water–hexamine instead of ethanol–hexamine under autogenous pressure conditions at 85 °C. In either case, ZnO/BC foams with sphere-string morphologies and loosely packed ZnO spheres were obtained (Fig. S2C, S2D†). We concluded that reflux in a non-aqueous polar medium provides the most conducive environment for the controlled hydrolysis to take place, leading to the ultimate formation and close packing of ZnO spheres surrounding BC nanofibers. Ethanol provides an ideal polar medium that modulates the hydrolysis of zinc acetate, and the nucleation and crystal growth of ZnO. The presence of hexamine23 warrants the close packing of ZnO spheres and the formation of the stringbeaded morphology, whose role appears to deviate from previous reports. The assembly process is schematically illustrated in Fig. 4A. Given that all the reactants in Step 1 are anhydrous, zinc acetate dihydrate may first be hydrolyzed by the chemisorbed water of the BC aerogel forming crown-ether-type complexes surrounding the BC nanofibers.24 This is followed by polycondensation among the adjacent hydrolyzed zinc species forming a ZnO network with partially crystallized ZnO.24b Seeding of wurtzite ZnO takes place in Step 1 (Fig. 2D). To establish a more insightful view of the assembly process, the concentration of Zn2+ ions in the solution of Step 2 was tracked using an electrochemical method. A control experiment was conducted by placing the intermediate product of

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Fig. 4 (A) Schematic illustration of the assembly process: (a) BC nanofibers; (b) the intermediate product of Step 1 resulting from the hydrolysis of zinc precursors by the chemisorbed water of BC; (c) the intermediate product of Step 1 containing ZnO seeds and amorphous nanoparticles; (d) the dots in the black dotted circle represent dissolved Zn2+ ions during the solvothermal crystallization of Step 2; (e) the formation of ZnO spheres containing aggregated ZnO nanocrystals; (f ) illustration of ZnO/BC bionanocomposite foams. (B) The concentration of dissolved Zn2+ ions in Step 2.

Fig. 5 Nitrogen adsorption–desorption isotherms: (A) BC aerogel with 0 wt% of ZnO. (B) 40 wt% ZnO/BC foam. (C) 70 wt% ZnO/BC foam. Inset shows the pore size distributions of (a) BC aerogel with 0 wt% of ZnO; (b) 40 wt% ZnO/BC foam; (c) 70 wt% ZnO/BC foam.

Step 1 in an autoclave containing a stoichiometric amount of fresh ethanol and hexamine. The results show that the concentration of Zn2+ ions was 0 mmol dm−3 at the beginning of Step 2, increasing to a maximum value of 0.25 mmol dm−3 at 3 h, followed by a rapid drop to 0.07 mmol dm−3 at 4 h and a gradual decrease to 0 mmol dm−3 at the end of Step 2 (Fig. 4B). These results suggest that the amorphous phase grown around the BC nanofibers of Step 1 dissolve under solvothermal conditions, and they serve as a Zn2+ precursor in the growth of ZnO crystals around the BC nanofibers. The nitrogen adsorption–desorption isotherms show that BC and the ZnO/BC foams contain broad-featured meso-macropores with average pore sizes shifting upwards in the order of BC < 40 wt% ZnO/BC < 70 wt% (Fig. 5, Inset). This suggests that the cooperative assembly of BC and ZnO consumes hydroxyl

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groups, causing the BC nanofibers to reassemble. As a result, the ZnO/BC foams replicate the modulated hierarchical porous architecture of BC. In the ZnO/BC foams, the packing of the ZnO nanocrystals generates mesopores while the macropores reflect the structural pores of the modulated architecture of BC. The BET surface areas of the BC aerogel, 40 wt% ZnO/BC foam and 70 wt% ZnO/BC foam are 166 m2 g−1, 80 m2 g−1 and 92 m2 g−1, respectively (Fig. 5). ZnO powder has a BET surface area of 14 m2 g−1 (Fig. S3†). The chemical nature of the 70 wt% ZnO/BC foam was characterized using IR and XPS. The presence of C–O–Zn linkages is implied by the increasing intensities of IR absorption at 554 cm−1 and 1577 cm−1 (Fig. S4A†), which are attributed to the Zn–O and Zn–O–C vibrations, respectively.25 A concurrent weakening in the absorption peak at 1650 cm−1, attributed to C–O(H) stretching vibrations,9 is observed. The weakening becomes more pronounced as the assembly proceeds, suggesting an increasing level of interactions at the interface of ZnO and BC. The XPS survey spectra of ZnO/BC foams match well with those of the ZnO powder at corresponding binding energies of Zn2+, suggesting a similar chemical environment in all three materials (Fig. S4B†). The O 1s peak of the ZnO/BC foams can be deconvoluted to two peaks at 530.8 eV and 532.8 eV, where the binding energy at 530.8 eV may be ascribable to O–Zn bonds and the binding energy at 532.8 eV attributed to C–O bond,25b implying interactions between ZnO and BC (Fig. S4C†).20b Antibacterial activity of the ZnO/BC foams The ZnO/BC foams with the combined merits of hierarchical porosity, large BET surface areas, high ZnO content and BC’s water retention capability may enhance the antibacterial activity of zinc oxide. Table 1 summarizes the reduction in viability towards E. coli under normal daylight conditions. The results are moni-

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Antibacterial activity towards E. coli in daylight

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E. coli Treatment time (h)

CFU mL−1

Reduction in viability (%)

0 1 3 5 0

7.0 × 107 1.7 × 108 3.6 × 108 1.0 × 109 7.0 × 107

0 0 0 0 0

ZnO powder

1 3 5 0

4.6 × 107 1.7 × 107 4.8 × 106 7.0 × 107

33.4% 75.7% 93.1% 0

40 wt% ZnO

1 3 5 0

4.2 × 107 1.2 × 107 2.3 × 106 7.0 × 107

39.9% 82.9% 96.7% 0

70 wt% ZnO

1 3 5

4.0 × 107 7.0 × 106 9.0 × 105

42.8% 90.0% 98.7%

Sample BC

tored by counting the number of CFU in saline suspensions to better represent the antibacterial effects in the natural aquatic environment. The data shows that the 40 wt% ZnO/BC foam and 70 wt% ZnO/BC foam inactivate E. coli by 96.7% and 98.7%, respectively after 5 h of treatment. The ZnO powder inactivates E. coli by 93.6% after 5 h and BC shows no antibacterial effects towards E. coli under daylight. The antibacterial activity shows a decreasing order of 70 wt% ZnO/BC > 40 wt% ZnO/BC > ZnO powder, which is likely due to the decreasing BET surface area, and the small size of the ZnO nanoparticles of the ZnO/BC foams.26 The results may suggest that ROS generated under daylight reacts with the DNA cell membranes and cellular proteins leading to cell death.16 The quantity of ROS relates to the specific surface areas of the antibacterial agents.26,27 The computed MIC of the 70 wt% ZnO foam based on the experimental data is 35 μg mL−1. To verify the proposed mechanism, the antibacterial activity of the ZnO/BC foams and ZnO powder towards E. coli was examined under the irradiation of UV light at 365 nm and in the dark as a control. As shown in Fig. 6A, the antibacterial activity of all ZnO materials significantly increased compared to those observed under daylight, in the order of 70 wt% ZnO/ BC > 40 wt% ZnO/BC > ZnO powder > BC aerogel. The ZnO/BC foams and the ZnO powder reach their respective maximum antibacterial activity in 1 h compared to 5 h under daylight (Table 1). This is likely due to the fact that UV light promotes the generation of ROS,16,17b which in turn enhances the antibacterial activity of ZnO materials. The antibacterial activity observed in the dark shows the same trend, however, at a much reduced magnitude suggesting that the generation of ROS is significantly suppressed (Fig. 6B). The antibacterial activity of BC observed under the irradiation of UV light at 365 nm may be caused by a weak germicidal effect of the UV light (Fig. 6A). No antibacterial activity of BC was observed in

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Fig. 6 (A) The antibacterial activity of all samples under the irradiation of UV light at 365 nm. (B) The antibacterial activity of all samples under the irradiation of UV light at 365 nm and in the dark for 10 min. The % survival = (A/B) where A is the number of surviving CFU of the test samples on the MH agar plates and B is the initial number of CFU on the untreated MH agar plates.

the dark confirming the germicidal effect of UV irradiation at 365 nm.17b

Conclusions We have developed a facile approach for the assembly of monolithic, flexible, lightweight and hierarchically porous zinc oxide bionanocomposite foams through the mediation of bacterial cellulose. The zinc oxide/bacterial cellulose foam with the maximum ZnO loading of 70 wt% has hierarchical porosity and an intriguing string-beaded morphology constructed from the spheres of aggregated wurtzite zinc oxide nanocrystals and bacterial cellulose. The wurtzite zinc oxide is mineralized through the mediation of bacterial cellulose aerogel, and it replicates its modulated hierarchical porous architecture. The zinc oxide/bacterial cellulose foams exhibit excellent antibacterial activity towards E. coli under both daylight and UV light. This could be a result of functional integration of the porous structures and small nanocrystal sizes of zinc oxide that afford them usefulness for water sterilization and wound dressing. The current contribution affords a facile and sustainable

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approach for the development of bionanocomposite foams for advanced applications.

Acknowledgements The authors are grateful to the funding agencies including the National Natural Science Foundation of China (21171067), Jilin Provincial Talent Funds (802110000412) and Tang Aoqing Professor Funds of Jilin University (450091105161, 1G3116651461). The authors wish to thank all anonymous contributors for their valuable suggestions and proof reading of the manuscript.

Notes and references 1 (a) M. Darder, P. Aranda and E. Ruiz-Hitzky, Adv. Mater., 2007, 19, 1309–1319; (b) M. Darder, P. Aranda, M. L. Ferrer, M. C. Gutiérrez, F. del Monte and E. Ruiz-Hitzky, Adv. Mater., 2011, 23, 5262–5267. 2 A. Nussinovitch, Mol. Nutr. Food Res., 2005, 49, 195–213. 3 R. T. Olsson, M. A. Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L. A. Berglund, O. Ikkala, J. Nogues and U. W. Gedde, Nat. Nanotechnol., 2010, 5, 584–588. 4 (a) C. Y. Khripin, D. Pristinski, D. R. Dunphy, C. J. Brinker and B. Kaehr, ACS Nano, 2011, 5, 1401–1409; (b) B. Wicklein and G. Salazar-Alvarez, J. Mater. Chem. A, 2013, 1, 5469–5478; (c) D. Klemm, F. Kramer, S. Moritz, T. Lindstrom, M. Ankerfors, D. Gray and A. Dorris, Angew. Chem., Int. Ed., 2011, 50, 5438–5466. 5 T. Zhang, W. Wang, D. Zhang, X. Zhang, Y. Ma, Y. Zhou and L. Qi, Adv. Funct. Mater., 2010, 20, 1152–1160. 6 C. Legnani, C. Vilani, V. Calil, H. Barud, W. Quirino, C. Achete, S. Ribeiro and M. Cremona, Thin Solid Films, 2008, 517, 1016–1020. 7 S. V. Costa, A. S. Goncalves, M. A. Zaguete, T. Mazon and A. F. Nogueira, Chem. Commun., 2013, 49, 8096–8098. 8 Z. Yang, S. Chen, W. Hu, N. Yin, W. Zhang, C. Xiang and H. Wang, Carbohydr. Polym., 2012, 88, 173–178. 9 D. Sun, J. Yang and X. Wang, Nanoscale, 2010, 2, 287–292. 10 N. Shah, M. Ul-Islam, W. A. Khattak and J. K. Park, Carbohydr. Polym., 2013, 98, 1585–1598. 11 Z. Jing and J. Zhan, Adv. Mater., 2008, 20, 4547–4551.

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Flexible and monolithic zinc oxide bionanocomposite foams by a bacterial cellulose mediated approach for antibacterial applications.

The use of self-assembled biomacromolecules in the development of functional bionanocomposite foams is one of the best lessons learned from nature. He...
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