Polydopamine-Based Surface Modification of ZnO Nanoparticles on Sericin/Polyvinyl Alcohol Composite Film for Antibacterial Application

Silk sericin (SS) is a type of natural macromolecular protein with excellent hydrophilicity, biocompatibility and biodegradability, but also has very poor mechanical properties. To develop sericin-based wound dressings, we utilized polyvinyl alcohol (PVA) to reinforce the mechanical property of sericin by blending PVA and sericin, then modified zinc oxide nanoparticles (ZnO NPs) on SS/PVA film with the assistance of polydopamine (PDA) to endow SS/PVA film with antibacterial activity. Scanning electron microscopy, energy dispersive spectroscopy and X-ray powder diffraction demonstrated ZnO NPs were well grafted on PDA-SS/PVA film. Fourier transform infrared spectra suggested PDA coating and ZnONPs modification did not alter the structure of sericin and PVA. Water contact angle and swelling tests indicated the excellent hydrophilicity and swellability of ZnO NPs-PDA-SS/PVA composite film. Mass loss analysis showed ZnO NPs-PDA-SS/PVA film had excellent stability. The mechanical performance test suggested the improved tensile strength and elongation at break could meet the requirement of ZnO NPs-PDA-SS/PVA film in biomaterial applications. The antibacterial assay suggested the prepared ZnO NPs-PDA-SS/PVA composite film had a degree of antimicrobial activity against Escherichia coli and Staphylococcus aureus. The excellent hydrophilicity, swellability, stability, mechanical property and antibacterial activity greatly promote the possibility of ZnO NPs-PDA-SS/PVA composite film in antibacterial biomaterials application.


Introduction
Wound dressing has a variety of functions, of which the first is to prevent continued bleeding through a certain degree of mechanical compression. In addition, wound dressing should have a certain degree of water absorption ability. It is also necessary for wound dressing to absorb the wound exudate in a timely manner, and at the same time to adequately maintain a moist environment to promote wound healing and reduce scarring formation [1]. Finally, some antibacterial properties are needed to prevent wound infection [2,3]. To meet the requirement of novel wound dressing, it is necessary to find natural materials with good hydrophilicity and biocompatibility that could be of ZnO NPs on the film. Antimicrobial assays were carried out to investigate the antimicrobial activity of the composite film against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

Results and Discussion
ZnO NPs-PDA-SS/PVA composite film was prepared through the blending of sericin and PVA solution followed by PDA coating and ZnO NPs modification. The antibacterial activity of the composite film was tested to assess its usability in antibacterial applications such as wound dressing. The schematic diagram was shown in Figure 1.

SEM, EDS and XRD
SEM micrograph showed the surface morphology of the films. Figure 2(a),(d) showed that SS/PVA film had a smooth surface, indicating the uniform blending of sericin and PVA. Figure 2(b) and (e) showed the surface morphology of PDA-SS/PVA film with a rough surface. PDA adsorption on the film is determined by monomer concentration, reaction time and temperature [14]. We carried out the experiment by constant stirring at 37 °C for 12 h in 2.0 mg/mL dopamine. PDA enhanced the interaction of sericin with ZnO NPs as a mediating agent. Figure 2(c) showed the surface morphology of ZnO NPs-PDA-SS/PVA composite film. The red arrows indicated ZnO NPs on the surface of PDA-SS/PVA film. Figure 2(f) showed the size distribution of ZnO NPs on the PDA-SS/PVA film. Most of ZnO NPs were in the size range of 60-120 nm, which implied its potential antibacterial activity.

SEM, EDS and XRD
SEM micrograph showed the surface morphology of the films. Figure 2a,d showed that SS/PVA film had a smooth surface, indicating the uniform blending of sericin and PVA. Figure 2b,e showed the surface morphology of PDA-SS/PVA film with a rough surface. PDA adsorption on the film is determined by monomer concentration, reaction time and temperature [14]. We carried out the experiment by constant stirring at 37 • C for 12 h in 2.0 mg/mL dopamine. PDA enhanced the interaction of sericin with ZnO NPs as a mediating agent. Figure 2c showed the surface morphology of ZnO NPs-PDA-SS/PVA composite film. The red arrows indicated ZnO NPs on the surface of PDA-SS/PVA film. Figure 2f showed the size distribution of ZnO NPs on the PDA-SS/PVA film. Most of ZnO NPs were in the size range of 60-120 nm, which implied its potential antibacterial activity.
Molecules 2019, 24 3 of ZnO NPs on the film. Antimicrobial assays were carried out to investigate the antimicrobial activity of the composite film against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

Results and Discussion
ZnO NPs-PDA-SS/PVA composite film was prepared through the blending of sericin and PVA solution followed by PDA coating and ZnO NPs modification. The antibacterial activity of the composite film was tested to assess its usability in antibacterial applications such as wound dressing. The schematic diagram was shown in Figure 1.

SEM, EDS and XRD
SEM micrograph showed the surface morphology of the films. Figure 2(a),(d) showed that SS/PVA film had a smooth surface, indicating the uniform blending of sericin and PVA. Figure 2(b) and (e) showed the surface morphology of PDA-SS/PVA film with a rough surface. PDA adsorption on the film is determined by monomer concentration, reaction time and temperature [14]. We carried out the experiment by constant stirring at 37 °C for 12 h in 2.0 mg/mL dopamine. PDA enhanced the interaction of sericin with ZnO NPs as a mediating agent. Figure 2(c) showed the surface morphology of ZnO NPs-PDA-SS/PVA composite film. The red arrows indicated ZnO NPs on the surface of PDA-SS/PVA film. Figure 2(f) showed the size distribution of ZnO NPs on the PDA-SS/PVA film. Most of ZnO NPs were in the size range of 60-120 nm, which implied its potential antibacterial activity.  To verify whether ZnO NPs were well grafted on the film, we performed energy-dispersive spectroscopy (EDS) analysis. The EDS spectrum of the selected area of the composite film (Figure 3a) showed the well-defined peaks of Zn and O, confirming the existence of ZnO on PDA-SS/PVA film ( Figure 3c). This result proved that ZnO NPs were well grafted on the composite film. We also noticed variant shape and size of ZnO NPs on the surface of the film (Figure 3b).
XRD showed sericin has an abroad and shallow diffraction peak at around 2θ = 19.2 • . A broad peak located at 2θ = 19.8 • indicated the semicrystalline structure of sericin ( Figure 3d). No typical diffraction peak of PDA was observed, indicating that PDA coating did not affect the crystal structure of SS/PVA composite film. The diffraction peaks observed at 2θ = 31.  [28].
Molecules 2019, 24 4 To verify whether ZnO NPs were well grafted on the film, we performed energy-dispersive spectroscopy (EDS) analysis. The EDS spectrum of the selected area of the composite film (Figure 3a) showed the well-defined peaks of Zn and O, confirming the existence of ZnO on PDA-SS/PVA film ( Figure 3c). This result proved that ZnO NPs were well grafted on the composite film. We also noticed variant shape and size of ZnO NPs on the surface of the film (Figure 3b).

FT-IR
FT-IR was performed to characterize the chemical composition of sericin, SS/PVA, PDA-SS/PVA, ZnO NPs-PDA-SS/PVA films, as shown in Figure 4. The amide I, II and III bands of sericin existed in all spectra, corresponding to the characteristic peaks located at 1621 cm −1 , 1518 cm −1 , and 1240 cm −1 [29]. PVA has characteristic O-H stretching vibration peaks at 3276 cm −1 (Figure 4a) and 3269 cm −1 (Figure 4b) [30]. The peak of 1606 cm −1 indicates the C=C stretching and N-H deformation vibration of indoles or indoline structures in PDA molecule [31], suggesting that PDA was successfully grafted on the surface of SS/PVA composite film. The FT-IR spectrum of ZnO NPs-PDA-SS/PVA film was similar to that of PDA-SS/PVA film, suggesting ZnO NPs modification does not affect the amide peaks of sericin and the characteristic peaks of PVA and PDA.

FT-IR
FT-IR was performed to characterize the chemical composition of sericin, SS/PVA, PDA-SS/PVA, ZnO NPs-PDA-SS/PVA films, as shown in Figure 4. The amide I, II and III bands of sericin existed in all spectra, corresponding to the characteristic peaks located at 1621 cm −1 , 1518 cm −1 , and 1240 cm −1 [29]. PVA has characteristic O-H stretching vibration peaks at 3276 cm −1 (Figure 4a) and 3269 cm −1 (Figure 4b) [30]. The peak of 1606 cm −1 indicates the C=C stretching and N-H deformation vibration of indoles or indoline structures in PDA molecule [31], suggesting that PDA was successfully grafted on the surface of SS/PVA composite film. The FT-IR spectrum of ZnO NPs-PDA-SS/PVA film was similar to that of PDA-SS/PVA film, suggesting ZnO NPs modification does not affect the amide peaks of sericin and the characteristic peaks of PVA and PDA. Molecules 2019, 24 5 29.8°, which was the smallest among all of the tested films, indicating that it has the best water absorption ability. PDA is an important factor to affect water absorption as it has excellent hydrophilicity [25]. The water contact angle of SS/PVA film was 56.7°, indicating the film is hydrophilic. After ZnO NPs modification, the water contact angle increased to 75.8°, but it was still hydrophilic. ZnO NPs modification covered the hydrophilic groups on the surface of PDA-SS/PVA film, which thus resulted in the decrease of water absorption property.

Wettability and Swellability
Figure 5a-c showed the instantaneous water-absorption of SS/PVA, PDA-SS/PVA and ZnO NPs-PDA-SS/PVA composite films. The water contact angle of PDA-SS/PVA film was 29.8 • , which was the smallest among all of the tested films, indicating that it has the best water absorption ability. PDA is an important factor to affect water absorption as it has excellent hydrophilicity [25]. The water contact angle of SS/PVA film was 56.7 • , indicating the film is hydrophilic. After ZnO NPs modification, the water contact angle increased to 75.8 • , but it was still hydrophilic. ZnO NPs modification covered the hydrophilic groups on the surface of PDA-SS/PVA film, which thus resulted in the decrease of water absorption property.    29.8°, which was the smallest among all of the tested films, indicating that it has the best water absorption ability. PDA is an important factor to affect water absorption as it has excellent hydrophilicity [25]. The water contact angle of SS/PVA film was 56.7°, indicating the film is hydrophilic. After ZnO NPs modification, the water contact angle increased to 75.8°, but it was still hydrophilic. ZnO NPs modification covered the hydrophilic groups on the surface of PDA-SS/PVA film, which thus resulted in the decrease of water absorption property.

Mechanical Property
The mechanical properties of SS/PVA, PDA-SS/PVA and ZnO NPs-PDA-SS/PVA films were shown in Figure 6. SS/PVA had the highest tensile strength among all of the tested films, while PDA-SS/PVA film was the lowest. ZnO-PDA-SS/PVA film had a tensile strength of about 8 MPa, which is suitable for application of wound dressings. The elongation at break is an indicator of the material's flexibility [32]. The results showed that PDA coating resulted in the increase of the elongation at break of SS/PVA film, while ZnO NPs modification reduced the elongation at break of PDA/PVA film (Figure 6b). PDA may interact with sericin/PVA through its amide and hydroxyl groups to form an extensive molecular network, and thus improved the elongation at break of SS/PVA film and decreased the tensile strength of SS/PVA film. ZnO NPs likely disrupted the molecular interaction between PDA and sericin/PVA partially, which resulted in the decrease of the elongation at break and the increase of the tensile strength. The elongation at break of these films ranged from 50% to 160%. The data showed ZnO NPs-PDA-SS/PVA film meets the requirements of wound dressing materials.

Mechanical Property
The mechanical properties of SS/PVA, PDA-SS/PVA and ZnO NPs-PDA-SS/PVA films were shown in Figure 6. SS/PVA had the highest tensile strength among all of the tested films, while PDA-SS/PVA film was the lowest. ZnO-PDA-SS/PVA film had a tensile strength of about 8 MPa, which is suitable for application of wound dressings. The elongation at break is an indicator of the material's flexibility [32]. The results showed that PDA coating resulted in the increase of the elongation at break of SS/PVA film, while ZnO NPs modification reduced the elongation at break of PDA/PVA film (Figure 6b). PDA may interact with sericin/PVA through its amide and hydroxyl groups to form an extensive molecular network, and thus improved the elongation at break of SS/PVA film and decreased the tensile strength of SS/PVA film. ZnO NPs likely disrupted the molecular interaction between PDA and sericin/PVA partially, which resulted in the decrease of the elongation at break and the increase of the tensile strength. The elongation at break of these films ranged from 50% to 160%. The data showed ZnO NPs-PDA-SS/PVA film meets the requirements of wound dressing materials.

Mass Loss Analysis
Sustained stability is one of the characteristics that a wound dressing is supposed to have. Here, we analyzed the mass loss of ZnO NPs-PDA-SS/PVA film under pH 4.0, 7.4 and 10.0 conditions to assess its stability. The cumulative mass loss of the composite film increased over time. Under pH 10.0, the mass loss of the film occurred faster than that under pH 4.0 and 7.4 conditions (Figure 7). This may be that sericin contains a number of acidic amino acids and has an isoelectric point of 3.8 [33]. While in an alkaline environment, sericin more easily reacts and can be more easily hydrolyzed [34]. The result showed that ZnO NPs-PDA-SS/PVA composite film has good stability.

Mass Loss Analysis
Sustained stability is one of the characteristics that a wound dressing is supposed to have. Here, we analyzed the mass loss of ZnO NPs-PDA-SS/PVA film under pH 4.0, 7.4 and 10.0 conditions to assess its stability. The cumulative mass loss of the composite film increased over time. Under pH 10.0, the mass loss of the film occurred faster than that under pH 4.0 and 7.4 conditions (Figure 7). This may be that sericin contains a number of acidic amino acids and has an isoelectric point of 3.8 [33]. While in an alkaline environment, sericin more easily reacts and can be more easily hydrolyzed [34]. The result showed that ZnO NPs-PDA-SS/PVA composite film has good stability.

Mechanical Property
The mechanical properties of SS/PVA, PDA-SS/PVA and ZnO NPs-PDA-SS/PVA films were shown in Figure 6. SS/PVA had the highest tensile strength among all of the tested films, while PDA-SS/PVA film was the lowest. ZnO-PDA-SS/PVA film had a tensile strength of about 8 MPa, which is suitable for application of wound dressings. The elongation at break is an indicator of the material's flexibility [32]. The results showed that PDA coating resulted in the increase of the elongation at break of SS/PVA film, while ZnO NPs modification reduced the elongation at break of PDA/PVA film (Figure 6b). PDA may interact with sericin/PVA through its amide and hydroxyl groups to form an extensive molecular network, and thus improved the elongation at break of SS/PVA film and decreased the tensile strength of SS/PVA film. ZnO NPs likely disrupted the molecular interaction between PDA and sericin/PVA partially, which resulted in the decrease of the elongation at break and the increase of the tensile strength. The elongation at break of these films ranged from 50% to 160%. The data showed ZnO NPs-PDA-SS/PVA film meets the requirements of wound dressing materials.

Mass Loss Analysis
Sustained stability is one of the characteristics that a wound dressing is supposed to have. Here, we analyzed the mass loss of ZnO NPs-PDA-SS/PVA film under pH 4.0, 7.4 and 10.0 conditions to assess its stability. The cumulative mass loss of the composite film increased over time. Under pH 10.0, the mass loss of the film occurred faster than that under pH 4.0 and 7.4 conditions (Figure 7). This may be that sericin contains a number of acidic amino acids and has an isoelectric point of 3.8 [33]. While in an alkaline environment, sericin more easily reacts and can be more easily hydrolyzed [34]. The result showed that ZnO NPs-PDA-SS/PVA composite film has good stability.

Antibacterial Property
The antibacterial property of ZnO NPs-PDA-SS/PVA, PDA-SS/PVA and SS/PVA films were analyzed against Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus), respectively. As shown in Figure 8, while compared to the control, the colonies number did not show a significant difference in the presence of SS/PVA and PDA-SS/PVA films. However, in the presence of ZnO NPs-PDA-SS/PVA film, the colonies number was much less than that of the control, indicating the good antibacterial property of the composite film against E. coli and S. aureus. The antibacterial activity of ZnONPs comes from three aspects: (a) Reactive oxygen species are produced from ZnO NPs, which destroy cell membrane, thus causing leakage of cytoplasmic contents, DNA damage, and cell death [35]; (b) Zinc ions from ZnO NPs can penetrate cell membrane to inhibit bacterial metabolic activity [32,36]; (c) The intracellular accumulation of ZnO NPs can destroy the cell wall of bacteria and affect DNA replication, leading to the death of bacteria [18].
Molecules 2019, 24 7 The antibacterial property of ZnO NPs-PDA-SS/PVA, PDA-SS/PVA and SS/PVA films were analyzed against Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus), respectively. As shown in Figure 8, while compared to the control, the colonies number did not show a significant difference in the presence of SS/PVA and PDA-SS/PVA films. However, in the presence of ZnO NPs-PDA-SS/PVA film, the colonies number was much less than that of the control, indicating the good antibacterial property of the composite film against E. coli and S. aureus. The antibacterial activity of ZnONPs comes from three aspects: (a) Reactive oxygen species are produced from ZnO NPs, which destroy cell membrane, thus causing leakage of cytoplasmic contents, DNA damage, and cell death [35]; (b) Zinc ions from ZnO NPs can penetrate cell membrane to inhibit bacterial metabolic activity [32,36]; (c) The intracellular accumulation of ZnO NPs can destroy the cell wall of bacteria and affect DNA replication, leading to the death of bacteria [18].

Bacterial Growth Assay
To further confirm the bacteriostasis of ZnO NPs-PDA-SS/PVA film, the bacterial growth curve in the presence of the composite film was presented by measuring bacterial OD600, as shown in Figure  9. The bacterial growth profile was very similar between SS/PVA, PDA-SS/PVA films and the control. However, the bacterial growth was significantly retarded in the presence of ZnO NPs-PDA-SS/PVA film, suggesting ZnO NPs-PDA-SS/PVA film has a certain inhibitory effect on the growth of E. coli and S. aureus. This result was consistent with that of the colony counting method.

Materials
Silkworm cocoons were supplied by the State Key Laboratory of Silkworm Genome Biology, Southwest University, Chongqing, China. PVA and dopamine hydrochloride were purchased from

Bacterial Growth Assay
To further confirm the bacteriostasis of ZnO NPs-PDA-SS/PVA film, the bacterial growth curve in the presence of the composite film was presented by measuring bacterial OD 600 , as shown in Figure 9. The bacterial growth profile was very similar between SS/PVA, PDA-SS/PVA films and the control. However, the bacterial growth was significantly retarded in the presence of ZnO NPs-PDA-SS/PVA film, suggesting ZnO NPs-PDA-SS/PVA film has a certain inhibitory effect on the growth of E. coli and S. aureus. This result was consistent with that of the colony counting method.
Molecules 2019, 24 7 The antibacterial property of ZnO NPs-PDA-SS/PVA, PDA-SS/PVA and SS/PVA films were analyzed against Gram-negative bacteria (E. coli) and Gram-positive bacteria (S. aureus), respectively. As shown in Figure 8, while compared to the control, the colonies number did not show a significant difference in the presence of SS/PVA and PDA-SS/PVA films. However, in the presence of ZnO NPs-PDA-SS/PVA film, the colonies number was much less than that of the control, indicating the good antibacterial property of the composite film against E. coli and S. aureus. The antibacterial activity of ZnONPs comes from three aspects: (a) Reactive oxygen species are produced from ZnO NPs, which destroy cell membrane, thus causing leakage of cytoplasmic contents, DNA damage, and cell death [35]; (b) Zinc ions from ZnO NPs can penetrate cell membrane to inhibit bacterial metabolic activity [32,36]; (c) The intracellular accumulation of ZnO NPs can destroy the cell wall of bacteria and affect DNA replication, leading to the death of bacteria [18].

Bacterial Growth Assay
To further confirm the bacteriostasis of ZnO NPs-PDA-SS/PVA film, the bacterial growth curve in the presence of the composite film was presented by measuring bacterial OD600, as shown in Figure  9. The bacterial growth profile was very similar between SS/PVA, PDA-SS/PVA films and the control. However, the bacterial growth was significantly retarded in the presence of ZnO NPs-PDA-SS/PVA film, suggesting ZnO NPs-PDA-SS/PVA film has a certain inhibitory effect on the growth of E. coli and S. aureus. This result was consistent with that of the colony counting method.

Materials
Silkworm cocoons were supplied by the State Key Laboratory of Silkworm Genome Biology, Southwest University, Chongqing, China. PVA and dopamine hydrochloride were purchased from

Materials
Silkworm cocoons were supplied by the State Key Laboratory of Silkworm Genome Biology, Southwest University, Chongqing, China. PVA and dopamine hydrochloride were purchased from Aladdin Corp (Shanghai, China). ZnO NPs (99.9%) was purchased from Guohua reagent (Shanghai, China). All other chemicals were of analytical grade and were directly used.

Fabrication of ZnO NPs-PDA-SS/PVA Film
Sericin was obtained from silkworm cocoons by means of autoclave at 121 • C for 30 min [37,38]. SS/PVA film was prepared as per our previous procedure [37,38]. Briefly, sericin solution (4%, w/t) and PVA solution (5%, w/t) were well mixed, and then dried at 65 • C in the Petri dishes to become SS/PVA film. SS/PVA film was immersed directly into fresh dopamine solutions (2.0 mg/mL, pH 8.5) at 37 • C for 12 h with continuous stirring. Then the PDA coated SS/PVA film was taken out and washed with MilliQ water to remove extra PDA. Furthermore, PDA-SS/PVA film was soaked in 61.7 mM ZnO NPs at 25 • C for 12 h. Finally, the ZnO NPs-PDA-SS/PVA composite film was produced after repetitive washing and was dried at 25 • C.

SEM, XRD, FT-IR and Mechanical Test
The surface morphology of ZnO NPs-PDA-SS/PVA, PDA-SS/PVA and SS/PVA films were imaged on a JSM-5610LV scanning electron microscopy (Tokyo, Japan) with a working voltage of 25 kV. The crystalline structure of the composite film was examined by PANalytical X'Pert, a powder X-ray diffraction system (Almelo, The Netherlands) over Bragg angles ranging from 10 • to 80 • . FT-IR spectra of the composite film were characterized on Thermo-Fisher Nicolet iz10 IR microscope (Framingham, MA, USA) over the wavenumber of 4000-400 cm −1 .
SS/PVA, PDA-SS/PVA, ZnO NPs-PDA-SS/PVA films were prepared with a dimension of 4 cm × 1 cm (length × width). These films' tensile properties were tested on a universal AG-X-plus testing machine (Shimadzu, Kyoto, Japan) equipped with a 1000 N loading cell. The crosshead speed was 5 mm/min. The thickness of the film was measured by SEM. To reveal the stress-strain relation, the recorded data was transformed into real stress (σ) and strain (ε) [39].

Hydrophilicity and Swellability
The hydrophilicity of ZnO NPs-PDA-SS/PVA, PDA-SS/PVA and SS/PVA were analyzed at 25 • C on a Krüss DSA100 system (Hamburg, Germany) via sessile drop contact angle. At five different positions, the process of water absorption was recorded by dispensing the water droplet on the surface of the sample.
The swelling of the composite films in phosphate buffer (PBS, pH 7.4) was measured by gravimetric method with a minor modification. The original films were weighed as W 1 . Then, the swelling films were immersed into phosphate buffer at 37 • C, then taken out at separate time intervals and gently removed by filter paper. The mass of swollen composite films was marked as W 2 . The swelling ratio (S) was calculated by the following equation: The same operation was performed at least three times under the same condition. The swellability of the film was presented by average data.

Mass Loss Analysis
A mass losing ratio analysis was conducted to analyze the stability of the composite films. First, the films were prepared with a dimension of 3 cm × 3 cm (length × width), and then soaked into PBS with different values (pH 4.0, 7.4, 10.0) at 25 • C, respectively. At given time intervals, the films were carefully taken out, washed, and weighed. The mass of the film before and after the treatment were recorded as W 3 and W 4 , respectively. The test was repeated five times. The mass losing ratio (R) was calculated using the following equation:

Antibacterial Test
The antibacterial activity of the composite film was assessed against gram-positive bacteria S. aureus and gram-negative bacteria E. coli as Pal's protocol [40]. First, the bacteria were cultured in 10 mL Luria-Bertani (LB) medium (pH 7.4) under a constant vibrating velocity of 2.8× g at 37 • C overnight. Then the bacteria suspension was diluted with LB medium to an optical density at 600 nm (OD 600 ) of 0.02, which was used as the control. The diluted bacteria suspension was cultured at 37 • C for 30 h in the presence of sterile ZnO NPs-PDA-SS/PVA, PDA-SS/PVA, SS/PVA films (1 cm × 1 cm), respectively.
The bacteria were cultured in 12-well plate in the presence of sterile composite films at 37 • C, and were collected at different time intervals to measure OD 600 . After 3 h, the bacterial suspension of each experiment was diluted for several times and then cultured on LB plate at 37 • C overnight. The antibacterial activity of the film was evaluated by counting the number of colonies on the plate. The assay was performed in triplicate for each independent experiment.

Conclusions
In this work, we prepared SS/PVA film with enhanced mechanical performance through blending sericin and PVA, and then grafted ZnO NPs on SS/PVA film via PDA to yield ZnO NPs-PDA-SS/PVA composite film. SEM confirmed the spherical shape ZnO NPs on PDA-SS/PVA film. The elemental composition and chemical composition of ZnO NPs-PDA-SS/PVA film were confirmed by EDS and FT-IR. The crystal planes of ZnO NPs were determined by XRD. Water contact angle and swelling analysis indicated the excellent hydrophilicity and swellability of ZnO NPs-PDA-SS/PVA film. The mechanical test validated the improved mechanical performance of the composite film. Mass loss analysis showed good stability of ZnO NPs-PDA-SS/PVA film under different pH conditions. ZnO NPs-PDA-SS/PVA film exhibited a certain degree of antibacterial effect against E. coli and S. aureus. The improved mechanical performance and antibacterial activity will greatly promote the application of ZnO NPs-PDA-SS/PVA composite film in antimicrobial biomaterials such as wound dressing.