The Enhanced Antibacterial and Food Simulant Activities of Silver

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The Enhanced Antibacterial and Food Simulant Activities of Silver Nanoparticles / Polypropylene Nanocomposite Films Guozhou Cao, Han Lin, Palanisamy Kannan, Chun Wang, Yingying Zhong, Youju Huang, and Zhiyong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03061 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Enhanced Antibacterial and Food Simulant Activities of Silver Nanoparticles / Polypropylene Nanocomposite Films

Guozhou Cao1,2, Han Lin1, Palanisamy Kannan4*, Chun Wang2, Yingying Zhong2, Youju Huang3* and Zhiyong Guo1*

1

The School of Materials Science and Chemical Engineering, Ningbo University, Ningbo-315211,

Zhejiang Province, P. R. China 2

Ningbo Academy of Inspection and Quarantine, Ningbo-315012, Zhejiang Province, P. R.

China 3

Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang

Province, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China 4

College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing,

314001, PR China.

* Corresponding Authors Emails: [email protected] (Kannan), [email protected] (Huang), [email protected] (Guo).

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Abstract In this work, we synthesize dodecyl mercaptan functionalized silver nanoparticles integrated with polypropylene nanocomposite (DM-AgNPs/PP) substrates by a simple in-situ melt blending method. The formation and distribution of AgNPs are confirmed by UV-visible spectroscopy, Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The existence of DM-AgNPs in PP film substrate enhances the thermal degradation and crystallization properties. Further, the antimicrobial activity of assynthesized DM-AgNPs/PP film substrate is studied using Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) as model microbes, which displayed significantly enhanced bacteriostatic activities under optimized composition and experimental conditions. Interestingly, PP substrate with 0.4% DM-AgNPs is drastically improved antibacterial property via release of oxygen reactive species and Ag ion diffusion processes; thus, the inhibition rates of E. coli and S. aureus obtained as 100% and 84.6%, respectively, which is higher than the conventional AgNPs. Finally, we demonstrate the migration study of Ag ions from DMAgNPs/PP film using different food simulant solutions by ICP–MS analysis and dissolved Ag ion content is estimated, which is a key prospect for the toxicity analysis. The overall Ag ions migration value between 1.8 and 24.5 µg/cm2 and displayed a lowest limit of Ag ions migration as 0.36 µg/cm2. Our work highlights the development of high performance nanocomposites as promising antibacterial and food simulant materials for biomedical and industrial applications.

Keywords: Silver Nanoparticle, Polypropylene Film, Antibacterial Property, Migration Rules.


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Introduction Compared to bulk systems, nanostructured materials have shown vibrant potential in medicine and clinical diagnostics due to their large active surface area, biocompatibility, stability and practical applicability.1,

2

Substantial interest has been focused on the noble metal

nanoparticles because of their low toxicity, high affinity with targeted biomolecules, and specificity for surface immobilization of a wide range of biomolecules.3, 4, 5 Thus, research on the recent nanobiotechnology has gear-up its progress with the use of bio-friendly noble metal nanoparticles.5,

6, 7

Of the interest, silver nanoparticles (AgNPs) have been fascinated huge

attention due to their resilient physical and chemical properties, which are intensively applied in catalysis, antimicrobial, and surface-enhanced Raman scattering.8, 9 It has been well-known that silver (Ag) and silver-based compounds were highly toxic to 16 major bacterial species.10, 11, 12 Ag is normally used as nitrate form to stimulate antimicrobial effect; however, the use of AgNPs resulted a huge enhancement in the active surface area accessible for the microbe to be exposed.13, 14, 15, 16, 17 The fact is that AgNPs penetrate into the bacterial cell wall causing damage by interacting with phosphorus- and sulfur-containing compounds such as DNA and proteins.18, 19 Particularly, Ag has less toxicity towards mammalian cells and doesn’t simply provoke microbial resistance.20,

21, 22, 23

Moreover, Ag has been

intercalated into plastics in various forms such as catheters, dental material, medical devices and implants, and burn dressings to protect against microbial infection.24,13, 25, 26 Polypropylene (PP) is considered as a most commercially available thermoplastic polymers with wide practical applications in automotive, household appliances, construction, food and packaging industry, etc.,27 Recently, studies were focused on the enhanced mechanical and antibacterial properties of polymers-silver composites in medical applications.28, 3 ACS Paragon Plus Environment

29, 30, 31

For

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instance, Aalaie et. al., reported that PP/AgNPs-zeolite plastics displayed higher antibacterial efficiency (depends on the AgNPs contents) against microbes such as E. coli and S. aureus.27 Fages et.al., reported polyvinyl pyrrolidone coated Ag NPs onto a PP matrix increased the protection against S. aureus and E. coli, and antimicrobial activity of about 1.5 against S. aureus and E. coli. On the other hand, AgNPs coated with oleic acid leads to strong protection against S. aureus with antimicrobial activity of about 2.5–3.3; but the overall protection against E. coli was very low (antimicrobial activity falls up to 0), probably due to the presence of a PP-g-MA compatibilizer agent.32 Later, Le et.al., reported PP/AgNPs-zeolite with various AgNPs content of 0, 40, 80 and 160 ppm in a twin-rotating-screw extruder for developing antimicrobial activity. Mechanical properties and antibacterial ability of PP/AgNPs-zeolite plastics were investigated. The higher antibacterial efficiency (η~100%) was observed for the PP/AgNPs-zeolite plastic samples containing from 80 to 160 ppm of AgNPs.33 Moreover, PP material incorporated AgNPs are extensively used for food packaging application, which enable the food safety by improving mechanical, heat resistance, antimicrobial and antifungal properties.34, 35, 36 The potential migration of AgNPs from packaging into food and drinks cause major risk to the consumers and environments, thus the accurate determination is practically necessary for health and environmental safety.37, 38, 39, 40, 41 In this study, we synthesize the dodecyl mercaptan functionalized Ag nanoparticles encapsulated within polypropylene nanocomposite (DM-AgNPs/PP) substrates by a simple in-situ melt blending method. Then, the antimicrobial activities of as-synthesized DM-AgNPs/PP are studied by using Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) microbes and displayed significant bacteriostatic activities. Finally, we demonstrated the migration study of Ag


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from DM-AgNPs/PP using different food simulant solutions and estimated the dissolved Ag content, which is a key prospect for toxicity analysis.

Experimental Section Materials Polypropylene resin (F401) was purchased from Sinopec Yangzi Petrochemical Co. Ltd. Sodium borohydride (purity≥98%), silver nitrate (purity ≥99.8%), and dodecyl mercaptan (purity ≥99.8%)were purchased from Aladdin Industrial Inc. Acetone (purity ≥99.5%), isopropyl alcohol (purity ≥99.7%), toluene (purity ≥99.5%), and cetyltrimethylammonium bromide (purity ≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All other reagents employed in this work were of analytical reagent grade and were used as received without further purification.

Preparation of DM-AgNPs Cetyltrimethylammonium bromide (CTAB; 1.8 g) was dispersed in 25 ml of toluene in a 250 ml round bottom flask, and 36 ml (3 mol/L) silver nitrate solution was slowly added and stirred for 4 h for phase transfer reaction. Next, 240 µl dodecyl mercaptan (DM) was added drop-by-drop into the above mixed solution with continuous stirring. Finally, 30 ml NaBH4 (0.44 mol/L) was added and a yellow brown solution was obtained. Then the organic phase was collected and washed with isopropyl alcohol, and acetone. Finally, the DM-AgNPs was obtained by centrifugation at 10000 r/min for 10 min and dried at 60◦C under vacuum for 12 h.


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Preparation of DM-AgNPs/PP nanocomposites film Varied amount of DM-AgNPs (0.05, 0.10, 0.40 and 0.70%) were added to polypropylene (PP) resins and treated with Brabender mixer at 200C for 8 min with a rotation speed of 50 rpm/min. The obtained nanocomposites were pressed to form film-like substrates at 200C under 10 MPa for 5 minutes, cooled down to room temperature and then cut into suitable size for testing.

Characterization of DM-AgNPs and DM-AgNPs/PP nanocomposites The UV–vis spectra were recorded using UV-vis–NIR spectrophotometer T10CS (Beijing Persee General Instrument Co., Ltd) China. DM-AgNPs were dissolved in 50 mL xylene and dispersed by ultrasonic till the solvent become red brown for UV spectral analysis. Fourier transform infrared spectroscopy (FT-IR) was carried out on a Nicolet 6700 FT-IR spectrometer (Thermo-fisher Scientific, USA). The measurement was carried out in the region of 4000–400 cm-1 with

KBr pellet as standard. Thermogravimetric analysis (TGA) was tested on a

TGA/DSC1 Analyzer (Mettler Toledo International Inc., Switzerland) at a heating rate of 10C/min in nitrogen (N2) and air atmospheres. The TGA analysis was conducted at the temperature between 50 to 600C for as-synthesized samples. The size and morphology of the DM-AgNPs/PP nanocomposite were examined by Tecnai F20 TEM instrument (Thermofisher Scientific, USA) operated at an acceleration voltage of 300 kV. The sections of about 50100 nm in thickness were prepared by ultramicrotome cutting with a diamond knife. TGA/DSC1 analyzer (Mettler Toledo International Inc., Switzerland) was used to analyze the crystallization and melting behavior of PP film. About 4–6 mg of PP samples were weighed accurately in the 6 ACS Paragon Plus Environment

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aluminum pans. The DMAQ800 dynamic mechanical analyzer (TA instruments, USA) was used to investigate the dynamic mechanical performance of DM-AgNPs/PP nanocomposite film. The samples were tested under stretching mode in N2 atmosphere at a heating rate of 3C/min. The testing frequency was 1Hz and size of the sample was 25mm×5mm×0.2mm (lengthwidththickness). The differential scanning colorimeter (DSC; Perkin-Elmer Model 7) equipped with a cooler was used under nitrogen flow. The DSC process was performed as follows: the samples were heated at 50 to 250 oC at 3C/min, then cooled at a rate of 10C/min to 25C, maintained for 3 min, and heated to 250C at a rate of 10C/min. Both the exothermic and endothermic curves were recorded. The onset melting temperature was derived from the intercept of baseline and maximum tangent of respective exothermic and endothermic peaks. From the DSC graphs, melting temperatures (Tm) of the DM-AgNPs/PP nanocomposite were obtained, and the apparent fusion enthalpies were calculated from the area of the endothermic peak. The degree of crystallinity of polypropylene was assessed using given equation (1): Hf X=

 100%

(1)

Hf where X is the degree of crystallinity, Hf is the enthalpy heat of fusion of PP film and Hf is the enthalpy heat of fusion of 100% crystalline PP taken as 209 J/g.42 The inductively coupled mass spectrometry (ICP-MS) analysis was carried out using Agilent Technologies (7700 series ICPMS) instrument. After removal of the DM-AgNPs–PP nanocomposite membrane, the migrated simulant was evaporated using water bath and then dissolved in 50 ml of 2% HNO3 for ICP-MS analysis.


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Antibacterial activity of DM-AgNPs/PP nanocomposites Antibacterial activity was tested according to ISO 22196 (2011). Escherichia coli (E. coli ATCC8739) and Staphylococcus aureus (S. aureus ATCC6538) were obtained from China Center of Industrial Culture Collection (CICC). The microbes were transferred from the stock culture to the slant culture medium and incubated at 35±1C for 24 h. From this culture, bacteria was transferred using a sterile inoculating loop into the fresh slant culture medium and incubated at 35±1C for 20 h. The concentration of bacterial suspensions (reference of OD600) was controlled between 2.5×105 CFU/ml and 10×105 CFU/ml. The above solution was used as the test inoculum for antibacterial activity. The DM-AgNPs/PP nanocomposite film was cut into the size of 5050 mm for antibacterial testing and control testing. Meanwhile, the PP film with a size of 4040 mm was prepared as a cover film. Before testing, all the film substrates were cleaned with 75% ethanol and rinsed with distilled water. An individual test film was placed into a separate sterile petri dish plate, and 0.2ml of as-prepared test inoculum was added on each test substrates and covered by the cover film. Afterwards, the petri dishes containing inoculated test specimens were incubated at a temperature of 35±1C with a relative humidity of not less than 90% for 24 h. Finally, viable bacteria count was determined by plate count method. The antibacterial analysis was independently repeated three times and obtained results were presented as an average with error bars as percentage difference for each data point.

Food simulant rules Food materials are complex matrices, not chemically uniform (composition), and determination of migrant substance is highly complicated. Thus, food simulants are used for 8 ACS Paragon Plus Environment

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better determination of migration. Council Directive 82/711/EEC dated 8th October 1982, and 85/572/EEC dated 19th December 1985, issued the list of simulants to be used for testing migration of constituents. (A) – distilled water (substitution of neutral food); (B) – diluted acid solution (e.g. 3% (w/v) acetic acid solution, substitution of acidic food); (C) – ethanol/water mixtures (e.g. 10% (v/v) ethanol solution, substitution of alcoholic food), and (D) – e.g. olive oil or isooctane (substitution of fatty food).

Migration study of DM-AgNPs/PP nanocomposites The DM-AgNPs/PP nanocomposite membrane was (surface area of 160 cm2) immersed in food simulation liquid (water, 4% acetic acid, 20% ethanol, and n-hexane) and sealed. The samples were left for definite time intervals at specific temperatures in an oven. Migration tests were performed at 20, 40, 50C, and migration times were set at 1, 2, 3, 4, 5, and 10 days. The determination of the amount of Ag in the food simulants at each temperature and time was carried out by ICP-MS (see above).

Results and discussion Characterization of DM-AgNPs, and DM-AgNPs nanocomposite The formation of AgNPs was monitored by UV–visible spectrophotometer, and the obtained results are shown in Figure 1A. The optical absorption spectra of as-synthesized DMAgNPs shows intense absorption bands at 471 nm with a small shoulder peak at 347 nm. The absorption band of plasmonic nanoparticle depends on the shape, size, and the surrounding medium.43, 44, 45 The spherical shaped AgNPs are known to exhibit characteristic plasmon bands, which are highly related to the collective oscillations of surface electrons (absorption band at 471 9 ACS Paragon Plus Environment

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nm). In addition, a small shoulder peak observed at 347 nm was related to the presence of excess dodecyl mercaptan (DM) in the AgNPs solution. However, the presence of excess DM in colloidal AgNPs was removed by successive washing steps using isopropyl alcohol and acetone. Further, capping of DM compound with AgNPs was characterized by FT-IR measurements. Figure 1B (curve a) shows the characteristic peaks obtained for DM functional groups at 2922 and 2850 cm-1 which were corresponding to the symmetrical stretching vibration and asymmetric stretching vibration bands of -CH3, respectively. The peaks at 1465 and 1287 cm-1 denotes the shearing deformation vibration and plane rocking vibration of -CH2- group. Also, a sharp band at 716 cm-1 indicates the stretching vibration of -C-S- (Figure 1B; curve a). After functionalization with Ag metal, these peaks were also found in DM-AgNPs. Moreover, the related peaks have slight translation indicating the strong interaction between dodecyl DM and Ag. The bands at 1452, 1365 and 1287 cm−1 were merged to give bands at 1385 and 1316 cm−1 for DM-AgNPs. The disappearance of band at 254 cm−1 corresponding to -SH stretching for DMT-AuNPs indicates the absence of –SH groups at the surface of AuNPs (Figure 1B; curve b).46, 47 The thermal degradation behavior of DM-AgNPs was studied by TGA analysis (Figure 1C). The TGA curve showed that DM-AgNPs starts losing mass at about 200C, which is due to the presence of

water or adsorbed impurities in the nanoparticle sample (dashed line i), and

approximately 5% weight loss was noted. Then, quick weight loss (T5wt%) occurred between 200C and 300C for DM-AgNPs, which is close to the boiling point of DM (dashed line ii). It has been reported that -SH capped noble metal nanoclusters displayed the weight loss, but very quickly progressed at the boiling point of the corresponding ligand.48 Further, the onset of weight loss at a higher temperature is a measure of strong chemisorption of DA on the Ag surface. We estimate the overall weight loss of DM in the surface of AgNPs was about 33% (dashed line iii). 10 ACS Paragon Plus Environment

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Figure 1. UV-vis absorption (A), FT-IR spectra (B), and TGA curve (C) obtained for DMAgNPs.

The size and morphology of the AgNPs capped with DM-compound were examined by TEM analysis, and it shows that nanoparticles are mostly spherical in shape with a size of 9.5 ± 2 nm and have a random size distribution (Figure 2A). Next, we have systematically analyzed the various amount of DM-AgNPs loading into the PP film substrates (Figure 2B). The PP film is highly transparent and colorless (Figure 2B; picture “a”), it turned dark yellow color on loading 0.1% of DM-AgNPs, though it can maintain the transparent nature (Figure 2B; picture “b”), and the pattern “Ag” appeared visibly to the naked-eye. The dark yellow color was got darkened (pale brown) on loading of 0.7% DM-AgNPs into the PP film substrate and still the pattern “Ag” 11 ACS Paragon Plus Environment

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was quite visible (Figure 2B; picture “c”). Finally, the color of PP film substrate darkened and pattern “Ag” was not visible on the PP film surface indicating the substantial amount of DMAgNPs present in the PP film matrix (Figure 2B; picture “d”).

A

B 100

C

Transmittance(%)

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D

80 60 40

PP PP

0.1% DM-AgNPs/PP PP/0.1MAg

20

0.4% DM-AgNPs/PP PP/0.4MAg

0.7% DM-AgNPs/PP PP/0.7MAg E 0 1000 900 800 700 600 500 400 300 200

Wavelength (nm)

Figure 2. The TEM image of DM-AgNPs solution (A); Photos of transparent PP film substrate (B-a), PP film with 0.1% of DM-AgNPs (B-b), PP film with 0.4% of DM-AgNPs (B-c), PP film with 0.7% of DM-AgNPs (B-d); TEM images of PP film substrate with 0.4% of DM-AgNPs (C), and PP film substrate with 0.7% of DM-AgNPs (D). UV–vis transmission spectra obtained for various content of DM-AgNPs in the PP film substrates (E).

We further examined the various amount of DM-AgNPs loading into the PP film substrate by TEM and UV-vis spectral analysis. Particularly, 0.1% of DM-AgNPs content was hard to disperse in PP film substrate. Thus, optimal amount of nanoparticles loading was required to obtain uniform dispersion of DM-AgNPs into the PP film substrate. As can be seen in Figure 2C, the TEM image displayed that DM-AgNPs (0.4% of loading) were randomly distributed on the PP film matrix without any aggregation or coalescence; however, 0.7% of DM-AgNPs loading 12 ACS Paragon Plus Environment

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into the PP film matrix clearly evidenced the aggregation of DM-AgNPs/PP nanocomposites. This indicates the optimized loading of DM-AgNPs (0.4%) into the PP film matrix results in uniform dispersion, which is highly optimal for antibacterial and food packing applications. Finally, we followed-up the content of DM-AgNPs loading in PP film matrix by UV–vis transmission spectral analysis, as shown in Figure 2E. The UV-vis transmittance spectrum of PP film was not responsive to UV-vis region; however, a predominant surface plasmon resonance (SPR) band at 480 nm was noticed and gradually increased while incorporating various amounts (0.1%, 0.4%, and 0.7%) of DM-AgNPs. For instance, SPR band intensity of 0.1% DM-AgNPs in PP film was about 32%, however 0.7% DM-AgNPs in PP film showed as high as 63% due to the maximum loading of AgNPs contents,49 and the obtained results were closely matched with UVvis absorption spectra of colloidal DM-AgNPs solution.

Thermal degradation behaviors of DM-AgNPs nanocomposite The thermal degradation behaviors of DM-AgNPs film in N2 were studied by TGA. The weight loss at 1%, 5%, 10%, 20% and 50% against the temperatures were denoted as T1wt%, T5wt%, T10wt%, T20wt% and T50wt%, respectively. Figure 3A, and corresponding expanded view (Figure 3B) displayed a single-step thermal degradation of DM-AgNPs/PP nanocomposite. The T1wt% for DM-AgNPs nanocomposite has lower thermal stability than PP film (Figure 3B) and the degradation temperatures of DM-AgNPs nanocomposite above T20wt% were notably higher than PP film substrate. In the view of overall analysis, TGA curves of DM-AgNPs nanocomposite gradually shifted to higher temperature (Figure 3B) with respect to the Ag content in the nanocomposite. On the other hand, the DTG responses of DM-AgNPs nanocomposite were also clearly matching with TGA results i.e., the thermal stability gradually 13 ACS Paragon Plus Environment

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increased with respect to the amount of DM-AgNPs in PP film substrate. We summarized the thermal degradation results derived under N2 atmosphere in the supporting information (see Table S1). The obtained results showed that loading of DM-AgNPs into the PP film evidently improved the thermal stability of the nanocomposite substrate.

A

80 60 PP 0.1% DM-AgNPs/PP 0.4% DM-AgNPs/PP 0.7% DM-AgNPs/PP

40 20

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500

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-4 200

430

440

450

Temperature (C)

D -2

DTG

-1

-3

420

600

C

0

-2

PP 0.1% DM-AgNPs/PP 0.4% DM-AgNPs/PP 0.7% DM-AgNPs/PP

70

0 100

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Weight (%)

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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

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300

400


-3

500

Temperature (C)

600

440

450

460

470

480

Temperature (C)

Figure 3. TGA (A and B) and DTG (C and D) analysis showing the thermal degradation behaviors of PP film with 0%, 0.1%, 0.4%, and 0.7% of DM-AgNPs under N2 (nitrogen) atmosphere.

We have also studied the thermal degradation behaviors of PP/M-Ag film in air by TGA analysis. Noticeably, thermal degradation behaviors of all the samples have two-step process (onset degradation at 250C and 450C), which is completely different from those obtained for N2 atmosphere (Figure 4A, and corresponding expanded view shown in Figure 4B). Moreover, the effect of DM-AgNPs on degradation temperature of PP was very evident i.e., Stability of 14 ACS Paragon Plus Environment

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nanocomposite increases with increase in content of DM-AgNPs. All degradation temperatures of DM-AgNPs/PP film from 1 to 50% were comparably higher than the PP film. Furthermore, the degradation temperature gap between PP film and DM-AgNPs/PP film was larger (27C) than that in N2 (6C). 100

A

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-1.5

-2.0

-2.5

100 200 300 400 500 600 700 800

250

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Figure 4. TGA (A and B) and DTG (C and D) analysis showing the thermal degradation behaviors of PP film with 0%, 0.1%, 0.4%, and 0.7% of DM-AgNPs under air (oxygen) atmosphere. For example, T20wt% of 0.4% DM-AgNPs/PP is 5oC higher than that of PP in N2, while T20wt% of 0.4% DM-AgNPs/PP is 14C higher than that of PP in air. In addition, the broad DTG peaks indicate the slower degradation rate (Figure 3C and D). We summarized the thermal degradation results derived under air atmosphere in the supporting information (see Table S2). The obtained 15 ACS Paragon Plus Environment

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results indicate that DM-AgNPs hinders the thermal oxidation of PP matrix, and it plays vital role in air for enhancing the thermal stability.

Melting and crystallization behaviours of DM-AgNPs/PP film The melting and crystallization behaviors of DM-AgNPs/PP film were studied by DSC method due to the semi-crystalline nature of PP film. The melting and crystallization curves were obtained for PP film, 0.1% DM-AgNPs/PP and 0.4% DM-AgNPs/PP nanocomposites (Figure S1A). All the samples displayed melting point (Tm) at 161oC with a very small shift (2-3C) observed at higher temperature due to the presence of 0.1% and 0.4% of DM-AgNPs into the PP film. The addition of M-Ag does not change the Tm greatly. On the other hand, we observed similar trend of results for crystallization temperature (Tc; Figure S1B). The above observation suggested that existence of optimal DM-AgNPs in PP film does not promote the heterogeneous nucleation and growth of PP chains.

Dynamic mechanical properties of DM-AgNPs/PP film The storage modulus (energy stored during deformation due to stress), loss modulus (the energy dissipated as heat, representing the viscous portion), and damping factor Tan δ (the ratio of energy loss and energy stored during deformation) of PP film and DM-AgNPs/PP nanocomposite as the function of temperature were shown in Figures S2. The storage modulus of DM-AgNPs/PP is higher than that of PP at low temperature evidently confirmed that the DMAgNPs hinder the motion of PP film at low temperature. With the content of DM-AgNPs increasing the storage modulus increases systematically (Figure S2A). The glass transition temperature (Tg) is a crucial property of the polymer because it is often applied for the determination of its suitability in various applications. The Tg was obtained from loss modulus 16 ACS Paragon Plus Environment

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curves, i.e., the Tg of DM-AgNPs/PP nanocomposite film was considerably lower than PP film indicating the chain segment of PP is easy to move. The DM-AgNPs increases the distance between PP molecular chains, which provides more space for the small chain segment movement resulting in lower Tg (Figure S2B). Next, the Tan δ peak was associated with the soft segment Tg. The peak of Tan δ shifted towards lower glass transition temperature while increasing the content of DM-AgNPs in PP film (Figure S2C).50, 51 The Tg increase was perhaps due to the interaction between the PP film and DM-AgNPs, which slightly hindered the mobility of polymeric chains with resulting minor increase in stiffness.

Antibacterial property of DM-AgNPs/PP film The antibacterial efficiency of DM-AgNPs/PP nanocomposite was examined against Gram negative (E. coli), and Gram positive (S. aureus) microorganisms because of its vital clinical interest. It is well known that E. coli is the furthermost characterized bacterium which can cause gastroenteritis, urinary tract infections, and neonatal meningitis; on the other side, S. aureus can cause skin infections including abscesses, respiratory infections, food poisoning; major bone and joint infections such as osteomyelitis, and septic arthritis.52, 53 Characteristically, the DM-AgNPs/PP nanocomposite was found to show potential antibacterial activity against E. coli, and S. aureus microorganisms. Known volume (0.2 ml) of bacterial inoculum was added on the DM-AgNPs/PP nanocomposite film substrate and incubated at 35C for 24 h. The antimicrobial activity of the nanocomposites was summarized in Table 1. It was found that the bacterial growth was slightly inhibited in both bacterial strains using 0.1% DM-AgNPs/PP film; while 0.4% of DM-AgNPs in PP film resulted in a remarkable activity against E. coli and S. aureus. 17 ACS Paragon Plus Environment

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Scheme 1. (A) Schematic representation of E. coli or S. Aureus approached with DM-AgNPs/PP nanocomposite substrate (A1), and further illustrating the release of Ag ions from DMAgNPs/PP nanocomposite substrate by contacting with bacterial cell membrane, followed by cell death (A2). (B) Schematic representation showing the food simulant solution approaching with DM-AgNPs/PP nanocomposite substrate (B1), and following migration of Ag ions into the food simulant solution (B2).


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Table 1. Antibacterial activities of DM-AgNPs/PP film against E. coli, and S. aureus. Average number of colonies after exposure to 24 h

Samples

R(%)

Ut

At

PP (control)

3.0x105

-

-

E. coli

0.05% DM-AgNPs/PP

-

2.6x105

13.3

(ATCC8739)

0.1% DM-AgNPs/PP

-

1.3x105

56.7

(CFU/piece)

0.4% DM-AgNPs/PP

-

0

100

0.7% DM-AgNPs/PP

-

0

100

PP (control)

2.6x105

-

-

S. aureus

0.05% DM-AgNPs/PP

-

1.1x105

57.7

(ATCC6538)

0.1% DM-AgNPs/PP

-

7.1x104

72.7

(CFU/piece)

0.4% DM-AgNPs/PP

-

4.0x104

84.6

0.7% DM-AgNPs/PP

-

2.7x104

89.6

We have calculated the antibacterial efficiency (rate) using below equation (2);

R = (Ut-At)/Ut×100%

(2)

Here, R is the antibacterial activity rate (%); Ut is the average number of colonies for control sample recovered after exposure to 24 h (CFU/piece); At is the average number of colonies for tested sample recovered after exposure to 24 h (CFU/piece); It is understanding that higher R value denotes the better antibacterial property of the applied system. Notably, 0.4% of DM19 ACS Paragon Plus Environment

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AgNPs in PP film showed the best antibacterial performances towards both microbes i.e., 100% inhibition rate for E. coli and 84.6% for S. aureus; this difference is due to the following reason. It is well known that the cell walls of Gram-positive and Gram-negative microbes are different in their physiology.18 Principally, peptidoglycan is a mesh-like thick layer in the plasma membrane of Gram-positive bacteria, which forms a thicker cell walls than Gram negative ones. Such thicker cell wall of S. aureus was playing a vital protecting role for the cell from penetration of Ag into the cytoplasmic membrane. For E. coli, the Ag ion can easily penetrate the cell wall and subsequently interaction with the thiol groups of bacteria proteins, which facilitating the release of oxygen reactive species.18, 54 Prolong interaction of Ag ions with thiol groups of bacteria, the DNA loses its replication capability and the proteins were inactivated.18 Thus the inhibition of respiratory enzymes by AgNPs leads to the formation of oxygen reactive species such as hydroxyl radicals, superoxide, and hydrogen peroxide through oxidative decomposition process of the cellular components, resulting damage to the proteins and followed by cell death (Scheme 1, Route-A). It is worthwhile to compare our results with literatures describing incorporation of AgNPs into polymer films.32, 55, 56, 57

Migration of Ag in food simulants We have systematically investigated the relationship between the migration of Ag vs. temperature and time in food simulants such as water, 20% ethanol, n-hexane, and 4% acetic acid (Figure S3; supporting information). Under optimized conditions, the migration amount of Ag in four food stimulants is given in order; 4% acetic acid > n-hexane > 20% ethanol > water, which means that acid food > oily food > alcoholic food > water food. Particularly, Ag rapidly dissolved in 4% acetic acid and increase in temperature enhanced the solubility rate, thus the 20 ACS Paragon Plus Environment

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concentration of Ag in acid simulant was much higher than other simulant analogs. Notably, dissolution of propylene from DM-AgNPs/PP nanocomposite in n-hexane released Ag contents rapidly. Ethanol is an organic solvent, but it can be a polar protic substance like water, thus nano Ag has relatively low solubility in water, and ethanol.

Figure 5. Calibration plots showing the migration amount of Ag in the food simulants, water (A), 20% ethanol (B), n-hexane (C), and 4% acetic acid (D) at 20℃, 40℃, and 50℃. The time account was referred as 1, 2, 3, 5, and 10 days. According to the Fick’s laws of diffusion, the amount of diffusion is directly related to temperature and time. Thus, the temperatures such as 20℃, 40℃ and 50℃ were selected for comparative Ag migration performances using food simulant solution, and the obtained results are shown in Figure 5. As can be seen from the plots, the relationship between the migration amount of Ag and time in the food simulants were gradually increased for all the food simulants 21 ACS Paragon Plus Environment

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at different temperature conditions (Scheme 1, Route-B). Based on the plots, we have estimated Ag migration values ranging between 1.8 and 24.5 µg/cm2, and the lowest migration limit was found to be 0.36 µg/cm2, which is comparable with the reported literatures.58, 59, 60, 61

Conclusions In summary, we have synthesized DM-AgNPs/PP nanocomposite substrate by a simple insitu melt blending method with good thermal stability, melting and crystallization properties. The DM-AgNPs/PP nanocomposite shows antibacterial activity toward E. coli, and S. aureus via release of oxygen reactive species and Ag ion diffusion mechanism; thus, the inhibition rates enhanced and obtained as 100% and 84.6%, respectively, which is higher than the conventional AgNPs. Next, the migration study displayed significant food simulant property for food packing application in the following order; acid food > oily food > alcoholic food > water food. The total Ag migration values were ranging between 1.8 and 24.5 µg/cm2, and the lowest migration limit was found to be 0.36 µg/cm2. In the follow-up work, biotoxicity analysis of migratory nanoparticles will be simulated to further improve the biosafety research using nano additives in food contact materials. Thus, the DM-AgNPs/PP nanocomposite is considered as potential candidate, and capable of attracting diverse biological/clinical and industrial applications.

Acknowledgments We gratefully acknowledge the Natural Science Foundation of China (Grant Nos. 51873222, 51473179, 81773483, 41576098), Natural Science Foundation of Zhejiang Province, China (Grant No. LY13E01004) Fujian province-Chinese Academy of Sciences STS project (2017T31010024) and Youth Innovation Promotion Association of Chinese Academy of Science (2016268). 22 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

Supporting Information DSC melting and crystallization curves of PP film and DM-AgNPs/PP nanocomposites; Temperature dependent storage modulus, loss modulus, and Tan δ of DM-AgNPs/PP nanocomposites; Bar plots of Ag migration in the food simulants; TGA analysis of DMAgNPs/PP nanocomposites under N2 and oxygen (air) atmospheres.

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Table of Content


The Enhanced Antibacterial and Food Simulant Activities of Silver

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