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Research Article

Synthesis of multi‑walled carbon nanotubes (MWCNTs) from plastic waste & analysis of garlic coated gelatin/MWCNTs nanocomposite films as food packaging material

Akshata Pattanshetti1 · N. Pradeep1 · V. Chaitra1 · V. Uma1

Received: 4 October 2019 / Accepted: 6 March 2020 © Springer Nature Switzerland AG 2020

AbstractPlastic waste is considered as major source of air, water and soil pollution, due to its non-biodegradable nature. Its direct disposal has negative impact on the environment. So here we present nanotechnological aid to solve the problem of plastic waste. As carbon nanotubes offer wide range of applications, conversion of plastic waste into high value carbon nanotubes (CNTs) could be a beneficial option. Multi-walled carbon nanotubes (MWCNTs) were synthesized from waste plastic bottles using T-CVD method. Synthesized MWCNTs were characterized using XRD, SEM and EDAX. The CNTs were found to be of 30–45 nm in diameter. Garlic microparticles were successfully synthesized using planetary ball milling and characterized using SEM. Gelatin/MWCNT nanocomposite films with varying concentration of MWCNTs (1%, 2% and 3%) were prepared. The improved water and oil resistance as well as antibacterial study suggest that the prepared nanocomposite films can be used as potential food packaging material. Coating these nanocomposite films with garlic microparticles not only enhances their antibacterial activity but also can avoid the interaction of food with MWCNTs and solve the problem of the migration.

Keywords Plastic waste · MWCNTs · Nanocomposite · Garlic microparticles · Gelatin · Food packaging

List of symbolsMWCNT Multi walled carbon nanotubesCNTs Carbon nanotubesT-CVD Thermal chemical vapour depositionXRD X-ray diffractionSEM Scanning electron microscopeEDAX Energy dispersive X-ray spectroscopyMW Molecular weightWVP Water vapour permeabilityPET Polyethylene terephthalateHCl Hydrochloric acidPVDP Polyvinylidene fluorideNMP N-Methyl-2-pyrrolidone

1 Introduction

Plastic waste (mainly made of polymeric carbon containing materials) is considered as the major source of air, water, soil and marine pollution [1]. It is non-biodegradable and has non-dissociation property. Thin plastic bags have no recyclable value and are disposed by public directly. It is used by the shopkeepers and vendors for packaging pur-poses, as

it is cheap. The lifespan of plastic waste on earth can be thousands of years before biodegradation or oxidation is complete [1]. Therefore it is necessary to have mechanism in place for regulating its use, recycling and disposal to remove the plastics from the environment.

The conversion of plastic waste into valuable and sig-nificant products such as carbon nanotubes (CNTs) could

* Akshata Pattanshetti, akshatapattanshetti10@gmail.com; * V. Uma, umamcciyer@gmail.com | 1Department of Nanoscience and Technology, Mount Carmel College, Bangalore 560052, India.

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be a great and beneficial option for industry as well as environmental protection. Due to its high tensile strength, mechanical, electrical and chemical properties, CNTs pro-vide a great ability for various applications ranging from textile, electronics, actuator to biomedical [2]. CNTs can be synthesized by various methods such as arc discharge [3], pyrolysis [4], laser ablation [5], plasma assisted deposi-tion [6] and thermal chemical vapor deposition [7]. Among them, chemical vapor deposition (CVD) is most widely used method due to its simple instrumentation, easy pro-cessing, economical viability and capability of producing large yield of CNTs in short time.

Gelatin (a denatured collagen peptide with MW = 100 kDa) is a soluble protein obtained by partial hydrolysis of collagen, which is the main insoluble fibrous protein constituent of bones, cartilages and skins. It has potential applications in food and pharmaceutical indus-tries due to its gel/film forming properties [8]. Due to its highly hydrophilic nature, gelatin shows poor mechani-cal as well as barrier properties. Thus, the current trends in designing gelatin-based biodegradable materials for packaging and biomedical applications are focused on developing the films with improved mechanical and water resistance properties by combining gelatin with other biopolymers, synthetic polymer, nanofiller, plasticizer as well as cross linking agents [9–11].

Incorporating nanofillers into gelatin has been the topic of much research, among which the carbon nano-tubes have gained much attraction. Nano packaging is being designed to enable materials to interact with food, exhibiting antibacterial and antioxidant properties, and to alter the properties of food, including its texture, heat tolerance and shelf life [9]. Gelatin-based nanocomposite films reinforced by low concentrations of MWCNTs can be synthesized in order to obtain improvements in water sol-ubility, water swelling, water uptake, water vapor perme-ability (WVP), tensile strength, elongation at break, Young’s modulus and antibacterial properties. These nanocompos-ite films can be used in food and biomedical applications as an alternative to conventional synthetic polymer [12].

Garlic (Allium sativum L.) which belongs to the liliaceae family contains carbohydrates, proteins, free amino acids, phenolic compounds, fiber, minerals, saponins, moderate levels of selenium, vitamin A, C and B complex. Addition-ally it is an important source of antioxidants [13]. Garlic has been used for centuries in various societies to combat infectious disease. More recently, garlic has been proven to be effective against a plethora of gram-positive, gram-negative, and acid-fast bacteria. These include Salmonella, Escherichia coli, Pseudomonas, Proteus, Staphylococcus aureus, Salmonella, Klebsiella, Micrococcus, Bacillus subtu-lis, Clostridium, Mycobacterium and Helicobacter. The anti-bacterial activity of garlic is widely attributed to allicin. It is

known that allicin has sulfhydryl modifying activity and is capable of inhibiting sulfhydryl enzymes [14]. Allicin anti-microbial action is due to its activity as a strong inhibitor of some enzymes such as cysteine- proteases and alcohol dehydrogenases, which are responsible of the infections caused by bacteria, fungi and virus [15]. Many fungi are sensitive to garlic, including Candida, Torulopsis, Tricho-phyton, Cryptococcus, Aspergillus, Trichosporon, and Rhodotorula. Garlic extracts have been shown to decrease the oxygen uptake, reduce the growth of the organism, inhibit the synthesis of lipids, proteins, and nucleic acids and damage membranes [14]. Garlic can be available in various forms such as powder, extracts and essential oil. Garlic particles were obtained by the reduction and selec-tion method using maceration and sieve technique [16]. Incorporation of garlic microparticles can enhance the antibacterial activity of the packaging material.

In this report, multi-walled carbon nanotubes (MWC-NTs) are synthesized using waste plastic as precursor using chemical vapor deposition (CVD) method and were char-acterized using XRD and SEM. These synthesized MWCNTs were used as nanofillers to develop the gelatin/MWCNT nanocomposite films. The water solubility, water swelling, oil swelling and antibacterial properties of these films were examined [12]. Garlic microparticles are synthesized using ball milling technique. As toxicity of carbon nanotubes appears still controversial in the literature, garlic micro-particles were coated on the prepared gelatin/MWCNT nanocomposite films in order to avoid the interaction of the food with the carbon nanotubes. To the best of our knowledge this is the first work done on the synthesis of garlic microparticles using ball milling technique and coating them on gelatin/MWCNTs nanocomposite films. The antibacterial activity of the prepared gelatin/MWCNT nanocomposite films as well as garlic microparticles were examined.

2 Experimental

2.1 Synthesis of MWCNTs from plastic waste using thermal chemical vapour deposition (CVD) method

For the synthesis of MWCNTs, waste plastic bottles (PET) were collected. These bottles were cut, washed and dried completely. These plastic pieces were melted on crucible at 3000 C for 15 min in muffle furnace. For the synthesis of MWCNTs, ball milled nickel metal powder was used as the catalyst. 10 g of plastic and 1 g of Ni catalyst were placed in two quartz tubes separately. Both these boats were placed inside the CVD chamber in zone 1 and zone 2 respectively. The synthesis process involves heating of the catalyst at

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10,000 ° C and plastic at 8000 °C in the tube furnace. This thermal treatment was performed under controlled flow of inert gas such as Argon Ar (90 ml/min.) and process gas such as Hydrogen H2 (10 ml/min.) for an hour (Fig. 1).

2.2 Purification of synthesized MWCNTS

For the purification of synthesized CNTs, they were taken in the Whatmann filter paper. Diluted hydrochloric acid (HCl) was added to the CNTs in the filter paper. We obtain green coloured solution which indicates the removal of traces of nickel catalyst. Then it was again filtered with dis-tilled water to remove HCl and then followed by air drying.

2.3 Synthesis of garlic microparticles

For the synthesis of garlic microparticles, firstly garlic was purchased from the local market. The garlic cloves were separated. These garlic cloves were peeled off. The peeled garlic cloves were chopped into small pieces. This chopped garlic was dried in microwave at 350 W for 20 min. This dried garlic pieces were then grinded in the mixer grinder

to form the garlic powder. The formed garlic powder is then subjected to planetary ball milling. The garlic powder is placed in the ball milling cylinder along with the balls. The ratio of ball to powder is 10:2. The ball milling was set at 200 rpm and for 5 h. After completion of 5 h of ball mill-ing, garlic powder was converted to garlic microparticles (Fig. 2).

2.4 Preparation of gelatin/MWCNTs nanocomposite films coated with garlic microparticles

For the preparation of the gelatin/MWCNTs nanocompos-ite films, 10 ml distilled water was taken in three beakers each. 1 g of gelatin was added to each beaker. The gela-tin was allowed to completely swell for some time. To the swollen gelatin, different concentrations of MWCNTs (1%, 2% and 3%) were added and mixed properly. These beak-ers were then placed in hot water bath to dissolve the gelatin. The solution of gelatin and MWCNTs was properly stirred. The solution of gelatin and MWCNTs was poured on the aluminium foil placed in the petri dish for film cast-ing. The petri plates were placed at room temperature and the films were allowed to dry. After completely drying, the films were peeled off from the aluminium foil (Fig. 3).

Due to the toxicity issues involved with CNTs, the pre-pared nanocomposite films were coated with garlic micro-particles on one side in order to avoid the interaction of food with CNTs. To form the coat of garlic microparticles on the prepared nanocomposite films, 1 g garlic microparti-cles were taken in the mortar. To this 4% of polyvinylidene fluoride (PVDF) was added and grinded together. To this few drops of N-Methyl-2-pyrrolidone (NMP) are added to form a paste. This paste of garlic microparticles is then coated onto the prepared nanocomposite films using doc-tor blade method (Fig. 4).

Fig. 1 MWCNTs synthesized from plastic waste using T-CVD

Fig. 2 Ball milled garlic microparticles Fig. 3 Film casting of gelatin/MWCNTs solution

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2.5 Water swelling test of gelatin/MWCNTS films

The prepared gelatin/MWCNT films were cut into 2 cm × 2 cm. Each of this cut film was weighted in air-dried condi-tions, which is taken as W1. Then the film is immersed in deionized water (250° C) for 2 min. The wet sample were taken out of water and wiped between the filter paper to remove excess water from the surface of the film. Again the weight of the wet sample was calculated, which is taken as W2. The percentage of weight gaining was taken as swelling test calculated by formula given in Eq. (1) [17].

Equation (1) gives the formula for water swelling test

2.6 Oil swelling test of gelatin/MWCNT films

The prepared gelatin/MWCNT films were cut into 2 cm × 2 cm. Each of this cut film was weighted in air-dried condi-tions, which is taken as W1. Then the film is immersed in vegetable oil (25° C) for 2 min. The wet sample were taken out of oil and wiped between the filter paper to remove excess oil from the surface of the film. Again the weight of the oil dipped sample was calculated, which is taken as W2. The percentage of weight gaining was taken as swelling test calculated by formula given in Eq. (2).

Equation (2) gives the formula for oil swelling test

2.7 Antibacterial activity

The antibacterial activity of the gelatin/MWCNT nano-composite films with 1%, 2% and 3% MWCNT, garlic powder and garlic microparticles was carried out. Their antibacterial activity was tested against Escherichia coli

(1)Water swelling(%) =w2 − w1

w1

100

(2)Oil swelling(%) =w2 − w1

w1

100

(gram negative bacteria) and Staphylococcus aureus (gram positive bacteria). To investigate the antibacterial activity, the disk diffusion method was used. The disks with 1 cm size were cut and coated with garlic powder and garlic nanoparticles. Even the gelatin/MWCNT films were cut accordingly. The bacterial suspensions of E. coli and Staphylococcus aureus were spread on the nutrient agar plates. The prepared disks and films were placed in direct contact with the agar medium. These plates were incubated at 370° C for 24 h. The bare disk without any coating was used as control. After incubation, the clear inhibition zone was observed around the disks and films. The diameters of clear inhibition zones, including the diameter of disk and length of films was measured using scale, and their antibacterial activity was evaluated.

3 Result and discussion

3.1 X‑ray diffraction (XRD) analysis

XRD analysis was carried for pure MWCNT’s. XRD pat-tern of MWCNT is shown in the Fig. 5. The XRD pattern of the MWCNT shows three diffraction peaks at 2θ = 26°, 44° and 53.63° which can be indexed as (002), (100), and (004) reflections of graphite. The prominent peak about 2θ = 260 indexed to (002) indicates the reflection of car-bon [2].

3.2 SEM analysis

The morphology of both MWCNTs and garlic nanoparti-cles were characterized by scanning electron microscope

Fig. 5 XRD pattern of MWCNT. The Major peaks were obtained at 26°, 44° and 53.63° for the hkl (002), (100) and (004) planes

Fig. 4 Garlic coated gelatin/MWCNTs nanocomposite films with (a) 1% MWCNT (b) 2% MWCNT (c) 3% MWCNT

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(SEM). The morphology of MWCNTs obtained from plas-tic waste is shown in Fig. 6. It can be observed that the diameter of the MWCNTs is about 30–45 nm. The tubes were lengthy and coiled together.

The morphology of ball milled garlic microparticles is shown in Fig. 7. The morphology of garlic microparticles were found to be of irregular shape.

3.3 Energy dispersive analysis of X‑rays (EDAX)

EDAX analysis gives the elemental composition of the sam-ple. The EDAX analysis spectrum of MWCNTs is show in Fig. 8. It shows the presence of highest content of carbon (C) that is 86.45%, oxygen (O) content of 6.98% which indi-cates the presence of MWCNTs in the sample. It also shows the presence of gold (Au) which may come from gold coat-ing done during sample preparation for SEM analysis. The elemental composition of MWCNTs is given in Table 1.

The EDAX analysis spectrum of garlic microparticles is shown in Fig. 9. It shows the presence of carbon (C) that is 47.76% and oxygen (O) content of 52.24%. The elemental composition of garlic microparticles is given in Table 2.

3.4 Water swelling capacity of gelatin/MWCNT films

The water swelling capacity of gelatin film was highest that is 186.66%. Incorporation of MWCNTs to the gelatin films caused significant decrease in swelling of the films. The water swelling capacities of nanocomposite film with 1%, 2% and 3% MWCNT was observed to be 160.30%, 150.52% and 116.92% respectively (Table  3). Gelatin, due to its hydrophilic nature, absorb molecules of water.

Fig. 6 SEM image of MWCNTs

Fig. 7 SEM image of garlic microparticles

Fig. 8 Spectrum of MWCNTs by EDAX analysis

Table 1 Elemental composition of MWCNTs

Element Weight (%) Atomic (%)

C K 86.45 93.87O K 6.98 5.69Au M 6.56 0.43Totals 100 100

Fig. 9 Spectrum of garlic microparticles by EDAX analysis

Table 2 Elemental composition of garlic microparticles

Element Weight (%) Atomic (%)

C K 47.76 54.91O K 52.24 45.09

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MWCNTs are hydrophobic in nature. Incorporation of MWCNTs reduced the water swelling capacity of the gela-tin films because of increase in the hydrophobicity (water resistance) of the gelatin matrix. This phenomenon may be due to increase in the cross-linking density between the MWCNTs and gelatin. The hydrophobic domains of gelatin can essentially interact with the nanotubes through hydro-phobic interaction. This event saturates gelatin network with MWCNT, consequently water molecules cannot dif-fuse through the gelatin network, thereby decreasing the swelling capacity [12].

3.5 Oil swelling capacity of gelatin/MWCNT films

The oil swelling capacity of gelatin film was highest that is 5.13%. Incorporation of MWCNTs to the gelatin films caused significant decrease in oil swelling of the films. The oil swelling capacities of nanocomposite film with 1%, 2% and 3% MWCNT was observed to be 4.69%, 3.98% and 2.97% respectively (Table 3). The decrease in the oil swell-ing capacity of the gelatin/MWCNT nanocomposite films might be due to the hydrophobic nature of MWCNTs.

3.6 Antibacterial studies

The results of antibacterial studies of gelatin/MWCNT nanocomposite films, garlic powder and garlic micro-particles are summarized in Table 4. The initial diameter of all films and disks was fixed at 10 mm. The diameters of the clear inhibition zones were used determining the antibacterial activity. According to the results obtained,

the gelatin film with 1% MWCNT showed absence of clear inhibition zone that is no antibacterial activity. Gelatin films with the 2% and 3% MWCNTs showed the inhibition zones of diameter 13 mm and 16 mm for E. coli bacteria respectively and 13 mm and 15 mm for Staphylococcus aureus. Garlic powder and garlic microparticles showed inhibition zones with diameter 18 mm and 20 mm respec-tively for E. coli and 16 mm and 17 mm respectively for Staphylococcus aureus. It was shown that carbon nano-tubes exhibit significant antibacterial activity to E. coli through cell membrane damage and expression of stress-related gene. It was also suggested that carbon nanotubes are relatively flexible and interact with cell membranes and penetrate various microorganisms and consequently cause cell death [12]. The antibacterial activity of garlic is widely attributed to allicin [18]. From the results it can be seen that garlic microparticles showed largest diameter of inhibition zone that is highest antibacterial activity.

4 Conclusions

The novelty of this work is to synthesize garlic micro-particles using ball milling technique and form gelatin/MWCNTs nanocomposite films coated with these garlic microparticles. The water swelling and oil swelling capac-ity of the films were investigated. The nanocomposite films showed decrease in water and oil swelling capacity as compared to the gelatin films. This showed that MWCNTs inhibited the swelling due to its hydrophobic interaction with the gelatin. The lowest water and oil swelling capacity was shown by nanocomposite films with 3% MWCNTs. This shows that as the concentration of MWCNTs increases in the nanocomposite films, its water and oil swelling capac-ity decreases.

All the nanocomposite films, garlic powder and garlic microparticles showed significant antibacterial activities against gram positive (Staphylococcus aureus) and gram negative (E. coli) bacteria. Among the nanocomposite films, the film with 3% MWCNT showed the higher antibac-terial activity. Compared to garlic powder, garlic micropar-ticles showed highest antibacterial activity since the garlic microparticles have large surface to volume ratio. Hence, coating garlic microparticles on the composite films can enhance the antibacterial activity of the films.

In summary, MWCNTs can be synthesized from plas-tic waste which can remove the plastic waste from envi-ronment and can be used for various applications. The improved water and oil resistance as well as antibacterial study suggest that the prepared gelatin/MWCNT nano-composite films can be used as potential food packaging material. Also coating these nanocomposite films with garlic microparticles not only enhances their antibacterial

Table 3 Water swelling and oil swelling of gelatin films incorpo-rated with MWCNTs

Films Water swelling (%) Oil swelling (%)

Gelatin 186.66 5.13Gelatin + MWCNT 1% 160.30 4.69Gelatin + MWCNT 2% 150.52 3.98Gelatin + MWCNT 3% 116.92 2.97

Table 4 Diameter of inhibition zones of gelatin/MWCNTs nanocom-posite films, garlic powder and garlic microparticles

Films/disks Escherichia coli Staphy-lococcus aureus

Gelatin + MWCNT 1% 0 0Gelatin + MWCNT 2% 13 mm 13 mmGelatin + MWCNT 3% 16 mm 15 mmGarlic powder 18 mm 16 mmGarlic microparticles 20 mm 17 mm

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activity but also solves the problem related to toxicity of MWCNTs due to migration. Migration is the unintentional transfer of packaging material into the food. Hence, gar-lic microparticles coat on these nanocomposite films can avoid the interaction of food with MWCNTs and solve the problem of the migration.

Acknowledgements The authors would like to thank Dr. Sister Arpana Principal of Mount Carmel College for allowing us to access the major research lab for experimentation. I would also like to thank Nitisha Mehrotra for her assistance in processing of food packaging films.

Author’s contribution AP completed the paper work as her master’s thesis project. Dr. UV guided the team to work in the direction. PN and CV supervised the work and helped in the data analysis.

Compliance with ethical standards

Conflict of interest The authors declare no conflict of interest.

References

1. Tripathi PK, Durbach S, Coville NJ (2017) Synthesis of multi-walled carbon nanotubes from plastic waste using a stainless-steel CVD reactor as catalyst. Nanomaterials 7(10):284

2. Mishra N, Das G, Ansaldo A, Genovese A, Malerba M, Povia M, Sharon M (2012) Pyrolysis of waste polypropylene for the syn-thesis of carbon nanotubes. J Anal Appl Pyrol 94:91–98

3. Cui S, Scharff P, Siegmund C, Schneider D, Risch K, Klötzer S, Schawohl J (2004) Investigation on preparation of multiwalled carbon nanotubes by DC arc discharge under N2 atmosphere. Carbon 42(5–6):931–939

4. Liu BC, Lee TJ, Lee SH, Park CY, Lee CJ (2003) Large-scale synthe-sis of high-purity well-aligned carbon nanotubes using pyroly-sis of iron (II) phthalocyanine and acetylene. Chem Phys Lett 377(1–2):55–59

5. Kuo TF, Chi CC, Lin IN (2001) Synthesis of carbon nanotubes by laser ablation of graphites at room temperature. Jpn J Appl Phys 40(12R):7147

6. Chhowalla M, Teo KBK, Ducati C, Rupesinghe NL, Amaratunga GAJ, Ferrari AC, Milne WI (2001) Growth process conditions of

vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition. J Appl Phys 90(10):5308–5317

7. Reddy NK, Meunier JL, Coulombe S (2006) Growth of carbon nanotubes directly on a nickel surface by thermal CVD. Mater Lett 60(29–30):3761–3765

8. Rawdkuen S, Sai-Ut S, Benjakul S (2010) Properties of gelatin films from giant catfish skin and bovine bone: a comparative study. Eur Food Res Technol 231(6):907–916

9. Sorrentino A, Gorrasi G, Vittoria V (2007) Potential perspectives of bio- nanocomposites for food packaging applications. Trends Food Sci Technol 18(2):84–95

10. Cao N, Yang X, Fu Y (2009) Effects of various plasticizers on mechanical and water vapor barrier properties of gelatin films. Food Hydrocoll 23(3):729–735

11. Gomes SR, Rodrigues G, Martins GG, Henriques CMR, Silva JC (2013) In vitro evaluation of crosslinked electrospun fish gelatin scaffolds. Mater Sci Eng, C 33(3):1219–1227

12. Kavoosi G, Dadfar SMM, Dadfar SMA, Ahmadi F, Niakosari M (2014) Investigation of gelatin/multi-walled carbon nanotube nanocomposite films as packaging materials. Food Sci Nutr 2(1):65–73

13. Lawrence R, Lawrence K (2011) Antioxidant activity of garlic essential oil (allium sativum) grown in north Indian plains. Asian Pac J Trop Biomed 1(Suppl 1):S51–S54

14. Bayan L, Koulivand PH, Gorji A (2014) Garlic: a review of potential therapeutic effects. Avicenna J Phytomedicine 4(1):1

15. Tsai CW, Chen HW, Sheen LY, Lii CK (2012) Garlic: health benefits and actions. Biomedicine 2(1):17–29

16. Figueroa-López KJ, Torres-Vargas OL, Prías-Barragán JJ, Ariza-Calderón H (2015) Optical and structural characterization of Allium sativum L. nanoparticles impregnate in bovine loin. Acta Agronómica 64(1):54–60

17. Cao N, Fu Y, He J (2007) Mechanical properties of gelatin films cross-linked, respectively, by ferulic acid and tannin acid. Food Hydrocoll 21(4):575–584

18. Harris J, Plummer S, Lloyd D (2001) Antigiardial drugs. Appl Microbiol Biotechnol 57(5–6):614–619

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