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Department of Chemistry, Indian Institute

Shibpur, Howrah-711103, West Bengal, Ind

2668 6464

† Electronic supplementary information (and treated charcoal. See DOI: 10.1039/c4

Cite this: RSC Adv., 2014, 4, 30162

Received 2nd May 2014Accepted 18th June 2014

DOI: 10.1039/c4ra04022h

www.rsc.org/advances

30162 | RSC Adv., 2014, 4, 30162–3016

Involuntary graphene intake with food andmedicine†

Manav Saxena and Sabyasachi Sarkar*

Graphene is found in charred roasted meat and also in plant charcoal, which is present in the infant's gripe

water. Graphene as graphene oxide (GO) is produced on charring the surface of meat on a barbecue

forming nitrogen doped GO originating from the pyrolysis of protein in air. The unintentional intake of

such nano carbon stained cooked meat is a traditional delicacy. The presence of graphene and nano

carbon particles in plant charcoal, used in the branded formulation of gripe water applied for stomach

ailments in infants as a medicine, strongly refutes the toxicity of such nano carbons in humans. The

isolation of graphene and nano particulates from both the sources is described here. These are

characterized by elemental analysis, Raman spectroscopy, PXRD and by FESEM, TEM and SADP image

analysis. The intake of nano carbon contaminated roasted food since the discovery of fire possibly trails

mutation to evolve resistance in humans. This work suggests that graphene and other nano carbon

particles produced by the pyrolysis of bio-products in air are non-toxic to humans.

1. Introduction

The benecial or deleterious effect of nano-materials present inour ecosystem has been debated. Such issues have gainedimportance because of the recent rise in nano-materialresearch.1–6 One aspect of nano material–human interactionmay be exterior as external application7,8 that may be readilyavoided if found harmful. However, interior involvement ofnano-materials may have serious consequences, which can onlybe traced by in vitro or by in vivo studies.1,5,6 Current research isaimed at developing biocompatible nano-materials to avoidpossible adverse effects.9,10 However, the question still remainsas how long these are in trial use to justify them as absolutelysafe by current health protocols. Such application virtuallyrequires an in depth investigation similar to what is required fora new drug to conrm its safety. Therefore, the introduction ofany new nano-material for the benet of humans requiresseveral queries to be answered that may involve a long waitingtime for systematic procedural tests to conclude. Further, ‘safeuse’ is a relative term in the time domain and any adverse effectmay crop up even aer several decades of use.

Carbon nanomaterials have emerged as alternate biocom-patible materials. The fundamental queries regarding theproperties of these carbon nanomaterials lie in their safe in vivoapplications.9,10 However, to waive off the toxicity menace ofthese nano-materials, the demonstration of the interaction of

of Engineering Science and Technology

ia. E-mail: abya@iitk.ac.in; Tel: +91 33

ESI) available: S1, S2 SEM image of rawra04022h

7

carbon nano-materials with a bio-system for an extended periodof time is required. Humans explore newer rituals and in theseprocesses have gained enough observation. In search of such aritual we focused on the food roasted on an open re. Barbecueremains synonymous to roasted food since the inception of rein the Mesolithic age where roasting on an open re wasinvariably associated with some charring on the uneven surfaceof meat (Fig. 1). Therefore, our investigations were focussed onprocedures such as the barbeque. In addition, we extended oursearch for nano carbon materials in plant charcoal present in‘Colic Calm’ branded gripe water (NDC 13992-001-01), USA,which is generally used for stomach ailments in infants.11

Roasted meat is one of the preferred delicacies with a smokyaroma. Here, we show that the charred parts of the roasted meatcontain graphene in the form of graphene oxide along withnano carbon particles. Interestingly, elemental analyses of suchcarbogenic materials show the presence of an appreciablequantity of nitrogen. Under mild treatment with dilute mineralacid the graphene oxide aggregates undergo layer separationyielding heteroatom doped graphene oxide (GO) sheets. Thepresence of graphene oxide in the charred parts of animalproteins suggest that no special conditions are required for itssynthesis as considered earlier.12 Earlier the formation of only

Fig. 1 The charred parts (yellow circles) on the surface of roastedchicken.

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nano carbon particles but no graphene have been shown duringthe pyrolysis of vegetable materials.13,14 We specically exploredthe content of plant charcoal that is added in the medicallyapproved ‘Colic Calm’ branded baby gripe water. Such charcoalis used to achieve relief from gas, indigestion and abdominaldistension with cramping pains in infants.11 To our surprise wecould identify the presence of graphene in Colic Clam.

2. MethodsA Charred part experiment

In a typical experiment, freshly prepared dressed chickenprocured from the local market was used directly for roasting onan open re without the application of external oil, butter orspices to simulate the mesolithic period of cooking. Aerroasting, the charred parts were mechanically separated outfrom the surface of the roasted chicken, crushed in a mortar–pestle and sieved to separate the carbon powder from anyadhered cooked meat. The separated charred part was dividedinto two parts.

Experiment 1. One part of the charred material was chewedin the mouth for several minutes to allow salivary enzymes towork and then the mass was transferred into a beaker. Theslurry mass was washed with distilled water twice and nally 20mL HCl (pH¼ 2) was added into it to simulate stomach pH. Themixture was stirred at 50 rpm speed for 3 h followed by soni-cation for 15 min and nally the dispersed material was allowedto settle. The liquid containing the dispersed carbon wasltered using a cellulose acetate membrane lter of 0.1 mmpore-size and the carbon mass was washed with distilled waterto collect it as digested charred carbon and subjected tocharacterization.

Experiment 2. The charred powder was repeatedly washed bya Soxhlet extractor with petroleum ether followed by acetone toremove any soluble organic matter formed during the roastingof the meat. The washed charred carbon was dried in air andlabelled as ‘raw charred carbon’. The raw charred carbon wastreated in HNO3 : H2O (1 : 1) for 3 h at room temperature andthe excess acid was removed by the evaporation of the mixtureusing a boiling water bath.15 The obtained yellow-brown masswas dried in air and termed as ‘treated charred carbon’.

B Baby's gripe water

10 mL of branded Colic Calm baby's gripe water was removedaer shaking the bottle well. The content was centrifuged andthe residue was washed repeatedly three times with distilledwater. The washed charcoal was dried in air as ‘raw charcoal’.For acid treatment of the raw charcoal a similar procedure wasfollowed as described above, and the obtained mass was termedas ‘treated charcoal’.

Fig. 2 Representation of the reaction to mimic the effect of acid onthe charred parts of meat in the stomach.

3. Characterization

The eld emission scanning electron microscopic (FESEM)study was carried out using a SURA 40VP eld emission scan-ning electron microscope (Carl Zeiss NTS GmbH, Oberkochen,

This journal is © The Royal Society of Chemistry 2014

Germany) in high vacuum mode operated at 10 kV. Trans-mission electron microscopy (TEM) analysis was carried outusing a EI Technai 20 U twin transmission electron microscopeoperated at 200 kV using a 400 mesh size carbon coated coppergrid to deposit sample procured from Electron MicroscopySciences, Hateld, PA. For FESEM and TEM images the sampleswere dispersed in isopropanol. Powder X-ray diffraction (PXRD)was recorded on a Bruker AXS diffractometer with Cu-Ka (l ¼1.54 A) radiation using PANalytical X'Pert HighScore Plus so-ware. Raman spectra were recorded on a Raman spectrometer,WITEC MODEL, with 514.5 nm laser excitation. Elementalanalyses for carbon, hydrogen and nitrogen were performedusing a Perkin-Elmer 2400 micro-analyzer.

4. Results

The charred carbon was chewed in the mouth to allow theinteraction with the enzymes present in saliva. The chewedcarbon instead of swallowing was taken out and treated withdilute HCl pH (�2) to mimic digestion in the stomach.16

1 Chewed charred part

Salivary glands in the mouth secrete an array of enzymes andsubstances mainly lingual lipase, amylase, mucin and lysozymeto pre-treat the food to facilitate digestion.16 The charredcarbonaceous part from meat was chewed to allow its interac-tion with the saliva, and the saliva treated carbon mass wassubjected to acid hydrolysis, as shown schematically in Fig. 2.

The FESEM of the washed residue of the chewed part termedas ‘digested charred carbon’ aer acid hydrolysis is shown inFig. 3a. It conrms the presence of a sheet type structure ofcarbon along with nano particles. Fig. 3b shows the high reso-lution image of the white dotted area in Fig. 3a. The TEM image(Fig. 3c) corroborates the presence of thin sheet type structuressimilar to the FESEM image. The black dots present in the TEMimage are due to the presence of carbon nano particles. TheRaman spectrum of the digested chewed part shows theappearance of D- and G- bands at 1357 and 1570 cm�1 due tothe presence of sp3 and sp2 hybridized carbon, respectively(Fig. 3d).

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Fig. 3 Acid digested chewed charred carbon (a) FESEM image. (b)High resolution FESEM image of the area marked in (a). (c) TEM imageand (d) Raman spectrum.

Fig. 4 FESEM and TEM images of: (a and d) raw charred carbon; (b, cand e) treated charred carbon. (f) SADP of (e). Inset of (f) is the dif-fracted intensity taken along the line.

Table 1 Analysis of charred parts of meat and plant charcoal

Elemental analysis of different samples

%C %N %H %Oa

Raw charred part 55.84 9.44 4.16 30.56Treated charred part 48.94 9.53 4.31 37.22Raw charcoal 89.44 1.02 7.59 1.95Treated charcoal 83.36 1.92 8.03 6.69

a Calculated by difference.

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The characterization of saliva treated charred parts aer acidhydrolysis led to sheet type structures with sp2 and sp3 carbonatoms in the network, which were well separated on acidictreatment.

2 Treated charred part

The FESEM image (Fig. 4a) of the raw charred carbon shows thepresence of multi-layered graphene sheets under turbostraticstate along with carbon nanoparticles. Elemental analyses(Table 1) of the raw charred carbon show the presence of N,around 9.53%, suggesting that these carbons are doped with ahigh percentage of nitrogen atoms. The corresponding TEMimage (Fig. 4d) conrms the presence of the multi-layered sheettype structure. Fig. 4b and c are the FESEM images of the treatedcharred carbon showing the presence of semi-transparent totransparent graphene oxide (GO) sheets of large area along withcarbon nanoparticles. The GO sheets shown in Fig. 4c aretransparent and visible only due to its appearance in a crumpleform. The transparent GO sheets appear to create a blanketcover over the carbon nano particles (Fig. 4c). The TEM image ofthe treated charred carbon shown in Fig. 4e conrms thepresence of transparent GO sheets along with carbon nano-particles (dense part below the triangular area). The lightestpart in the triangular area of Fig. 4e shows the presence of asingle layer of GO sheet. The SADP of the treated charred carbon(Fig. 4f) is characteristic of a carbon structure with hexagonalsymmetry. The inset of Fig. 4f shows the diffracted intensity ofbright spots in SADP along the line. The SADP with a brighterdiffraction intensity of the spots in the inner circle is charac-teristic of the single layered graphene (Fig. 4f and inset).17

Therefore, the FESEM, TEM and SADP of the treated charredcarbon support the separation of the layers of turbostratic gra-phene sheets simply by treatment with dilute nitric or hydro-chloric acid. The presence of heteroatoms in the turbostraticstate readily separates the layers under dilute acid treatment.

30164 | RSC Adv., 2014, 4, 30162–30167

In the PXRD of raw charred carbon (Fig. 5a), the peaks at 2q¼ 21.3, 28.44, 29.43 and 31.72� correspond to the d-spacing ¼4.17, 3.13, 3.02 and 2.82 A, respectively. The d-spacing of 4.17 Ais indicative of a lower degree of crystallization18 and the higherinterplanar distance related to the stacking of disordered gra-phene sheets.18 In the PXRD of treated charred carbon (Fig. 5b),the peaks at 2q ¼ 14.38, 20.80, 26.67, 29.50 and 30.28� corre-spond to the d-spacing of 6.17, 4.26, 3.34, 3.02 and 2.95 A,respectively. Here, the new peaks at 2q of 14.38 and 14.92� wereobserved on high resolution of the red circled area, as shown inthe inset of Fig. 5b, and correspond to the d-spacing of 6.17 and5.93 A, respectively. The higher d-spacing (6.17, 5.93 and 4.26)conrms the separation of layers as independent sheets in thetreated charred carbon supporting the FESEM and TEM studies,

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Fig. 5 PXRD of (a) raw and (b) treated charred carbon (the inset is thezoomed image of the red circled area). Raman spectrum of (c) raw and(d) treated charred carbon.

Fig. 6 (a) Gripe water. FESEM and TEM image of: (b and d) rawcharcoal, and (c and e) treated charred carbon, respectively. (f)SADP of (e).

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as shown in Fig. 4. The peaks at 14.38 and 14.92� are at differentpositions than the GO only peak position. The presence ofheteroatoms in the sheets with a change in chemical structuremight be responsible for such shiing in the peak positions.19

In the Raman study of the raw charred carbon (Fig. 5c), thecharacteristic D, G and 2D bands are positioned at 1345, 1586and 2695 cm�1, respectively. For the treated charred carbon(Fig. 5d), the D, G and 2D bands are at 1367, 1590 and 2730cm�1 respectively, which are shied toward the higher wavenumber when compared with those of the raw charred carbon.The 2D band in the treated charred carbon is unexpectedlybroad with the maxima at 2730 cm�1. This broadening isbecause of the uorescent nature of the treated charred carbondue to the presence of uorescent GO and carbon nano particles(not shown).

Fig. 7 The PXRD of (a) raw and (b) treated charcoal. Raman spectrumof (c) raw and (d) treated charcoal.

3 Charcoal from baby's gripe water

Colic Calm branded baby's gripe water contains black sus-pended plant charcoal (Fig. 6a). Fig. 6b is the FESEM image ofthe raw charcoal, in which the sheet type structure marked witha white dotted line and nearby ribbon type structures were dueto disordered (turbostratic) graphene oxide. The arrow shows anindentation present on the brass stub used for the study, wherea part of the graphene oxide sheet was deposited. The TEMimage of raw charcoal (Fig. 6d) conrms the presence of agraphene oxide sheet. The disordered layers of graphene oxidepresent in raw charcoal upon dilute acid treatment undergoseparation to yield crumbled GO sheets, as shown in Fig. 6c. TheTEM image of treated charcoal (Fig. 6e) conrms the presenceof highly transparent GO sheets. The SADP (Fig. 6f) of Fig. 6edisplays the typical six-fold symmetry of graphene oxide. Twinspots in this SADP suggest an orientation mismatch at the fol-ded site in Fig. 6e. The higher intensity of the inner circle spotsin the SADP supports the presence of a single layer of GO.17

Along with a graphene structure, amorphous carbon and poly-hedral shaped carbon structures are also present in the raw and

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treated carbon, respectively (Fig. S1 and S2†). The elementalanalyses of raw and treated charcoal (Table 1) from gripe waterconrms the presence of nitrogen in trace amounts, suggestingthat turbostratic graphene from plant sources contains lessnitrogen as it is mainly based on cellulose material incomparison with the turbostratic graphene from meat, which isenriched in proteins.

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Fig. 7a is the PXRD of raw charcoal (from gripe water) havingpeaks at 2q of 8.61, 16.50 and 22.65�, which corresponds to thed-spacing of 10.20, 5.37, and 3.92 A, respectively. In the PXRD oftreated charcoal, the peaks shied towards the lower angle at 2qof 8.55, 15.92 and 22.16� corresponding to the d-spacing of10.33, 5.55 and 4.00 A, respectively (Fig. 7b). The shiing of thePXRD peaks suggests an increase in the d-spacing and thepresence of loosely bound layers in the raw charcoal, whichupon acid treatment undergo separation, as shown in Fig. 6(cand e). The much higher inter-layer spacing in the raw plantcharcoal suggests the presence of spaced graphene sheets in theplant charcoal, as suggested earlier.20–25

In the Raman study of the raw charcoal (Fig. 7c), the char-acteristic D and G bands were positioned at 1335 and 1587cm�1, respectively, which shi to a higher wave number and arepositioned at 1347 and 1598 cm�1 for treated charcoal (Fig. 7d).Such shis are similar to that observed previously for charredcarbon. The 2D band is similar, as reported earlier.26,27

5. Discussion

Meat contains mainly proteins, which upon heating undergoMaillard caramelization and polymerization reactions toproduce heteroatom containing cyclic compounds.28–34 Undercharring conditions, the ve membered hetero rings may rear-range to six membered rings35 followed by condensation togenerate graphene units doped with heteroatoms at the edgesand basal planes for stability.35–37 In addition, peripheral oxogroups are introduced at the edges of the graphene layer duringcarbonization in air.20 The heteroatoms at the edges or in thebasal planes mismatch the standard interlayer spacing ingraphite to yield the turbostratic form.20 The elemental analysesof the charred materials of meat and plant charcoal are tabu-lated in Table 1. The presence of a signicant amount of Nsupports the presence of nitrogen doped graphene oxide sheetsespecially from charred meat. However, note that the materialssubjected to elemental analysis does contain carbon nanoparticles in part and therefore the absolute value of nitrogenanalysis should be taken with caution and the possibility for theexistence of the nitrogen doped carbon nano particles may notbe ruled out.

The disordered state of graphene in plant charcoal could berelated to the formation of turbostratic graphene similar to thecase of natural re in forests.21–25 The interplanar distance ofgraphite is not achieved because of the formation of heteroatomfunctionalities, which prevents the close packing of sheets.20

The formation of a graphitic structure requires a temperature ofaround 3500 �C,20 which is signicantly higher than thetemperature used in roasting the meat and also in the synthesisof plant charcoal. The interaction of chewed parts with diluteHCl is not ideally akin to the natural process in digestion butprovides a close chemistry regarding the fate of the charredparts in the human stomach. The FESEM, TEM of the chewedcharred part aer standard cleaning (Fig. 3a–c) conrms thepresence of a layer of graphene sheets doped with heteroatoms,as conrmed by the elemental analyses (Table 1). The graphenesheets present in loose bundles are formed due to heteroatom

30166 | RSC Adv., 2014, 4, 30162–30167

incorporation, which are separated out readily to single layerGO sheets upon treatment with dilute HNO3 or HCl acid, asshown in Fig. 4 and 6(c and e). The intensity of the bright spotsin SADP (Fig. 4 and 6(f)) conrms the presence of a single layerof GO sheet.17 Such single layer sheet separation of hetero GOsheets on treatment with dilute acid is noteworthy because thesynthesis of GO from graphite requires laborious and harshacidic conditions.19,38–41

6. Conclusion

The practice of barbecuing food can be traced back to thediscovery of re. This study revealed the existence of hetero-atoms doped graphene oxide with carbon nano particles in foodprepared by barbecuing. Surprisingly, graphene derivatives andcarbon nano particles are also present in plant charcoal used inbrand medicine approved for infants to cure stomach ailments.The evolution of humans since the Mesolithic age parallel to thediscovery of re has continued without showing any ill effectcaused by the intake of such an inadvertent contaminant inroasted food. If mutation occurred under the stress of such foodintake in human, our evolution has acquired full immunityagainst the use of such nano carbon materials.

Acknowledgements

M.S. acknowledges the University Grant Commission, India forDr D. S. Kothari Post Doctoral Fellowship. S.S. acknowledgesCromoz Inc., USA for nancial support for the study and aRamanna Fellowship from DST, New Delhi to sustain research.Both the authors thank Master Sagneic Biswas of Boston, USAfor loaning Colic Calm gripe water used in this study.

Notes and references

1 C. Buzea, I. I. Pacheco and K. Robbie, Biointerphases, 2007, 2,MR17.

2 E. L. K. Chng, Z. Sofer and M. Pumera, Chem. – Eur. J., 2014,20, 6366.

3 D. B. Warheit, Toxicol. Sci., 2008, 101, 183.4 W. H. De Jong and P. J. Borm, Int. J. Nanomed., 2008, 3, 133.5 N. Lewinski, V. Colvin and R. Drezek, Small, 2008, 4, 26.6 J. Wang, Z. Hu, J. Xu and Y. Zhao, NPG Asia Mater., 2014, 6, 1.7 N. Sozer and J. L. Kokini, Trends Biotechnol., 2009, 27, 82.8 L. A. DeLouise, J. Invest. Dermatol., 2012, 132, 964.9 M. Ghosh, S. K. Sonkar, M. Saxena and S. Sarkar, Small, 2011,7, 3170.

10 S. K. Sonkar, M. Ghosh, M. Roy, A. Begum and S. Sarkar,Mater. Express, 2012, 2, 105.

11 http://www.coliccalm.com/gripe-water.htm.12 G. Ruan, Z. Sun, Z. Peng and J. M. Tour, ACS Nano, 2011, 5,

7601.13 M. P. Sk, A. Jaiswal, A. Paul, S. S. Ghosh and

A. Chattopadhyay, Sci. Rep., 2012, 2.14 M. Saxena and S. Sarkar, Mater. Express, 2013, 3, 201.15 M. Saxena and S. Sarkar, Diamond Relat. Mater., 2012, 24, 11.

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16 H. R. Mahler and E. H. Cordes, in Biological Chemistry,Harper and Row, Publishers, Inc., N. Y, 2nd edn, 1971, p.762.

17 Y. Hernandez, et al., Nat. Nanotechnol., 2008, 3, 563.18 Y. Wu, et al., Nano Res., 2010, 3, 661.19 D. C. Marcano, et al., ACS Nano, 2010, 4, 4806.20 A. Downie, A. Crosky and P. Munroe, in Biochar for

Environmental Management:science and technology, ed. J.Lehmann and S. Joseph, Earthscan, VA, USA, 2009, pp. 17–18.

21 I. Cohen-Ofri, R. Popovitz-Biro and S. Weiner, Chem. – Eur. J.,2007, 13, 2306.

22 R. Qadeer, J. Hanif, M. Saleem and M. Afzal, J. Chem. Soc.Pak., 1994, 16, 229.

23 R. C. Bansal, J. B. Donnet and F. Stoeckli, Active Carbon, ed.Marcel Dekker, New York, NY, 1988.

24 J. Biscoe and B. Warren, J. Appl. Phys., 1942, 13, 364.25 B. E. Warren, Phys. Rev., 1941, 59, 693.26 G. Eda, et al., J. Mater. Chem., 2011, 21, 11217.27 W. Chen, L. Yan and P. Bangal, J. Phys. Chem. C, 2010, 114,

19885.28 W. Ni, L. McNaughton, D. M. LeMaster, R. Sinha and

R. J. Turesky, J. Agric. Food Chem., 2007, 56, 68.

This journal is © The Royal Society of Chemistry 2014

29 P. Mottier, V. Parisod and R. J. Turesky, J. Agric. Food Chem.,2000, 48, 1160.

30 E. Widmark, Nature, 1939, 143, 984.31 C. Perez, A. L. de Cerain and J. Bello, Mutagenesis, 2002, 17,

63.32 P. Issenberg and A. O. Lustre, J. Agric. Food Chem., 1970, 18,

1056.33 J. A. Maga, J. Agric. Food Chem., 1986, 34, 249.34 M. J. Friedman, J. Agric. Food Chem., 1996, 44, 631.35 J. Pels, F. Kapteijn, J. Moulijn, Q. Zhu and K. Thomas,

Carbon, 1995, 33, 1641.36 Y. N. Sazanov, A. Gribanov and V. Lysenko, Fibre Chem.,

2008, 40, 355.37 H. Marsh, in Carbon and Coal Gasication, ed. J. L.Figueiredo

and J. A. Moulijn, Martinus Nijhoff, Dordrecht, 1986, vol.105, p. 27.

38 B. C. Brodie, Philos. Trans. R. Soc. London, 1859, 149, 249.39 L. Staudenmaier, Ber. Dtsch. Chem. Ges., 1898, 31, 1481.40 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958,

80, 1339.41 H. P. Bohem, A. Clauss, G. O. Fischer and U. Hofman, Z.

Anorg. Allg. Chem., 1962, 316, 119.

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