curcumin additives loaded on porous carbon adsorbents from

favoured by the highly humid environment.9-11 Many studies on the design and .... nation with complex 2 results in a certain degree of pore blockage...
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Composite materials based on (cymene)Ru(II) curcumin additives loaded on porous carbon adsorbents from agricultural residues display efficient antibacterial activity Riccardo Pettinari, Francesca Condello, Fabio Marchetti, Claudio Pettinari, Mª Isidora Bautista-Toledo, Sergio Morales-Torres, Paul J. Dyson, and Francisco J. Maldonado-Hodar ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00035 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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ACS Applied Bio Materials

Composite materials based on (cymene)Ru(II) curcumin additives loaded on porous carbon adsorbents from agricultural residues display efficient antibacterial activity Riccardo Pettinari,*a Francesca Condello,a Fabio Marchetti,b Claudio Pettinari,a Ma Isidora Bautista-Toledo,c Sergio Morales-Torres,c Paul J. Dyson,d Francisco J. Maldonado-Hódar*c a

School of Pharmacy, Chemistry Section, University of Camerino, Via S. Agostino 1, 62032 Camerino MC, Italy, email: [email protected]; bSchool of Science and Technology, Chemistry Section, University of Camerino, Via S. Agostino 1, 62032 Camerino MC, Italy; cDepartment of Inorganic Chemistry. Faculty of Sciences. University of Granada. 18071. Granada. Spain; dInstitut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland. KEYWORDS: Ru-complexes, curcumin, activated carbon, porous composite materials, antibacterial materials.

ABSTRACT: Alternative composite materials with antibacterial activity have been developed by combining the porosity of carbon materials and the antibacterial activity of half sandwich (cymene)Ru-curcumin complexes. Different types of activated carbon were used, from commercial to synthetic and also ones derived from agricultural waste residues (almond shells), to study the influence of their physicochemical properties. Half sandwich (cymene)Ru(II) curcumin complexes were selected on the basis of solubility and toxicity. The stability of the Ru complexes on carbon surfaces was analysed by different techniques. The combination of low soluble Ru complexes with high porous carbon materials prevents leaching into solution and leads to new composite materials with high porosity, high adsorption capacity and efficient antibacterial activity. All composite materials are able to maintain their surface free of gram negative bacteria even after three days in contact with a highly concentrated bacteria cultivation media. These features can be relevant to the use of such carbon composites in air filters or aqueous treatment applications.

INTRODUCTION Carbon materials are widely used in different kind of filters because their porosity and surface area induce a very high adsorption capacity. Specific adsorbent-pollutant interactions can also be favoured by modification of their surface chemistry. The pore size distribution and the nature of the chemical species on surface can be easily modified by different activation or functionalization treatments, in order to improve adsorption ability and consequently filter performance.1-2 Carbon filters are normally used for environmental purposes, with the goal to eliminate pollutants from air or water streams by adsorption processes.3-5 They are also used as supports of active metal phases to develop catalytic systems able to transform them into nontoxic compounds,6 or as bacteria supports in biofilters.7-8 In addition, there are some specific applications in the food and pharmaceutical industries, such as in air conditioning equipment especially for hospitals, operating rooms (ORs) etc., where the quality of treated air requires not only the removal of possible pollutants, but also the abatement of airborne bacteria in the treated air flow or of those bacteria that can grow on the filter, favoured by the highly humid environment.9-11 Many studies on the design and production of antibacterial materials are based on silver or ZnO nanoparticles.12-14 However, silver nanoparticles can be ineffective, depending on the nature of bacteria and experimental conditions.15 Moreover, the use of nanoparticles has been also pointed out as a source of pollution with potential

health risks, with consequences that today are mostly unexplored.16-17 Carbon materials are non-toxic compounds, in fact they are traditionally used in poisoning treatments. Thus, bacteria can grow on their surface and can even be used for the development of biofilters in water or air treatments.18-19 As many small metal complexes are known to display antimicrobial activity,20-21 and based on a long standing interest of some of us in the design and synthesis of metal complexes with antibacterial activity,22-27 it seemed interesting to develop alternative composite materials with antibacterial behaviour based on a synergistic effect of carbon materials and antibacterial metal complexes. Low cost carbon materials can in fact provide the appropriate porosity and a large surface area where the active metal complexes can be supported in a small loading, affording the adequate toxicity for antibacterial uses, but preserving the porosity of the adsorbent. Ruthenium complexes are well known candidate prodrugs for their anticancer potentials,28-29 but they also display interesting antimicrobial activity.30-33 We have recently reported that (arene)Ru(II) complexes with curcuminoid ligands show in vitro cytotoxicity toward a number of tumour cell lines,34-38 with a synergistic effect between the organometallic moiety and curcumin, the latter being the major bioactive ingredient extracted from the rhizome of the plant Curcuma longa (turmeric) with several biological properties, among them also antimicrobial and antifungal activity.39-43 In

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this work, we extend our investigation on the biological potential of Ru-curcumin complexes by supporting them on different carbon materials, to afford novel composites exhibiting efficient antibacterial activity, which could be suitable candidates as filters in air and water treatment applications. Commercial activated carbons (ACs) are prepared mainly from coconuts (or alternatively from mineral coal). Some of us have considerable experience in obtaining ACs from many different waste residues, almond shells, olive stones, rest of pruning, etc.44-47 In this work we have chosen three types of ACs, one from commercial sources (M), a second obtained by polymerization of resorcinol and formaldehyde to give carbon aerogels that are subsequently carbonized in N2 (R), and a third prepared from almond shells by direct activation in CO2, (A) which is more relevant with respect to environmental sustainability. RESULTS AND DISCUSSION Synthesis of the carbon composites The half sandwich Ru complexes 1 and 2 were synthesized and characterized according to reported procedures (Figure 1).34-37

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Table 1 Percentage of V. fischeri inhibition after contact with Ru-complex suspensions. Sample

I15(%)

I30(%)

1

9

24

2

93

95

Complex 2 was therefore selected to impregnate different types of AC materials indicated as A, M and R (see experimental for details) using a saturated ethanol solution. After the impregnation of AC materials with complex 2 the stability of the supported complex was evaluated. TG experiments on pure complex 2 and on the RRu composite (composite of complex 2 supported on carbon sample R) were recorded, see Figure 2. Complex 2 is stable below 200 °C, there is, however, a small weight loss in this temperature range corresponding to the humidity. This event induces a sharp peak on the DTG profile. Nevertheless, the decomposition of 2 is fast from 200 to 500 °C, and although from this temperature the weight loss continues, the decomposition is slower. Thus, the maximum decomposition rate occurs at 440 °C, as shown in the DTG curve.

MeO

MeO HO

HO O

Ru Cl O

[(p-cymene)RuCl2]2 + curcH

O

1) AgSO3CF3 2) PTA

Ru P

N N

O

OMe

OMe

1

OH

SO3CF3-

N

- AgCl

2

OH

Figure 1 Synthetic procedure and chemical structures of Ru complexes 1 and 2. Complex 1 is a neutral molecular system in which the bioactive curcumin ligand is coordinated to the ruthenium metal centre through the carbonyl oxygen atoms. Complex 2 is the corresponding ionic species obtained by metathesis reaction using AgSO3CF3 as silver salt to replace the chloride with PTA (1,3,5-triaza-7-phosphaadamantane). The water-soluble PTA assures a better solubility in polar solvents such as water and alcohols, and cytotoxic effects toward some cancer cell lines in vitro have been found much higher for cationic (arene)Ru(II) complexes with PTA than the complementary neutral precursors.36 The maximum solubility of complex 2 in water is 40 mg/L, which has been determined by preparing solutions of different concentrations until saturation and analysing them by UV (λmax = 415 nm). Results were fitted by the LambertBeer equation to obtain the corresponding calibration curve. The toxicity of the solution has been determined through a test based on the photoluminescent Vibrio fischeri,48 formerly known as Photobacterium phosphoreum. According to recommendations of the European Guideline ISO 11348-2:2007,49 the toxicity was found to be I15(%) = 55.5 and I30(%) = 58.6, corresponding to the percentage of V. fischeri bacteria that die during the first 15 and 30 min, respectively. The neutral complex 1 is unfortunately essentially insoluble in water. Thus, to compare their toxicity, a modification of this method was carried out, where the percentage inhibition was followed by suspending 2 mg of each complex in 1 mL of bacteria suspension in NaCl medium. The Ru complexes remain undissolved at the bottom of the tube in contact with the bacteria suspension and the percentage inhibition was followed in a similar way. The obtained results are shown in Table 1. Therefore, complex 2 is not only water soluble, but also the most toxic. It is, however, noteworthy that the toxicity of this supersaturated suspension is around twice the toxicity of the saturated solution. In the presence of undissolved solid complex, near-complete inhibition of bacteria growth is reached after 30 min of contact.

Figure 2 TG/DTG profiles of (a) complex 2 and (b) the carbon composite of complex 2 supported on AC R. When 2 is supported on AC material R, this peak is clearly visible at the same position, in spite of the low amount of loaded complex (1% wt). These results indicate that 2 maintains its structure after loading on the carbon surface. The TG/DTG profiles slowly change because the release of oxygenated groups present on the carbon surface occurs simultaneously with the complex decomposition.1 Similar results were obtained for A-Ru and M-Ru composites. The degree of leaching of 2 from the doped carbon samples was then determined, and should ideally be avoided in order to maintain their antibacterial activity. Different suspensions of doped carbon samples were prepared and sacked at different pH values for 48 h at room temperature. After filtering the carbon particles, the UVsignal at 415 nm of all samples were negligible, independently of the pH used, i.e. no complex desorption takes place.

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ACS Applied Bio Materials

(a)

(b)

(c)

Figure 3 N2 adsorption isotherms of carbon supports M (a), A (b) and R (c) before and after impregnation with complex 2 at 1% loading. The toxicity of the water solution was I15(%) = 6.3 and I30(%) = 6.6, denoting the low solubility of the complex. Such a low solubility of complex 2 in water and the high interaction with the carbon phase helps to avoid leaching during the application of these materials. After the confirmation of the stability of complex 2 in the impregnated carbon samples, we determined if the composites maintain an adequate porosity to be employed as pollutant filters. Thus, the textural characteristics of carbon samples before and after impregnation were compared (Table 2 and Figure 3). The different AC materials A, M and R were selected because they present different porous textures and chemical properties. In all cases, impregnation with complex 2 results in a certain degree of pore blockage. Nevertheless, as observed in Figure 3, the isotherms of doped and undoped carbon materials are practically parallel, i.e. the pore blockage takes place mainly in the narrowest microporosity, as indicated by analyzing the blockage degree. Thus, it is noteworthy that both microporous activated carbons A and M present a decrease of VT between 17-20%, while this parameter is only of 11% for sample R, which is mainly mesoporous.

ganic content (1.7 %wt) has a pHPZC value of 6.8. After impregnation with 2, the pHPZC decreases around two units in all the samples. The Ru-distribution on the carbon supports was investigated by SEM-EDS analysis and was found to be reasonably homogeneous. Antibacterial tests Bacteria adsorption experiments on each sample were carried out as described in the experimental, using E. coli as a typical Gram negative bacterium, usually related to food poisoning, diarrhea, and haemolytic uremic syndrome.50 Moreover, the E. coli strain is the reference recommended by CLSI (Standards). The colony forming units (CFU) from the liquid phase were counted directly after plating in TSA medium (Figure 4), and this parameter compared with the blank (Table 3).

Table 2 Textural characteristics of carbon supports before and after impregnation with complex 2. Sample

SBET

Smicro

W0

L0

Vmeso

VT

(m2/g)

(m2/g)

(cm3/g)

(nm)

(cm3/g)

(cm3/g)

A

913

558

0.380

1.36

0.142

0.522

A-Ru

768

441

0.319

1.44

0.100

0.419

R

840

631

0.346

1.10

1.258

1.604

R-Ru

597

311

0.245

1.58

1.176

1.421

M

978

512

0.388

1.51

0.147

0.535

M-Ru

735

377

0.288

1.53

0.154

0.442

The blockage of the narrowest microporosity produces a micropore widening (L0), however, all the impregnated carbon samples present values around 1.5 nm. All samples maintained SBET > 600 m2/g and significant values of mesoporosity (external surface area). Therefore, they can be considered suitable for application as adsorbent filters. The acid/basic character of these samples is determined by the presence of inorganic impurities. The ash content of supports was determined by burning some portions of these samples. Sample R is a pure carbon phase, being synthesized from pure reactants, and presents a low oxygen content because of the high temperature of carbonization, thus pHPZC = 10.3. A similar value (pHPZC = 10.6) is observed for sample A, which present an inorganic content of only 0.1%. However, sample M, with a significantly higher inor-

Figure 4 Colonies of bacteria on TSA medium from (a) blank and (b) A-Ru suspension. Variations in the CFUs on carbon samples are due to the adsorbed bacteria. It has been observed that the decrease in the bacteria concentration on the adsorbent surface vary in the order A > R > M. Due to the large size of bacteria, their adsorption on carbon samples and doped carbon composites seems to be more related with the pHPZC than with the porosity of carbon, being preferentially adsorbed on the external surface of basic materials A and R with respect to the slightly acidic sample M. This is in agreement with previous studies that showed a negative effect of the acidic support on the development of biofilms.51 Table 3 Colony forming units (CFU) of E. coli detected in suspension after 24 h in contact with the different undoped and doped carbon samples, and toxicity of the suspension against V. fischeri measured as percentage inhibition I15(%) and I30(%).

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Sample

CFU/mL

I15(%)

I30(%)

Blank

2.6x108

-

-

R

1.4x106

-

-

R-Ru

2.3x105

34,63

42,49

5

A

6.0x10

-

-

A-Ru

2.0x105

44,93

58,32

M

3.0x107

-

-

M-Ru

8.8x105

26,15

37,47

interaction of bacteria with the doped surface (i.e. bacteria are retained or adsorbed on the doped-carbon surface), they progressively die. Within 3 days of cultivation (i.e. favouring the bacteria growth by feeding with TSB) the adsorbent surface was found to be free of bacteria (Figure 6) and their polysaccharide fibres, and dead bacteria were sometimes detected (R-Ru sample in Figure 6).

Figure 6 SEM analysis of E. coli biofilms after 3 days of contact on samples A (top left), A-Ru (top right), M (middle left), M-Ru (middle right), R (bottom left), and R-Ru (bottom right).

Figure 5 Biofilms formed on samples A and A-Ru after 24 h of bacteria adsorption. After impregnation with complex 2, the concentration of bacteria in suspension decreases in comparison to the undoped carbon samples, but now the effect is due to the combination of the adsorption and toxicity. Small CFU values were observed also for doped RRu and A-Ru samples. A similar effect can be described for the degree of inhibition of V. fischeri observed in the corresponding suspension of solids: again, the higher inhibition degree was observed with the A-Ru sample. After E. coli adsorption experiments, the recovered carbon particles were also analysed by SEM. As an example, the micrographs obtained for A and A-Ru samples are showed in Figure 5. A very high concentration of bacteria with the formation of a well adhered biofilm is formed on the undoped carbon surface. In the case of A-Ru, the filaments produced by bacteria on the supports are still visible, but the number of bacteria on this support after 24 h in contact with the saturated bacteria suspension is significantly less than for A alone. These filaments are biosynthesized polysaccharides that help the microorganism to grow colonies attached to the supports. These structures can provide the microorganisms with additional capacities, such as electron transfer processes,52 and in some cases these polysaccharides have been extracted and purified because they can present some interesting properties and applications.53 Analogous observations were made for the other doped-undoped carbon materials. Complex 2 on the carbon surface inhibits bacteria growth thus, in spite of an initial

Only some isolated bacteria were still detected on the M-Ru surface (M-Ru is the less toxic sample). In the absence of leaching of 2 from the carbon composites, their antibacterial activity is likely related to the cationic nature of the supported Ru complex, which is able to interact and disrupt the negatively charged bacterial cell membrane, thus provoking bacterial death, through a mechanism similar to that of polymers functionalized with quaternary ammonium end groups.54 CONCLUSIONS Two half sandwich Ru complexes, one neutral and one cationic, were used to impregnate three different AC materials (one available commercially and two prepared from different raw materials including almond shells. While the cationic Ru complex is reasonably soluble in water, the neutral one is essentially insoluble, thus the method to determine their toxicity was modified. The water soluble Ru complex was found to be the most toxic and was supported on the different AC materials by impregnation. In the obtained composite materials the Ru complex is homogenously distributed and strongly interacts with the carbon surface preventing leaching. After impregnation, all AC samples retain high volume of pores and surface area, being blocked mainly at the narrowest micropores. All undoped carbon samples have a strong interaction with E. coli, showing a high ability to adsorb bacteria and inhibiting the development of biofilms. Nevertheless, bacteria die after contact with the Ru doped composites and, in general, the surface of the doped carbon samples remains free of bacteria, even after exposure for days at high concentrations of bacteria in solution. These

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ACS Applied Bio Materials results indicate a high antibacterial efficiency (high bacterial adsorption capacity and enough toxicity by contact) and provide good perspectives for their use in air filters or aqueous treatment applications.

plating in Tryptone Soy Agar (TSA) medium with appropriate dilutions.

Experimental

E. coli ATCC 25922 was used in these experiments. To immobilize the bacteria on the different supports they were first incubated at 37 °C, using a buffered media at pH 7 of Tryptic Soy Broth TSB, Difco Lab. Bacteria were then supported on the different solid samples adding 1 mL of this suspension to 0.4 g of carbon sample suspended in 20 mL of TSB, and the mixture was shaken at 37 °C for 3 days. After this time, the colonized carbon supports were filtered and washed repeatedly with sterilized distilled water.51 The formation of bacterial colonies on the different carbon materials was examined by SEM using a LEO Carl Zeiss GEMINI-1530 microscope.

Synthetic procedures Complexes 1 and 2 were synthesized as previously reported.34-37 Three carbon materials were selected as supports for this study, here indicated as A, M and R samples. Two of them (samples A and R) were prepared in our laboratory from different raw materials, in order to have different porosity characteristics. Briefly, sample A was prepared from almond shells by direct activation in CO2 flow at 1123 K for 5 h. Sample R is a carbon aerogel produced by polymerization of resorcinol and formaldehyde using Na2CO3 as polymerization catalyst and carbonized at 1123 K for 5 h in N2 flow.55 The third carbon support (sample M) is a commercial AC, purchased from Merck and used as reference material. The antibacterial composite materials were prepared by impregnation: complex 2 (1% with respect to the carbon support) was dissolved in ethanol and the solution slowly added drop by drop to the carbon surface. After impregnation, the composites were maintained at room temperature for 24 h and then dried at 120 °C in a conventional furnace. Chemical and textural characterization of composites Thermogravimetric experiments were used to establish the stability of complex 2 on the surface of carbon sample R. Complex decomposition before and after deposition on the carbon support was analyzed by following the corresponding TG/DTG curves. Experiments were carried out in N2 flow and using a thermobalance SHIMADZU mod. TGA-50H was programmed with a heating ramp of 20 °C/min. Textural characterization of doped and undoped carbon samples was carried out by N2 adsorption at −196 °C using a Quantachrome Autosorb-1 equipment. From these isotherms, parameters such as the BET surface area,56-57 the micropore volume and width by applying the Dubinin–Radushkevich and Stoeckli equations,58 and the mesopore size distribution, by applying the Barrett-Joyner-Halenda BJH method,59 were obtained. The total pore volume VTotal was considered as the volume of N2 adsorbed at P/P0 = 0.95. The pHPZC (point of zero charge) measurements were carried out according to a previously reported procedure.2,60 Each sample was suspended on 4 mL of distilled water previously degasified. Suspensions were stirred and thermostated at 25 ºC measuring the pH periodically till constant measurement. The final pH obtained this way was considered as the pHZPC for each sample. Antibacterial activity of composites Different experiments, using two types of bacteria, E. coli and V. fischeri, were carried out to determine the interaction of doped and undoped carbon materials with bacteria, their adsorption capacity and their ability to develop/kill the colonies of bacteria. All tests were done in triplicate following the methodology previously used.61 Briefly, E. coli ATCC 25922 strain was cultivated overnight at 37 °C in Tryptic Soy Broth (TSB) medium (Difco Lab). This bacteria suspension was centrifuged and washed with sterile distilled water and finally re-dispersed in 20 mL of this water. This suspension was used as a blank. Adsorption experiments on the different carbon samples were carried out using 0.25 g of adsorbent suspended in 5 mL of the bacteria suspension, stirred with a vortex mixer and then maintained in an orbital sacker (45 rpm) at room temperature. The remaining bacterial cells in suspension were determined by counting the colony forming units (CFU) at 24 h by

Biofilms formation on composite surfaces

Toxicity measurements of composites The toxicity of solid suspensions of doped and undoped carbon samples was measured based on inhibition of the luminescence intensity of marine bacteria V. fischeri NRRL-B-11177, in accordance with the European guideline ISO 11348-2:2007.49 A suspension of bacteria was prepared according to this procedure and the initial luminescence was determined. The solid supports were removed from their suspensions by centrifugation, to avoid interferences in the determination of the light intensity, and water recovered was used to dilute the bacteria suspension by mixing in a 1:1 ratio. After 15 min exposure, bioluminescence was measured by LUMISTOX system. In all measurements, the percentage inhibition I(%) was obtained by comparing the response of a control saline solution with that of the sample. Toxicity was expressed as the percentage inhibition I(%) of bacterial growth as a function of treatment time. Acknowledgements SMT acknowledges the financial support from University of Granada (Reincorporación Plan Propio). REFERENCES (1) Vivo-Vilches, J. F.; Bailón-García, E.; Pérez-Cadenas, A. F.; CarrascoMarín, F.; Maldonado-Hódar, F. J. Tailoring the Surface Chemistry and Porosity of Activated Carbons: Evidence of Reorganization and Mobility of Oxygenated Surface Groups. Carbon 2014, 68, 520-530. (2) Perez-Cadenas, A. F.; Maldonado-Hodar, F. J.; Moreno-Castilla, C. On the Nature of Surface Acid Sites of Chlorinated Activated Carbons. Carbon 2003, 41 (3), 473-478. (3) Vivo-Vilches, J.; Bailón-García, E.; Pérez-Cadenas, A.; CarrascoMarín, F.; Maldonado-Hódar, F. Tailoring Activated Carbons for the Development of Specific Adsorbents of Gasoline Vapors. J. Hazard. Mater. 2013, 263, 533-540. (4) Sidheswaran, M. A.; Destaillats, H.; Sullivan, D. P.; Cohn, S.; Fisk, W. J. Energy efficient indoor VOC air cleaning with activated carbon fiber (ACF) filters. Build. Environ. 2012, 47, 357-367. (5) Capelo-Neto, J.; Buarque, N. M. S. Simulation of Saxitoxins Adsorption in Full-Scale GAC Filter Using HSDM. Water Res. 2016, 88, 558-565. (6) Morales-Torres, S.; Carrasco-Marín, F.; Pérez-Cadenas, A. F.; Maldonado-Hódar, F. J. Coupling Noble Metals and Carbon Supports in the Development of Combustion Catalysts for the Abatement of BTX Compounds in Air Streams. Catalysts 2015, 5 (2), 774-799. (7) Moreno-Castilla, C.; Bautista-Toledo, I.; Ferro-Garcıa, M.; RiveraUtrilla, J. Influence of Support Surface Properties on Activity of Bacteria Immobilised on Activated Carbons for Water Denitrification. Carbon 2003, 41 (9), 1743-1749. (9) Esteves, B. M.; Rodrigues, C. S.; Boaventura, R. A.; Maldonado-Hódar, F. J.; Madeira, L. M. Coupling of Acrylic Dyeing Wastewater Treatment by Heterogeneous Fenton Oxidation in a Continuous Stirred Tank Reactor with Biological Degradation in a Sequential Batch Reactor. J. Environ. Manag. 2016, 166, 193-203.

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