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PVA-zeolite doped with noble metal cations
Food Packaging and Shelf Life 22 (2019) 100378
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Antimicrobial packaging film based on biodegradable CMC/PVA-zeolite doped with noble metal cations
T
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H.F. Youssefa, Mehrez E. El-Naggarb, , F.K. Foudac, Ahmed M. Youssefd a
Ceramics, Refractories and Building Materials Department, National Research Centre (NRC), Dokki, Egypt Textile Research Division, National Research Centre, 33 El Bohouth st., Dokki, Giza, Egypt c Medical Biochemistry Department, National Research Centre, Doki, Giza, Egypt d Packing and Packaging Materials Department, National Research Centre, 12622, Dokki, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biodegradable polymers Carboxymethyl cellulose Film Zeolite Silver cation Gold cation Food packaging
The current study aimed to enhance the antimicrobial, mechanical and vapor transmission rate of food packaging film by producing films-based zeolite doped with silver (Ag+) or gold (Au+3) cations. Thus, nanopowders of sodium zeolites-A (ZA) and Faujazite-X (ZX) were hydrothermally synthesized using microwave. Sodium cations in both zeolites were exchanged with either Ag+ or Au+3. Films of CMC/PVA were then prepared to support film formation of zeolites containing Ag+ or Au+3. The obtained data indicated that the synthesized films from CMC/ PVA possessed tensile strength equal to 2.05 kgf/mm2. This value was improved and reached to 8.69 kgf/mm2 upon the addition of ZA-Au. Finally, the water vapor transmission (WVTR) and gas transmission rate (GTR) of the prepared nanocomposite film enhanced with the addition of ZA-Au or ZA-Ag. Furthermore, the increase of ZA-Ag or ZA-Au concentration, augmented and boosted both mechanical and antimicrobial properties.
1. Introduction Latest foodborne microbial outbreaks are the main authority to search for such innovative ways to restrain, inhibit and stop the growth of microbial species in food without adversely affecting its worth and safety (Havelaar et al., 2010). It is well known that the traditional film used for food preservation is an easy target for contamination with hazard microbes, which are able to diffuse from the film surface to the food (Biji, Ravishankar, Mohan, & Gopal, 2015). Finding a new appropriate and self-protracting films is the guarantee for having a wellprotected food stuff. Thus, it is a must to use active packaging films incorporated with antimicrobial agent. Silver or gold in the nano size or cation form can be utilized to achieve the goal. The produced film should indicate mechanical properties and air permeability, both of which could be achieved by adding a hard nanoporous filler or enhancer such as zeolitic materials to afford such improvement (Pina et al., 2011). There are many research studies in the literature addressed such point and employed polymers like caroxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) as film forming material. Carboxymethyl cellulose (CMC) is a biodegradable and biocompatible anionic polymer processed from natural cellulose by chemical derivatization. CMC is used for keeping moisture, and improving the structural consistency of bakery products as it is usually applied by
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blending with other stabilizers and gums owing to its high water-absorbing capacity (El-Newehy, Saleh Alotaiby, El-Hamshary, Moydeen, & Al-Deyab, 2016; Radwan et al., 2018; Yu, 2009). On the other hand, polyvinyl alcohol (PVA) is a hydroxyl rich, non-toxic, biocompatible, semi-crystalline plastic and water-soluble polymer (Gaaz et al., 2015). It is extensively used not only in food packaging, but also in many applications, like household and construction sectors, owing to its excellent properties, such as mechanical performance solvent resistance, and biocompatibility (Leja & Lewandowicz, 2010; Yang et al., 2016). The low biodegradation rate and high moisture absorption of PVA are the most common attributes that favors its application in food packaging (Youssef, Assem, Salama, & Abd El-Salam, 2017). Hence, in order to enhance the performance and environmental properties of PVA, it is frequently mixed with certain biopolymers and/ or biobased reinforcements (REF). Further, the problems associated with PVA films concerning shortcomings in both mechanical properties and water permeability are to be relieved via the addition of zeolites. Zeolites are crystalline natural or synthetic solids. They acquire ring structure, which is brought about by linking silicate (SiO4) and aluminate (AlO4) tetrahedra in a porous skeleton which, in turn, entrap an open channel network with interconnected uniform voids and pores in the molecular dimensions of 0.3–1.4 nm range. Some pore size ranges are 3.5–4.5 Å for LTA zeolite, 4.5–6.0 Å for ZSM-5 and 6.0–8.0 Å for
Corresponding author. E-mail addresses: [email protected], [email protected] (M.E. El-Naggar).
https://doi.org/10.1016/j.fpsl.2019.100378 Received 3 July 2019; Received in revised form 27 July 2019; Accepted 2 August 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
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refined kaolin rock, supplied by Middle East Mining Company, Egypt. The main constituents of the rock were 53.25 SiO2, 42.94 Al2O3, 0.41 Fe2O3, 1.62 TiO2 and some other minor amounts of MgO, CaO, K2O. Sodium hydroxide pellets (NaOH) were of analytical grade reagent with the composition of 98.6% NaOH + 0.4% Chloride (Sigma-Aldrich; USA). Ludox AS-40 colloidal silica, 40 wt% suspension, Aldrich, is used as the additional SiO2 source for adjusting the kaolin composition to match that of the targeted Faujasite. Silver nitrate (BDH chemicals—England) and Gold III Chloride, Sigma Aldrich, were used for zeolite modification with Ag+ and Au3+. Carboxymethyl Cellulose (CMC; Mw: 240.208 g/mol) was purchased from Kelong Chemical Agent Factory, Chengdu, China. Polyvinyl alcohol (PVA; Mw: 98,000 with purity more than 99%) was purchased from Sigma-Aldrich Chemicals.
zeolite X and Y (Van Bekkum, Flanigen, Jacobs, & Jansen, 2001). Zeolites are nontoxic with thermal and chemical stabilities. They possess long-term biological properties (Hrenovic, Milenkovic, Ivankovic, & Rajic, 2012; Sabbani et al., 2010), and have ability to reversibly bind small molecules and cations proving that zeolites can be successfully utilized in agriculture, detergency, adsorbents, catalysis, food preservation, as well as medical and pharmaceutical industries (El-Magrabi et al., 2018). It is also reported that introduction of microwave energy during hydrothermal synthesis acts in favor of chemical processes and selectively accentuate chemical reaction kinetics for ceramic powders, gels and metal powders by more than one order of magnitude. In fact, the manufactured zeolites are advantageous for industrial applications than those of natural zeolites because of their high purity, good crystallinity and their tailoring nature. Antimicrobial activities due to cations are introduced for the sake of immobilization or attachment onto zeolite structures (Hrenovic et al., 2012). Incorporating some metals onto zeolite cavities causes structural disinfectionability (Sabbani et al., 2010; Shameli, Ahmad, Yunus, Ibrahim, & Darroudi, 2010). Silver is one of the most common ions to combine in zeolite frameworks for attaining biological activity by virtue of its high stability and broad spectrum against different types of bacteria, viruses, germs, and fungi (Hernández-Sierra et al., 2008; Nirmala Grace & Pandian, 2007; Youssef, Abdel-Aziz, & Fouda, 2017). Silver cations (Ag+) or gold cations (Au+3) are replacing part of the framework of Na+ cations via the exchange process by impregnation of the zeolite solid powder into Ag- or Au-containing liquids. Similarly, Ag and Au in the cations state or nanoform exert a great bactericidal effect on several ranges of microorganisms; their bactericidal effect depends on the size and the shape of the particle (Elshaarawy, Seif, El-Naggar, Mostafa, & El-Sawi, 2019; Hebeish, ElNaggar, Tawfik, Zaghloul, & Sharaf, 2019; Medhat et al., 2017). Also, such cations attain antibacterial and antifungal action, by virtue of their ability to interact with microorganisms (Abdelgawad, El-Naggar, Eisa, & Rojas, 2017; El-Naggar, Shaheen, Fouda, & Hebeish, 2016; Rai, Yadav, & Gade, 2009; Shaheen et al., 2016; Sharma, Yngard, & Lin, 2009; Yan, Abdelgawad, El-Naggar, & Rojas, 2016). This interaction causes structural changes and damage, markedly disturbing vital cell functions, such as permeability, causing pits and gaps, depressing the activity of respiratory chain enzymes, and finally leading to cell death (Kuge & Calzaferri, 2003; Munthali, Elsheikh, Johan, & Matsue, 2014). The target of our work is to design a novel film for food packaging that have several advantages over the commercially available films. Most of the commercial films have low mechanical properties and easily infected with microbes which, in turn, contaminate the preserved foods. In our work, the designed films contain zeolite doped with silver (Ag+) or gold (Au+3) cations possessed high mechanical properties, low water vapor transmission rate and gas transmission rate and excellent antimicrobial properties. Thus, the work is undertaken with a view to firstly prepare nano-powders of sodium zeolite-A (ZA) and Faujasite-X (ZX) hydrothermally using lab microwave. Then, sodium cations in both zeolite types are exchanged with either silver (Ag+) or gold cations (Au+3). The obtained zeolites containing the latter cations are evaluated. Films of CMC/PVA are then prepared to act as a support for film formation of zeolites containing Ag+ or Au+3. The morphological features, particle size and shape, crystallinity and the elemental analysis of the as-synthesized zeolites nanoconmposites are evaluated by making use of TEM, SEM, XRD and EDX. This in-depth assessment of the zeolite nanocomposites is sustained to encompass water vapor transmission rate (WVTR), Gas transmission rate (GTR), and mechanical as well as antimicrobial properties.
2.2. Methods 2.2.1. Microwave hydrothermally preparation of zeolite The two zeolites under investigation were formed using the microwave hydrothermal heating, following the method of Youssef, AbdelAziz, et al. (2017). Zeolite-A (ZA) was hydrothermally derived from kaolin through the alkali attack on the thermally treated kaolin (metakaolinite), whereas, Faujasite-X (ZX) was prepared after the modification of kaolin by additional of silica (Ludox 40). Zeolite-A (ZA) was prepared from a fresh gel which is directly processed by attacking the calcined kaolin by 100 ml of caustic soda of 3.0 M composition and stir for 15 min at room temperature. Whereas, ZX was derived from two separate solutions, one is containing 40 gm of Ludox 40 dissolving in 50 ml of 3.0 M caustic soda solution in a Teflon beaker. The other solution is formed by adding 5 g of the starting material to another 50 ml of the caustic soda with the same previous concentration. Lab microwave digestive system, model MARS 5, XP-1500 vessels, CEM Corp., Matthews, NC), operating at 2.45 GHz of frequency and from 1 to 100% power of 1600 W, represents the hydrothermal tool used for the zeolite crystallization from the previously obtained gels. ZA was heated for 1 h at 70 °C whereas, ZX was heated for 2 h at 110 °C. Reaction products were then collected, washed several times with distilled water (up to pH = 7–8), dried overnight at 100 °C in an electric oven. The materials and their ratios for the zeolites under investigations; ZA are S/Al (1–2), NaOH (2.5–4 M) and H2O/SiO2 (100–150), meanwhile, for ZX are the same as presented in ZA but with a higher Si/Al ratio in the range of 4–8 and H2O/SiO2 (100–200). 2.2.2. Preparation of silver and gold doped zeolite nanocomposite The cation exchange process was used to incorporate the silver and gold cations into the zeolite structure by the immersion method. Typically, zeolite powders (5 g) were impregnated into 100 ml of 0.1 M of either AgNO3 or HAuCl4 for 2 and 10 h, respectively. Then, the stained zeolites resulted from the cation replacement between zeolitecontained sodium and the cations of silver or gold from the exchanging solutions were collected, washed several times with distilled water and finally dried at 50 °C for 2 h and kept for further investigation. 2.2.3. Preparation of CMC/PVA/Zeolite doped with Ag or Au cations A homogenous viscous solution of carboxymethyl cellulose (CMC) was prepared by dissolving 3 g of CMC in 100 ml of deionized water at 70 °C under magnetic stirring. Similarly, polyvinyl alcohol solution (PVA) was prepared by adding 3 g of PVA to 100 ml of deionized water at 70 °C and kept under magnetic stirring till complete dissolution. Then, 50:50 (v/v) of CMC: PVA solution was prepared by mechanical mixing the solutions of the two polymers and kept under mechanical stirring for 6 h. The as-prepared CMC/PVA nanocomposites were mechanically mixed with different concentrations of ZA-Ag and ZA-Au or ZX-Ag, ZX-Au solutions (1%, 3% and 5% w/v) followed by sonication for 1 h at room temperature. The homogeneous CMC/PVA/zeolite nanocomposites were poured onto the Teflon dish in order to evaporate
2. Materials and method 2.1. Materials The starting materials used for zeolite synthesis were MEMCO 2
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Scheme 1. Schematic representation of the formation of CMC/PVA film before and after mixing with ZA doped with Ag or Au cations.
The antimicrobial properties of zeolite films were investigated as follows: Disc agar plate method was established to evaluate the antimicrobial activities of the as synthesized CMC/PVA loaded with zeolites containing the said nobel cations. Four different test microbes, Staphylococcus aureus (G + ve bacterium), Escherichia coli (G-ve bacterium), Candida albicans (Yeast) and Aspergillus niger (fungus) were selected to evaluate the antimicrobial activities. The bacterial and yeast test microbes were grown on a nutrient agar medium. whilst, Aspergillus niger was cultivated on potato dextrose agar (PDA) medium. The culture of each test microbe was diluted by sterilized distilled water to 107–108 colony forming units (CFUs)/ml, and then 1 ml of each was used to inoculate 1 L Erlenmeyer flask containing 250 ml of solidified agar media. These media were put onto previously sterilized Petri dishes (10 cm diameter having 25 ml of solidified media). 10 mm discs of films were placed on the surface of inoculated petri under sterilized conditions. The plates seeded with test microbes were incubated for 24 h–48 h, at the appropriate temperature of each test organism. Antimicrobial activities were recorded as the diameter of inhibition zones (including the disc itself) that appeared around the discs. Neomycin (100 μg/disc) and cyclohexamide (100 μg/disc) were used as antibacterial and antifungal standards, respectively.
the solvent and to obtain the ultimately product in the form of a film. The Teflon dishes were left without interruption at room temperature for 72 h. The prepared nanocomposite films were peeled off from the mold and made ready for characterizations.
2.3. Characterization The obtained zeolites were characterized before and after the cation exchange process for their mineralogical composition using X-rays diffraction analysis (XRD), Bruker D8 Advance-Germany using a Ni filter, Copper Target, at V = 40 Kv and A = 40 mA. Meanwhile, the grain morphology and particle size of the nanopowders were monitored using transmission electron microscopy (TEM) model (JEOL JEM-1230, Tokyo, Japan). The surface of the resulting films was investigated by scanning electron microscopy (SEM) at an acceleration voltage of 20 kV using a Quanta 400 SEM (Oxford, UK). Energy Dispersive X-ray analysis (EDX) was used for analyzing the chemical composition of zeolite crystallites, Oxford Instrument INCAx-sight is a part added to Joel TXA840A electron probe micro analyzer. Water vapor transmission rate (WVTR) was carried out using GBI W303 (B) Water Vapor Permeability Analyzer (China) using the cup method. The water vapor permeability was measured as the amount of water vapor passes through the tested film. Also, WVTR was measured as the mass of water vapor transmitted throughout a unit area in a unit time under controlled conditions of temperature (38 °C) and humidity (4%) according to the following Standards (ASTM E96, ISO 2528, ASTM D1653, TAPPI T464, DIN 53122-1, JIS Z0208). Also, the gas transmission rate (GTR, (O2 and CO2) are measured by N530 Gas Permeability Analyzer (China), according to the following StandardsISO2556-2001, GB/T1038-2000, ASTM D143 4-8 2(2003). The mechanical properties of the prepared films containing CMC/ PVA blended with zeolite doped with cations were measured according to the ASTM D638-91 standard using a universal testing machine LK10k (Hants, UK) fitted with a 5 kN load cell, and operated at a rate of 5 mm/ min on the samples.
3. Results and discussion To start with, the colors of CMC/PVA films before and after submission to zeolite bearing silver and gold cations are inspected. It was observed that the films display white color before being blended with zeolite/Ag+ or zeolite/Au+3. On the other hand, the films after such blending assume optically semitransparent yellowish color. No difficulty is encountered while peeling the films from the dish; the formed films are easily separated from the dish. It was further noticed that the color of the films under investigation become more yellowing after casting these films. This could be accounted for by reduction of traces of Ag+ or Au+3 found in the cavity of zeolite to nanoparticles under the influence of reducing groups located on the CMC backbone (El-Naggar 3
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Fig. 1. XRD patterns for the prepared Ag+ and Au+3 doped (A) ZA and (B) ZX.
with the chemical formula of Na92Al92Si100O384.27H2O. Meanwhile, Fig. 2B exhibits the XRD patterns for the nano-forms of Faujasite-NaX and its ZXAg & ZXAu versions that are comparable with the PDF# 832319, which assigned for sodium aluminum silicate hydrate zeolite Xsynthetic, with a Na7.4 Si7 Al5O24.5.2H2O representative chemical composition. It is as well to emphasize that, XRD patterns of the parent zeolites (ZA and ZX) are preserved in the cations-containing species of both types after the cation exchange process, but ZXAg displays some reduction in its peak intensities. The latter effect could be traced back to the corrosive action of the AgNO3 on the much smaller crystallites of ZX, vis-à-vis those of the bigger crystallites of ZA (Youssef, Abdel-Aziz, et al., 2017). The measurement of the specific surface area (BET) for the two parent zeolites are determined and the data obtained are recorded. Values refer to 374 m2/g and 462 m2/g for ZA and ZX, respectively. As an integral part of the above characterization, the particle shape of the two zeolites in question, before and after doping with Ag and Au cations, was examined using TEM technique. Fig. 2A and B exhibit the fine particle identity of ZX in its free and corresponding Ag-exchanged form. In a similar manner, the nanostructure of bigger particles of Aufree and Au-replaced-ZA are displayed in Fig. 2C and D.
et al., 2016; El-Rafie et al., 2011; Hebeish, El-Rafie, El-Sheikh, Seleem, & El-Naggar, 2014). However, mention should also be made that the calcination temperature above 400 °C is not advocated due to the formation of cracks at this temperature (Youssef & El-Sayed, 2018). Films formed from CMC/PVA/ZA and doped with Ag or Au cations are represented in Scheme 1. Obviously, the color of original zeolite before adding the noble cations is white, its color turned to grey and yellowish only after introducing the cations of Ag and Au into its structure. This speaks of the formation of Ag or Au inside the cavity of zeolite as cation exchange or nanosized forms. The nanocomposite of zeolite doped with Ag or Au cations is fully characterized using XRD, BET, TEM, SEM-EDX techniques. This is done before film characterization
3.1. Characterization of zeolite-A and zeolite-X loaded with and without Ag and Au cations Fig. 1A and B depict the XRD profiles for the currently synthesized nanozeolite powders of ZA and ZX with their Ag (ZAAg & ZXAg) and Au (ZAAu & ZXAu) derivatives, respectively. Clearly, Fig. 1A shows the complete match between the peak positions and their relative intensities of the PDF#76-1041 standard card for the synthetic zeolite 4A, 4
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Fig. 2. TEM of the as-prepared (A) ZA, (B) ZX, (C) Ag incorporated ZA, (D) Ag incorporated ZX, (E) Au incorporated ZA and (F) Au incorporated ZX.
containing Ag+ and Au+3. Fig. 3 shows the images which represent the output of this study. Fig. 3A and B illustrate the internal texture under microwave heating of ZA and ZX respectively. It can also be easily realized that all over the scanned area a homogenous grain size distribution exists reflecting the development of uniform nucleation and crystallization process. When these types of zeolites (ZA and ZX) are immersed independently in solutions of Ag and Au salts, the ZA containing Ag+ and Au+3 (Fig. 3C and E) and ZX (Fig. 3D and F) imply brighter tents than their original species. The implication of this is that the exchange process is successful, and can be interpreted in terms of the interaction involving the secondary electrons of the microscopic and the incorporated Ag+ and Au+3 within zeolite pores and channels. A close examination of the output of current evaluation would reveal that both zeolites (ZA and ZX) undergo decrement in particle size after the cation exchange process because of particle decay of aluminosilicate zeolites and their instability under acidic media (Hrenovic et al., 2012). The onset images in Fig. 3 assess the elemental analysis of the scanned ZA and ZX before and after both zeolites are impregnated in the salts of Ag+ and Au+3. This is, indeed confirm the appearance of Ag and Au after impregnation thereby approving the successful
Based on TEM examination, it is obvious that the population of developed crystallites of both zeolites (ZA and ZX) are uniformly disseminated and imply a narrow range of particle size distribution of (100–125 nm) and (200–300 nm) for ZX and ZA, in respective order. ZA and ZX containing Ag and Au are synthesized followed by analyzing their colloidal solution by making use of TEM. Images obtained are shown in Fig. 2. Accordingly, the silver and gold- loaded zeolites of both types (Fig. 2C, D) acquire, to some extent, the homogenous replacement process resulted in a homogeneity in the tent of the crystal color under TEM conditions. Whereas, those exchanged by gold (Fig. 2E, F) were spotty, probably due to the common difficulty encountered in the replacement process of the sodium cations of zeolite by Au ones, ascribed to the instability of Au ions without legends (Weitkamp & Puppe, 2013). Particularly visualized is that both ZA and ZX nanopowder presented more Ag cations in causing staining as compared with Au species, this high affinity of Ag towards zeolite-A will be explained in the next SEM-EDX part. SEM-EDX technique is devoted to evaluate the morphological structure and chemical analysis of the scanned samples of ZA and ZX 5
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Fig. 3. SEM and EDX of (A) ZA, (B) ZX, (C) Ag incorporated ZA, (D) Ag incorporated ZX, (E) Au incorporated ZA and (F) Au incorporated ZX.
formulation of zeolites incorporated with Ag+ and Au+3. A significant reduction in the amounts of sodium appears in the case of the Ag+ exchange process, accompanied by a remarkable presence of incorporated Ag in both types of zeolites. It seems that the evolution of Ag+ in zeolite compositions was established at the expenses of the depleted sodium. Table 1 contains the average amounts of Ag and Au incorporated in ZA and ZX; expressed in weight (wt%) and atomic (At%); keeping in mind the consideration that these data are taken from the average 5
crystallites in SEM images. Results of Table 1 displays that, the amount of Ag+ and Au+3 incorporated ZA is much higher than that of the cations incorporated ZX. This can be interpreted in the light of the cation exchange capacity (CEC) of each zeolite type. CEC is the reflection of its number of exchangeable sites that can facilitate the incorporation of the Ag+ and Au+3 at the expenses of sodium cations (replaceable cation). Table 1 shows also that the amount of Au in zeolite constituents is so low with nearly slight decrease in sodium contents of both zeolites. This is rather expected since ZA has a low Si/Al- zeolite and much more exchangeable sites (12 Na+/unite cells) than those of ZX species (8 Na+/unite cells). ZA has much compensating Na+ cations within its framework (12 Na+ / unite cells), compared to that of ZX (only 8 Na+ /unite cell) (Barrer, Rees, & Shamsuzzoha, 1966; Moissette, Hureau, Col, & Vezin, 2016).
Table 1 EDX micro chemical analysis for amounts of Ag and Au in zeolites. Quantity
Weight % Atomic%
Ag
Au
ZA
ZX
ZA
ZX
27.31 6.81
17.46 3.85
2.13 0.22
1.54 0.17
6
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Fig. 4. SEM of (A) CMC/PVA, (B) CMC/PVA/ZA-Ag(1%), (C) CMC/PVA/ZA-Ag(3%), (D) CMC/PVA/ZA-Ag (5%), (E) CMC/PVA/ZA-Au (1%), (F), CMC/PVA/ZA-Ag (3%), (G) CMC/PVA/ZA-Ag(5%).
3.2. Characterization of CMC/PVA/ZA and CMC/PVA/ZX films incorporated with different concentrations of Ag and Au cations
movement of water vapor from the surrounding atmosphere into food products or moisture losses from the food to surrounding atmosphere exert significant influence on the packaged food products from the quality and stability point of view during the shelf-life time (Youssef, Youssef, Ayad, & Sarhan, 2015). Table 2 shows the WVTR for the CMC/PVA/ZA nanocomposite films under investigation. The results signify that CMC/PVA films display higher WVTR as compared to CMC/PVA/ZA nanocomposite films. Increasing ZA-Ag or ZA-Au in the CMC/PVA/ZA nanocomposites decreases the WVTR value of the film. The addition of (1%, 3% and 5%) of ZA-Ag decreases the WVTR value of the film to (1692.05, 1416.13 and 616.15 g/(m2 day), respectively. This is in comparison to CMC/PVA films which acquire a value of 2014.56 g/(m2 day). Also, in the case of using ZA-Au, the WVTR value of the prepared nanocomposites film decreases by increasing the concentration of ZA-Au in the nanocomposite film (1%, 3% and 5%); the WVTR attain values of (1919.82, 1260.43 and 623.95 g/(m2 day), respectively. It is also of interest that the GTR of the prepared CMC/PVA/ZA nanocomposite films is accentuated by the addition of modified zeolite through either Ag+ and Au+3. Table 2 shows that the GTR (CO2 or O2) of the prepared nanocomposites enhances by raising the ratios of ZA-Ag or ZA-Au in the CMC/PVA matrix. This is due to the characteristic features of zeolite as porous materials. Hence the permeability of (CO2 or O2) increased move through the prepared nanocomposites films as
The surface and fracture morphology of the cast CMC/PVA and CMC/PVA/zeolite films containing Ag+ or Au+3 was studied using the SEM technique as shown in Fig. 4. It is observed that the microstructural image of CMC/PVA film (Fig. 4A) is smooth, compact and homogeneous surface and fracture. Meanwhile, the morphological features of CMC/PVA undergo changes owing to mixing with zeolite incorporated with Ag+ as shown in Fig. 4(B–D) or Au+3 (Fig. 4E–G). As previously mentioned, the zeolite was firstly prepared using three different concentrations of Ag and Au cations (1%, 3% and 5%). Fig. 4(B, E) concerned with the film blended with zeolite/Ag (1%) and zeolite Au (1%) reveal that the surface becomes slightly rough with the appearance of small particles due to the presence of zeolite/Ag and zeolite/Au particles in the fracture. By increasing the concentration to 3% of Ag+ and Au+3 attached to zeolite before mixing with CMC/PVA solution as well as before film formation, the surface of the fabricated film becomes rough with remarkable presence of particles on the surface of the film as illustrated in Fig. 4(C and F) for Ag- and Au- incorporated zeolite respectively. Further increase in the concentration to 5% of cation incorporated zeolite solution, the surface becomes very rough with well-defined occurrence of aggregated particles on the surface of CMC/PVA film as shown in Fig. 4(D and G). However, there are no visible cracks or holes formed with films of CMC/PVA blended with zeolite incorporated with different concentrations of Ag+ and Au+3. Since there is no significant difference in the SEM characterization of CMC/PVA mixed with ZA or ZX nanocomposites, the as-prepared films based on CMC/PVA/ZA containing different concentrations of Ag+ and Au+3 is only selected for further investigation.
Table 2 The water vapor permeability as well as the gas transmission rate (O2 and CO2) of the prepared nanocomposites films. Samples
CMC/PVA Zn-Au (1%) Zn-Au (3%) Zn-Au (5%) Zn-Au (1%) Zn-Au (3%) Zn-Au (5%)
3.3. Gas transmission rate (GTR) and water vapor transmission rate (WVTR) of the newly prepared nanocomposites films It is well established that the water vapor transmission rate (WVTR) is an essential measure for advocating the suitability of packaging materials that are used in food packaging applications. Accordingly, 7
GTR, (cc/M2 day)
WVTR, g/(m2 day)
O2
CO2
15.31 27.37 38.79 83.54 30.7 40.86 67.84
0.05 0.09 1.89 29.17 0.60 1.96 31.28
2014.56 1692.05 1416.13 616.15 1919.82 1260.43 623.95
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antimicrobial properties against many species of microbes (bacteria; Fungi and Yeast) as shown in Fig. 6. The Films incorporated with Ag or Au cations in the zeolite framework display antibacterial activity against microbes. Because of their concentration, the cations of Ag or Au can easily reach the nuclear content of microbes and present the greatest surface area. It is also found that the diameter of the inhibition zone increases by increasing the content of cation inside the pores of zeolite. As well known, metal nanoparticles such as silver or gold nanoparticles play a key role in antimicrobial activities. Thus, there are two possible mechanisms for killing the microbes using AgNPs or AuNPs nanoparticles loaded CMC/PVA/Zeolite film. The first mechanism relies on the high oxidation state of AgNPs or AuNPs than can produce atomic oxygen around it. Along these lines, the formed atomic oxygen can react with the double bonds of bacterial cell wall lipids and penetrates into the inside of the microbe, interact with proteins and lipopolysaccharides, which consequently changes the permeability of the bacterial cellular membrane leading to bacterial death. The second hypothesis might be assigned to the capability of AgNPs or AuNPs to disturb the cell wall of microbes by means of direct contact to the cell membrane through electrostatic interaction, prompting to the adjustment of its permeability and respiration chain or by indirect contact throughout penetration into microbe cell and thenceforth release the ions of Ag or Au which interacts with thiol groups and/or phosphates moieties of DNA or protein in microbes (Dakal, Kumar, Rita, & Yadav, 2016; Mohamed, El-Naggar, Shaheen, & Hassab, 2017). By and large, the data obtained are promising due to the super antimicrobial properties of the produced films. Thus, the fabricated film can be used for realistic food packaging applications like food containers, chairs, vials, etc.
Table 3 The mechanical properties of the prepared CMC/PVA/ZA nanocomposite films. Samples
% ZA-Ag or ZA-Au
Tensile strength kgf/mm2
Elongation (%)
PVA CMC CMC/PVA CMC/PVA CMC/PVA CMC/PVA CMC/PVA CMC/PVA
0.0 0.0 ZA-Au (1%) ZA-Au (3%) ZA-Au (5%) ZA-Ag (1%) ZA-Ag (3%) ZA-Ag (5%)
1.43 1.82 3.24 5.74 8.69 2.56 4.12 6.85
150.53 3.67 60.54 68.98 75.70 55.70 68.28 80.15
compared with CMC/PVA matrix. 3.4. Mechanical properties of the newly prepared nanocomposites film The mechanical properties of the newly prepared nanocomposites films are of prime importance and vital for films used for packaging applications (Avella et al., 2005; Rhim, 2012). These films can be evaluated for the tensile strength and modulus of elasticity, yield strength, ultimate tensile strength, and toughness. Table 3 discloses that the mechanical properties of the prepared CMC/PVA/ZA nanocomposite films are based on the (ZA-Au or ZA-Ag). These latter two components increase by increasing the addition of (ZAAu and ZA-Ag) as per the following ratios (1, 3 and 5%) based on CMC/ PVA matrix. The tensile strength improves from 2.05 kgf/mm2 for CMC/PVA matrix to attain values of (3.24, 5.74 and 8.69 kgf/mm2) upon increasing the concentration of ZA-Au in the matrix from (1, 3 and 5%), respectively. This effect could be accredited to the fact that at minor concentrations of ZA-Au, the good interfacial adhesion is attained. This tolerates the interfacial configuration of the fabricated CMC/PVA/ZA nanocomposite to let part of the tensile strength. Additionally, by increasing the amount of ZA-Au in the CMC/PVA/ZA nanocomposite films, the elongation enhances to 75.32 by addition of 5% ZA-Ag in the nanocomposite films compared to 60.54% elongation for CMC/PVA matrix. Also, the addition of ZA-Ag in different amounts (1, 3, and 5%) to the CMC/PVA matrix leads to the improvement of the tensile strength of the prepared nanocomposites by (2.56, 4.12 and 6.85 kgf/mm2) respectively. The elongation of the prepared nanocomposite films enriches further to attain a value of 80.15 by addition of 5% ZA-Ag in the CMC/PVA/ZA nanocomposite films as compared to 60.54% elongation for CMC/PVA matrix. This enhancement in both tensile strength and elongation speaks of good interaction between the CMC/PVA matrix and the (ZA-Au or ZAAg) as filler. As a result of the higher contact surface area between inorganic filler and polymer matrix is developed.
4. Conclusion The present research was designed to tackle serious problems encountered with biodegradable CMC/PVA blend films upon their use in food packaging applications. Hence, novel nanocomposite films with superior antimicrobial properties were fabricated comprise zeolite doped with a silver (Ag+) or gold cation (Au+3). Nanopowders of sodium zeolite-A (ZA) and Faujazite-X (ZX) were hydrothermally synthesized using microwave. Sodium cations of zeolites were replaced with Ag+ or Au+3. To this end, films of CMC/PVA were prepared to act as a support for film formation of zeolites containing Ag+ or Au+3 cations. TEM images displayed that the particle shape of crystals of both zeolites were uniformly distributed and acquired narrow range of sizes with average less than 200 nm. SEM microstructural images of surface fracture morphology of the cast CMC/PVA/zeolite films containing Ag+ and Au+3 exhibited smooth compact and homogeneous surface and fracture. No visible holes or cracks were formed with the formed films. Energy dispersive X-ray (EDX) proved the existence of metal cations doped zeolite nanopowder and precipitated on the surface of the asprepared film. The obtained data indicated that the as-synthesized films based on CMC/PVA had a tensile strength equal to 2.05 kgf/mm2. This value has been improved and reached to 8.69 kgf/mm2 with the addition of ZA-Au or ZA-Ag. Water vapor transmission (WVTR) for nanocomposite films (CMC/PVA/ZA-Au displayed high values (616 g/ (m2 day) than does the CMC/PVA films (2014 g/m2 day). Additionally, the value of gas transmission rate was increased from 15 (cc/M2 day) for CMC/PVA film to 83 (cc/M2 day) with the addition of ZA-Au (5%). The mechanical properties of CMC/PVA film was increased with the addition of ZA-Au to the composite before the formation of films. Finally, the CMC/PVA/ZA containing Ag+ and Au+3 displayed excellent antimicrobial activity. It is envisaged that the outputs and other deliverables of current research will constitute a platform for development of novel nanocomposite films particularly food packaging application.
3.5. Antimicrobial properties of the currently prepared film of CMC/PVA/ ZA incorporated with different concentrations of Au and Ag against many types and species of microbes The target of the current work is to fabricate an environmentally biocompatible film used for food preserving. Thus, the antimicrobial evaluation for these prepared films is very imperative (Imran, ElFahmy, Revol-Junelles, & Desobry, 2010; Li et al., 2016). Fig. 5 represents the antimicrobial activity of the fabricated nanocomposite film doped with different concentrations of Ag+ and Au+3. Films fabricated from CMC/PVA/ZA containing Au at different concentrations (1%, 3% and 5%) and CMC/PVA/ZA containing Ag at two different concentrations; namely; (3% and 5%) are chosen for the evaluation of the antimicrobial activity. It is observed that the composite film prepared with a concentration of Ag (less than 3%) does not give any noticeable antimicrobial activity (data not given). On the contrary, all samples of the nanocomposite films containing Ag+ and Au+3 exhibit super 8
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Fig. 5. Antimicrobial activity of films containing CMC/PVA/ZA, CMC/PVA/ZA incorporated with different concentrations of Au 1%, 3% and 5%) and Ag (3% and 5%) against microbial species; S. aureus, E. coli, C. albicans and A. niger.
Declaration of Competing Interest
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Antimicrobial packaging film based on biodegradable CMC/PVA-zeolite doped with noble metal cations
T
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H.F. Youssefa, Mehrez E. El-Naggarb, , F.K. Foudac, Ahmed M. Youssefd a
Ceramics, Refractories and Building Materials Department, National Research Centre (NRC), Dokki, Egypt Textile Research Division, National Research Centre, 33 El Bohouth st., Dokki, Giza, Egypt c Medical Biochemistry Department, National Research Centre, Doki, Giza, Egypt d Packing and Packaging Materials Department, National Research Centre, 12622, Dokki, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biodegradable polymers Carboxymethyl cellulose Film Zeolite Silver cation Gold cation Food packaging
The current study aimed to enhance the antimicrobial, mechanical and vapor transmission rate of food packaging film by producing films-based zeolite doped with silver (Ag+) or gold (Au+3) cations. Thus, nanopowders of sodium zeolites-A (ZA) and Faujazite-X (ZX) were hydrothermally synthesized using microwave. Sodium cations in both zeolites were exchanged with either Ag+ or Au+3. Films of CMC/PVA were then prepared to support film formation of zeolites containing Ag+ or Au+3. The obtained data indicated that the synthesized films from CMC/ PVA possessed tensile strength equal to 2.05 kgf/mm2. This value was improved and reached to 8.69 kgf/mm2 upon the addition of ZA-Au. Finally, the water vapor transmission (WVTR) and gas transmission rate (GTR) of the prepared nanocomposite film enhanced with the addition of ZA-Au or ZA-Ag. Furthermore, the increase of ZA-Ag or ZA-Au concentration, augmented and boosted both mechanical and antimicrobial properties.
1. Introduction Latest foodborne microbial outbreaks are the main authority to search for such innovative ways to restrain, inhibit and stop the growth of microbial species in food without adversely affecting its worth and safety (Havelaar et al., 2010). It is well known that the traditional film used for food preservation is an easy target for contamination with hazard microbes, which are able to diffuse from the film surface to the food (Biji, Ravishankar, Mohan, & Gopal, 2015). Finding a new appropriate and self-protracting films is the guarantee for having a wellprotected food stuff. Thus, it is a must to use active packaging films incorporated with antimicrobial agent. Silver or gold in the nano size or cation form can be utilized to achieve the goal. The produced film should indicate mechanical properties and air permeability, both of which could be achieved by adding a hard nanoporous filler or enhancer such as zeolitic materials to afford such improvement (Pina et al., 2011). There are many research studies in the literature addressed such point and employed polymers like caroxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) as film forming material. Carboxymethyl cellulose (CMC) is a biodegradable and biocompatible anionic polymer processed from natural cellulose by chemical derivatization. CMC is used for keeping moisture, and improving the structural consistency of bakery products as it is usually applied by
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blending with other stabilizers and gums owing to its high water-absorbing capacity (El-Newehy, Saleh Alotaiby, El-Hamshary, Moydeen, & Al-Deyab, 2016; Radwan et al., 2018; Yu, 2009). On the other hand, polyvinyl alcohol (PVA) is a hydroxyl rich, non-toxic, biocompatible, semi-crystalline plastic and water-soluble polymer (Gaaz et al., 2015). It is extensively used not only in food packaging, but also in many applications, like household and construction sectors, owing to its excellent properties, such as mechanical performance solvent resistance, and biocompatibility (Leja & Lewandowicz, 2010; Yang et al., 2016). The low biodegradation rate and high moisture absorption of PVA are the most common attributes that favors its application in food packaging (Youssef, Assem, Salama, & Abd El-Salam, 2017). Hence, in order to enhance the performance and environmental properties of PVA, it is frequently mixed with certain biopolymers and/ or biobased reinforcements (REF). Further, the problems associated with PVA films concerning shortcomings in both mechanical properties and water permeability are to be relieved via the addition of zeolites. Zeolites are crystalline natural or synthetic solids. They acquire ring structure, which is brought about by linking silicate (SiO4) and aluminate (AlO4) tetrahedra in a porous skeleton which, in turn, entrap an open channel network with interconnected uniform voids and pores in the molecular dimensions of 0.3–1.4 nm range. Some pore size ranges are 3.5–4.5 Å for LTA zeolite, 4.5–6.0 Å for ZSM-5 and 6.0–8.0 Å for
Corresponding author. E-mail addresses: [email protected], [email protected] (M.E. El-Naggar).
https://doi.org/10.1016/j.fpsl.2019.100378 Received 3 July 2019; Received in revised form 27 July 2019; Accepted 2 August 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
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refined kaolin rock, supplied by Middle East Mining Company, Egypt. The main constituents of the rock were 53.25 SiO2, 42.94 Al2O3, 0.41 Fe2O3, 1.62 TiO2 and some other minor amounts of MgO, CaO, K2O. Sodium hydroxide pellets (NaOH) were of analytical grade reagent with the composition of 98.6% NaOH + 0.4% Chloride (Sigma-Aldrich; USA). Ludox AS-40 colloidal silica, 40 wt% suspension, Aldrich, is used as the additional SiO2 source for adjusting the kaolin composition to match that of the targeted Faujasite. Silver nitrate (BDH chemicals—England) and Gold III Chloride, Sigma Aldrich, were used for zeolite modification with Ag+ and Au3+. Carboxymethyl Cellulose (CMC; Mw: 240.208 g/mol) was purchased from Kelong Chemical Agent Factory, Chengdu, China. Polyvinyl alcohol (PVA; Mw: 98,000 with purity more than 99%) was purchased from Sigma-Aldrich Chemicals.
zeolite X and Y (Van Bekkum, Flanigen, Jacobs, & Jansen, 2001). Zeolites are nontoxic with thermal and chemical stabilities. They possess long-term biological properties (Hrenovic, Milenkovic, Ivankovic, & Rajic, 2012; Sabbani et al., 2010), and have ability to reversibly bind small molecules and cations proving that zeolites can be successfully utilized in agriculture, detergency, adsorbents, catalysis, food preservation, as well as medical and pharmaceutical industries (El-Magrabi et al., 2018). It is also reported that introduction of microwave energy during hydrothermal synthesis acts in favor of chemical processes and selectively accentuate chemical reaction kinetics for ceramic powders, gels and metal powders by more than one order of magnitude. In fact, the manufactured zeolites are advantageous for industrial applications than those of natural zeolites because of their high purity, good crystallinity and their tailoring nature. Antimicrobial activities due to cations are introduced for the sake of immobilization or attachment onto zeolite structures (Hrenovic et al., 2012). Incorporating some metals onto zeolite cavities causes structural disinfectionability (Sabbani et al., 2010; Shameli, Ahmad, Yunus, Ibrahim, & Darroudi, 2010). Silver is one of the most common ions to combine in zeolite frameworks for attaining biological activity by virtue of its high stability and broad spectrum against different types of bacteria, viruses, germs, and fungi (Hernández-Sierra et al., 2008; Nirmala Grace & Pandian, 2007; Youssef, Abdel-Aziz, & Fouda, 2017). Silver cations (Ag+) or gold cations (Au+3) are replacing part of the framework of Na+ cations via the exchange process by impregnation of the zeolite solid powder into Ag- or Au-containing liquids. Similarly, Ag and Au in the cations state or nanoform exert a great bactericidal effect on several ranges of microorganisms; their bactericidal effect depends on the size and the shape of the particle (Elshaarawy, Seif, El-Naggar, Mostafa, & El-Sawi, 2019; Hebeish, ElNaggar, Tawfik, Zaghloul, & Sharaf, 2019; Medhat et al., 2017). Also, such cations attain antibacterial and antifungal action, by virtue of their ability to interact with microorganisms (Abdelgawad, El-Naggar, Eisa, & Rojas, 2017; El-Naggar, Shaheen, Fouda, & Hebeish, 2016; Rai, Yadav, & Gade, 2009; Shaheen et al., 2016; Sharma, Yngard, & Lin, 2009; Yan, Abdelgawad, El-Naggar, & Rojas, 2016). This interaction causes structural changes and damage, markedly disturbing vital cell functions, such as permeability, causing pits and gaps, depressing the activity of respiratory chain enzymes, and finally leading to cell death (Kuge & Calzaferri, 2003; Munthali, Elsheikh, Johan, & Matsue, 2014). The target of our work is to design a novel film for food packaging that have several advantages over the commercially available films. Most of the commercial films have low mechanical properties and easily infected with microbes which, in turn, contaminate the preserved foods. In our work, the designed films contain zeolite doped with silver (Ag+) or gold (Au+3) cations possessed high mechanical properties, low water vapor transmission rate and gas transmission rate and excellent antimicrobial properties. Thus, the work is undertaken with a view to firstly prepare nano-powders of sodium zeolite-A (ZA) and Faujasite-X (ZX) hydrothermally using lab microwave. Then, sodium cations in both zeolite types are exchanged with either silver (Ag+) or gold cations (Au+3). The obtained zeolites containing the latter cations are evaluated. Films of CMC/PVA are then prepared to act as a support for film formation of zeolites containing Ag+ or Au+3. The morphological features, particle size and shape, crystallinity and the elemental analysis of the as-synthesized zeolites nanoconmposites are evaluated by making use of TEM, SEM, XRD and EDX. This in-depth assessment of the zeolite nanocomposites is sustained to encompass water vapor transmission rate (WVTR), Gas transmission rate (GTR), and mechanical as well as antimicrobial properties.
2.2. Methods 2.2.1. Microwave hydrothermally preparation of zeolite The two zeolites under investigation were formed using the microwave hydrothermal heating, following the method of Youssef, AbdelAziz, et al. (2017). Zeolite-A (ZA) was hydrothermally derived from kaolin through the alkali attack on the thermally treated kaolin (metakaolinite), whereas, Faujasite-X (ZX) was prepared after the modification of kaolin by additional of silica (Ludox 40). Zeolite-A (ZA) was prepared from a fresh gel which is directly processed by attacking the calcined kaolin by 100 ml of caustic soda of 3.0 M composition and stir for 15 min at room temperature. Whereas, ZX was derived from two separate solutions, one is containing 40 gm of Ludox 40 dissolving in 50 ml of 3.0 M caustic soda solution in a Teflon beaker. The other solution is formed by adding 5 g of the starting material to another 50 ml of the caustic soda with the same previous concentration. Lab microwave digestive system, model MARS 5, XP-1500 vessels, CEM Corp., Matthews, NC), operating at 2.45 GHz of frequency and from 1 to 100% power of 1600 W, represents the hydrothermal tool used for the zeolite crystallization from the previously obtained gels. ZA was heated for 1 h at 70 °C whereas, ZX was heated for 2 h at 110 °C. Reaction products were then collected, washed several times with distilled water (up to pH = 7–8), dried overnight at 100 °C in an electric oven. The materials and their ratios for the zeolites under investigations; ZA are S/Al (1–2), NaOH (2.5–4 M) and H2O/SiO2 (100–150), meanwhile, for ZX are the same as presented in ZA but with a higher Si/Al ratio in the range of 4–8 and H2O/SiO2 (100–200). 2.2.2. Preparation of silver and gold doped zeolite nanocomposite The cation exchange process was used to incorporate the silver and gold cations into the zeolite structure by the immersion method. Typically, zeolite powders (5 g) were impregnated into 100 ml of 0.1 M of either AgNO3 or HAuCl4 for 2 and 10 h, respectively. Then, the stained zeolites resulted from the cation replacement between zeolitecontained sodium and the cations of silver or gold from the exchanging solutions were collected, washed several times with distilled water and finally dried at 50 °C for 2 h and kept for further investigation. 2.2.3. Preparation of CMC/PVA/Zeolite doped with Ag or Au cations A homogenous viscous solution of carboxymethyl cellulose (CMC) was prepared by dissolving 3 g of CMC in 100 ml of deionized water at 70 °C under magnetic stirring. Similarly, polyvinyl alcohol solution (PVA) was prepared by adding 3 g of PVA to 100 ml of deionized water at 70 °C and kept under magnetic stirring till complete dissolution. Then, 50:50 (v/v) of CMC: PVA solution was prepared by mechanical mixing the solutions of the two polymers and kept under mechanical stirring for 6 h. The as-prepared CMC/PVA nanocomposites were mechanically mixed with different concentrations of ZA-Ag and ZA-Au or ZX-Ag, ZX-Au solutions (1%, 3% and 5% w/v) followed by sonication for 1 h at room temperature. The homogeneous CMC/PVA/zeolite nanocomposites were poured onto the Teflon dish in order to evaporate
2. Materials and method 2.1. Materials The starting materials used for zeolite synthesis were MEMCO 2
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Scheme 1. Schematic representation of the formation of CMC/PVA film before and after mixing with ZA doped with Ag or Au cations.
The antimicrobial properties of zeolite films were investigated as follows: Disc agar plate method was established to evaluate the antimicrobial activities of the as synthesized CMC/PVA loaded with zeolites containing the said nobel cations. Four different test microbes, Staphylococcus aureus (G + ve bacterium), Escherichia coli (G-ve bacterium), Candida albicans (Yeast) and Aspergillus niger (fungus) were selected to evaluate the antimicrobial activities. The bacterial and yeast test microbes were grown on a nutrient agar medium. whilst, Aspergillus niger was cultivated on potato dextrose agar (PDA) medium. The culture of each test microbe was diluted by sterilized distilled water to 107–108 colony forming units (CFUs)/ml, and then 1 ml of each was used to inoculate 1 L Erlenmeyer flask containing 250 ml of solidified agar media. These media were put onto previously sterilized Petri dishes (10 cm diameter having 25 ml of solidified media). 10 mm discs of films were placed on the surface of inoculated petri under sterilized conditions. The plates seeded with test microbes were incubated for 24 h–48 h, at the appropriate temperature of each test organism. Antimicrobial activities were recorded as the diameter of inhibition zones (including the disc itself) that appeared around the discs. Neomycin (100 μg/disc) and cyclohexamide (100 μg/disc) were used as antibacterial and antifungal standards, respectively.
the solvent and to obtain the ultimately product in the form of a film. The Teflon dishes were left without interruption at room temperature for 72 h. The prepared nanocomposite films were peeled off from the mold and made ready for characterizations.
2.3. Characterization The obtained zeolites were characterized before and after the cation exchange process for their mineralogical composition using X-rays diffraction analysis (XRD), Bruker D8 Advance-Germany using a Ni filter, Copper Target, at V = 40 Kv and A = 40 mA. Meanwhile, the grain morphology and particle size of the nanopowders were monitored using transmission electron microscopy (TEM) model (JEOL JEM-1230, Tokyo, Japan). The surface of the resulting films was investigated by scanning electron microscopy (SEM) at an acceleration voltage of 20 kV using a Quanta 400 SEM (Oxford, UK). Energy Dispersive X-ray analysis (EDX) was used for analyzing the chemical composition of zeolite crystallites, Oxford Instrument INCAx-sight is a part added to Joel TXA840A electron probe micro analyzer. Water vapor transmission rate (WVTR) was carried out using GBI W303 (B) Water Vapor Permeability Analyzer (China) using the cup method. The water vapor permeability was measured as the amount of water vapor passes through the tested film. Also, WVTR was measured as the mass of water vapor transmitted throughout a unit area in a unit time under controlled conditions of temperature (38 °C) and humidity (4%) according to the following Standards (ASTM E96, ISO 2528, ASTM D1653, TAPPI T464, DIN 53122-1, JIS Z0208). Also, the gas transmission rate (GTR, (O2 and CO2) are measured by N530 Gas Permeability Analyzer (China), according to the following StandardsISO2556-2001, GB/T1038-2000, ASTM D143 4-8 2(2003). The mechanical properties of the prepared films containing CMC/ PVA blended with zeolite doped with cations were measured according to the ASTM D638-91 standard using a universal testing machine LK10k (Hants, UK) fitted with a 5 kN load cell, and operated at a rate of 5 mm/ min on the samples.
3. Results and discussion To start with, the colors of CMC/PVA films before and after submission to zeolite bearing silver and gold cations are inspected. It was observed that the films display white color before being blended with zeolite/Ag+ or zeolite/Au+3. On the other hand, the films after such blending assume optically semitransparent yellowish color. No difficulty is encountered while peeling the films from the dish; the formed films are easily separated from the dish. It was further noticed that the color of the films under investigation become more yellowing after casting these films. This could be accounted for by reduction of traces of Ag+ or Au+3 found in the cavity of zeolite to nanoparticles under the influence of reducing groups located on the CMC backbone (El-Naggar 3
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Fig. 1. XRD patterns for the prepared Ag+ and Au+3 doped (A) ZA and (B) ZX.
with the chemical formula of Na92Al92Si100O384.27H2O. Meanwhile, Fig. 2B exhibits the XRD patterns for the nano-forms of Faujasite-NaX and its ZXAg & ZXAu versions that are comparable with the PDF# 832319, which assigned for sodium aluminum silicate hydrate zeolite Xsynthetic, with a Na7.4 Si7 Al5O24.5.2H2O representative chemical composition. It is as well to emphasize that, XRD patterns of the parent zeolites (ZA and ZX) are preserved in the cations-containing species of both types after the cation exchange process, but ZXAg displays some reduction in its peak intensities. The latter effect could be traced back to the corrosive action of the AgNO3 on the much smaller crystallites of ZX, vis-à-vis those of the bigger crystallites of ZA (Youssef, Abdel-Aziz, et al., 2017). The measurement of the specific surface area (BET) for the two parent zeolites are determined and the data obtained are recorded. Values refer to 374 m2/g and 462 m2/g for ZA and ZX, respectively. As an integral part of the above characterization, the particle shape of the two zeolites in question, before and after doping with Ag and Au cations, was examined using TEM technique. Fig. 2A and B exhibit the fine particle identity of ZX in its free and corresponding Ag-exchanged form. In a similar manner, the nanostructure of bigger particles of Aufree and Au-replaced-ZA are displayed in Fig. 2C and D.
et al., 2016; El-Rafie et al., 2011; Hebeish, El-Rafie, El-Sheikh, Seleem, & El-Naggar, 2014). However, mention should also be made that the calcination temperature above 400 °C is not advocated due to the formation of cracks at this temperature (Youssef & El-Sayed, 2018). Films formed from CMC/PVA/ZA and doped with Ag or Au cations are represented in Scheme 1. Obviously, the color of original zeolite before adding the noble cations is white, its color turned to grey and yellowish only after introducing the cations of Ag and Au into its structure. This speaks of the formation of Ag or Au inside the cavity of zeolite as cation exchange or nanosized forms. The nanocomposite of zeolite doped with Ag or Au cations is fully characterized using XRD, BET, TEM, SEM-EDX techniques. This is done before film characterization
3.1. Characterization of zeolite-A and zeolite-X loaded with and without Ag and Au cations Fig. 1A and B depict the XRD profiles for the currently synthesized nanozeolite powders of ZA and ZX with their Ag (ZAAg & ZXAg) and Au (ZAAu & ZXAu) derivatives, respectively. Clearly, Fig. 1A shows the complete match between the peak positions and their relative intensities of the PDF#76-1041 standard card for the synthetic zeolite 4A, 4
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Fig. 2. TEM of the as-prepared (A) ZA, (B) ZX, (C) Ag incorporated ZA, (D) Ag incorporated ZX, (E) Au incorporated ZA and (F) Au incorporated ZX.
containing Ag+ and Au+3. Fig. 3 shows the images which represent the output of this study. Fig. 3A and B illustrate the internal texture under microwave heating of ZA and ZX respectively. It can also be easily realized that all over the scanned area a homogenous grain size distribution exists reflecting the development of uniform nucleation and crystallization process. When these types of zeolites (ZA and ZX) are immersed independently in solutions of Ag and Au salts, the ZA containing Ag+ and Au+3 (Fig. 3C and E) and ZX (Fig. 3D and F) imply brighter tents than their original species. The implication of this is that the exchange process is successful, and can be interpreted in terms of the interaction involving the secondary electrons of the microscopic and the incorporated Ag+ and Au+3 within zeolite pores and channels. A close examination of the output of current evaluation would reveal that both zeolites (ZA and ZX) undergo decrement in particle size after the cation exchange process because of particle decay of aluminosilicate zeolites and their instability under acidic media (Hrenovic et al., 2012). The onset images in Fig. 3 assess the elemental analysis of the scanned ZA and ZX before and after both zeolites are impregnated in the salts of Ag+ and Au+3. This is, indeed confirm the appearance of Ag and Au after impregnation thereby approving the successful
Based on TEM examination, it is obvious that the population of developed crystallites of both zeolites (ZA and ZX) are uniformly disseminated and imply a narrow range of particle size distribution of (100–125 nm) and (200–300 nm) for ZX and ZA, in respective order. ZA and ZX containing Ag and Au are synthesized followed by analyzing their colloidal solution by making use of TEM. Images obtained are shown in Fig. 2. Accordingly, the silver and gold- loaded zeolites of both types (Fig. 2C, D) acquire, to some extent, the homogenous replacement process resulted in a homogeneity in the tent of the crystal color under TEM conditions. Whereas, those exchanged by gold (Fig. 2E, F) were spotty, probably due to the common difficulty encountered in the replacement process of the sodium cations of zeolite by Au ones, ascribed to the instability of Au ions without legends (Weitkamp & Puppe, 2013). Particularly visualized is that both ZA and ZX nanopowder presented more Ag cations in causing staining as compared with Au species, this high affinity of Ag towards zeolite-A will be explained in the next SEM-EDX part. SEM-EDX technique is devoted to evaluate the morphological structure and chemical analysis of the scanned samples of ZA and ZX 5
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Fig. 3. SEM and EDX of (A) ZA, (B) ZX, (C) Ag incorporated ZA, (D) Ag incorporated ZX, (E) Au incorporated ZA and (F) Au incorporated ZX.
formulation of zeolites incorporated with Ag+ and Au+3. A significant reduction in the amounts of sodium appears in the case of the Ag+ exchange process, accompanied by a remarkable presence of incorporated Ag in both types of zeolites. It seems that the evolution of Ag+ in zeolite compositions was established at the expenses of the depleted sodium. Table 1 contains the average amounts of Ag and Au incorporated in ZA and ZX; expressed in weight (wt%) and atomic (At%); keeping in mind the consideration that these data are taken from the average 5
crystallites in SEM images. Results of Table 1 displays that, the amount of Ag+ and Au+3 incorporated ZA is much higher than that of the cations incorporated ZX. This can be interpreted in the light of the cation exchange capacity (CEC) of each zeolite type. CEC is the reflection of its number of exchangeable sites that can facilitate the incorporation of the Ag+ and Au+3 at the expenses of sodium cations (replaceable cation). Table 1 shows also that the amount of Au in zeolite constituents is so low with nearly slight decrease in sodium contents of both zeolites. This is rather expected since ZA has a low Si/Al- zeolite and much more exchangeable sites (12 Na+/unite cells) than those of ZX species (8 Na+/unite cells). ZA has much compensating Na+ cations within its framework (12 Na+ / unite cells), compared to that of ZX (only 8 Na+ /unite cell) (Barrer, Rees, & Shamsuzzoha, 1966; Moissette, Hureau, Col, & Vezin, 2016).
Table 1 EDX micro chemical analysis for amounts of Ag and Au in zeolites. Quantity
Weight % Atomic%
Ag
Au
ZA
ZX
ZA
ZX
27.31 6.81
17.46 3.85
2.13 0.22
1.54 0.17
6
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Fig. 4. SEM of (A) CMC/PVA, (B) CMC/PVA/ZA-Ag(1%), (C) CMC/PVA/ZA-Ag(3%), (D) CMC/PVA/ZA-Ag (5%), (E) CMC/PVA/ZA-Au (1%), (F), CMC/PVA/ZA-Ag (3%), (G) CMC/PVA/ZA-Ag(5%).
3.2. Characterization of CMC/PVA/ZA and CMC/PVA/ZX films incorporated with different concentrations of Ag and Au cations
movement of water vapor from the surrounding atmosphere into food products or moisture losses from the food to surrounding atmosphere exert significant influence on the packaged food products from the quality and stability point of view during the shelf-life time (Youssef, Youssef, Ayad, & Sarhan, 2015). Table 2 shows the WVTR for the CMC/PVA/ZA nanocomposite films under investigation. The results signify that CMC/PVA films display higher WVTR as compared to CMC/PVA/ZA nanocomposite films. Increasing ZA-Ag or ZA-Au in the CMC/PVA/ZA nanocomposites decreases the WVTR value of the film. The addition of (1%, 3% and 5%) of ZA-Ag decreases the WVTR value of the film to (1692.05, 1416.13 and 616.15 g/(m2 day), respectively. This is in comparison to CMC/PVA films which acquire a value of 2014.56 g/(m2 day). Also, in the case of using ZA-Au, the WVTR value of the prepared nanocomposites film decreases by increasing the concentration of ZA-Au in the nanocomposite film (1%, 3% and 5%); the WVTR attain values of (1919.82, 1260.43 and 623.95 g/(m2 day), respectively. It is also of interest that the GTR of the prepared CMC/PVA/ZA nanocomposite films is accentuated by the addition of modified zeolite through either Ag+ and Au+3. Table 2 shows that the GTR (CO2 or O2) of the prepared nanocomposites enhances by raising the ratios of ZA-Ag or ZA-Au in the CMC/PVA matrix. This is due to the characteristic features of zeolite as porous materials. Hence the permeability of (CO2 or O2) increased move through the prepared nanocomposites films as
The surface and fracture morphology of the cast CMC/PVA and CMC/PVA/zeolite films containing Ag+ or Au+3 was studied using the SEM technique as shown in Fig. 4. It is observed that the microstructural image of CMC/PVA film (Fig. 4A) is smooth, compact and homogeneous surface and fracture. Meanwhile, the morphological features of CMC/PVA undergo changes owing to mixing with zeolite incorporated with Ag+ as shown in Fig. 4(B–D) or Au+3 (Fig. 4E–G). As previously mentioned, the zeolite was firstly prepared using three different concentrations of Ag and Au cations (1%, 3% and 5%). Fig. 4(B, E) concerned with the film blended with zeolite/Ag (1%) and zeolite Au (1%) reveal that the surface becomes slightly rough with the appearance of small particles due to the presence of zeolite/Ag and zeolite/Au particles in the fracture. By increasing the concentration to 3% of Ag+ and Au+3 attached to zeolite before mixing with CMC/PVA solution as well as before film formation, the surface of the fabricated film becomes rough with remarkable presence of particles on the surface of the film as illustrated in Fig. 4(C and F) for Ag- and Au- incorporated zeolite respectively. Further increase in the concentration to 5% of cation incorporated zeolite solution, the surface becomes very rough with well-defined occurrence of aggregated particles on the surface of CMC/PVA film as shown in Fig. 4(D and G). However, there are no visible cracks or holes formed with films of CMC/PVA blended with zeolite incorporated with different concentrations of Ag+ and Au+3. Since there is no significant difference in the SEM characterization of CMC/PVA mixed with ZA or ZX nanocomposites, the as-prepared films based on CMC/PVA/ZA containing different concentrations of Ag+ and Au+3 is only selected for further investigation.
Table 2 The water vapor permeability as well as the gas transmission rate (O2 and CO2) of the prepared nanocomposites films. Samples
CMC/PVA Zn-Au (1%) Zn-Au (3%) Zn-Au (5%) Zn-Au (1%) Zn-Au (3%) Zn-Au (5%)
3.3. Gas transmission rate (GTR) and water vapor transmission rate (WVTR) of the newly prepared nanocomposites films It is well established that the water vapor transmission rate (WVTR) is an essential measure for advocating the suitability of packaging materials that are used in food packaging applications. Accordingly, 7
GTR, (cc/M2 day)
WVTR, g/(m2 day)
O2
CO2
15.31 27.37 38.79 83.54 30.7 40.86 67.84
0.05 0.09 1.89 29.17 0.60 1.96 31.28
2014.56 1692.05 1416.13 616.15 1919.82 1260.43 623.95
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antimicrobial properties against many species of microbes (bacteria; Fungi and Yeast) as shown in Fig. 6. The Films incorporated with Ag or Au cations in the zeolite framework display antibacterial activity against microbes. Because of their concentration, the cations of Ag or Au can easily reach the nuclear content of microbes and present the greatest surface area. It is also found that the diameter of the inhibition zone increases by increasing the content of cation inside the pores of zeolite. As well known, metal nanoparticles such as silver or gold nanoparticles play a key role in antimicrobial activities. Thus, there are two possible mechanisms for killing the microbes using AgNPs or AuNPs nanoparticles loaded CMC/PVA/Zeolite film. The first mechanism relies on the high oxidation state of AgNPs or AuNPs than can produce atomic oxygen around it. Along these lines, the formed atomic oxygen can react with the double bonds of bacterial cell wall lipids and penetrates into the inside of the microbe, interact with proteins and lipopolysaccharides, which consequently changes the permeability of the bacterial cellular membrane leading to bacterial death. The second hypothesis might be assigned to the capability of AgNPs or AuNPs to disturb the cell wall of microbes by means of direct contact to the cell membrane through electrostatic interaction, prompting to the adjustment of its permeability and respiration chain or by indirect contact throughout penetration into microbe cell and thenceforth release the ions of Ag or Au which interacts with thiol groups and/or phosphates moieties of DNA or protein in microbes (Dakal, Kumar, Rita, & Yadav, 2016; Mohamed, El-Naggar, Shaheen, & Hassab, 2017). By and large, the data obtained are promising due to the super antimicrobial properties of the produced films. Thus, the fabricated film can be used for realistic food packaging applications like food containers, chairs, vials, etc.
Table 3 The mechanical properties of the prepared CMC/PVA/ZA nanocomposite films. Samples
% ZA-Ag or ZA-Au
Tensile strength kgf/mm2
Elongation (%)
PVA CMC CMC/PVA CMC/PVA CMC/PVA CMC/PVA CMC/PVA CMC/PVA
0.0 0.0 ZA-Au (1%) ZA-Au (3%) ZA-Au (5%) ZA-Ag (1%) ZA-Ag (3%) ZA-Ag (5%)
1.43 1.82 3.24 5.74 8.69 2.56 4.12 6.85
150.53 3.67 60.54 68.98 75.70 55.70 68.28 80.15
compared with CMC/PVA matrix. 3.4. Mechanical properties of the newly prepared nanocomposites film The mechanical properties of the newly prepared nanocomposites films are of prime importance and vital for films used for packaging applications (Avella et al., 2005; Rhim, 2012). These films can be evaluated for the tensile strength and modulus of elasticity, yield strength, ultimate tensile strength, and toughness. Table 3 discloses that the mechanical properties of the prepared CMC/PVA/ZA nanocomposite films are based on the (ZA-Au or ZA-Ag). These latter two components increase by increasing the addition of (ZAAu and ZA-Ag) as per the following ratios (1, 3 and 5%) based on CMC/ PVA matrix. The tensile strength improves from 2.05 kgf/mm2 for CMC/PVA matrix to attain values of (3.24, 5.74 and 8.69 kgf/mm2) upon increasing the concentration of ZA-Au in the matrix from (1, 3 and 5%), respectively. This effect could be accredited to the fact that at minor concentrations of ZA-Au, the good interfacial adhesion is attained. This tolerates the interfacial configuration of the fabricated CMC/PVA/ZA nanocomposite to let part of the tensile strength. Additionally, by increasing the amount of ZA-Au in the CMC/PVA/ZA nanocomposite films, the elongation enhances to 75.32 by addition of 5% ZA-Ag in the nanocomposite films compared to 60.54% elongation for CMC/PVA matrix. Also, the addition of ZA-Ag in different amounts (1, 3, and 5%) to the CMC/PVA matrix leads to the improvement of the tensile strength of the prepared nanocomposites by (2.56, 4.12 and 6.85 kgf/mm2) respectively. The elongation of the prepared nanocomposite films enriches further to attain a value of 80.15 by addition of 5% ZA-Ag in the CMC/PVA/ZA nanocomposite films as compared to 60.54% elongation for CMC/PVA matrix. This enhancement in both tensile strength and elongation speaks of good interaction between the CMC/PVA matrix and the (ZA-Au or ZAAg) as filler. As a result of the higher contact surface area between inorganic filler and polymer matrix is developed.
4. Conclusion The present research was designed to tackle serious problems encountered with biodegradable CMC/PVA blend films upon their use in food packaging applications. Hence, novel nanocomposite films with superior antimicrobial properties were fabricated comprise zeolite doped with a silver (Ag+) or gold cation (Au+3). Nanopowders of sodium zeolite-A (ZA) and Faujazite-X (ZX) were hydrothermally synthesized using microwave. Sodium cations of zeolites were replaced with Ag+ or Au+3. To this end, films of CMC/PVA were prepared to act as a support for film formation of zeolites containing Ag+ or Au+3 cations. TEM images displayed that the particle shape of crystals of both zeolites were uniformly distributed and acquired narrow range of sizes with average less than 200 nm. SEM microstructural images of surface fracture morphology of the cast CMC/PVA/zeolite films containing Ag+ and Au+3 exhibited smooth compact and homogeneous surface and fracture. No visible holes or cracks were formed with the formed films. Energy dispersive X-ray (EDX) proved the existence of metal cations doped zeolite nanopowder and precipitated on the surface of the asprepared film. The obtained data indicated that the as-synthesized films based on CMC/PVA had a tensile strength equal to 2.05 kgf/mm2. This value has been improved and reached to 8.69 kgf/mm2 with the addition of ZA-Au or ZA-Ag. Water vapor transmission (WVTR) for nanocomposite films (CMC/PVA/ZA-Au displayed high values (616 g/ (m2 day) than does the CMC/PVA films (2014 g/m2 day). Additionally, the value of gas transmission rate was increased from 15 (cc/M2 day) for CMC/PVA film to 83 (cc/M2 day) with the addition of ZA-Au (5%). The mechanical properties of CMC/PVA film was increased with the addition of ZA-Au to the composite before the formation of films. Finally, the CMC/PVA/ZA containing Ag+ and Au+3 displayed excellent antimicrobial activity. It is envisaged that the outputs and other deliverables of current research will constitute a platform for development of novel nanocomposite films particularly food packaging application.
3.5. Antimicrobial properties of the currently prepared film of CMC/PVA/ ZA incorporated with different concentrations of Au and Ag against many types and species of microbes The target of the current work is to fabricate an environmentally biocompatible film used for food preserving. Thus, the antimicrobial evaluation for these prepared films is very imperative (Imran, ElFahmy, Revol-Junelles, & Desobry, 2010; Li et al., 2016). Fig. 5 represents the antimicrobial activity of the fabricated nanocomposite film doped with different concentrations of Ag+ and Au+3. Films fabricated from CMC/PVA/ZA containing Au at different concentrations (1%, 3% and 5%) and CMC/PVA/ZA containing Ag at two different concentrations; namely; (3% and 5%) are chosen for the evaluation of the antimicrobial activity. It is observed that the composite film prepared with a concentration of Ag (less than 3%) does not give any noticeable antimicrobial activity (data not given). On the contrary, all samples of the nanocomposite films containing Ag+ and Au+3 exhibit super 8
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Fig. 5. Antimicrobial activity of films containing CMC/PVA/ZA, CMC/PVA/ZA incorporated with different concentrations of Au 1%, 3% and 5%) and Ag (3% and 5%) against microbial species; S. aureus, E. coli, C. albicans and A. niger.
Declaration of Competing Interest
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