Bio-nanocomposite-based packaging materials have gained interest due to their possible applications in food packaging. Cellulose acetate (CA) is a biopolymer obtained by acetylation of cellulose and has characteristics such as biocompatibility, biodegradability and high transparency. Introducing Metal-Organic Frameworks (MOFs) offer good mechanical strength, unique surface area and both chemical and thermally stable, making them great supporting materials in the development of polymer-based packaging materials. Among them, Fe-MIL-88A an iron-based MOF, integrated with CA and spinach extract was added to the prepared material in different compositions and cast as a film. The Spinach loaded, Fe-MIL-88A integrated cellulose acetate film significantly enhanced the tensile strength, water vapour permeability, and anti-microbial activity. The prepared film is then characterized using a scanning electron microscope (SEM), Fourier transforms infrared spectrometer (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Further, studies on mechanical properties, degradation test and real-time applications of the prepared films were carried out.
Research Article
Novel synthesis of metal-organic-framework-based film with enhanced mechanical properties, biodegradability and antibacterial performance for packaging applications
https://doi.org/10.21203/rs.3.rs-1947579/v1
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Cellulose acetate
Fe -MIL 88A
Spinach
Biodegradation
In recent years, bacterial food contamination has been a major source of concern. To address these concerns and problems, a new class of environmentally friendly and biodegradable packaging materials have emerged. One of the possibilities for reducing human influence on the environment is the creation of biodegradable materials [1]. For the past few decades, several bio-based materials and their novel applications in food packaging have received a lot of attention [2]. Biodegradable polymers such as cellulose acetate (CA), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), polyester, pectin, starch, chitosan, poly(vinyl alcohol) (PVA), etc. have emerged. Out of which CA is the most interesting of all the cellulose derivatives. It is water-insoluble, which is an important property for packaging applications. Furthermore, it has a wide range of applications, such as reverse osmosis membranes, paper coating, filters, etc. [3].
However, CA has significant drawbacks, which include low ductility and flexibility, and poor water vapour barrier properties, which all prohibit their application in the food packaging industry. Nanofillers like nano clay, inorganic nanoparticles, carbon nanotubes, and graphene oxide nanosheets, etc. can be incorporated into CA to enhance its mechanical and functional properties [4]. Metal-organic frameworks (MOFs) are a broad category of hybrid organic-inorganic supramolecules made up of metal ions connected together by organic bridge ligands. MOFs are ideal candidates for a wide range of applications, including catalysis, energy storage, bio-imaging, medication delivery, food packaging, etc. due to appealing features like high surface area, acceptable biocompatibility, biodegradability, flexible porosity, etc. MOFs like MIL-53, ZIF-67, MOF-1, ZIF-7, MOF-2, ZIF-8, and have been reported in various studies [5–8]. Fe-MIL-88A is one of the most promising materials in these MOFs. Fe-MIL-88A has a three-dimensional framework with interconnected pores and cages, consisting of oxocentered trimers of iron (III) octahedrally connected by fumarate linkers with high flexibility. Since both constituents are naturally present in the human body, the regular hexagonal-shaped particles in this material are biocompatible and nontoxic [9]. The main objective of this work was to develop high-performance and biodegradable packaging material with high mechanical properties. Therefore, to overcome the drawbacks of CA it was reinforced with Fe-MIL-88A to enhance the above-mentioned properties. The influence of MOF on the CA/Fe-MIL-88A composite was evaluated by studying the structural, thermal and mechanical performance of the nanocomposites. Recent studies have revealed that when an adequate amount of antimicrobial agent (AM) is introduced into a polymer film used for packaging material can effectively suppress the bacterial growth which is responsible for the food rotting. In that case, AM packaging plays a major role in inhibiting the growth of specific microorganisms in foods, improving food safety and extending shelf life without sacrificing food quality [10].
Plants are the cheapest and safest alternative sources of antimicrobials and are used in therapeutic treatments for both phytochemical and antibacterial activity. Allmania nodiflora spinach, a widespread herb and green leafy vegetable species in the Amaranthaceae family which is commonly known as celosia. Allmania nodiflora spinach extract (ANE) contains a mixture of alkaloids, flavonoids, saponins, glycosides, steroids, polyphenols and these have antibacterial, phytochemical, fungicidal, and antiviral capabilities. ANE is robust against both Gram-positive and Gram-negative microorganisms [11]. However, the Allmania nodiflora spinach extract-loaded MOF/CA matrix has not been reported in the literature. Hence a spinach-loaded MOF/CA composite film was prepared as the MOF increases the film's mechanical properties while spinach increases the antibacterial activity. Spinach extract was well dispersed in MOF/CA composite film due to its hydrophilicity. With the addition of ANE in the CA/MOF matrix, the MOF-based degradable films were successfully prepared against E.coli and S.aureus. The ability of films to prolong the shelf life of food products was tested and it proved as a novel biodegradable material with antibacterial activity.
Materials
Cellulose acetate (CA, 39.8 wt. % of acetyl content, Mn ca. = 30 000 g.mol-1, DS = 2.45), ferric chloride hexahydrate, fumaric acid, acetone, and sodium hydroxide, DMF was purchased from Sigma-Aldrich, the selected medicinal plant Allmania nodiflora spinach was collected from the local market in Chinnalapatti, Dindigul district, Tamilnadu.
Synthesis of Fe-MIL 88A
Hydrothermal synthesis was followed, wherein 0.9744g fumaric acid and 2.2722g FeCl3.6H2O were added to a beaker containing 50 mL distilled water. The mixed solutions were stirred for 30 minutes with a magnetic stirrer before being placed in a 100mL Teflon-lined steel autoclave and heated at 105°C for 12 hours. The precipitates were recovered by centrifugation at 9000 rpm for 10 minutes and cooled until they reached room temperature. To remove any excess fumaric acid that did not participate in the reactions, the as-prepared solid was washed with ethanol and water several times. After centrifugation, the final precipitates were dried in a vacuum oven at 100°C for at least 10 hours [12].
Extraction of Allmania nodiflora spinach
50g Allmania nodiflora spinach leaves were washed with water until all the dust particles are removed and then dried. The dried leaves were ground to a fine powder, which was soaked in 200ml of ethanol in a 500ml beaker for about 24 hours. After that, the crude sample was filtered through Watmann filter paper. The collected sample was concentrated by a rotary evaporator [13].
Preparation of spinach-loaded CA/MOF matrix film
As follows, composite films were produced using a modified method as shown in Fig. 1.[14]. By dissolving cellulose acetate in acetone, the casting solution was prepared. The preceding solution was stirred at ambient temperature to attain a homogeneous solution using a magnetic stirrer. Then, a 1% solution of Fe-MIL 88A dispersed in acetone was mixed with CA and stirred for 4h to obtain CA/Fe-MIL 88A mixture. Different amounts of spinach extract relative to the total mass of the CA/Fe-MIL 88A mixture were also added and mixed to produce various concentrations of spinach extract in the films. A casting instrument has been used to uniformly cast a homogeneous spinach extract loaded CA/Fe-MIL 88A film-forming solution. The obtained film was placed in a coagulant wash & dried to form composite films. The films were kept for 24 hours in a humidifier at ambient temperature with a relative humidity of 50% for further use. The composed films that had been prepared were assigned names: CA(F1), CA/Fe-MIL-88A(F2), 2mL of ANE-CA/Fe-MIL 88A(F3), 4mL of ANE-CA/Fe-MIL 88A(F4), 6mL of ANE-CA/Fe-MIL 88A(F5), 8mL of ANE-CA/Fe-MIL 88A(F6), 10mL of ANE-CA/Fe-MIL 88A(F7).
Characterization details and Analysis
Functional groups in CA, Fe-MIL 88A and Allmania nodiflora extract reinforced composite films were analyzed using an FTIR (Fourier Transform Infrared Spectrometer) On a JASCO FTIR – 460 plus, in the 4000–400 cm-1 range. The morphologies of bare CA (F1), C A/Fe-MIL 88A (F2) and 2mL of ANE-CA/Fe-MIL 88A(F3), 4mL (F4), 6mL (F5) of ANE-CA/Fe-MIL 88A, films were analyzed by SEM. XRD characterization was performed to evaluate the crystal structure of Fe-MIL 88A. The prepared films' thermal stability was analyzed using A SEIKO model TG/DTA 6200 being used for TGA. The prepared films' water contact angles were measured by goniometry (rame - hart instrument. Co, USA). The average of three measurements was used to measure each composite film's water contact angle. To investigate the mechanical strength of composite films, a universal testing machine (UTM, Tec Sol India) was used at 23 °C. ASTM D638 with a cross head speed of 5 mm/min. The testing was performed with test films having a thickness of 150 mm x 25 mm (in mm) dimension [15]. The composite film’s thickness was measured using a digital screw gauge at five distinct places, and the average values were given as the thickness values for the films.
Water vapour permeability
A slight modification of the standard procedure E96-E95 (ASTM method 1995) [16] has been used to assess the water vapour permeability (WVP) of films. The sample films were firmly placed on the head of a 15cm3 glass vial, and Calcium chloride anhydrous was pre-weighed using a standard procedure. The vials were retained at 75 ± 5% RH and kept in a desiccator with a saturated sodium chloride solution at 30 ± 2 °C. In the desiccator, which was tightly closed, the vials were measured before and after 1 day. The film's WVP was calculated based on the formula Eq. (A.1),
where A (g/h) is the slope of the linear regression of the weight in time, Δx (mm) is the film thickness, (p1 – p2) (kPa) is the difference in partial pressures inside and outside the cell, and S (m2) is the surface area of the film.
Transparency of the film
A UV–vis spectrophotometer was used to analyze the polymeric films’ absorbance between 280 and 600 nm, respectively, to determine their transparency. The test film was divided into rectangular pieces and put in a sample container for the spectrophotometric technique. The film's opacity was determined according to the method in Eq. (B.2),
Where T denotes transparency, Abs600 refers to the film’s absorbance around 600 nm, while X represents the film's thickness in mm [17].
Biodegradation tests
Biodegradation tests were carried out as described in the literature [18]. With reference to that, films that were free from moisture were taken as (W0). The weighted films (F1, F2, F3, F4, and F5) were placed onto the soil for 60 days in a squeezed atmosphere. The aforementioned setup was maintained at room temperature following regular water additions. Changes according to by were gravimetrically measured. Films that were already deposited were cleaned properly. Afterward, all films were reweighed (Wt) to estimate the average weight reduction (% W L) [19]. To find % of weight reduction W L values using Eq. (C.3) the preceding methods were performed three times.
Antibacterial activity of food packaging films
Antibacterial activity of films was evaluated using the diffusion method against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) [20]. Before the testing, the samples were sterilized in nutrient agar. Following sterilization, the produced mixture was poured onto Petri plates and allowed to solidify. After the agar plate has solidified, the sample microorganisms (5% of the lower inoculum) were scrubbed on its surface. All films were clipped into 10 mm-diameter circle parts, which were then placed on an inoculation agar plate and incubated for 24 hours at 37 °C. After incubation, the microbial inhibition distance was measured.
Mechanical properties
TS (tensile strength) and EAB (elongation at break) results of different samples are shown in Fig. 2. TS and EAB values of F1 (CA) film were 5.47 MPa and 22.55 %, respectively. By gradually increasing the amount of antibacterial agent as 2mL, 4mL and 6mL for the films F3, F4 and F5 respectively, along with MIL 88A, increases the TS of composite film from 15.9 MPa to 22.78MPa whereas EAB values gradually declined from 20.57% to 7.94%. The strong binding nature of CA/MIL 88A and the antibacterial agent could probably be the reason for the change in mechanical properties. Herein, MIL 88A with an antibacterial agent could have acted as a cross-linker and the molecules packed densely to enhance tensile strength. On the other hand, the composite film’s flexibility was decreased which is reflected in reduced EAB values. The addition of an antibacterial agent up to 6mL did not cause a considerable change in mechanical property, which further increase in the antibacterial agent affected the tensile strength of composite films, due to higher hydrophilicity of films F6 and F7. So the optimized ratio of the antibacterial agent is fixed at 6mL (F5) film.
SEM
The morphology of the prepared film was studied using SEM and the image of CA (F1) film Fig. 3a. indicated a slightly porous and cracked nature. Whereas, Fig. 3b. Fe-MIL 88A exhibits a regular hexagonal rod-like structure. Fe-MIL 88A well embedded onto the CA film as seen in Fig.3c. While Fig 3d, 3e, 3f respectively of films F3, F4, F5 show the different ratios (2mL, 4mL, 6mL of ANE antibacterial agent blended with CA/Fe-MIL 88A mixture show different morphologies like smooth, flat surface and without of pores. Hence, the prepared Fe-MIL 88A and Antibacterial agent like Allmania nodiflora extract were well mixed and dispersed onto the CA film. SEM images and corresponding EDAX mapping images show only the presence of Fe, C and O elements in the F3 film.
FTIR
In the F1 spectrum illustrated in Fig. 4, peaks for functional group C=O, OH, CH3, and ether C – O – C and identified at 1734, 3453, 1369/1219, and 1029 cm -1 respectively. This is supported by earlier data [21]. Peaks in the Fe-MIL-88A spectrum are slightly shifted to higher wavenumbers and a new peak at 551 cm -1 is assigned to Fe–O stretching in the Fe-MIL-88A structure. The formation of this bond demonstrates that the organic ligands are attached to the inorganic metal of the cluster showing the successful synthesis of the Fe-MIL 88A [22]. The absorption peaks around 1390 and 1549 cm-1, respectively, are attributed to the carboxyl's vibration modes in the fumarate as the bridging ligand of Fe-MIL-88A [23]. Absorption peaks of all reinforced composites (F2, F3, F4, F5) with 1 % Fe-MIL 88A and ANE loading remain the same, showing that the chemical structure of the films is unchanged by the addition of Fe-MIL 88A and ANE. The broad and bending vibration of the hydrogen bond (–OH group) of CA occurred at 3500–3000 cm-1 due to the stretching frequency of water. Increasing antibacterial agents in composite films cause no discernable difference was noted in the position of peaks at different spectra, showing that the MOF integrated antibacterial agent also has no effect on the film's functional groups.
XRD
The XRD patterns of the Fe-MIL 88A, F1, F2, F3, F4, and F5 composite films as-synthesized are shown in Fig. 5. For Fe-MIL 88A, all bands are in accordance with previous literature, [24] confirming that the prepared Fe-MOF is pure and crystalline in nature. Furthermore, two peaks at around 10.2° and 11.8° could be attributed to (100) and (101) planes. For pure CA (F1) film, one weak diffraction While peak observed at 8.7° for F1 film corresponds to the semi-crystalline acetylated cellulose [25]. Fe-MIL 88A after dispersing into the CA matrix, the XRD spectra of CA/Fe-MIL 88A composites show Fe-MIL 88A characteristic peaks in the F2 film, as well as F3, F4, and F5 films within Fe-MIL 88A peak, proving the good compatibility between Fe-MIL 88A and CA.
TGA analysis
The thermal stability of the composite is analyzed by using the TGA technique. Fig. 6. shows DTG (derivative thermogravimetric) DTG and TGA curves of F1 and Fe-MIL 88A impregnated F2 and Fe-MIL 88A with antibacterial agent blended film F3.
The first degradation stage of CA (F1) film was between 30 to 190 °C, with a weight loss of 48 wt%. The first degradation of Fe-MIL 88A incorporated film (F2) was observed between room temperature and 100 °C (13 wt%), and the first degradation of antibacterial agent blended film (F3) was obtained between room temperature that is 30 to 100 °C with a weight loss of 4 wt%. The probable reason may be due to the loss of water molecules absorbed in the prepared films, as evidenced by the exothermic DTA peak.
The second degradation occurred at temperatures ranging from 190-300 °C for F1, F2 and F3 films (51.5 wt%), 100-290 °C (20.2 wt%), and 100-300 °C (18.7 wt%), which corresponds to the acetyl groups decomposition and cellulose backbones respectively. Similar results had been previously reported in the literature [26].
The third decomposition of the organic ligand present in the prepared CA/Fe-MIL 88A and CA/ Fe-MIL 88A/ANE blended films occurred at the range 290 -390 °C (54.8 wt%), 300 -500 °C (46.7 wt%). Previous literature had observed similar results [27]. Table 1. shows the thermogravimetric curves of CA (F1) and Fe-MIL 88A and antibacterial agent-coated F2 and F3 films.
Water contact angle (WCA) analysis
Water contact angle measurements were used to determine surface hydrophobicity and wettability of films. The hydrophilic and hydrophobic nature of the material is described by surface wettability character. Fig. 7. shows the WCA results for (a) F1 (b) Fe-MIL 88A (c) F2 (d) F3 (e) F4 and (f) F5.
The contact angle degrees are 68.6°, 84.5°, 75.9°, 53.7°,48.7° and 46.5° for F1, Fe-MIL 88A, F2, F3, F4 and F5 respectively. The contact angle of the CA/Fe-MIL 88A composite film was 75.9°, which was slightly hydrophobic than the bare CA film (68.6°). But the water absorbed by F3, F4 and F5 films spread on the substrate, thus making it hydrophilic. That is, with increased content of ANE, the WCA value declined from 75.9° to 46.5°.
Water vapour permeability (WVP)
The WVP test was used to check whether the moisture could easily permeate and flow through the material. Fig. 8. Shows the WVP results of composite films. The WVP of CA film was 0.3267 g mm/(kPa h m2) while it slightly increased to 0.4311 g mm/(kPa h m2 ) for CA blend Fe-MIL 88A film. The value of WVP values varied from 1.2913 to 1.7562 g mm/(kPa h m2 ) with the increasing content of ANE and Fe-MIL 88A in the composite films. The more the ANE and Fe-MIL 88A were, the better WVP of the composite films displayed. It may be due to the hydrophilic nature of Fe- MIL 88A, which prefers to absorb water molecules even though ANE was present. The porous Fe-MIL 88A nanocage served as a water vapour channel in this investigation, and the addition of more ANE-Fe MIL 88A created more water vapour channels that were far more suitable for water vapour transfer.
Transparency of films
Table 2. shows the transparency values of films measured at 600 nm. While F1 film, with a transparency value of 8.86, was the least transparent (highly opaque), than the F2, F3, F4, and F5 films with the obtained transparency values of 5.64, 0.85, 0.55, and 0.28, respectively. The addition of Fe-MIL 88A/antibacterial agent to the CA film improved the film's transparency.
Antibacterial test
Fig. 9. show the inhibitory effect of F1, Fe-MIL 88A, F2, F3, F4, and F5 against common bacterial strains such as (Escherichia coli) E. coli and Staphylococcus aureus (S. aureus). The inhibitory effect was computed based on the disc and clear solution zone measurements. The inhibitory effect was absent in F1 and F2 film. When compared to F1 and F2, the inhibitory zones for F3, F4, and F5 films against E.coli and S. aureus were much higher. Accordingly, incorporating ANE in Fe-CA/MIL88A films (F3, F4, F5) enhances antibacterial activity.
The inhibition zone of modified films against the food pathogenic bacteria, gram-positive (S. aureus) and gram-negative (E. coli) is shown in Table 3.
Biodegradation test
The degradation rate of different composing films, F1, F2, F3, F4, and F5 was monitored for 90 days in soil. The setup's atmospheric state was kept constant. The weight loss percentage in each film was represented in Fig. 10. The weight loss percentage was correlated with the original film. According to the findings, pure F1 film degraded after 60 days (50 wt %), whereas Fe-MIL 88A integrated film F2 degraded 30 wt %, and F3, F4, and F5 films showed degradation of 65, 73 and 89 wt %, respectively. When compared to F1 and F2 film, the % weight loss of F3, F4, and F5 film was significantly lower. As a result, the prepared films are biodegradable.
Realtime application
The antibacterial efficiency of the composite films was tested to protect cucumber pieces for the purpose of food packaging. As shown in Fig. S1. fresh cucumber pieces with increased water content were covered with different composite films (F1, F3, F4, F5) and kept for five days at 25 °C with 50% relative humidity. The cucumber piece covered with F1 film appeared dark brown with microbes on the surface, even on the next day. The addition of an antibacterial agent as in the case of films F3 and F4 showed slight water loss and a brownish appearance. While film with 4mL ANE/Fe-MIL 88A/CA (F5) looked relatively fresh with minimum water loss and is free from bacterial attack.
Cellulose acetate-based composite films were made in this study with Fe-MIL 88A as the nanocarrier to load allmania nodiflora extract. The hydrophilicity of Fe-MIL 88A improved the compatibility of allmania nodiflora extract inside the CA/Fe-MIL 88A matrix substantially. The incorporation of Fe-MIL 88A into the film greatly enhanced their mechanical strength, water vapour permeability and transparency. The addition of allmania nodiflora extract up to 6 mL exhibited effective antibacterial activity against E.coli and S.aureus. Further addition of antibacterial agent, beyond the optimum level, decreased the mechanical property of the composite film. Fresh cucumber pieces covered with F5 composite films were looking better, relatively fresh with minimum water loss and without bacterial contamination on comparing with F1, F3 and F4 composite films. This work provides a novel strategy for the preparation of antibacterial food packaging film.
Ethical Approval
The study in question was not submitted/evaluated by the ethics committee because tests on animals and humans were not performed.
Competing interests
On behalf of all the authors of the manuscript entitled “Novel synthesis of metal-organic-framework-based film with enhanced mechanical properties, biodegradability and antibacterial performance for packaging applications”, I confirm that we have no conflict of interest.
Author contribution
Anitha Pius helped in the supervision of the work
Ananthi P carried out the experiments and wrote the manuscript.
Hemkumar K conceptualized the present idea.
Subasini S carried the technical work.
Funding
No funding was obtained for this study
Availability of data and materials
Ananthi P | [email protected]
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Table 1
TGA and DTG values of F1, F2 and F3 films
|
Film |
Thermal degradation step |
Tonset(oC) |
Tendset(oC) |
Weight loss(%) |
DTA characteristics |
|
F1 |
I |
30 |
190 |
48 |
Exothermic |
|
II |
190 |
300 |
51.5 |
||
|
F2 |
I |
30 |
100 |
13 |
Exothermic |
|
II |
100 |
290 |
20.2 |
||
|
III |
290 |
390 |
54 |
||
|
F3 |
I |
30 |
100 |
4 |
Exothermic |
|
II |
100 |
300 |
18.7 |
||
|
III |
300 |
500 |
46.7 |
Table 2
Transparency of the films F1, F2, F3, F4 and F5\
|
Sample code |
Average film thickness (mm) |
Absorbance at 600 nm |
Transparency value |
|
F1 |
0.022 |
0.195 |
8.86 |
|
F2 |
0.070 |
0.395 |
5.64 |
|
F3 |
0.152 |
0.130 |
0.85 |
|
F4 |
0.018 |
0.01 |
0.55 |
|
F5 |
0.070 |
0.02 |
0.28 |
Table 3
Bacterial inhibitory zone for F1, F2, F3, F4 and F5 against E. coli, S. aureus
|
Sample code |
Inhibition Zone (mm) |
|
|
S. aureus |
E. coli |
|
|
F1 |
0.3 |
0.2 |
|
F2 |
1.2 |
1.0 |
|
F3 |
1.6 |
1.5 |
|
F4 |
2.0 |
1.9 |
|
F5 |
2.0 |
2.5 |
No competing interests reported.