Antibacterial activity of poly (vinyl alcohol)—biogenic silver nanocomposite film for food packaging material

Nanotechnology is the study of engineering and manipulating materials which have at least one of their dimensions in the range of 1–100 nm [1]. When brought down to nanometer scale, the particles fall into the quantum mechanics regime and exhibit a very high surface area to volume ratio. These phenomena are responsible for the altered chemical and physical properties observed in nanomaterials as compared to their bulk counterparts. The unique and tunable properties exhibited by nanomaterials have intrigued researchers all over the world and their potential is being explored in fields such as medical therapeutics and diagnostics [2], drug delivery [3–5], antibacterial nanomedicine [6–16], photocatalysis [19], catalysis [20–24] and energy production [25]. Another such field where nanomaterials have slowly gained importance is the food industry [26]. Nanomaterials are being currently explored for pesticide and fertilizer delivery [27], food quality enhancement [28] and food packaging [29].

Food packaging is an essential component of the food industry as it ensures the quality and stability of processed food till it is consumed. With the growing public awareness about disease transmission caused by microorganisms in food, there has been an increasing demand for antibacterial packaging materials which can prevent bacterial contamination in food without affecting their stability [30]. The conventional food packaging made up of polymeric films of polypropylene, polystyrene and polyethylene terephthalate provide useful gas barrier properties and mechanical properties but they lack biodegradability [31]. Polysaccharide (starch) polymers obtained from biological sources have gained popularity due to their biodegradability but their oxygen transmission rate shows great variation with humidity, thus limiting their use commercially [32].

Several polymer nanocomposites have been explored to create the ideal food packaging material by dispersing nanoscale fillers in polymer matrices. Clay and silicate nanoplatelets, ${\rm Si}{{{\rm O}}_{2}}$ nanoparticles [33–35], graphene [36], starch nanocrystals [37] and cellulose-based nanofibers [38] have been used as nanofillers for enhancing polymer barrier properties. Although these fillers have enhanced various properties such as flame resistance, thermal properties, wettability and hydrophobicity but these nanocomposites are unable to prevent contamination of food product by microorganisms.

Antibacterial packaging membranes can be synthesized by incorporating nanomaterials such as silver, titanium dioxide or copper nanoparticles into the polymer films [39, 40]. Silver nanoparticles (AgNPs) have been preferred for such antibacterial applications over other molecular antibacterial due to their broad spectrum antimicrobial activity. They have also been shown to be effective against antibiotic-resistant bacterial strains [41]. AgNP/Polymer nanocomposites have been reported to exhibit antimicrobial activity against Aspergillus niger, Escherichia coli, Staphylococcus aureus and Candida albicans which are common food contaminants [42–44].

Several methods have been reported for silver nanoparticle synthesis and subsequent incorporation into polymer matrices. However most of these methods use environmentally and biologically hazardous chemicals, such as sodium borohydride and formaldehyde as reducing agents [45–47]. Currently extensive research is being carried out to replace such conventional reducing agents with biological extracts and the use of bio-compatible polymers for stabilization of silver nanoparticles [48]. The use of polymers such as poly vinyl alcohol (PVA), chitosan, carboxy methyl cellulose, sodium alginate and starch along with green synthesized silver nanoparticles can facilitate the synthesis of biodegradable polymer films with antibacterial properties for food packaging [49–51].

In this study, we have developed a biodegradable poly vinyl alcohol- biogenic silver nanocomposite film as an antibacterial packaging material. The silver nanoparticles were synthesized using Ficus benghalensis leaf extract [19]. The antibacterial activity of nanocomposite film was analyzed against Salmonella typhimurium by means of disk diffusion methods.

2.1. Materials

2.2. Synthesis silver nanoparticles and PVA-silver nanocomposites

2.3. Antibacterial activity of nanocomposite films

3.1. Biosynthesis silver nanoparticles (AgNPs)

Silver nanoparticles have already been synthesized by biological route using Ficus benghalensis [1]. UV-vis spectra were recorded as from 200 to 800 nm to analyze the surface plasmon resonance (SPR). Figure 1(a) shows that the absorbance was increased with increasing time of reaction which indicates the formation of nanoparticles. The SPR was occurred at 410 nm which is in the good agreement of SPR of silver nanoparticles. Figure 1(b) shows the x-ray diffraction (XRD) pattern of biosynthesized silver nanoparticles. A characteristic peak occurred at $2\theta =80.02{}^\circ$ which corresponding to the (222) set of lattice plane, matching with JCPDS file No. 04-0783. TEM micrograph reveals that the synthesized nanoparticles are biosynthesized in the range of 20–90 nm with spherical shape (figure 2(a)). The micrograph also indicates that the presence of bio-capping layer which provide stability. EDX mapping and pointer both show the presence of elemental silver in biosynthesized nanoparticles (figures 2(b) and (c)). The presence of copper signal in EDX pointer is depicting due to use of copper grid.

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3.2. Atomic force microscopy of Ag NP/PVA

A thin film of the silver nanoparticles-Poly vinyl alcohol solution was casted on a glass slide and analyzed by AFM to determine the size of the AgNPs and the morphology of the nanocomposite thin film. AFM micrograph depicted the incorporation of the AgNPs within the PVA layer and with spherical morphology (figures 3(a)–(c)). The average particle size was found to be 32 nm (figure 3(d)). Furthermore, AFM indicated that the particles were not agglomerated within the PVA matrix.

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3.3. Analysis of antibacterial activity

The antibacterial activity of PVA-silver nanocomposite film was tested against Salmonella typhimurium by disk diffusion method. After 24 h of incubation period, regions with no bacterial growth were observed around the nanocomposite film disks in the culture plates. The antibacterial activity of the nanocomposite film was evaluated by analyzing the diameter of these inhibition zones. The diameter of the inhibition zones ranges from 9 mm to 18.3 mm and was seen to increase with increasing the concentration of silver nanoparticles in the film (figure 4). The maximum inhibition zone of 18.3 mm was observed at ${\rm 500}\mu {\rm l}$ silver nanoparticles.

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In this study, biogenic silver nanoparticles were synthesized using F. benghalensis leaf extract. This method is rapid, cost effective, and environmental friendly. Synthesized silver nanoparticles were mixed with the 8% of PVA solution to develop nanocomposite film. The developed nanocomposite film was characterized by atomic force microscopy (AFM). The antibacterial activity of nanocomposite films was evaluated by the means of disk diffusion method. It was found that the nanocomposite films show excellent antibacterial activity against Salmonella typhimurium. The inhibition zones were increased with increasing concentration of biogenic silver nanoparticles in the film. The developed nanocomposite films could be prominent materials for food packaging.

RMT and RNP are grateful to the Amity University Uttar Pradesh, Noida, India, for providing excellent facility for the above work.