Silver nanoparticles blended PEG/PVA nanocomposites synthesis and characterization for food packaging

2.1 Capparis zeylanica

Capparis zeylanica is commonly known as Indian caper a climbing scandant shrub found throughout India. Capparis zeylanicais belonging to the family capparidaceae as shown in Fig. 1 (Amit et al., 2010; Saranyaadevi et al., 2014a). Plants are in 2–3 m in height, armed with 3–6 mm long recurved thorns, branched, leaves are elliptic or broadly lanceolate, base rounded, apex mucronate (Saranyaadevi et al., 2014a). Flower profase, pinkish white, later turning pink, berries are globular or elliposide, 3–4 cm in diameter, and are globase, embedded in white pulp. Leaves and seeds contain thioglucosides, glucocapparin, n-tricontane, α-and β-amyrin, an alkaloid, a phytosterol, a mucilaginous substance and a water-soluble acid, capricacid (Amit et al., 2010). Leaves are used as a counter irritant and as a cataplasm in boils, swellings, piles and rheumatism. Capparis zeylanica has shown antibacterial activity, mainly against Gram-negative bacteria (Saranyaadevi et al., 2014a). Extracts of C. zeylanica leaves have shown antioxidant and hepatoprotective properties in pharmacological models. Athondai has shown some invitro anticancer, antiestrogenic, immunomodulatory, and antiamoebic activity (particularly against Entamoebahistolytica) (Saranyaadevi et al., 2014a; Dhabale et al., 2012). Capparis zeylanicaleaves were collected from surroundings areas of Mannargudi, Tamilnadu, India.

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Fig. 1

Capparis zeylanica.

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2.2 Preparation of Capparis zeylanica leaf broth

AR grade silver nitrate (AgNO₃) was purchased from Sigma-Aldrich chemicals and fresh Capparis zeylanica leaf extract used for the reduction of Ag+ ions to AgNPs was prepared by taking 20 g of thoroughly washed finely cut leaves in 500 ml flask along with 100 ml of distilled water and the mixture boiled at 60 °C for 20 min. Then the extract was filtered with Whatman No. 1 filter paper and stored at 4 °C (Saranyaadevi et al., 2014a).

2.3 Synthesis of nanoparticles

In a typical reaction procedure, 5 ml of plant leaves extract was added to 20 ml of AgNO₃ (1 mM) solution at room temperature. The resulting solution become dark brown in colour after 60 min, indicating the formation of AgNPs. Then the extract was centrifuged at 8500 rpm for 35 min and subsequently dispersed in distilled water to get rid of any uncoordinated biological materials (Saranyaadevi et al., 2014a).

2.4 Preparation of PVA/PEG blend with nanoparticles

PVA (50%)/PEG(50%) films were prepared as follows: For each film, 2 g PVA was dissolved in 50 ml double distilled water under 80 °C and stirring for 2 h till the complete dissolution of PVA. 2 g PEG was dissolved in 50 ml of water at room temperature then mixed with PVA solution. The mixed solution stirred for another 1 h at room temperature. Then the nanoparticles samples were measured with different concentration were added into blend and stirred over complete mixing and it has been casted into glass Petri dishes and placed in an oven at 50 °C for 36 h in Hot air oven. Fig. 2 (a) shows the PVA/PEG thin film and (b) PVA/PEG with AgNPs (see Fig. 3).

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Fig. 2

(a) PVA/PEG and (b) PVA/PEG blended with AgNPs.

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Fig. 3

FTIR analysis of AgNPs + Polymer blend.

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2.5 Characterization

Synthesized silver nanoparticles were confirmed at different time intervals and the absorption maxima was scanned by UV–Vis spectrophotometer at the wavelength of 300–800 nm on Perkin-Elmer Lambda 25. XRD pattern was measured by drop coated films of AgNO₃ on glass plate and employed with X-ray diffractometer (INEL X-ray diffractometer) of characteristic Co-kα1 radiation (λ = 1.78 A°) in the range of 10–70° at a scan rate of 0.05°/min with the time constant of 2 sec.

FTIR spectrum was recorded in the range of 4000–400 cm⁻¹ using Thermo Nicolet Nexus 670 spectrometer in the diffuse reflectance mode operating at resolution of 4 cm⁻¹. SEM observations were carried out on a ZEISS EVO 40 EP Electron microscope. Morphology and size of the silver nanoparticles were investigated by TEM images using HITACHI H-7650 at an operating voltage of 80 kV instrument.

Addition of salts, plasticizers and fillers to the polymer host results in interactions and/or complexation of the host polymer matrix. This will influence the local structure of polymer back- bones and significantly affects the immobility. In this context, infrared spectroscopy studies will give the evidence of possible complexation and interactions. Infrared spectra were recorded with a Nicolet 5 DXC spectrometer. Thin films were prepared by the solution casting method. All the samples were annealing at 140 °C for 2 h to remove the residual solvent and to make the polymer blend mixing complete. A minimum of 32 scans was signal averaged with a resolution of 2 cm⁻¹ at 150 °C. The hydroxyl stretching region of the IR spectrum was analyzed for the varied phenolic blends.

Thermal behaviour of the composites were analysed by Differential Scanning Calorimeter Perkin– Elmer DSC-7. Samples with a mass of about 10 mg were used with heating step from 0 to 260 °C at a rate of 10 °C/min followed by an isothermal heating for 10 min, a first cooling from 260 to 0 °C, and a second heating in the same range. All DSC runs were carried out under nitrogen atmosphere to minimize the oxidative degradation.

Moisture Absorption test is used to determine the moisture absorptivity of the material. In order to determine the moisture absorption, samples of square pieces of the polymer films (2 cm diameter) were placed on a desiccator’s tray which contains initially CaCl₂. After 24 h the weight of the samples was measured and taken it as initial weight. Then the Ca(NO₃)₂ was taken instead of CaCl₂ for the complete moisture absorption of the biopolymer composites. The weight of the samples was measured at various time interval. The films were then quickly placed back on the desiccator’s tray. The process was repeated until the films attained constant weight after a certain period. The percentage of moisture absorption was calculated from the equationgiven below, $$\mathit{M}\mathit{A}\left(\mathbf{\%}\right)=\left(\frac{{\mathit{M}}_{\mathit{F}}-{\mathit{M}}_{\mathit{I}}}{{\mathit{M}}_{\mathit{I}}}\right)\times 100$$ where MA is moisture absorption in %, M_F is final weight of the polymer in g and M_I is initial weight of the polymer in g respectively.

The antimicrobial activity of the developed Agbio-films was tested by disc diffusion method against Escherichia coli, Pseudomonas Aeruginosa, Staphylococcus aureus. In disc diffusion method, the films are cut into a disc shape with 5 mm diameter, sterilized by autoclaving for 30 min at 120 °C, and placed on different cultured agar plates. The plates are incubated for 2 days at 37 °C in an incubation chamber maintaining with 5% CO₂ flow and the inhibition zone is then measured.

3.1 FTIR – spectroscopy analysis

FTIR spectra of AgNPs blend shown in Fig. 7. The peak at 3200–3650 cm⁻¹ indicates the presents of OH group is engaged in the formation of hydrogen bond or complex with metal particles (Pencheva et al., 2012; Sahaa et al., 2014; Bryaskova et al., 2010; Bryaskova et al., 2010; Filippo et al., 2009). The presents of C-H and CH₂ bonds in Alkanes which are in the PVA structure was confirmed with the intense bonding peaks around 2700–3000 cm⁻¹ and 1550–1650 cm⁻¹ respectively (Pencheva et al., 2012). Absorption band due to C⚌O stretching of the Ester group of the (blend) PVA/PEG can be seen at peaks around 1050–1300 cm⁻¹.

3.2 DSC analysis of composites

The glass transition temperature (T_g) of the polymer blend films were determined by Differential Scanning Calorimeter. The scan rate was 20 °C/min within a temperature range of −70 to 250 °C. The measurements were made with 3 mg samples on DSC sample plates after the specimens were quickly cooled to −70 °C following the first scan. This procedure was adopted to ensure a complete mixing of the polymer blends and to remove the residual solvent and water in the specimen completely. The glass transition temperature and melting point of the PVA/PEG blend was 78.8 and 187.6 °C shows from the Fig. 4. The T_g and melting point values was increased when the nanoparticles are added into the polymer blend (Saini et al., 2013; Ghaffari-Moghaddam and Eslahi, 2014; Gautama and Ramb, 2010; Ali, 2013; Abdul Kareem and Anukaliani, 2011).

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Fig. 4

DSC analysis of PVA/PEG, PVA/PEG + AgNPs.

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3.3 Moisture absorption test

Moisture Absorption test is used to determine the moisture absorptivity of the material. From the Fig. 5moisture absorption value of the composite increased with higher composition of nanoparticles in the biopolymer. The result shows that the composite is thicker when the nanoparticles are added into the biopolymer blend which indicates the percentage of moisture absorption depends the concentration of nanoparticles in blend.

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Fig. 5

Moisture absorption of AgNPs in biopolymer.

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When the silver nanoparticles composition increases the moisture absorption content initially increases, usually when the moisture content increases, the permeability of microbes on the polymers which allows the degradation of biopolymers usually. But in case of AgNPs blend increases the antimicrobial activity also increases. Hence less/or no degradation could have occur, which will leads to self-life of the biopolymers and increases the food self-life also.

3.4 Antimicrobial study of composites

Bacterial sensitivity to antibiotics is commonly tested using a Disk diffusion method. The antimicrobial activity was done on various pathogenic bacteria like Escherichia coli, Pseudomonas Aeruginosa, Staphylococcus aureus. Nutrient agar medium was used to cultivate Bacteria. The pure cultures of bacteria were sub-cultured on nutrient agar medium. After solidification of medium, the species are spread over the medium using L-rod. Then the disks were placed on the medium. Freshly prepared Silver bio composite samples were added into the disks that were made in the seeded plates. The samples were incubated for 24 h at 37 °C. Positive test results were scored when a zone of inhibition was observed around the disk after the incubation period. The zone of inhibition values of blend with silver nanoparticles were shown in Fig. 6. The maximum zone of inhibition value was obtained for Pseudomonas Aeruginosa with 1.5 ml concentration of AgNPs (Saranyaadevi et al., 2014a).

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Fig. 6

Antimicrobial activity of AgNPs + blend - E-Coli (a), Pseudomonas Aeruginosa (b), Staphylococcus Aureus (c).

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