Synthesis of Silver Nanoparticles Through Orange Peel Powder for Antibacterial Composite Filler Applications

Environmental concerns and the positive aspects of biowaste materials gained the attention of researchers to use them as filler materials in fabricating of green composites along with polymer matrices, but most of them are not suitable for extensive applications in high thermal applications. In most of the natural particulate materials are not having the ability to fight against pathogens. To overcome such barriers, a modification of biowaste—orange peel powder (OPP) by the generation of silver nanoparticles (AgNPs) is prepared with the one-step hydrothermal process. The modified Orange Peel Powder (MOPP), is then characterized by FESEM, EDX, FT-IR, XRD, and Thermal analyses. The presence of AgNPs in the MOPP is confirmed through FESEM & EDX analysis. FT-IR spectral analysis pronounced the non mutate functional groups in MOPP as compared with OPP. The generation of AgNPs in MOPP is confirmed through the XRD peaks of reflection planes at (1,1,1), (2,0,0), (2,2,0) & (3,1,1). Thermal Analysis results of TGA and DSC show the MOPP has increased thermal stability up to 363 °C. Antibacterial test against Gram-negative and Gram-positive bacteria for OPP & MOPP shows the inclusion of Ag strongly objects the pathogens. Eventually, the MOPP can be utilized as filler material along with the polymer matrix in high thermal as well as antibacterial composite filler applications.


Introduction
Majority of the agricultural wastes as well as municipal solid waste left unused and create problems to the environment and municipal solid waste management in countries like India and European countries [1]. Studies revealed that agricultural wastes of 200 million tons were generated in India alone and 998 million tons globally [2]. In that organic fruit waste of Cítricos del Andévalo Orange Juice processing company located in Spain contributes 400 tons every day even though they can be reused [3]. Organic fruits like Orange posses 50-60% of their total weight as waste in forms of peels, seeds and membrane residue. Orange peels contain 17.5% of cellulose, 8.5% of hemicellulose and Pectin [4]. The cognate chemical composition materials were reported by the researchers for various applications like Ocimum sanctum leaves with silver nanoparticles to form films for packaging application [5], Chitosan in plated with silver nanoparticles for the application of the natural rubber latex [6], Acacia leucophloea fibers for the design of lightweight materials [7], Tamarind leaf with silver nanoparticles used for the antibacterial textile materials [8], and Pennisetum purpureum grass formulated with copper nanoparticles for the application of dressing materials in the medical field [9]. With the above incentive, Orange Peels in the Powder form can be utilized for packaging applications in addition to the polymers matrices as a filler. To improve its utilization in large scope it can be modified by adding silver nanoparticles to involve the same in high thermal applications.. Silver nanoparticles (AgNPs) were generated from the aqueous silver nitrate solution (AgNO 3 ) and they possess good chemical as well as thermal properties [10,11]. From the literatures, the lack of compatability analyse of the biowaste materials are found, to utilize as a filler material especially along with silver nano particles. The anti bacterial activity of the natural waste materials can be improved with the addition of AgNPs in order to improve its utilization as a filler material along with polymer matrix for the application of packaging films. By this inclusion of AgNPs the thermal properties of this biowaste materials can also be improved and also the utilization in the high thermal applications like packing foods to improve their shelf life.
In this investigation, OPP has been modified with silver nanoparticles by the hydrothermal process and the modified OPP (MOPP) is characterized and suggested that the MOPP can be used as a filler material for the high thermal green composite applications.

Materials
Orange Peel Powder (OPP) was procured from MG Naturals, Chennai, Tamil Nadu with the size of less than 50 µm.
The ultrapure grade of silver nitrate (AgNO 3 ) was purchased from Modern Scientific company, Madurai, Tamil Nadu and used as procured.

Preparation of Modified Orange Peel Powder (MOPP)
The preparation of MOPP was done by the procedure described elsewhere [12,13]. Briefly, The 3 g of OPP was added to the 50 ml of 5 mM aqueous solution of AgNO 3 and stirred with a magnetic stirrer for a period of 24 h at the temperature of 80 °C. The modification of OPP can be observed by coloration change from pale yellow to dark brown. Then the MOPP separated and dried in the oven for 3 h at the temperature of 100 °C.

Microscopic Analysis (FESEM)
The morphological study of AgNPs in MOPP had been evaluated by the images of FESEM and EDX spectra through Carl Zeiss-Sigma with Gemini column at a magnification of 1 µm and operated at 20 kV. The experiment is to analysis the distribution of silver nanoparticles in the MOPP.

FTIR Analysis
The functional groups in the MOPP & OPP are determined through the FT-IR spectrum recorded by Perkin-Elmer 783. The spectrophotometer in the range of 4000 cm −1 to 500 cm −1 with 4 cm −1 resolution and 45 scans for both the case using KBr pellets.

X-Ray Diffraction Analysis (XRD)
To record the X-ray diffractogram of MOPP and OPP, to analyse the materials present as well as plane of reflection in both OPP and MOPP the EXPERT-PRO diffractometer system was used with the generator setting of 30MA, 40 kV. The anode material was Cu and the position in the range of 2θ = 10° to 80°. The reflection planes of AgNPs presented in the MOPP had been calculated by, Bragg's law d = 2sin and

Thermogravimetric Analysis (TGA)
The thermogravimetric analysis for MOPP & OPP was carried out on NETZSCH STA 2500 Regulus instrument with the temperature range of 30 °C to 550 °C at 10 °C/min heating rate at an nitrogen atmosphere with a flow rate of 20 ml/ min. From the TGA, thermal stability of OPP and MOPP were obtained. To obtain the derivative form of thermo gram the differentials of primary thermogram was calculated by central finite difference method [14] i.e., where Wt + Δt & Wt − Δt are residual weight samples at t + Δt and t − Δt time, and Δt is time for reading sample residual weight.

Kinetic Activation Energy
The Broido's kinetics equation was used to determine the Kinetic activation energy through [15,16], where R universal gas constant (8.314 J/mol K), T temperature in K, Ea kinetic activation energy, K reaction rate constant.

Differential Scanning Calorimetry (DSC)
DSC was performed to find glass transition temperature of both OPP & MOPP, on NETZSCH STA 2500 Regulus instrument at a heating rate of 10 K/min under Nitrogen atmosphere by following ASTM E-967 ASTM 2008a and ASTM E-968 ASTM2008b standards [10,17]. Figure 1a shows the scanning electron microscopic image of MOPP. Figure 1b represents the EDX spectra recorded from the SEM image. In Fig. 1a, the spherical shape particles indicates the insitu generates AgNPs which are marked in arrow heads. From Fig. 1b the EDX spectra peak corresponding to 2.98 keV ~ 3 keV value in the X axis confirms the presence of Ag in the MOPP along with C and O. The presence of C and O is confirmed through the peaks at 0.2 and 0.3 keV respectively [18,19].

FTIR Analysis
The FTIR spectra ranges from 4000 to 500 cm −1 for MOPP and OPP are shown in Fig. 2 to observe the chemical groups present in them. In this observation, the spectra of MOPP and OPP showed a broad band between 3500 and 3100 cm −1 assigned to O-H stretching (hydroxyl groups). The peak at 2992 cm −1 wave number recognized to -C-H-stretching vibration of alkenes [20]. The absorption bands at 1741 cm −1 and 1622 cm −1 represented C=O stretching present in hemicelluloses [7] and -C=C-stretching respectively. The 1521 cm −1 arise due to the C=C ring stretching (Aromatic group). 1438 cm −1 peak observed due to carbonyl stretching raise in lignin [21]. The bands at finger print region, 1016 cm −1 clarified the C-O-C vibrational stretching of polysaccharides [12] and 838 cm −1 confirmed the presence of glucosidic (β) linkages between hemicelluloses and cellulose sugar units  [7]. The C-OH bending in cellulose observed through peak at 598 cm −1 [22,23]. From the Fig. 2 it is conspicuous that there is slight shift in the peaks of MOPP as compared to the OPP such as 2924 cm −1 to 2992 cm −1 , 1743 cm −1 to 1741 cm −1 , 1612 cm −1 to 1622 cm −1 and 1016 cm −1 to 1018 cm −1 likewise the intensity of those peaks lowered in the MOPP due to reduction of silver ions to AgNPs (Ag + to Ag) but it clearly evident that the minor shift in peaks does not affected the functional groups present in the OPP during synthesis of AgNPs. The shift in the peaks as well as reduction in the intensity of the peaks are due to the involvement of functional groups in the synthesis of AgNPs. The results of FTIR spectral also revealed that no chemical interaction between AgNPs and OPP during preparation of MOPP [24].

Thermogravimetric Analysis (TGA)
TGA analysis is for analyzing the thermal stability based on the weight loss of the material with the increase of temperature. DTG curves are derived using Eq. (2). Figure 4a and b showed the TGA and DTG thermo grams of MOPP and OPP. Three stages of thermal degradations observed in both the powders, clearly through DTG curves. The initial degradation found to be identical for both MOPP & OPP at 100 °C with weight loss of 4.27% for MOPP and 6.22% for OPP with the effect of moisture evaporation. The intermediate degradation observed at 232 °C for OPP & 246 °C for MOPP with the higher weight loss by MOPP with 7.96% where as OPP exhibited 5.87% due to the degradation of pectin, hemi cellulose and links of cellulose [26]. The final degradation between 250 and 400 °C, the higher mass reduction of the MOPP & OPP observed because of complete degradation of cellulose. The mass loss observed at 400 °C for MOPP is 48.7% and for OPP is 54.1%. There is a change in the point of inflection observed in MOPP as compared to OPP due to the inclusion of thermally stable AgNPs. The MOPP thermally stable up to 363 °C where as thermal stability of OPP is 357 °C. The residual mass of MOPP observed as 35% but OPP exhibited 24% of residual mass at 548 °C. The higher residual masses of MOPP is due to the inclusion of high thermal withstanding capacity AgNPs.

Kinetic Activation Energy
It is the minimum energy required to start the degradation of the samples and are calculated from the Eq. (3), the activation energy for the thermal degradation of the MOPP & OPP in the temperature region of 200 °C to 375 °C was evaluated [27]. Figure 5a and b shows the plot between

Antibacterial Test
Both OPP and MOPP were tested against P. aeruginosa, E.Coli (Gram-negative) and S. aureu, S. oralis  Fig. 7. The large zone of inhibition relates to the strong antibacterial activity of the samples [33,34]. The inhibition zones of OPP and MOPP against pathogens are listed in Table 1. The OPP powder posses a partial inhibition zone in the range of 13.5 mm to 17.4 mm whereas the MOPP showed the strong inhibition zones from 37.4 to 39.0 mm of diameter because of the silver nanoparticles present in them. This implied that the MOPP has the ability against pathogens due to the antibacterial activity of AgNPs nature.

Conclusions
Owing to stretch the applications of biowaste filler materials, the modification of the OPP was done with the hydrothermal process for the in-situ generation of silver nanoparticles (AgNPs). The MOPP was characterized using SEM, EDX, FT-IR, XRD and Thermal Analysis. The presence of silver nanoparticles in MOPP was identified through SEM and EDX. Further, no change in the functional groups of MOPP was observed during the generation of AgNPs by FT-IR analysis. The XRD result confirmed the presence Ag by the peaks mentioned by the ICDD. The generated AgNPs improved the thermal behavior of the MOPP by increased thermal stability up to 363 °C and Kinetic activation energy of 91.10 kJ/mol. Likewise, the glass transition temperature is also improved to 102 °C. The zone of clearance produced against gram-negative and gram-positive bacteria by MOPP represents the strong antibacterial activity as compare to OPP. With the above results the MOPP can be act as a filler material along with the polymer matrices for the fabrications of green composites in high thermal as well as antibacterial applications.