Green synthesis of Ag NPs using olive pomace

Special Issue for The 2nd Annual Conference on Theories and Applications of Basic and Biosciences ♦ September, 1st, 2018 Green synthesis of silver nanoparticles using olive pomace extract Abdounasser Albasher Omar1*, Haneen Meftah Alkelbash2, Yosra Fathi Alhasomi 2, Omymah Mohammed Al-muntaser2, Samia Sallah Eldien Elraies1, Abdurrahman Abuabdalla Khalifa1 2 1 Chemistry Department, Faculty of Science, Gharyan University Undergraduate student, Chemistry Department, Faculty of Science, Gharyan University * Correspomndence: [email protected] Abstract: this study reports a simple, low-cost method for green synthesis of sliver nanoparticles (AgNPs). This was achieved by reacting silver nitrate solutions with the aqueous extract of olive pomace (OP). Visible spectrophotometry and monitoring of colour change were used as proofs of AgNPs synthesis. The formed amount of AgNPs was found to increase by increasing pH, time, temperature and OP extract volume, while the optimum concentration was found to be 2.5 mM. In conclusion, the aqueous extract of OP represents a good candidate for green synthesis of AgNPs, and through controlling the factors that affect the proposed method more amount of AgNPs, with smaller particles, could be produced. Keywords: silver nanoparticles, olive pomace extract, green synthesis, Vis spectrophotometer. :‫الملخص‬ ‫ لقد تم هذا‬.‫هذه الدراسة تُبين طريقة بسيطة وغير مكلفة لتحضير جسيمات الفضة النانوية بالتخليق االخضر‬ ‫ واستخدمت المطيافية المرئية‬،)‫بالتفاعل بين محلول نترات الفضة والمستخلص المائي لثفل الزيتون (الفيتورة‬ ‫ لوحظ أن كمية جسيمات الفضة النانوية تزداد بزيادة‬.‫والتغير في اللون إلثبات تحضير جسيمات الفضة النانوية‬ ‫ بينما كان افضل تركيز من‬،‫االس الهيدر وجيني والزمن ودرجة الحرارة وحجم المستخلص المائي من الفيتورة‬ ‫ من خالل الدراسة نستنتج أن المستخلص المائي للفيتورة يمثل مادة مناسبة‬.‫ مللي موالري‬0.5 ‫نترات الفضة هو‬ ‫لتحضير جسيمات الفضة النانوية ومن خالل التحكم في العوامل المؤثرة في الطريقة المقترحة في هذه الدراسة‬ .‫يمكن تحضير كمية أكبر من جسيمات الفضة النانوية وباحجام صغيرة‬ .‫ جهاز طيف االشعة المرئية‬،‫ التخليق األخضر‬،‫ مستخلص الفيتورة‬،‫ جسيمات الفضة النانوية‬:‫الكلمات المفتاحية‬ Introduction Nanoparticles (NPs) are those particle with size range of 1-100 nm, and their sizes make them intermediate materials; between bulk materials and materials on molecular or atomic level[1]. Due to their size, shape and structure, NPs have unique chemical, mechanical, electrical, structural, morphological, and optical properties[2] making them suitable for different applications in pharmaceutical, food, agricultural industries[3] water treatment, catalysis, harvesting solar energy[4] electronics, cosmetics, mechanics and optics space industries[5]. Among the nanoparticles, metallic (such as silver, gold, nickel and platinum) NPs have found great interest, and have been synthesized by different physical and chemical methods. The physical methods include laser ablation, lithography and high-energy irradiation, whereas the chemical methods include chemical reduction, electrochemistry, and photochemical reduction[6]. However, these conventional methods suffer from some shortcomings including high cost, high energy consumption, using toxic chemicals and introducing chemical pollutants to the environment[7]. To overcome these shortcomings, many researchers have focused their efforts on green synthesis of metallic NPs, a simple eco-friendly method with low cost and low energy consumption. Another advantage 662 Special Issue:The 2nd Annual Conference on Theories and Applications of Basic and Biosciences ♦ September, 1st, 2018 for the green synthesis method is that the reduction of metal ions and forming the metallic NPs is accomplished by bioactive compounds, exist in plant extracts, without the need to use capping agents as in the case of the aforementioned conventional methods. The bioactive compounds believed to be responsible for green synthesis of NPs include phenolic acid, alkaloids, terpenoids[3,6], proteins, sugars and polyphenols[6]. Many studies have reported using the extract of different parts from plants for green synthesis of metallic NPs[8-12]. Silver nanoparticles (AgNPs) is one of these metallic NPs that have attracted great attention because of its various applications which include, but not limited to, antibacterial activity[13], antifungal activity[14], antioxidant activity, catalytic activity[15] anticancer activity[16] and dye degradation[17]. It is reported that AgNPs have been green synthesized using the extract of different parts of various plants such as Ocimum tenuiflorum, Solanum tricobatum, Syzygium cumini, Centella asiatica, Citrus sinensis[18], Pine, Persimmon, Ginkgo, Magnolia, Platanus[19], Ceratonia silique[20], Garcinia mangostana[21], Coriandrum sativum[22], Acalypha indica [23], Olive tree[24,25]. According to our knowledge, no previous study has reported the green synthesis of any metallic NPs by the extract of olive pomace (OP), an agricultural by-product of olive oil industry known to contain polyphenols[26] which are considered as potential reducing agents in green synthesis of metallic NPs[3,6]. Thus, the aim of the current study was to green synthesize AgNPs by reacting the aqueous extract of OP with silver nitrate (AgNO3) aqueous solution. Also, some factors known to affect the proposed method were studied, these factors were: pH, reaction time, AgNO3 concentration, temperature and the volume of OP extract. In addition, the green synthesis of AgNPs and the role of the above-mentioned factors were monitored by colour change and a visible spectrophotometer. Materials and Methods Chemicals: AgNO3 solution of 0.1 M (purchased from Winlab, UK) was used to prepare all the required AgNO3 solutions. To adjust the pH of reaction solutions, sodium hydroxide (NaOH) and Hydrochloric acid (HCl), purchased from BDH, UK, were used after solutions with a concentration of 0.1 M of both chemicals were prepared. And for preparation of all solutions and the OP extract deionized water was used. Preparation of OP aqueous extract: A fresh sample of OP was collected directly from the outlet of an olive mill located in Gharyan, Libya (March, 2018). The collected sample was transferred to the lab in a clean, sealed plastic bag. At the lab, 10 grams of the collected OP were added to 100 mL of deionized water in a beaker, shaken to homogenize the mixture and then sealed with a sheet of parafilm. The beaker, with its contents, was kept in a dark place for 24 hours. Later, the mixture was filtered using Whatman filter paper no. 1, and the filtrate (extract) was received into a dry clean conical flask and then stored at 4°C. This obtained extract was used for green synthesis of AgNPs. Green synthesis of AgNPs using OP extract: One mL of OP extract was added to 10 mL of 1 mM aqueous solution of AgNO 3, after the pH of the solution was adjusted to 9.5, the solution was left standing without stirring for 1 hour at room temperture, then it was checked for any change of colour and analysed with visible spectrophotometry to confirm AgNPs formation. 663 Green synthesis of silver nanoparticles using olive pomace extract Factors affecting green synthesis of AgNPs: Effect of pH: To investigate the role of pH on AgNPs synthesis by OP extract, five solutions were prepared as follows: 1 mL of OP extract was added to 9 ml of 1 mM aqueous solution of AgNO3 and the pH of these solutions was adjusted to the desired pH (5,7, 8, 9 or 10) by drops of either NaOH or HCl solutions. The resulting solutions were mixed then left at room temperature for 1 hour, and after that the colour change of each solution was observed and its visible spectrum was recorded. Effect of reaction time: Ten mL of OP extract was added to 90 mL of aqueous solution of AgNO3 (1 mM), and the pH was adjusted to 9 by NaOH solution, then mixed, and the mixture allowed to react at room temperature. From this solution, aliquots of 5 mL was pipetted after 15, 30, 60, 120 minutes and 24, 48, 72 hours for recording the Vis spectrum and monitor the reaction with respect to time intervals. Effect of AgNO3 Concentration: One mL of OP extract was added to 4 sample vials, each one containing 10 mL of AgNO3 with different concentrations, which were: 0.075, 0.75, 1, 2.5 and 5 mM. Each solution was mixed then left at room temperature for 1 hour after its pH was adjusted to 9.5 by adding drops of NaOH solution. Later, the visible spectrum of each solution was recorded. Effect of OP extract volume: Different volumes (0.5, 1, 1.5, 2 mL) of OP extract were added to four sample vials, each containing 10 mL of 1 mM aqueous solution of AgNO3. After mixing each solution, the pH was adjusted to pH=9.5, then left at room temperature for 1 hour. Later, the visible spectrum of each solution was recorded. Effect of temperature: Into four sample vials 1 mL of OP aqueous extract was added to 8 mL of AgNO 3 solution with a concentration of 1 mM, then the pH of each solution was adjusted to 9. One of the resulting solutions was kept at 20°C while the three others solutions were heated either to 30, 40 or 50° C. After 30 minutes the Vis spectrum of each solution was recorded. Monitoring of green synthesis of AgNPs: Vis spectra of AgNPs, in the range 370-600 nm, were recorded by a single beam Vis Spectrophotometer (Jenway 6300 spectrophotometer, Staffordshire, UK) using deionized water a blank. Whenever the need arises the AgNPs solution was diluted with deionized water, and the absorbance was multiplied by the dilution factor. Results and Discussion Synthesis of AgNPs by OP extract: A colour change was noticed in the solution (pH=9.5) resulted from mixing 1 mL of OP extract with 10 mL of 1 mM AgNO3. As the reaction time increased, the colour of the solution changed gradually from pale yellow to brown figure (1). This colour change confirms the formation of AgNPs. 664 Special Issue:The 2nd Annual Conference on Theories and Applications of Basic and Biosciences ♦ September, 1st, 2018 Figure (1) colour change due to AgNPs formation. In addition to that, the visible spectrum of AgNPs revealed a peak at 430 nm, which was not observed in the visible spectrum of the OP extract figure (2). This peak is attributed to the surface plasmon resonance (SPR) of free electrons in AgNPs[27,28], and its appearance confirms AgNPs synthesis. Similar observations, concerning the colour change and the appearance of the SPR peak as evidences for AgNPs synthesis, have been reported in many previous studies[18,20,24]. Figure (2) Proof of AgNPs formation by Vis spectrum. Effect of pH: It was noticed that the colour of the solutions became darker with increasing the pH, suggesting that the green synthesis in this study favours basic medium. Furthermore, the visible spectrum of these solutions with different pH showed that as the pH increased the absorbance of the SPR peak increased, with the highest absorbance recorded at pH = 10 as shown in figure (3). Figure (3) increasing absorbance of SPR peak as the pH of the solution increased. Several previous studies have demonstrated that pH plays an important role in green synthesis of AgNPs; enhanced by basic medium and suppressed in acidic medium[21,24,29-31]. It is crucial to note that at pH =5 there was no colour change and 665 Green synthesis of silver nanoparticles using olive pomace extract the SPR peak was not observed, indicating that AgNPs was not formed in the acidic medium. This could be ascribed to that some functional groups, such as OH in polyphenols, are not ionized in acidic medium; thus, could not perform their role in reducing Ag+ ions and converting them into AgNPs. Effect of reaction time: Figure (4) shows that as the time increased the absorbance of SPR peak increased and become sharper, reflecting that more AgNPs were formed. After 48 hours a maximum absorption of the SPR peak was observed indicating completion of the reaction. After 1 week there was only a slight change in the visible spectrum of this solution reflecting its stability. These findings are in agreement with the results of other studies[21,24,29]. Figure (4) increasing absorbance of SPR peak as the time increased. Effect of AgNO3 Concentration: In case of using AgNO3 with concentrations of 0.75, 1 and 2.5 mM to synthesize AgNPs the SPR peak appeared, with the maximum absorption resulted from using AgNO3 solution with a concentration of 2.5 mM. This indicates that increasing the AgNO3 concentration increased the produced amount of AgNPs, however; the SPR peak was not observed when the used AgNO3 concentration increased to 5 mM Figure (5), and this was combined with the formation of a colloidal pale brown solution. The suggested reasons are that silver oxide was formed instead of AgNPs, because using high concentrations of AgNO3 led to the reaction of Ag+ ions with OH- ions (from NaOH) instead of the reaction of Ag+ ions with OH functional groups present in polyphenols and other similar bioactive compounds, which exist in OP extract and believed of being responsible of AgNPs synthesis. Moreover, the SPR peak did not appear when the used AgNO3 concentration was 0.075 mM, and there was no colour change. This could be attributed to the small number of Ag+ ions available for reduction of Ag+ ions and formation of AgNPs. Figure (5) effect of AgNO3 Concentration. 666 Special Issue:The 2nd Annual Conference on Theories and Applications of Basic and Biosciences ♦ September, 1st, 2018 Effect of OP extract volume: As shown in figure (6) increasing the volume of OP extract increased the absorbance of the SPL peak which is an indication for formation of more amounts from AgNPs as more volumes of OP extract were used. Also, as the volume of OP extract increased, the SPR peak became sharper, which is an indication of forming more AgNPs with smaller sizes. This may be due to that increasing the extract volume provides more bioactive compounds responsible for synthesizing and capping AgNPs, subsequently the reaction occurs faster and the number of AgNPs with smaller particle size increases. In similar studies[29,30], in which other plants were used for green synthesis of AgNPs, it has been shown that AgNPs quantity increased with increasing the volume of plant extract. Figure (6) increasing absorbance of SPR peak with increasing of OP extract volume. Effect of temperature: As shown in figure (7) the absorption of the SPR peak increased with increasing temperature, with a remarkable increase when the temperature was 60°C, which indicates that the reaction is endothermic. In addition to that, as the reaction temperature increases, the reaction rate increases and the SPR peak became sharper, suggesting that more AgNPs with smaller particle size were formed[19,21,24,29-31]. Figure (7) increasing absorbance of SPR peak as temperature increased. Conclusion OP extract represents a good candidate for green synthesis of AgNPs, and through controlling the factors that affect proposed method (such as: time, temperature and OP extract volume) more amount of AgNPs, with small particle size, could be produced. 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