Green Synthesis of Silver/Silver Chloride Nanoparticles using Shallot Peel Extract as Reducing Agent

Precious metal nanoparticles, especially silver, can be an alternative antibacterial agent that is more promising than antibiotics, which tends to induce resistance over time. Unfortunately, most Silver nanoparticle (AgNP) manufacturing methods involve hazardous substances, extreme conditions, high energy, and complicated separations. Reduction of AgNO3 solution with plant extracts can serve as a ‘green’ synthesis pathway, especially if using food processing waste or other biological materials. Reducing substances that are commonly found in plant extracts, such as flavonoids, are widely contained in shallots (Allium cepa L. var. aggregatum) which are commonly used as food ingredients in Indonesia, but the processing waste is still not widely used. This paper describes the synthesis of silver nanoparticles (AgNP) with the AgNO3 solution reduction method, using onion peel waste extract as a reducing agent. Experiments were carried out with variations in the ratio of AgNO3 solution to onion peel extract. The resulting AgNPs were characterized by TEM and UV-vis.


Introduction
Nanomaterials is a field that studies materials with dimensions of 100 nm or less.At this scale, material properties can change because the ratio of surface to volume increases, so that surface properties dominate over bulk properties, thus the chemical properties appear to change even though the chemical composition remains the same.In addition, various phenomena which only occur on a quantum scale, such as localized surface plarmon resonance (LSPR) begin to appear, and these can affect both physical and chemical properties [1][2][3].
Currently, a lot of research are being done on metal nanoparticles for purposes such as catalysts, electronics and instrumentation, medical, optical, and drug delivery.One application of metal nanoparticles for medical purposes includes antibacterial and antiviral agents to prevent infection between patients [4][5][6][7].Metal nanoparticles offer an advantage over antibiotics by being far less susceptible to inducing bacterial resistance to it [6].
Silver is one of the commonly researched antibacterial nanometals.Main synthesis pathways of silver nanoparticles (AgNPs) can be categorized into physical, chemical, and biological/'green'.
Physical methods include lasers and high voltages, while chemical methods involve reducing agents such as hydrazine and sodium borohydride [8].To address the high energy usage or hazardous chemicals of aforementioned methods, green synthesis pathways involving living organisms or extracts thereof are among the major interests in current research.These methods usually involve the reduction of silver nitrate solution into silver nanoparticles.Among the reducing agents include flavonoids, such as quercetin [2,4].
One potential source of reducing agents is shallot (Allium cepa L. var aggregatum).Research has been carried out with the main meat of the shallot itself [2], but it can lead to supply competition because it is also needed for food.This study aims to investigate the use of shallot peel waste extract as bioreductor in silver nanoparticle production from silver nitrate.Shalllot peel waste is usually set aside, but still contains a high enough active substance to reduce AgNO3 to AgNP.

Collection of Shallot Peel
Shallot peel were obtained from local markets in Bandung and Cicalengka, West Java, Indonesia.They were rinsed with water thrice, followed by sun drying, and stored in dark and dry places with absorbent contaners such as paper bag or cardboard.

Preparation of Shallot Peel Extract
Dried peel were added to distilled water (Amidis, Indonesia) with 1:8 weight ratio, then blended for 40 seconds per batch using MEDEENALUX chopper.The resulting extract was filtered and squeezed from peel, then separated in Allegra X-30 centrifuge for 3 minutes at 9000 RPM.The supernatant is taken and stored in refrigerator for later stages of the synthesis as the reducing agent.

Synthesis of AgNPs
AgNO3 1 mM was prepared using Sigma Aldrich silver nitrate and IPA API sterile water.Then, shallot peel extract was added at a 15:100 extract:precursor volume ratio.The mixture was heated until its color was observed to change.The resulting nanoparticles were separated with centrifuge then washed with sterile water.

Characterization of Synthesized AgNPs
The optical characterization was carried out with UV-vis absorption using in the range of 200-700 nm at room temperature.Particle size and morphology were characterized with TEM.

Synthesis of Nanoparticles
The mixture of AgNO3 1 mM and shallot peel extract started to change color after it reached 75 °C, at which point its color turned from yellow to red, indicating the formation of silver nanoparticles.Heating was continued and held at 85 °C for 45 minutes.The resulting nanoparticles were separated with centrifuge,at 9000 RPM, then washed with water 3 times.

Characterization of Nanoparticles
UV-vis was carried using Drawell spectrophotometer.The sample was first diluted wth distilled water with a 10:250 volume ratio.Results showed a strong peak at 416 nm, indicating formation of silver nanoparticles, with the color being possible to appear due to localised surface plasmon resonance (LSPR) which only appears within suspended nanoscale silver particles [9].

Figure 1 UV-vis absorption spectroscopy result for the synthesized AgNPs
A portion of the sample was sonicated, then later dried on a substrate for TEM analysis using Hitachi TEM System.TEM imaging was carried out at 120 kV with x120,000 magnification.The resulting image displays the nanoparticles as darker regions of the image, which showed spherical particles within 100 nm range, thus fits the definitions of nanoparticles, and matches the LSPR phenomenon from the UV-vis results.The sample showed a high distribution of nanoparticle sizes, some bigger than 200 nm, while most are smaller than 50 nm.This drastic difference might indicate the presence of a capping action, which keeps nanoparticles separate until they collide strongly enough, at which point they suddenly aggregate due to the thermodynamic instability of nanoparticles.Both the nanoscale and aggregated particles show a spheroid morphology.

Conclusion
Silver nanoparticles can be synthesized using reducing agents derived from the extract of shallot peel, a common food waste in Indonesia.The synthesized silver particles were well within 100 nm, thus count as nanoparticles.The AgNPs are spherical in shape.There is a drastic size difference between nanoparticles smaler than 50 nm and 200 nm aggregates, indicating a capping action.It is possible to perform other characterization methods such as XRD, FTIR, and SEM for more thorough analysis.

Figure 2
Figure 2 TEM image of the synthesized AgNPs