UV-assisted green synthesis of silver nanoparticles and their characterizations

Silver nanoparticles are receiving discernible attention because of their potential applications in different fields of science and technology. However, chemical synthesis methods leave behind harmful byproducts. Here, we report an alternative method for the rapid synthesis of silver nanoparticles through radiation-assisted green synthesis. Aqueous leaf extract of Ocimum sanctum is used as a reducing and stabilizing agent with silver nitrate solution under UV-B irradiation. UV-visible spectroscopy (UV-Vis), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX), photoluminescence spectroscopy (PL) and fourier transform infrared spectroscopy (FTIR) collectively revealed that bio reduced silver nanoparticles were polydisperse, polycrystalline, spherical in shape, had an average diameter of 40.1 nm, and were stable more than six months. These observations suggest that radiation-assisted green synthesis is a quick and eco-friendly method for the large-scale production of stable silver nanoparticles without any harmful byproducts.


Introduction
Silver nanoparticles (AgNPs) are one of the noble metal nanoparticles attaining tremendous applications in many branches of science and technology, including photovoltaics, bioengineering and biomedical science, electronics, textile industrial science, and environmental science [1][2].These are due to the unique physical, chemical, mechanical, optical, and electrical properties of AgNPs [3][4].The AgNPs can be synthesized through different approaches, such as chemical reduction, sonochemical, microwaveassisted, and physical [5].However, these approaches include several disadvantages that may have harmful effects on the environment [6].Thus, to avoid harmful reagents, the safest alternative is biological synthesis, as this method leads to less toxicity, is low-cost, and is more eco-friendly [7][8].Biological synthesis of AgNPs can be achieved using various precursors like algae, microorganisms, lichens, and plant-based products [9].These biological entities are suitable for the synthesis of nanoparticles at ambient temperature and a regulated pH.The biochemicals present in them serve a dual role as reducing and stabilizing agents [10].Among these entities, plant-based synthesis has more benefits, as the production cost is lower and the purification processes are also simple [11].
Electromagnetic radiation is another key parameter that affects synthesis rates.Different types of radiation, namely microwave, visible, UV, combinations of UV and visible, and gamma rays, are used for the synthesis of AgNPs [12][13][14][15][16].The use of radiation for the production of semiconductor and nanometal particles has become very popular in chemistry, but the role of irradiation in nanoparticle biosynthesis has not yet been systematically studied [17].Studies have reported the synthesis of AgNPs under microwave irradiation using Musa paradisiaca, Ficus racemosa, Elephantopus scaber, and Biophytum sensitivum leaf extracts [18][19][20][21].AgNPs were also successfully synthesized through sunlight irradiation utilizing the leaf extracts of Nagia nagi, Sida retusa, Jasmine subtriplinerve blume, and Polygonatum graminifolium [22][23][24][25].Bulent et al. employed combinations of UV and visible radiation for the synthesis of AgNPs using Ficus carica leaf extract [26].Pradeep et al. used Lantana camara leaf extract in the UV-assisted production of AgNPs [27].
Ocimum sanctum (OS) is a diploid plant belonging to the Lamiaceae family, commonly known as Tulasi.It has been regarded as a divine herb in the Indian Ayurvedic system of traditional medicine.Tulasi extract is widely used in tonics as immune stimulators, anti-oxidants, neuromodulators, and for the treatment of various types of heart diseases, poisoning, inflammation, malaria, the common cold, and cough.Recently, studies reported the synthesis of AgNPs using Tulasi leaf extract, and eugenol, quercetin, luteolin, apigenin, naringin, and kaempferol are a group of flavonoids found in the extract that function as reducing and capping agents [28][29].
Considering all these things, the present study was carried out to synthesize simple, low-cost, ecofriendly, and easily renewable AgNPs using Tulasi leaf extract under UV-B irradiation.

Materials:
The chemicals silver nitrate (>99%) and sodium hydroxide pellets, that were employed in this study were acquired from Sigma Aldrich and used directly without additional purification.Fresh and healthy OS leaves were gathered from Bela village in Kasaragod, Kerala, India.To remove particulate debris, leaves were washed numerous times with tap water, finally being washed with doubledistilled water.To remove excess water from their surface, leaves were shade dried for thirty minutes.

Preparation of OS leaf extract:
10g of weighted leaves were pasted with 5mL of double-distilled water using a mortar and pestle.The solution was diluted up to 300mL, boiled for thirty minutes, and then filtered through Whatman No.1 filter paper.For the synthesis of AgNPs, clear OS leaf extract is utilized.
2.3 Synthesis of OS-AgNPs: 0.0016g of silver nitrate was dissolved in 10mL of double-distilled water to prepare a solution of 1mM silver nitrate [40].2mL of leaf extract was combined with 4mL of freshly made 1mM silver nitrate solution.By adding 0.1N NaOH solution, pH of the mixture was maintained at 9. The reaction mixture was exposed to UV-B (wavelength=280-315nm) irradiation for the quick and complete conversion of Ag + to Ag 0 .

Characterizations of synthesized AgNPs:
The formation of AgNPs was confirmed by the Shimadzu UV-1800 (Japan) UV-visible spectrophotometer.Further, they were characterized by the Titan Themis 300kV from FEI transmission electron microscope and the IR prestige-21 FTIR instrument.The PL study was carried using HITACHI Japan model F7000 spectrophotometer.The Oxford instrument JSM6390LV scanning electron microscope with energy dispersive spectrum detector is used for EDX analysis.

Results and Discussions
3.1 UV-Vis.absorbance analysis: With the addition of silver nitrate solution and UV-B irradiation, leaf extract underwent a color change from clear brown to black.The reason for this is surface plasmon resonance (SPR) phenomenon [30].It generated a peak having center close to 419nm (figure 1).The widening of the SPR curve denotes the creation of polydisperse nanoparticles [31].3.2 TEM analysis: TEM analysis revealed that biosynthesized AgNPs were spherical in shape, almost non-agglomerated, and polydisperse, having an average diameter of 40.1nm (figures 3 and 4).According to the selected area electron diffraction (SAED) pattern, synthesized AgNPs were polycrystalline, and the calculated d-spacing for five concentric circles matches JCPDS file number 00-004-0783 (figure 4).

EDX analysis:
A spectral signal in the silver region was found by EDX analysis (figure 5).Silver made up 0.40 of the total atomic percentage.Additionally, a spectral signal in the oxygen region is noticed.This is because extracellular molecules that were adsorbate on the surface of AgNPs and were derived from the leaf extract [32].Also, a peak for carbon is detected because of the grid utilized for the analysis.The spectrum also revealed two additional peaks at 906.28cm -1 and 756.62cm -1 .These results are from out-plane bending of aromatic -CH bonds [33].
It has been investigated the stability of the OS-AgNPs colloidal solution.Even after six months of storage at room temperature, no color or visual aggregate has been seen.Additionally, there was no change in the SPR peak position as indicated by the absorbance spectrum (figure 8).Thus, AgNPs formed using this approach are extremely stable.

Conclusion
In summary, we have demonstrated a simple method for the quick synthesis of AgNPs employing an aqueous leaf extract of Ocimum sanctum as the reducing and stabilizing agent.When compared to chemical synthesis, the remarkable repeatability of these nanoparticles without the need for extra chemicals will be advantageous.The findings showed that radiation-assisted green synthesis provides an eco-friendly, efficient, and alternative method for producing AgNPs on a large scale without creating hazardous by-products