Green Synthesis of Silver Nanoparticles by Using Various Plants Leaves

Abstract

In the present experimental work, a rapid and simple method was applied for the synthesis of silver nanoparticles using different types of medicinal plants, viz. Sphagneticola trilobata, Catharanthus roseus, Azadirachta indica, and Dalbergia sissoo. The aqueous leaf extracts of these plants were used as both the reducing agent and the capping agent for silver nanoparticles synthesis. The green synthesis of silver nanoparticles (AgNPs) was characterized by UV-Visible spectroscopy. The UV-Visible spectrophotometer showed surface plasmon absorbance peaks in the range of 420-480 nm, with peaks at 420 nm, 444 nm, 430 nm, and 425 nm for Sphagneticola trilobata, Catharanthus roseus, Azadirachta indica, and Dalbergia sissoo, respectively.

Introduction

Nanotechnology is a field of modern technology dealing with the synthesis, strategy and manipulation of a particle's structure within a 1-100 nm size range. Within this size range, all the fundamental properties (chemical, physical, and biological) change as compared to their bulk molecules or atoms (Ahmed et al., 2014). The novel applications of nanoparticles and nanomaterials are growing rapidly on various fronts due to their absolutely new and enhanced properties based on their size, distribution, and morphology. Nanotechnology is rapidly gaining innovation in a large number of fields such as healthcare, cosmetics, biomedical, food and feed, drug-gene delivery, environment, health, mechanics, optics, chemical industries, electronics, space industries, energy science, catalysis, light emitters, single electron transistors, nonlinear optical devices and photo-electrochemical applications.

For all the aforesaid purposes, metallic nanoparticles are considered the most promising due to their remarkable antibacterial properties. These properties are attributed to their large surface area to volume ratio, which is of interest for researchers due to the growing microbial resistance against metal ions, antibiotics, and the development of resistant strains (Khalil et al., 2013). Among all noble metal nanoparticles, silver nanoparticles are a leading product from the field of nanotechnology. They have gained boundless interests due to their unique properties such as chemical stability, good conductivity, catalytic, and most importantly, antibacterial, antiviral, and antifungal in addition to anti-inflammatory activities. These properties allow silver nanoparticles to be incorporated into composite fibers, cryogenic superconducting materials, cosmetic products, food industry, and electronic components (Ahmed et al., 2016).

A number of methods are available for the synthesis of silver nanoparticles, such as ion sputtering, chemical reduction, and sol gel, etc. (Bindhu & Umadevi; 2015). Unfortunately, many of these nanoparticle synthesis methods involve the use of harmful chemicals, which can be difficult and often involve wasteful purifications (Ikram & Swami; 2015). Thus, any method followed will likely lead to chemical contamination during the synthesis process or subsequent applications, along with associated limitations. However, one cannot deny their ever-growing applications in daily life. Hence, it becomes essential to emphasize an alternate, synthetic route that is not only cost-effective but also environmentally friendly. With an eye to aesthetics, green synthesis procedures are proving their potential as a leading method. The techniques for obtaining nanoparticles using naturally occurring reagents such as sugars, biodegradable polymers (like chitosan), plant extracts, and microorganisms as reductants and capping agents are considered attractive for nanotechnology (Devi et al., 2015). Green synthesis of nanoparticles also provides advancement over other methods as they are simple, one-step, cost-effective, eco-friendly, relatively reproducible, and often result in more stable materials (Mittal et al., 2014). Microorganisms can also be utilized to produce nanoparticles, but the rate of synthesis is slow compared to routes involving plant-mediated synthesis (Ahmed et al., 2015). Although the potential of higher plants as a source for this purpose is still largely unexplored. Plant extracts from marigold flower (Padalia et al., 2014), Ziziphora tenuior (Sadeghi & Gholamhoseinpoor, 2015), Abutilon indicum (Ashokkumar, Ravi, & Velmurugan, 2013), Solanum tricobatum (Logeswari et al.,2013), Erythrina indica (Sre et al., 2015), beetroot (Bindhu & Umadevi, 2015), Spirogyra varians (Salari et al., 2014), olive (Khalil et al., 2013), leaf extract of Acalypha indica with high antibacterial activities (Krishnaraj et al., 2010), and of Sesuvium portulacastrum (Nabikhan et al., 2010) are emerging in literature as sources for the synthesis of silver nanoparticles as an alternative to conventional methods. Considering the immense potential of plants as sources, this work aims to apply a biological green technique for the synthesis of silver nanoparticles as an alternate to conventional methods. Silver nanoparticles can be produced in low concentration of leaf extract, avoiding the use of any additional harmful chemical or physical methods. The effect of concentration of metal ions and leaf extract quantities was also evaluated to optimize the route to synthesize silver nanoparticles. The method applied here is simple, cost-effective, easy to perform, and sustainable.

It is clear from past research that biologically synthesized silver nanoparticles find various applications in the field of biomedicine. Therefore, the present research work planned to explore four different plants for the biosynthesis of silver nanoparticles. The work was carried out with the following objectives: synthesizing silver nanoparticles and characterizing them using UV-Vis spectroscopy.

Materials and Methods

Typically, a plant extract-mediated bioreduction involves mixing the aqueous extract with an aqueous solution of the appropriate metal salt. The synthesis of nanoparticles occurs at room temperature and completes within a few minutes to overnight incubation.

Preparation of plant extract

Sphagneticola trilobata, Catharanthus roseus, Spilanthes paniculata, Azadirachta indica, and Dalbergia sissoo leaves extracts were used to prepare silver nanoparticles on the basis of cost-effectiveness, ease of availability, and their medicinal properties. Fresh leaves were collected from the college campus in the month of October. They were surface cleaned with running tap water to remove debris and other contaminated organic content, followed by distilled water, and air-dried at room temperature. About 10 gm of finely cut leaves were kept in a beaker containing 100 ml distilled water and boiled for 30 min. The extract was cooled down and filtered with Whatman filter paper No.1 (Cat.No. 1001125), and the extract was stored at 4ºC for further use (Harekrishna Bar et al., 2009).

Green Synthesis of Silver Nanoparticles

100 ml of a 1 mM solution of silver nitrate was prepared in a 250 ml conical flask. Then, 12 ml of silver nitrate solution was added to 88 ml of plant extract. This reaction mixture was incubated in a dark chamber to minimize photo-activation of silver nitrate at room temperature, and observed for reaction to color change (Obaiad et al., 2015).

Characterization of synthesised silver nanoparticles

UV-Vis spectral analysis was done using a UV-Visible spectrophotometer (Systronics, UV-1300). Three ml of sample mixture was taken and subjected to a test in a UV-Visible absorption spectrophotometer. A resolution of 1 nm between 200 and 800 nm was used.

Results and discussion

In our experiment, the addition of plant leaf extracts of Sphagneticola trilobata, Catharanthus roseus, Spilanthes paniculata, Azadirachta indica, and Dalbergia sissoo into the flasks containing an aqueous solution of silver nitrate led to a change in the color of the solution from yellowish to reddish-brown (shown in Fig. 1). This color change occurred within the reaction duration due to the excitation of surface plasmon vibrations in silver nanoparticles (Veerasamy et al., 2011). Upon adding different concentrations (1-5mL) of leaf extracts to the aqueous silver nitrate solution—keeping its concentration at 10 mL (1 mM)—the color of the solution changed from faint light to yellowish brown, and finally to colloidal brown, indicating the formation of silver nanoparticles. Silver nanoparticles synthesized from different leaf extracts, using 1 mM of silver nitrate, were analyzed by UV spectra of the Plasmon resonance band observed at 420-480 nm.

A similar study done by Afrah Eltayeb Mohammed (2013) used the aqueous extract of E. Camaldulensis leaf and synthesized silver nanoparticles. The absorption value for the extract ranged between 400-450 nm and the color change to dark brown corresponded to the plasmon absorbance of AgNPs.

Geethalakshmi and Sarada (2010) synthesized silver nanoparticles by using Trianthema decandra leaf extract. The nanoparticles were characterized using UV-Vis spectroscopy. The absorption spectra formed in the reaction media showed an absorbance peak at 450 nm.

Conclusion

A simple green synthesis of silver nanoparticles using Sphagneticola trilobata, Catharanthus roseus, Spilanthes paniculata, Azadirachta indica, and Dalbergia sissoo leaf extract at room temperature was reported in this study. Synthesis was found to be efficient in terms of reaction time as well as the stability of the synthesized nanoparticles, which exclude external stabilizers/reducing agents. It proves to be an eco-friendly, rapid green approach for the synthesis, providing a cost-effective and efficient way for the synthesis of silver nanoparticles. Therefore, this reaction pathway satisfies all the conditions of a 100% green chemical process. Benefits of using plant extract for synthesis include energy efficiency, cost-effectiveness, protection of human health and environment, and theoretically less waste and safer products. This eco-friendly method could be a competitive alternative to the conventional physical/chemical methods used for the synthesis of silver nanoparticles.

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