A New Method for Synthesis of Silver Nanoparticles

In the present experimental work, a rapid and simple method was applied for the synthesis of silver nanoparticles using different types of medicinal plants, namely Sphagneticola trilobata, Catharanthus roseus, Azadirachta indica, and Dalbergia sissoo. Aqueous leaf extracts were used as reducing and capping agents for the synthesis of silver nanoparticles. The green synthesis of silver nanoparticles was characterized by UV-visible spectroscopy, which showed surface plasmon absorbance peaks in the range of 420-480 nm. The obtained peaks were 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 that deals with the synthesis, strategy, and manipulation of particle structures within the size range of 1-100 nm. Within this size range, all the fundamental properties change compared to their bulk molecules or atoms. Novel applications of nanoparticles and nanomaterials are growing rapidly in various fields due to their new and enhanced properties based on their size, distribution, and morphology. It is rapidly gaining traction in numerous fields such as healthcare, cosmetics, biomedicine, 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.

The metallic nanoparticles are considered the most promising for all the aforementioned purposes, as they have remarkable antibacterial properties due to their large surface area-to-volume ratio. This is of interest to researchers due to growing microbial resistance against metal ions, antibiotics, and the development of resistant strains. Among all noble metal nanoparticles, silver nanoparticles are a significant product from the field of nanotechnology. They have gained immense interest because of their unique properties such as chemical stability, good conductivity, catalytic activity, and most importantly, antibacterial, antiviral, antifungal, and anti-inflammatory activities. They can be incorporated into composite fibers, cryogenic superconducting materials, cosmetic products, food industry products, and electronic components.

Several methods are available for the synthesis of silver nanoparticles, such as ion sputtering, chemical reduction, sol-gel, etc. Unfortunately, many of these nanoparticle synthesis methods involve the use of harmful chemicals, which are difficult and include wasteful purifications. Thus, the scenario is that whatever method is followed, it will always lead to chemical contamination during their synthesis procedures or later applications with associated limitations. Yet, one cannot deny their ever-growing applications in daily life. Hence, it is becoming a responsibility to emphasize an alternative synthetic route that is not only cost-effective but also environmentally friendly. Keeping in view the aesthetic sense, green synthesis is proving itself as a key procedure and demonstrating its potential. Techniques for obtaining nanoparticles using naturally occurring reagents, such as sugars, biodegradable polymers, plant extracts, and microorganisms as reductants and capping agents, could be considered attractive for nanotechnology.

Green synthesis of nanoparticles also provides an advancement over other methods as they are simple, one-step, cost-effective, eco-friendly, and relatively reproducible, often resulting in more stable materials. Microorganisms can also be utilized to produce nanoparticles, but the rate of synthesis is slow compared to routes involving plant-mediated synthesis. Although the potential of higher plants as a source for this purpose is still largely unexplored, very recently plant extracts of marigold flower, Ziziphora tenuior, Abutilon indicum, Solanum tricobatum, Erythrina indica, beetroot, Spirogyra varians, olive, leaf extract of Acalypha indica with high antibacterial activities and of Sesuvium portulacastrum have also been reported in the literature with nanoparticle sizes ranging from 5 to 20 nm, as an alternative to conventional methods for the synthesis of silver nanoparticles.

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 alternative to conventional methods. Silver nanoparticles can be produced at a low concentration of leaf extract without using any additional harmful chemical or physical methods. The effect of the concentration of metal ions and the quantity of leaf extract 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 the study of research done in the past that biologically synthesized silver nanoparticles find various applications in the field of biomedicine. Therefore, the present research work was planned to explore four different plants for the biosynthesis of silver nanoparticles, and the whole work was carried out under the following objectives: synthesis of Silver nanoparticles and its characterization by using UV-Vis spectroscopy.

Materials and methods

Typically, a plant extract-mediated bio-reduction 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 trilobate, Catharanthus roseus, Spilanthes paniculate, 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 property. 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 contents, 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, and the extract was stored at 4ºC for further use.

Green synthesis of silver nanoparticles

A 100 ml, 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 a reaction to color change.

Characterization of Synthesized Silver Nanoparticles

UV-Vis spectral analysis was performed using a UV-Visible spectrophotometer. Three milliliters of the sample mixture were taken and subjected to testing in a UV-Visible absorption spectrophotometer with a resolution of 1 nm between 200 and 800 nm.

Results and Discussion

In our experiment, the addition of plant leaf extracts from 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 within the reaction duration due to excitation of surface plasmon vibrations in silver nanoparticles. By adding different concentrations of leaf extracts to an aqueous silver nitrate solution while keeping its concentration at 10 mL constant, 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 plasmon resonance bands observed at 420-480 nm.

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

Geetha Lakshmi and Sarada synthesized silver nanoparticles using Trianthema decandra leaf extract and characterized them using UV-Vis spectroscopy. Absorption spectra formed in the reaction media show 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 extracts at room temperature was reported in this study. The synthesis was found to be efficient in terms of reaction time as well as stability of the synthesized nanoparticles, which excludes 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. The benefits of using plant extract for synthesis are that it is energy-efficient, cost-effective, protects human health and the environment, leading to lesser 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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