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The Nanoparticles are being viewed as the building blocks for nanotechnology. The most important property is that they exhibit large surface area to volume ratio. Nanoscale inorganic particles (NIPs) have unique properties such as catalytic, optical, magnetic and electrical. Chemical reduction, photochemical reduction, electrochemical reduction, heat evaporation etc., chemical and physical methods are used for NIPs synthesis, which are not environmentally friendly. This study interested in bio-synthesis of silver nanoparticles, AgNPs.
This study is divided into three chapters.
Chapter one: Introduction and literature review:This chapter includes general introduction on nanoparticles and nanotechnology. The chemical, physical and biological methods especially the methods using plant extract for preparations are reviewed.
Chapter Two: This chapter includes the different experimental techniques that have been employed to study the structure, thermal and optical characterization of AgNPs samples with their preparation procedures.
Chapter three: This chapter was divided to three parts:
The first part deals with the biosynthesis of nanosilver using Eugenia uniflora leaf extract. The reason for selecting plant for biosynthesis is because they contain reducing agents such as citric acid, ascorbic acids, flavonoids, reductases and extracellular electron shuttlers that may play an important role in biosynthesis of metal nanoparticles. Unlike chemical methods which demands expensive instruments and results in release of inimical and hazardous chemicals, biological method is more facile, eco-friendly and results in more monodispersed nanoparticles. The most important results can be summarized as follows:
1) Addition of different concentration of the Eugenia uniflora leaf extract (from 0.2to 4mL) (20g/L) to the Ag+ solution (1.3x10-3M) leads to change in the color of the solution to yellow dark brown and is characteristic for the surface plasmon resonance (SPR) of different size of AgNPs. Increasing the extract concentrations leads to a slight decrease in the absorbance indicating that 4 mL of (2%) extract is enough for reduction of Ag+ to Ag0 and capping of the produced nanoparticles.
2) The Uv-vis spectra of the Ag+ solution after the addition of (4ml) extract as a function of time showed that the reaction ended in two hours and the nanoparticles are stable for 3 days without change in the absorbance at 430 nm.
3) The pH of the extract solution has an important effect on the size and shape of the nanoparticles. The absorbance at 450 nm at pH 5 shifted to 420 nm at pH 6 and the blue shift in the spectra suggested decrease in the particle size. Further increase in the pH lead to slight decrease in the absorbance. TEM measurements showed that at low pH solution, different shapes of nano-structured were obtained while at high pH 8, only spherical particles were obtained. Also, the particles formed in acidic medium were unstable and precipitated within 8 h while the particles prepared in basic medium are stable for at least one week.
4) FTIR measurements were carried out to identify the potential biomolecules in Eugenia uniflora leaf responsible for the reduction, capping of and efficient stabilization of the bio-reduced silver nanoparticles.
5) TGA plot of the capped gold nanoparticles in the temperature range of 100°–580°C showed weight loss of 6.74% due to the desorption of bioorganic compounds capped AgNPs. This suggest that using Eugenia uniflora in the synthesis gave high yield of AgNps.
6) Histochemical study on the Eugenia uniflora detected the presence of phenolic compounds.
The secondpart includes the biosynthesis of nanosilver using Psidium guajava leaf extract as source of reducing agent (antioxidant) capable of converting Ag+ to Ag nanoparticles. Silver nitrate (silver salt) when exposed to aqueous extracts of plant have resulted in the intra-cellular as well as extra cellular formation of metal nanoparticles. The rate of formation for nanoparticles was followed using Uv-Vis spectrophotometer. The data showed that size of the nanoparticles could, to an extent, be manipulated by controlling parameters such as pH, temperature, substrate concentration and exposure time to the extract. The most important results can be summarized as follows:-
1) Using guava leaf extract (20g/L) and the same silver nitrate solutions as the Eugenia uniflora it is found that the best concentration used is 6 mL of the extract. By increasing the concentration of the extract, the absorption increased with a blue shift from 428 to 422 nm. TEM measurements showed that the particle size in the range of 34-40 nm when 0.4 mL of the extract used, and of 14-34 nm when 5 mL of the extract was used.
2) Increase in the pH to pH 8 decrease the contact time needed for reduction to 52 min.
3) Increase the reaction temperature increase the rate of nanoparticles formation and the highest absorbance was observed at 90 0C which is much higher than Eugenia uniflora which was 40 0C.
4) FTIR measurements of guava leafand the capped AgNPs showed the presence of biomolecules in guava leaf responsible for the reduction, capping of and efficient stabilization of the bio-reduced silver nanoparticles. TGA measurements of the AgNPs showed that the amount of the biomolecules on the surface of the AgNPs was 32% four times higher than obtained with Eugenia uniflora.
5) The mechanism of reduction of Ag+ by aqueous guava leafextract and formation of AgNPs was suggested with the help of leaf histochemical study.
The third part also deals with biosynthesis of nanosilver using Syzygium jambos leaf extract following the same green chemistry eco-friendly protocol used in the synthesis of the AgNPs by the guava and Eugenia uniflora leaf extract. Again this plant extract has been chosen because of its value in the medicinal applications. The high phenolic content of the hot water extract of the Eugenia jambos leaf having strong anti-oxidant properties helped in the reduction of silver cations to Ag NPs.
This study proved that a crude (untreated) extract from any of the used plants contains novel, structurally diverse chemical compounds that are able to reduce Ag+ ions and form AgNPs. It will be not surfeit to say that nature is a natural nanofactory which can help to synthesize a variety of nanoparticles.