Evaluation of Natural gums as potential precursors for the synthesis of gold and silver Nanoparticles their characterization and applications

Abstract

Herbal medicines are recognized as multi-component remedies for the treatment of complicated diseases because of their better therapeutic value and fewer side effects compared to modern medicine. The work presented in this thesis consists of “Evaluation of natural gums as potential precursors for synthesis of gold and silver nanoparticles: their characterization and applications” which includes the synthesis, optimization, characterization, applications and bioactivity evaluation of gold and silver nanoparticles stabilized with different gums. Four different gums of plants origin and one gum of microbial origin were selected for the synthesis of gold and silver nanoparticles. Gums of plants origin were collected from three different plant families i.e. family Fabaceae (genus: Acacia), family Rosaceae (genus: Prunus), family Bixaceae (genus: Cochlospermum), while the microbial “Xanthan gum” was obtained from Xanthomonas campestris. In this study gold and silver nanoparticles (Au- and Ag-NPs) were biosynthesized by using the aqueous extracts of different gums. Synthesized nanoparticles were characterized with different spectroscopic and microscopic techniques such as UV–Vis spectroscopy, Fourier Transform Infrared (FTIR), Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD) and atomic absorption (AA). The effects of gum, metal ions concentration, reaction temperature and time on the synthetic stability of nanoparticles was studied along with their post synthetic stability against varying pH and salt concentrations, long term storage and extreme of temperature. Formation of Au- and Ag-NPs was confirmed from the surface Plasmon resonance in the range of 500-560 and 400-450 nm respectively and was further corroborated from the results of FTIR, EDX, XRD and Atomic absorption. SEM analysis showed that Au- and Ag-NPs stabilized with different gums were mostly spherical and in the size range of 5–50 nm. FTIR analysis revealed that the abundance of hydroxyl, carboxylate, amide and acetyl groups of the gum biopolymers are involved in the reduction and stabilization of metal cations. The EDX analysis confirmed the presence of Au and Ag in the respective gum stabilized nanoparticles. No peaks were observed in the XRD spectrum of pure gums, thus indicating their non crystalline nature; while the XRD pattern of gum stabilized Au- and Ag-NPs have characteristic peaks, and so are crystalline in nature. Atomic absorption analysis showed almost complete reduction of the metal ions into the respective nanoparticles. The TGA spectrum further confirmed their thermal stability, where three successive weight losses were observed in the temperature range of 50–800 oC. It was observed that reaction temperature of 80oC and reaction time of 4 h was most suitable for the efficient synthesis of Au- and Ag-NPs. The results of post-synthetic stability revealed that Au- and Ag-NPs were stable in different concentrations of NaCl (1–3 M), neutral to acidic pH (7–2) and without any long term storage (eight months) or thermal (100 oC) induced degradable changes. Gold and silver nanoparticles stabilized with different gums were evaluated for their prospective anticancer, antibacterial, urease inhibition, anti-inflammatory and analgesic properties. Gold and silver nanoparticles possessed mild to moderate antibacterial activity and can observed from their zone of inhibition against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Gold nanoparticles significantly alleviated the acetic acid induced writhes at much lower doses of 40 mg/kg (P <0.01) and 80 mg/kg (P < 0.001) compared to that of gum extract at 200 and 400 mg/kg (P <0.001). At similar doses, Au-NPs also significantly inhibited the carrageenan induced paw edema during the 1st h (P < 0.05) and 2–5 h (P <0.001) of the study duration. Different gums stabilized Au- and Ag-NPs were tested to evaluate their cytotoxic potential against human cervical cancer cells (HeLa). Potential anticancer effect was showed by the respective gold and silver nanoparticles. Biosynthesized Au- and Ag-NPs were also able to inhibit urease with better affinity.