In this study Cassia angustifolia (senna) is used for the environmentally friendly synthesis of silver nanoparticles. Stable silver nanoparticles having symmetric surface plasmon resonance (SPR) band centred at 420 nm were obtained within 10 min at room temperature by treating aqueous solutions of silver nitrate with C. angustifolia leaf extract. The water soluble components from the leaves, probably the sennosides, served as both reducing and capping agents in the synthesis of silver nanoparticles. The nanoparticles were characterized using UV-Vis, Fourier transform infrared (FTIR) spectroscopic techniques and transmission electron microscopy (TEM). The nanoparticles were poly-dispersed, spherical in shape with particle size in the range 9-31 nm, the average size was found to be 21.6 nm at pH 11. The zeta potential was -36.4 mV and the particles were stable for 6 months. The crystalline phase of the nanoparticles was confirmed from the selected area diffraction pattern (SAED). The rate of formation and size of silver nanoparticles were pH dependent. Functional groups responsible for capping of silver nanoparticles were identified from the FTIR spectrum. The synthesized silver nanoparticles exhibited good antibacterial potential against Escherichia coli and Staphylococcus aureus.

Utilization of bioresources for the synthesis of metal nanoparticles is the latest field in green chemistry. The present work reports the utilization of the aqueous bark extract of Plumbago zeylanica for the biosynthesis of Ag and Au NPs. The Ag and Au NPs thus obtained were characterized by UV-Vis, FT-IR, TEM, XRD, and EDAX analysis. The water-soluble components of the extract were responsible for the reduction of Ag ? and Au 3? ions. FT-IR spectra revealed that the -OH and [C=O groups present in the biomolecules were responsible for reduction and stabilization of nanoparticles. TEM images showed the existence of spherical Ag and Au NPs with average size of 28.47 and 16.89 nm, respectively, which was further substantiated by XRD analysis. The presence of elemental Ag and Au along with C and O from the attached biomolecules was proved by EDAX analysis. The antimicrobial, antioxidant, and in vitro cytotoxic activities of the synthesized nanoparticles were studied by disc diffusion, DPPH, and MTT assay methods, respectively. The free radical inhibition was found to be 78.17 and 87.34 % for Ag and Au nanoparticles, respectively. The Ag and Au NPs showed 61.56 and 65.61 % toxicity against DLA cell line, respectively. The DNA binding ability of Ag and Au NPs were investigated using CT-DNA. The hyperchromism shift inferred the groove binding of nanoparticles with CT-DNA.

Solar light induced photo catalysis by plasmonic nanoparticles such as Au and Ag is an important field in green chemistry. In this study an environmental benign method was investigated for the rapid synthesis of colloidal Ag and AuNPs using the extract of Hydrocotyle asiatica, as a reducing and stabilizing agent under sunlight irradiation. The nanoparticles were formed in few seconds and were characterized by UV-Vis., FT-IR, TEM, EDAX, XRD, DLS and Zetasizer. The nanoparticles were stable in aqueous solution for more than 6 months. TEM analysis established that the Ag and AuNPs were predominantly spherical with average size of 21 and 8 nm, respectively. The flavonoids and glycosides from the extract of H. asiatica were proved to be responsible for the reduction and capping through FT-IR analysis. The antimicrobial studies of AgNPs showed effective inhibitory activity against the clinical strains of gram-negative and positive bacteria. The localized surface plasmon resonance of AgNPs was used for the photo-driven degradation of cationic dyes (malachite green and methylene blue). Thus, this green technique can be used for bulk production of AgNPs, and thus prepared nanoparticles may be used for removal of dyes from effluent. Graphical Abstract

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Research on plasmonics and their applications in diverse fields has increased significantly over the last decade. The present work primarily aimed to bio-fabricate silver and gold plasmonic nanoparticles and to study their potential as photocatalysts and as DNA binders. To achieve this, Ag and Au NPs were derived employing Peristrophe paniculata and their optical, morphological and structural properties were investigated by standard analytical techniques. The formation rate constant k was 2.58 × 10 À 2 min À 1 and 3.56 × 10 À 2 min À 1 for Ag and Au NPs correspondingly. FT-IR analysis showed that the water-soluble alkaloid, 5-amino-3,4-dihydro-2H-pyrrole-2-carbonitrile was responsible for the stabilization of nanoparticles. Transmission electron microscopic studies disclosed the formation of uniform spherical AgNPs and irregularly shaped AuNPs and the diameter was 9.32 nm and 27.85 nm respectively which was again corroborated by XRD studies (16.64 nm and 36.08 nm). The interaction of ct-DNA with Ag and Au NPs was investigated using absorption spectroscopy in the UV-Vis range and the binding constant values were 2.2 × 10 5 M À 1 and 8.1 × 10 6 M À 1 respectively. The degradation of methylene blue and malachite green by AgNPs was almost 100 % and 65 % respectively in 3 h under sunlight and neutral pH (pseudo first order rate constants were 7.9 × 10 À 3 min À 1 and 1.9 × 10 À 3 min À 1 ). Moreover, the derived AgNPs exhibited mitigate activity against targeted pathogens. Thus, this green technique could be implicit for the eco-friendly synthesis of Ag and Au NPs, and the prepared nanoparticles may be explored as photocatalysts for the degradation of cationic dyes without any added oxidants.