Green Synthesis Of Capped Silver Nanoparticles And Their Hybrids: Antimicrobial And Optical Properties

ABSTRACT

Unique properties of silver (Ag) and its allied nanoparticles are of great interest in the fields of chemistry and technology. However, the widespread applications of nanoparticles call for synthesis routes involving eco-friendly procedures. Silver nanoparticles (Ag NPs) and their hybrids nanoparticles (Ag-Ni and Ag-Co) were synthesized using locally available biodiversity plants as alternative method to the expensive and toxic chemicals previously used. Fresh plants were collected and extracted in water by cold extraction. Phytochemical screening was carried out on the plant extracts and green plant-mediated reduction method was employed. Nanoparticles formation and growth were monitored with UV-Vis spectrophotometer at time intervals. The nanoclusters were further characterized using photoluminescence (PL), scanning electron microscope (SEM), transmission electron microscope (TEM), fourier transmission infra-red (FTIR), x-ray diffraction (XRD) and selected electron area diffraction (SAED). Antimicrobial activity of the nanoparticles was investigated on clinically isolated pathogens. Formation of nanoparticles was indicated by colour change in the reaction medium when nucleation and growth commenced as early as 2 minutes in the reaction at 70°C, compared to the syntheses carried out at room temperature. Optical properties were displayed with surface plasmon resonance bands (SPR) above 400 nm in Ag NPs and their hybrids. The high intensities of absorption were characterized with red-shifted wavelengths, except the Ag NPs and the corresponding nanohybrids formed using the extract of S. occidentalis (327-350 nm). However, near uniform emission of fluorophores was exhibited by Ag NPs (438-466 nm) and Ag allied nanoparticles (581-778 nm) irrespective of the excitation wavelengths. The morphological characterization of Ag-Co nanohybrid using SEM and TEM revealed novel alloy structure in which Ag atoms were attached to the vertices of cobalt in a planar arrangement. Other morphologies obtained using C. indica leaf extract were cluster-in-cluster spherical arrangement, nanorod, nanocube with truncated edges, rectangle and quasi-spheres; all caged, thereby providing stability for the newly formed nanoparticles. Cubic structures with truncated/irregular edges and quasi-spherical morphologies were formed by monometallic Ag NPs using other plant extracts. However, the corresponding Ag-Ni and Ag-Co nanobimetallic derived from these plant extracts resulted in nanohybrid with Ag core and Ni shell arrangement, nanocubes with truncated edges and multiply-twinned nanoparticles. Evidence of capping of the newly formed nanoparticles by the biomolecules was demonstrated by the diagonal orientation of Ag xxxv

nanoparticles through mapping. Hexagonal shaped nanoparticles with truncated edges were observed in the Ag NPs obtained from L. inermis leaf extract. The formation of highly crystalline Ag dominated by face-centre cubic (FCC) was supported by p-XRD. SAED also supported formation of hybrid nanoparticles as crystallinity decreased when compared with the equivalent Ag monometallic nanoparticles. The antimicrobial activity of nanoparticles against clinical isolates was highly significant (6.25 mg/mL MIC, and 12.5 mg/mL MBC) in the hybrid nanoparticles which demonstrated higher growth inhibition of the test organisms as revealed in one-way analysis of variance (ANOVA) using SPSS statistical tool (p < 0.05). The outcome of these findings suggests the application of Canna indica-influenced Ag NPs as antibacterial against E. coli, S. pyogenes and antifungus against C. albicans. Ag-Co nanoparticles formed using the extract of C. indica is a potential material in optical devices. The use of Ag NPs with narrow size distribution can serve as conductive fillers in electronically conductive adhesives (ECA).