Environmentally sustainable synthesis of metal nanoparticle clusters and their prospective applications in biomedical and surface enhanced Raman scattering

Abstract In the present study, gold and silver nanoparticle clusters were successfully synthesized for the first time, to the best of our knowledge, using an environmentally friendly green synthesis method. Both gold and silver nanoparticles exhibit characteristic plasmon resonance peaks at 530 nm and 420 nm respectively with additional peaks at higher wavelengths (620 nm for gold and 580 nm for silver) suggesting the formation of clusters or assemblies of nanoparticles. Scanning Electron Microscopy and Transmission Electron Microscopy analyses reveal that the synthesized gold nanoparticles and silver nanoparticles are predominantly spherical, with average sizes of 10–20 nm for gold nanoparticles and 15–30 nm for silver nanoparticles, along with observable nanoparticle clustering. The Fourier Transform Infrared spectroscopy analysis shows that the functional groups in the Azadirachta indica leaf extract, such as O–H and C–H bonds, participate in the reduction and stabilization of gold and silver nanoparticles. The synthesized gold nanoparticles and silver nanoparticles (showing stronger inhibition) exhibited dose-dependent antibacterial activity against Escherichia coli and antioxidant activity in 2,2-diphenyl-1-picrylhydrazyl and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical scavenging assays, with silver nanoparticles showing higher scavenging efficiency compared to gold nanoparticles. Further, the surface-enhanced Raman spectroscopy analysis of methyl orange showed significant signal enhancement with silver nanoparticles and gold nanoparticles attributed to inter-plasmon coupling and the creation of hot spots in clustered nanostructures.