Optical, structural, and antibacterial properties of biosynthesized Ag nanoparticles at room temperature using Azadirachta indica leaf extract

We report an enhancement of antibacterial properties of Ag nanoparticles (NPs) synthesized at room temperature using leaf extract of Azadirachta indica (Neem) following green synthesis route. To study such antibacterial properties Ag NPs of sizes within 9 nm to 17 nm were synthesized by varying the concentration of Neam leaf extract (NLE). The NP size and size distribution were seen to increase and decrease, respectively, with increase in NLE concentration. Also Ag NPs having a fixed size (similar to 26 nm) was also synthesized by varying the precursor (AgNO3) concentration. It is noticed that concentration of NLE has significant effects on the control of NP size as well as size distribution whereas there is almost no role of precursor concentration of the NP size. All the Ag NPs are found to have face-centred-cubic crystal structure with preferential growth along (111) plane which is stable one. The peak of X-ray diffraction at similar to 32.4 degrees (2 theta value), which is prominent for low concentrations of NLE and precursor, is identified as (101) plane of Ag crystal. The generation and growth of Ag NPs had also been confirmed using electron microscopic studies. These Ag NPs show prominent surface plasmon resonance (SPR) absorption at similar to 420 nm confirming again the genesis of Ag NPs. The SPR peak shifts towards longer wavelength (redshift) with a corresponding reduction in full width at half maximum with increase in NP size. All of the samples containing Ag NPs show a broad blue photoluminescence (PL) emission at similar to 471 nm. Emission peak is seen to redshift with increase in NP size and is consistent with the optical absorption data. Such PL emission is argued as due to interband transition or plasmon luminescence. Being biocompatible of the green synthesis process, antibacterial properties of these Ag NPs were studies in details considering all the samples (with varied NP size for one set and with fixed NP size for other set of samples). As per our knowledge this is the first report of size related total study of Ag NPs, showing increased antibacterial effect as size decreased and equal antibacterial effect as size equals. It is found that smaller Ag NPs has enhanced antibacterial effects due to large surface area to volume ratio in comparison with bigger sized Ag NPs.