Green synthesis of capped gold nanoparticles and their effect on Gram-positive and Gram-negative bacteria

Aim: We report synthesis of capped gold nanoparticles (C-AuNPs) of approximate to 20-30 nm by reducing HAuCl4 with flower and leaf extracts of Ocimum tenuiflorum, leaves of Azadirachta indica and Mentha spicata and peel of Citrus sinensis plants. Methods: Atomic force microscopy (AFM) and transmission electron microscopy (TEM) determined their size, shape and topographical structures. The C-AuNPs with UV-Vis spectrophotometer produced a maximum absorption within 530-535 nm wavelengths. Their Fourier transform IR stretching frequencies, from 450 to 4000 cm(-1), have inferred HAuCl4 reduction to Au. Results: The 512 and 600 mu g ml(-1) C-AuNP MICs were expressed on antimicrobial strains Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella pneumoniae, respectively. Conclusion: The chosen plant extracts have reduced the Au3+ to Au-0 with simultaneous in situ capping with bacteria inhibiting activities. Green routes for C-AuNP synthesis could be an asset for several other biomedical and bioengineering applications. Lay abstract: Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella pneumoniae are pathogenic bacteria that can be very dangerous for humans. Their growth is inhibited by gold nanoparticles (AuNPs) capped with chemical agents from the flowers and leaves of Ocimum tenuiflorum, leaves of Azadirachta indica and Mentha spicata and peel of Citrus sinensis. Capped AuNPs expressed higher antibacterial activities as compared with Ag, Cu and Se metallic nanoparticles obtained by chemical reductions contrary to green routes. Capped AuNPs synthesized through green routes could also be an asset for several other biomedical applications.