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

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Aim: We report synthesis of capped gold nanoparticles (C-AuNPs) of ≈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 μgml-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 Au0 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.

Scientific interest into use of natural resources to synthesize metallic nanoparticles (NPs) has been rapidly growing, especially concerning Cu, Ag and Au NPs as biocompatible nanomaterials. Metallic NPs expressing antimicrobial activities are being widely applied in several fields of nanoscience and nanotechnology. The use of these nanomaterials could supersede the activities of routinely used medicines [1–3], along with new industrially useful nanoformulations. Capped gold nanoparticles (C-AuNPs) are extensively used as an efficiently refined catalyst for medical and gene therapies, and for diagnostic, biomedical and biological purposes [4–6]. In these applications, C-AuNPs of ≤50 nm size have been preferred for effective uses. Despite wider applications, the synthetic methods for C-AuNPs are resource consuming with a multistep processes and there is a need to develop a new synthetic route with natural resources to achieve their desired sizes. Therefore, it was our aim to prepare C-AuNPs of ≈20–30 nm stable for longer durations.

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