African leafy vegetables as bio-factories for silver nanoparticles: A case study on Amaranthus dubius C Mart. Ex Thell

• We aimed to synthesise and characterise Ag nanoparticles using fresh and freeze-dried organs of Amaranthus dubius.
• The antimicrobial activity of synthesised Ag nanoparticles was assessed using minimum inhibition concentration method.
• Characteristics of the nanoparticles differed based on the organ used and whether the plant material was fresh or freeze-dried.
• Ag nanoparticles synthesised from fresh stem extracts displayed the highest antimicrobial activity.

Nanoparticles are used across many scientific and pharmaceutical fields and are found in products that come into close contact with the human body. There is a growing need for ‘green synthesis’ of silver (Ag) nanoparticles and plant-mediated synthesis is becoming increasingly popular. The current study aimed to firstly synthesise Ag nanoparticles using fresh and freeze-dried leaves, stems and roots of the African leafy vegetable, Amaranthus dubius. The synthesised Ag nanoparticles were subsequently characterised using UV–visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) spectral analysis. The bioactivity (antibacterial and antifungal) of the synthesised Ag nanoparticles was also assessed using the minimum inhibition concentration (MIC) method. The results suggest that A. dubius plant extracts can serve as environmentally benign bio-factories for the synthesis of bioactive Ag nanoparticles. However, the characteristics of these nanoparticles differed based on the organ used to prepare the extract and whether the plant material was fresh or freeze-dried. Silver nanoparticle yield was greatest in the freeze-dried and fresh leaf extracts of A. dubius. However, EDX analysis revealed nanoparticles produced using freeze-dried and fresh stem extracts to contain the most elemental Ag. Silver nanoparticles synthesised from the different plant organs all displayed a spherical shape; however, Ag nanoparticles synthesised from the stem extracts (30–35 nm) were significantly larger than those synthesised from leaf and root extracts (18–21 nm). FTIR analysis confirmed the presence of carbonyl groups, proteins and aldehydes on nanoparticles produced using all extract types. The Ag nanoparticles synthesised from fresh stem extracts displayed the highest antimicrobial activity compared with those synthesised from the other plant organs. Fresh stem extracts of A. dubius appear to be most suitable for biosynthesis of Ag nanoparticles, yielding the largest nanoparticles, with the highest elemental Ag content, and greatest inhibition of microbial growth.