Green synthesis of gold and silver nanoparticles by an actinomycete Gordonia amicalis HS-11: Mechanistic aspects and biological application

• A tropical soil isolate degrading n-hexadecane was identified as Gordonia amicalis.
• G. amicalis cell-free supernatants formed polydispersed nanoparticles.
• The component mediating nanoparticle synthesis was identified to be a glycolipid.
• The purified glycolipid mediated synthesis of monodispersed nanoparticles.
• The nanoparticles displayed in vitro free radical scavenging activities.

A bacterial isolate obtained from an oil-contaminated soil sample was identified as Gordonia amicalis on the basis of morphological features, biochemical traits and 16S rRNA gene sequence. The cell free supernatant (CFS) of this bacterium (cultivated on n-hexadecane) when incubated with 1 mM chloroauric acid (HAuCl4) or silver nitrate (AgNO3) at pH 9.0 in a boiling water bath mediated the synthesis of gold and silver nanoparticles, respectively. It was hypothesized that the CFS contained some thermostable biomolecule that mediated metal reduction reactions. Extraction of CFS with chloroform:methanol (2:1), subsequent column chromatography and thin layer chromatography led to the activity-guided purification of a glycolipid. The glycolipid was hydrolysed and the glycone (glucose) and aglycone components (three saturated fatty acids (myristic, palmitic and stearic) were identified by gas chromatography mass spectrometry. The reduction of glucose to gluconic acid under alkaline conditions provided the reducing equivalents needed for nanoparticle synthesis. The purified glycolipid and CFS mediated nanoparticles were characterized by using a UV–vis spectrophotometer, X-ray diffractometer and Transmission Electron Microscope. The nanoparticles displayed free radical scavenging activities. This paper thus highlights nanoparticle synthesis by a hitherto unreported actinomycete culture, identifies the biomolecule involved in the process and describes the associated antioxidant activity.

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