Among non-viral gene carriers with low toxicity and high transfection efficiency, the use of gold nanoparticles (AuNPs) is of particular interest due to their biocompatibility and special properties. This is the first time we attempted to functionalize the surface of the biological AuNPs in order to conjugate them with antimiR-135b through electrostatic interactions and knockdown the microRNA-135b gene expression inside the cells. A fungal strain, Fusarium oxysporum, was cultured in Sabouraud Dextrose Broth (SDB), centrifuged, and the mycelium-free supernatant was challenged with 1 mmol final concentration of HAuCl4.3H2O and incubated for 24 h at 37°C in a shake flask. AuNPs were characterized by visible spectrophotometry, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDS), and a zetasizer. The washed and sterilized AuNPs were used for cytotoxicity and conjugation assays. First transferrin (Tf) and then polyethylenimine (PEI) were used to functionalize and change the surface charge of the AuNPs and then antimiR-135b was conjugated to the AuNPs trough electrostatic interactions. Their association was confirmed by visible spectrophotometry and electrophoresis. Confocal microscopy was used to investigate the internalization of the AuNPs-antimiR-135b complex. The results proved the formation of AuNPs with a maximum absorption peak at 528 nm, round and oval shapes (15-20 nm), and average zeta potential of -21.02 mV. The AuNPs-antimiR-135b showed delayed electrophoresis unlike antimiR-135b or AuNPs alone. Functionalized AuNPs did not cause any toxicity in cell culture and confocal microscopy showed successful transfection of AuNPs-antimiR-135b into the vast majority of 4T1 cells. We concluded that the biological AuNPs were non-toxic and they could carry antimiR-135b to enable gene silencing.Keywords: Gold nanoparticles (AuNPs), Biological method, Conjugation, AntimiR-135b
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