Green synthesis of Ag and Au NPs decorated rGO nanocomposite for high impedimetric electrochemical sensor as well as enhanced antimicrobial performance against foodborne pathogens

In the present study, Allium cepa L. leaves extract mediated silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) decorated reduced graphene oxide (Ag/Au/rGO NC's) were synthesized and demonstrated their biological and electrochemical sensing applications. Interestingly, the synthesis of AgNPs and AuNPs was achieved rapidly within 1 h and 4 h, respectively. Whereas the production of both reduced graphene oxide (rGO) and Ag/Au/rGO NC's was achieved at 96 h. The achieved results from the UV–Vis spectrum of 432 nm for AgNPs and 550 nm of AuNPs and the shifting of the characteristic GO absorption band of 230 nm into 270 nm as well as an absence of 305 nm confirmed the rGO synthesis. In addition, HR-XRD analyses confirmed the JCPDS file No. 04–0783 and 01–1174 pertaining to Ag and Au NPs. Further, the Raman spectroscopy, FT-IR and EDS analyses confirmed the evidence of nanomaterials biosynthesis. A polydisperse, spherical shaped average size AgNPs of 1 to 30 nm, spherical and triangular shaped AuNPs with an average size of 1 to 120 nm, and silk like appearance of a thin layered graphene sheet in rGO were observed through TEM. Also, the recorded results evident the uniform decoration of Ag and Au NPs into the rGO sheet. The electrochemical properties of as-synthesized materials were studied by electrochemical impedance spectroscopic (EIS) and cyclic voltammetry (CV) techniques. Interestingly, the Ag/Au/rGO NC's modified electrode exhibited higher electrical conductivity and good electrocatalytic properties towards the electrooxidation of nitrite in a neutral medium than other nanomaterials. Eventually, Ag/Au/rGO NC's confirmed the enhanced antibacterial against Gram-negative bacteria and dose dependent antifungal inhibitory activities against foodborne fungal pathogens of Fusarium graminearum, Alternaria alternata, and phytopathogenic fungus of Curvularia lunata and Sclerotinia sclerotiorum at the maximum tested conc. of 2 mg/mL, compared to other nanomaterials. From the result of zebrafish embryotoxicity (ZET), the highest toxicity was observed for AgNPs at 0.19 μg/mL. But in the case of Ag/Au/rGO NC's the reduced toxicity was observed. Hence, the achieved results concluded the Ag/Au/rGO NC's could be suitable for development of both as biomaterials for biomedical and effective sensors for environmental applications.