A highly stable and magnetized citric acid (CA)-functionalized iron oxide aqueous colloidal solution (Fe3O4@CA) was synthesized by using a simple and rapid method of one-step co-participation via a chemical reaction between Fe3+ and Fe2+ in a NaOH solution at 65 °C, followed by CA addition to functionalize the Fe3O4 surface in 25 min. The NPs were synthesized at lower temperatures and shortened time compared with conventional methods. Surface functionalization is highly suggested because bare Fe3O4 nanoparticles (Fe3O4 NPs) are frequently deficient due to their low stability and hydrophilicity. Hence, 19 nm-sized Fe3O4 NPs coated with CA (Fe3O4@CA) were synthesized, and their microstructure, morphology, and magnetic properties were characterized using X-ray diffraction, transmission electron microscopy, Zeta potential, Fourier transform infrared spectroscopy, and vibrating sample magnetometer. CA successfully modified the Fe3O4 surface to obtain a stabilized (homogeneous and well dispersed) aqueous colloidal solution. The Zeta potential value of the as-prepared Fe3O4@CA increases from − 31 to − 45 mV. These CA-functionalized NPs with high magnetic saturation (54.8 emu/g) show promising biomedical applications.

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The field of nanotechnology is advancing rapidly due to its extensive applications in distinct fields of science and technology. Several methods have been used for the production of nanoparticles due to their immense functions in various fields. The limitations of the traditional methods and their toxic nature of the chemicals used during the synthesis or in their application hinder their usage in the biomedical field. This led to the development of the green synthesis of nanoparticles. Researchers have focused on developing simple, cost-effective, clean, non-toxic and eco-friendly procedures for synthesis of nanoparticles. Various biological agents like bacteria, fungi, plant extracts, etc. are used for the green synthesis of metal nanoparticles due to their biocompatibility. The dissolved metals ions are reduced into nano-metals by bio-agent in the green process. The manners and protocols of the green synthesis of gold nanoparticles are presented in this review. Recent literature on green synthesis of noble gold nanoparticles with their various applications in biomedical, antifungal/antibacterial, drug delivery, sensors and photocatalytic have also been discussed.

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