Nanotechnology, among emerging technologies, deals with structures of nano size (1 to 100 nm scale) and have innumerable novel and beneficial attributes as established by many researchers worldwide [1–3]. The wide-ranging technological appliances in the field of electronics, imaging, industrial, and health, among others include diagnostics, treatment, and new drug formulations in view of their unique and specific physico-chemical properties, thus embracing various disciplines namely physics, biological, and chemistry .
Cerium oxide nanoparticles, comprising cerium atoms surrounded by oxygen networks, have been widely deployed in polishing of mechanical chemical, solar cells, fuel oxidation analytical, corrosion protection, and automobile exhaust treatment. These nanoparticles also show imitation of superoxide dismutase, peroxidase, catalase, phosphatase activity and the inhibition of hydroxyl radical, proxy nitrite, and nitric oxide radicals .
A few studies have indicated the oxidative stress induction caused by nanoceria in vitro and in vivo where nanoceria has directly functioned as an antioxidant, and served as free radical scavenger as well; they interact with superoxide radical, hydroxyl and peroxide radical, and thus leading to cell death due to oxidative stress [5–7]. Consequently, CeO2 NPs can be utilized as potential drug agents in pharmacy setting, and biological scaffolding [8–10], the basis for these activities being the redox cycles between +3 and +4 states, and their specific ability in adsorption and released of oxygen [11, 12]. Although initially believed that both, the oxygen vacancy and redox cycles between these two states of cerium is involved in antioxidant activity, but now it has been accepted that redox cycle is primarily responsible for all the antioxidant properties [12, 13]; surface ratio of Ce+3/Ce+4 plays a key role in most of the biological activities of CeO2 NPs . CeO2 NPs also serve as oxidant at low pH and high doses, and based on synthesis method, concentration, and exposure time may have potentially cytotoxic function . So, careful optimization of appropriate synthetic parameters can produce relatively non-toxic CeO2 NPs, with oxidant or antioxidant properties .
The physico-chemical property of nanoparticles is determined largely by variation in synthetic methods which prompts us to explore the generation of CeO2 NPs with different size, morphology and aggregation. Recently, bio-inspired biological methods as stabilizing agents have garnered importance in the production of CeO2 NPs, because of the reduced concerns regarding their biocompatibility. Adhering to the green chemistry principles  has provided safer routes for the production of CeO2 NPs, which can be useful for drug applications; indeed, these methods offer cheaper and simpler options than the traditional chemical synthetic methods [17–21]. Another important factor for effective and important nanoparticles properties is the doping process as attested by several studies wherein the properties of doped nanoparticles with various metals have dramatic alterations compared to un-doped nanoparticles; synthetic methods for doped nanoparticles being similar [22–24].
Biebersteinia multifida is a perennial herbaceous plant with thick roots and stems with numerous longitudinal grooves that are covered with tuberous protrusions. This plant grows in many forms in Iran, Syria, Lebanon, Armenia, Central Asia, and Afghanistan and contains various alkaloids. The roots of this plant contain carbohydrates, saponins, flavonoids, and biopolymers such as monosaccharides, starches and dextrins while the aerial parts comprise triterpene saponins, essential oils, alkaloids and tannins . Herein, pure and cobalt doped cerium oxide nanoparticles were synthesized using B. multifida aqueous extract, and their cytotoxic effects were assessed on breast cancer cell (MCF7), colon cancer cell (CaCo2), and mouse embryo fibroblast cell (NIH-3T3) lines.