**1. Introduction**

In the twenty-first century, age-related diseases have become a major health concern worldwide. Ageing is a natural and progressive process which involves the degeneration of the functioning and structure of vital organs and is one of the risk factors responsible for numerous chronic diseases and accounts for the high mortality rate [1–4]. Among various theories thatunveil and elucidate the ageing process, the free radical theory holds an exceptional rank [5]. This theory states that the ageing occurs due to successive failure of the defense mechanism to resort the damage induced by the reactive oxygen species (ROS), especially in the mitochondria [6].

It is well comprehended that oxidative stress plays a significant role in degenerative senescence. ROS have been found to be involved in the pathogenesis of various cellular processes, and is also associated with numerous diseases like cardiovascular, cancer, neurodegenerative and respiratory diseases, as depicted in Figure 1 [7]. The rise in ROS concentration in cells have also been associated with ageing, however, it cannot be considered as the only determining factor responsible for ageing. Moreover, in age-related diseases, the elevated concentration of ROS has been involved in the impairment of mitochondria and cellular oxidative damage [2,8].

**Figure 1.** Side effects of oxidative stress on human body. COPD—Chronic obstructive pulmonary disease; ARDS—Acute respiratory distress syndrome; OCD—Obsessive-compulsive disorder; ADHD—Attention-deficit/hyperactivity disorder.

The production of ROS generally relies on both enzymatic as well as non-enzymatic reactions. The enzymatic reactions involved in various cellular processes, like phagocytosis, prostaglandin synthesis and respiratory chain system, are known to generate ROS [9–19]. The superoxide radical (O2•−) is synthesized during the activity of enzymes, like NADPH oxidase, peroxidase and xanthine oxidase, in various cellular processes. It has also been found that various other ROS, like hydrogen peroxide (H2O2), hydroxyl radical (OH•), hypochlorous acid (HOCl), peroxynitrite (ONOO−), etc., are also formed during enzymatic reaction, and the action of enzymes like xanthine oxidase and amino acid oxidase leads to the formation of H2O2. Furthermore, OH• is regarded as a highly reactive free radical species formed during the "Fenton reaction" between H2O2 and O2•−, in the presence Cu<sup>+</sup> or Fe2<sup>+</sup>, which acts as the catalyst [11–18]. On the other hand, the non-enzymatic reactions between organic compounds and oxygen, or when cells are exposed to ionizing radiations and during mitochondrial respiration, have also been found to be involved in ROS formation [14,15,18]. At present, extensive research is being conducted to explore the natural compounds that can control oxidative stress and improve the immune system [20]. The search for novel molecules with antioxidant properties is an effective way to promote healthy ageing and counteract oxidative stress. Hence, this review focuses on highlighting the effectiveness of antioxidants functionalized nanoparticles. The first section of the review discusses synergism between ROS and age-related diseases, antioxidants and sources of antioxidants. The section following that discusses the role of nano-antioxidants; antioxidant functionalized nanoparticles and challenges associate with them.
