*6.5. Zinc Oxide (ZnONPs), Selenium (SeNPs) and Nickel Oxide Nanoparticles (NiONPs)*

Zinc oxide is a metal oxide generally used in n-type of semiconductors. In recent decades, ZnONPs have drawn significant attention, owing to their widespread application in various fields like electronics, optical and the biomedical sector [147]. The process of production of zinc oxide nanoparticles is cost-effective, safe and easy, and ZnO has been given the status of generally recognized as safe (GRAS) by the US Food and Drug Administration [148,149]. ZnONPs are primarily known for their exceptional semiconducting properties, which can be attributed to the wide band gap of 3.37 eV, high catalytic activity, UV filtering properties and large exciton binding energy of 60 meV, and also have good optical, wound healing and anti-inflammatory properties [147]. ZnONPs have been comprehensively used in the cosmetic industry, in products like sunscreen lotions, for their intrinsic UV filtering properties [150]. They also have widespread applications in the biomedical sector, such as in drug delivery, and also exhibit antidiabetic, antibacterial anticancer and antifungal properties [147]. Chandra et al. (2019) used the leaf extract of *Berberis aristata,* a plant of medicinal importance, to synthesize zinc oxide nanoparticles [95]. Other studies have employed the yeast species, *Pichia kudriavzevii* for the extracellular biosynthesis of ZnONPs and the development of ZnONPs by using the fungal strain, *Aspergillus niger* [116,122].

In addition to these materials, selenium nanoparticles (SeNPs) are also attracting researchers, owing to their enhanced properties like semi-conduction, photoelectrical, photoconduction, catalytic etc., and their potential in optical and electronic instruments. They are known to have lesser toxic effects than selenium (Se) compounds, and are used in the medical field, as they show high therapeutic and anticancer properties [151]. Ramya et al. (2015) used an actinomycetes species, *Streptomyces minutiscleroticus* M10A62, obtained from magnesite mine, to develop SeNPs [98]. In 2012, Torres and his colleagues used *Pantoea agglomerans* isolated from the Camarones River to synthesize Se nanoparticles [113]. In 2013, Li and team members fabricated 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) coated surface-functionalized nanoparticles of Selenium (Se@Trolox) with antioxidant potential [152]. Moreover, Se@Trolox was found to block the activation of the AKT and MAPK signaling pathway, cisplatin-induced reactive oxygen species (ROS) accumulation, and DNA damage-mediated p53 phosphorylation in HK-2 cells [152].

Recently, nickel oxide has been found to perform different functions in various fields like biomedicine, electronics and magnetism, owing to its properties like anti-bacterial, anti-inflammatory, eco-friendliness, easy usage and high reactiveness [153]. Being highly reactive, it is readily used for catalyzing various organic reactions, like the α-alkylation of methyl ketone, the chemo-selective oxidative coupling of thiols, the hydrogenation of olefins, the synthesis of stilbenes from alcohol through Wittig-type olefination and the reduction of ketones and aldehydes [154–158]. Moreover, it is also found to catalyze inorganic reactions, such as the decomposition of ammonia [159]. Recently, it has been used for developing carbon nanotubes (CNTs) [160]. The plant derived nickel oxide nanoparticles showing antioxidant potential are shown in Table 3.
