**1. Introduction**

The enormous impact of nanobiotechnology on almost all life forms has intrigued researchers globally [1]. In 1959, Richard Feynman introduced the theoretical concept of miniaturization, and, for the first time, provided hidden hints on nanotechnology (directing towards technology, using materials that have dimensions of approximately 1–100 nm) [2]. Nowadays, microorganisms (bacterium, fungi, including mushroom, yeast) and green plants are used for green synthesis of metallic nanoparticles [3].

*Pleurotus* mushrooms, commonly known as oyster mushrooms, belong to the family of genus *Pleurotus,* and they are edible and nutritious in nature [4]. Oyster mushrooms are readily available and naturally grow in nearly all latitudes, tropical and subtropical forestry, except Antarctica [5]. The worldwide geographical distribution of different *Pleurotus* species, with optimum growth temperature, is shown in Table 1. The primary role of the fruiting bodies of oyster mushrooms is to absorb amino acids, proteins, vitamin B (niacin, thiamine, and riboflavin), vitamin D, carbohydrates, and mineral salts (iron, calcium, and phosphorus) [6,7]. Additionally, they show important antifungal, anti-inflammatory, antibacterial, and immunomodulatory activities.


**Table 1.** Geographical Distribution of *Pleurotus* spp.

Moreover, they have the ability to reduce sugar and cholesterol levels in the blood [11]. *P. ostreatus* contains β-1,3-D-glucan and pectin, which are water-soluble gel-forming substances, having the ability to bind with bile acids. They inhibit cholesterol-bile micelle formation, cholesterol absorption, and endogenous synthesis, while increase the removal of plasma cholesterol by reducing the production and secretion of very low density lipoproteins (VLDL) [12]. The content of the nutrients depends on the nature, age, and size of the fungus, as well as their growing conditions [13]. Several nutrients, such as carbohydrates (40–46%), protein (20–25%), fiber (10–21%), and amino acids (20–41%) are present in considerable amounts, while the content of fat is very low, ranging from 10–20% of the dry matter. Fungi fruiting bodies are a rich source of micro- and macro-elements, such as sodium, magnesium, phosphorus, calcium, manganese, potassium, iron, copper, and zinc. Mushrooms of the genus *Pleurotus* have their own importance, more than the commercially employed basidiomycetes, because they possess better superior nutritional, gastronomic, and medical properties than mushrooms (which can be easily cultivated on a broad range of substrates) [14]. Nowadays, mushrooms show significant potential in metal nanoparticle (NP) synthesis and multifaceted applications [15].

In recent decades, several reports on mycogenesis derived NPs have been published [11]. However, the precise mechanisms of synthesis of mycogenic nanomaterials with variable size dimensions and topologies are not well understood (yet). Members of the fungi kingdom include various heterotrophic multicellular eukaryotic organisms. These microbes play an essential role in diverse ecosystems, particularly in the nutrient cycling paradigms. Fungi can be reproduced by both processes, i.e.,

sexually as well as asexually, and have shown symbiotic relations with bacteria and plants. Fungi groups mostly consist of mildew, mold, rust, yeast, and mushrooms [16].

The benefits and relevant use of fungal cells, such as NP factories, are attributed to the release of high amounts of extracellular enzymes that can serve as bio-reducing (as well as stabilizing) agents for NP synthesis. Moreover, fungal-derived NPs are much better than the bacteria-derived NPs. Constituents, such as enzymes and metabolites secreted by fungal cells, play an important role in synthesizing metal NPs, which reduce the toxicity of substances [15]. Ions are often liable for toxicity. When metal ions in solution are exposed to bacterial cells, they become uniformly distributed in the environment surrounding the bacterial cell, with no specific localization. In contrast, NPs that interact with the bacterial cell wall produce a focal source of ions through continuous release of ions, and cause enhanced toxicity to the cells [17]. Positively charged metal ions can easily bind with the fungal cell surface containing negative charge through electrostatic and cell receptor-specific interactions. Both types of intracellular (as well as extracellular) fungi could be used to synthesize NPs. Fungi exhibit high metal-binding capabilities in comparison to bacteria, and, hence, fungal biomass has gained the attention of researchers, for the production of NPs, at a large-scale. Various metal NPs, such as PdNPs, AgNPs, AuNPs, CuNPs, FeNPs, ZnNPs, TiNPs, and PtNPs could be synthesized using their oxides, nitrides, sulfides, and fungal biomasses [18].

NPs derived from fungal biomasses exhibit distinct optical, physical, and chemical properties, such as high quantum yield, excellent biocompatibility, high photostability, and adequate near-infrared (NIR) light-absorbing capacity, owing to which, they can be used in various chemical and medical fields, such as sensing, medicine, catalysis, and food packaging [19,20]. Oyster mushrooms mediate myco NPs by using spent mushroom substrate (SMS), and have medical importance towards many pathogenic microorganisms, as reported for the first time in 2007 [3].

To the best of our knowledge, many review articles, based on the synthesis of metal NPs derived from diverse sources and their applications, have been reported. Still, few review articles have discussed the synthesis of distinct metal nanoparticles (MNPs) and several applications of oyster mushroom derived NPs, and we attempted to fill the gap. In this article, we intend to discuss NP synthesis derived from oyster mushrooms, using various techniques and different applications. First section discuses the synthesis of nanoparticles by using intra-and extra-cellular methods. Later, different applications involving antioxidant, anticancer, antibacterial, and catalysis, with possible mechanisms of action, have also been discussed.
