**1. Introduction**

Medicinal application of plants and the development of green nanotechnology stimulated the use of plant phytochemicals in the green synthesis of metallic nanoparticles (MNPs). Metals such as gold, silver, zinc, tin, platinum, lead, copper, palladium and so forth are easily bio-reduced to phyto-metallic nanoparticles (PM-NPs) in the range 1 nm and 100 nm by phytochemicals from plants through biosynthesis and green nanotechnology methods [1]. These methods are reported to eliminate the use of toxic solvents and chemicals, minimize cost, and the loss of atom

economy associated with chemical and physical methods [1, 2]. In the biosynthesis of nanoparticles, bio-nanomaterials are synthesized using natural products of plant and plant-derived compounds (phytochemicals), microbes (bacterial, yeast, fungi, viruses, and algae), and animals. Plant phytochemicals are prevalently used due to their bioavailability and relatively low experimental costs. The phytochemicals possess functional groups such as hydroxyl (–OH), amide (–NH2C=O), carbonyl (C=O), carboxylic (–COOH) etc. with reducing/stabilizing abilities through π-π dative bonding, hydrogen bonding, and electrostatic interactions [3]. This stabilization is reported to enhance the activities of a metal-based drug through size reduction, increase surface area, drug durability and biodistribution through tissue/cell binding [4]. During biosynthesis with phytochemicals, crude extract, or compounds isolated act as reducing/capping agents for single or bimetallic precursors to form their respective single PM-NPs or phyto-bimetallic nanoparticles (PBM-NPs) [1, 5]. These nanoparticles can interface with biological systems as drugs, diagnostic tools or as implants due to their physical, chemical, and biological properties.

For therapeutics and nanomedicine, metallic nanoparticles (MNPs) capped with plant extracts rich in phenolics, alkaloids, and terpenoids contents through biological methods have shown increased acceptance over chemical and physical methods [6]. The plant-based synthesis method of metallic nanoparticles as biomaterials is less expensive and eco-friendly. The method involves bio-reduction, nucleation and growth, capping and stabilization of metals and transition metals, notably gold (Au), silver (Ag), platinum (Pt), palladium (Pd), copper (Cu), iron (Fe), zinc (Zn) precursors using phytochemical constituents of plants. A unique property of noble metals is the surface plasmon resonance (SPR) phenomenon, due to the availability of free electrons oscillating at the metal surface near the visible frequency range (260–800 nm). Absorption of UV light in this region is responsible for the color changes observed from their aqueous solutions [7–9]. Interactions of surface electrons with reducing agents from phytochemicals cause the free electrons to collectively oscillate at a unique frequency specific to each metal. With the interaction between phytochemicals (capping agents) and metal precursors, the surface chemistry of the metals after synthesis is enhanced making PBM-NPs as good drug carrier for targeted drug delivery and improving biodistribution and clearance in the body [10, 11]. Biomedical applications of these materials in living systems require consideration of the toxicity level of the nanoparticles. The synergies provided by phytochemicals in the synthesized metallic nanoparticles are believed to reduce the inherent toxicity of metal nanoparticles with living cells and elicit selective cytotoxicity on bacterial and cancer cells [12]. Based on recent findings, schematic studies of PM-NPs in biomedical applications covers their activities as bactericidal, anticancer, antioxidants, antifungal, cytotoxicity, formation, and mechanism of action [13].
