## 1 Introduction  
In global standard norms, materials or particles that in at least one dimension have a length of 1 to 100 nanometers (nm) are termed nanoparticles (nanomaterials) [1,2]. Nanoparticles (NPs) are aggregates of between a few and many millions of atoms. Materials at the nano-scale (Figure 1) display novel biological and physicochemical properties that are never observed for their bulk counterparts [3]. The material in this size have high surface to volume ratio and they can pass across cell membranes by passive diffusion [4]. Nanoparticles (NPs) are sort of bridge between ions (atomic) and bulk phases of materials [5]. Nanomaterials based on structure, appearance, and their usage are defined by different names, but the common feature for all of them is to have at least one dimension in the nano-scale (1 to 100 nm) [6]; unique nano-specific phenomena of particulates are most likely to occur in the size range of 1 and 100 nm [7]. In view of the increased knowledge and understanding about the synthesis and the characterization of NPs, scientists have now succeeded in designing a useful newer hybrid class of NPs termed core shell (core-shell, core@shell or core/shell) NPs that can be defined as well-organized nanomaterials consisting of two, three, or more types of individual nanocomponents [8]. The surface of nanoparticles can be modified (functionalized) with a wide range of organic/inorganic materials such as polymers, biomolecules, silica, metals, and non-metal components. Stability, biocompatibility, newer specific performance, targeted delivery, treatment, or diagnosis of biological entities, such as cells, proteins, nucleic acids, enzymes, bacteria, etc., are some of the reasons for the surface modification of nanoparticles via functionalization. The coating of organic/inorganic materials that surrounds the NPs can actually be the shell or second shell, which are called core-shell nanoparticles. The value of the coating materials could be enormous when their presence not only protects the core, but also may play a significant functional role as exemplified in the protection afforded by the ZnO shell for the oxidation of the Fe core in the Fe@ZnO NPs. Core-shell nanoparticles are heterogeneous NPs composed of two or more materials (metal, element, or biomolecules); one nanomaterial acts as a core in the center while the other material/materials located around the central core (shell) (Figure 2). Essentially, core -shell NPs are a type of biphasic nanomaterial, which have an inner core and an outer shell made up of different components [9]. Figure 1: Comparison of macro, micro, and nano scales. Figure 2: Schematic of core-shell nanoparticles: core in the center (yellow part) and other material located around the central core (green part). A wide range of organic and inorganic nanomaterial can be used for forming both, the core and the shell comprising core-shell NP. A combination of several diverse materials, where the building components are easily distinguishable from each other, is referred to as composite or nanocomposite, when the dimensions are less than 100 nm [10]. The properties of nanocomposites need not necessarily originate from each individual part. Briefly, a component of nanocomposite may serve the role of an adhesive without which they may not stick together and the ensuing blend of components will not possess any new properties of the nanocomposite. The core-shell NPs can be labeled as nanocomposites but they may not necessarily be nanocomposites which tend to be uniform in microscopic view (mixed up components). In contrast, core-shell NPs usually have a separation that can be seen between the shell and the core. The nanocomposite properties are not expected from core-shell NPs which can be bimetallic or trimetallic [11; 12]. The synthesis of core-shell NPs has the ability to use a wide range of materials as core or shell that can impart desired and unique features and functions. The properties of core-shell NPs, such as physicochemical, biological, optical, etc.