**2. Colloidal metal nanoparticles as important nanomaterials for various applications**

In general terms, colloidal systems are heterogeneous systems in which very fine particles of one matter are scattered through another substance. Former is referred as "Dispersed Phase" while later as "Dispersion Medium" and both can be present in either of solid, liquid or gas states. Dispersed phase is completely insoluble in dispersion medium [25]. Colloidal NPs, as also called nanocolloids or solid colloidal particles, resemble a normal colloidal system where NPs act as dispersed phase. Being dispersed in the solvent medium, NPs are embroiled in some lively motions such as Brownian motions [26, 27]. As a consequence of their dominant characteristics over bulk correspondents the colloidal NPs, play vital role in number of applications [28]. The unique properties such as tunable size, configuration, structural arrangement, formulation, crystallinity and dimensions can deeply rectify the features of colloidal NPs according to the applications [29]. Colloidal NPs can be employed in prospective applications in the wide range of sectors including electronics, coatings, catalysis, packaging, biomedicine, biotechnology etc. In addition, the uses of colloidal NPs in biomedical field are increasing incredibly as they are being administrated with elegant attributes for healthier reactions with the biological circumstances and cope with on-demand requirements of *in vivo* diagnosis and therapies [30]. To boot, the fine size of NPs not only allows them to pass through the tissues or cells but also accesses them easily to target organs engrossing the novel biomedical applications at cellular level [31].

Magnetic NPs specifically iron oxide NPs are principally studied and utilized for their peculiar physicochemical, biological and magnetic features [32], remarkably stability, least perilous, significant magnetic vulnerability, severe saturation magnetism and biocompatibility [33]. Similarly, other magnetic NPs such as alloy, also known as bimetallic NPs of iron-cobalt (Fe-Co), iron-platinum (Fe-Pt) have high magnetic properties, super paramagnetism, high curie temperature [34, 35]. The exceedingly reported and mostly scrutinized uses of magnetic and bimetallic NPs are for target specific drug delivery [36, 37], in magnetic resonance imaging [38, 39] and to treat hyperthermia magnetically [40, 41].

Metallic NPs are the matter of curiosity that has been mesmerizing experts due to their extraordinary optical, electronic properties accompanied by its massive potential in nanotechnology. Nobel metal NPs of gold, silver, platinum, palladium, etc. have been used since ancient times for medicinal intents. Chemical inertness, ability to resist corrosion and oxidation even in moist air wholly justifies their uptake for biomedical applications [42]. Negative charge on the surface of gold NPs presents easy functionalization with organic compounds that offers further interactions with antibodies, drugs moieties or ligands for *in vitro* or *in vivo* drug delivery [43]. Likewise, silver NPs embrace distinct characteristics of being chemical inactivity, catalytic activity, high thermal and electrical conductive [44, 45]. The astonishing antimicrobial activity of silver NPs leads its utility in textile industries, wound healing dressings and as disinfectants [46, 47]. The employability of other metal NPs in bioimaging [48], biosensors [49], photothermal therapies [50] are growing day-by-day.

Metal oxide NPs such as titanium dioxide (TiO2) and zinc oxide (ZnO) NPs are markedly used in paints, coatings, food coloring, beauty products, sunscreens etc. Equating with other metal oxide NPs, ZnO confers minimal toxicity to living cells so that there is increase in biomedical applications namely in diabetes treatment, wound healing, anti-inflammation treatment, anti-aging products, antibacterial activities, etc. [23, 51–53].
