Multiple techniques are used in the physical synthesis of NPs, including vapor phase: Arc discharge, hydrogen plasma, laser pyrolysis, and chemical vapor condensation. Solid phase: Ball mill [71]. The high quantity of energy requirement is one of the main drawbacks associated with these approaches, as well as the substantial period needed to complete the entire process (for example, costly, limited manufacturing rate, enormous energy consumption for maintaining high pressure and temperature) [72].

The chemical substances that are most frequently used in the synthesis of MNPs include chemical reductants: Alcohol, molecular hydrogen, hydrazine, sodium tetrahydroborate, citrate, N,N-dimethyl formamide, polyols, ethylene glycol, and cyclodextrin are some of the ingredients in lithium aluminum hydrate. Sources of energy: Light, ultraviolet/visible light, electricity, heat, sonochemical energy, and X-rays are examples of photo energy. However, they are not considered green chemical reagents due to their potential environmental harm. Sodium borohydride is a well-known reducing agent that has numerous applications in chemical synthesis. As a result, it has been widely utilized to convert metal salts into NPs. The Brust-Schiffrin two-phase technique is one of the most widely used synthesis processes [73]. Chemical reduction occurs at an oil-water interface, followed by thiolated molecule adsorption and stabilization in the organic phase. This method has been widely used because of its simplicity and efficiency in expanding the understanding and applications of gold and AgNPs [74]. The use of sodium borohydride and colloidal stabilization is combined with capping molecules. Although occasionally mild compounds are used, such as derivatives of β-cyclodextrin [75] or clays [76], Citrate ions are used as both reducing and stabilizing agents, which is a common technique for the synthesis of spherical Au and AgNPs [77]. Carboxylic acids, polymers, aromatic and halogenated organic compounds, as well as surfactants, have been characterized as capping molecules and are relevant to the development of MNPs that have various morphologies [78, 79].

Nowadays, for the production of NPs, the use of biological synthesis has become a popular alternative to conventional techniques. Biosynthesis involves using unicellular and multicellular organisms such as actinomycetes [80], bacteria [81], fungi [82], plants [83], viruses [84], and yeast [85], entities in an environment-friendly green chemistry-based method. It is a non-toxic and eco-friendly method of NPs formation with a wide range of shapes, sizes, compositions, and physicochemical properties utilizing living organisms [86]. Green or biological NP synthesis prevents toxicity by using low pressure, temperature, and pH at a substantially lower cost [87]. Alkaloids, proteins, flavonoids, reducing sugars, polyphenols, and other compounds that are present in biomaterials work as capping and reducing agent for NPs from their metal salt predecessors [88]. Initial confirmation of the reduction of the metal salt precursor to its eventual NPs is aided by visualizing the color shift in the colloidal solution. Recently, several organisms, including unicellular and multicellular, have been employed for the green synthesis of NPs. The biological elements, including primary and secondary metabolites, perform as catalysts to promote metal ion reduction and the development of MNPs. On the surface of MNPs, these same reducing agents or other molecules may form a stabilizing layer, preventing or at least decreasing the capacity to assemble or become disordered throughout the production process [89]. Additionally, the production of MNPs made via biological methods can be influenced by experimental conditions such as temperature, pH, and reagent concentration [90].

UV-Vis is the most used method to characterize MNPs [128]. When synthesized from its specific metal, it produces a distinctive peak with significant visible-range absorptions [129]. The surface plasmon response (SPR) peak is well known for a range of MNPs, ranging in size, and various synthesized NPs have demonstrated that it is ideal for characterizing particles in the absorption band with a wavelength of about 200–800 nm [130]. In AgNPs, the valence and conduction bands are very close together. These bands provide a SPR absorption band by enabling unlimited electron migration. Particle size, dielectric medium, and the chemical environment all have an impact on how well AgNPs are absorbed. The stability of biologically generated AgNPs was studied for almost a year, and a SPR peak at the same wavelength was found using UV-Vis spectrophotometry [131].

XRD analysis methods are used for analyzing the structure of NPs, where MNPs show amorphous and crystalline nature, identified with the help of X-rays, which can penetrate deeply into the material [132]. The formation of crystalline NPs is verified by the diffraction pattern [133].

The Debye-Scherrer equation is used to quantify particle size from XRD data by estimating the width of the Bragg reflection peak according to the equation [134].

t = k λ β c o s θ

Where, t = Crystallite size, k = shape factor, λ = wavelength of the radiation, θ = Bragg’s angle, β = full width at half maximum.

To explore the structural characteristics of many materials, including polymers, glasses, biomolecules, and superconductors, XRD can be used. Additionally, XRD is an effective technique for researching nanomaterials [135].

FTIR can be used to analyze different capping agents, the involvement of biomolecules in the synthesis of MNPs, and the surface chemistry of synthesized MNPs [136]. In FTIR, the sample transmits photons; some of them are absorbed by the sample, and the rest pass through it. The resulting spectra show the transmission and absorption properties of the sample material [137]. It is affordable, appropriate, simple, and non-intrusive to evaluate the role of biological molecules in the conversion of silver nitrate to silver [138].

TEM is a particularly significant tool for characterizing by giving detailed information about their shape, size, internal morphology, and crystallographic structure [139]. TEM operates by transmitting a beam of electrons through an ultra-thin specimen; the interactions between the electrons and the atoms in the sample generate high-resolution images. Compared to SEM, TEM offers nearly 1,000 times higher resolution, allowing visualization at the atomic or molecular scale [140]. This makes it especially useful for observing the internal lattice structure, defects, and particle dispersion in nanomaterials. Additionally, TEM images yield more precise insights into the crystallinity, orientation, and morphological variations of NPs, making it an essential technique in nanoscience and biomedical research [141].

The particle size and its size distribution can be determined widely using the technique of DLS. In DLS parameters, zeta sizer and zeta potential have been used to describe NPs frequently and have been used to gauge the size. Additionally, DLS is also widely used to size MNPs in liquid form [142]. Its role in characterizing distinct types of NPs has been established. Because of Brownian motion, the size of NPs obtained through DLS is often larger than that determined by TEM; it is possible to estimate the average size of NPs in liquids using this technique [143].

In 1986, Binning, Quate, and Gerber developed the technology of the AFM to improve upon the drawbacks of the scanning tunnelling microscope (STM) [144]. AFM can provide three-dimensional (3D) topographic images with nanoscale resolution, and it makes the most efficient approach for morphological and structural investigation of polymeric nanocomposites under AFM [145]. The most significant development in AFM has been its ability to assess non-conductive samples' surface topography at sub-nanometer resolution [146]. Additionally, the AFM is useful since it requires less sample preparation and may be utilized in fields of natural settings. The sample does not need to be conductive or metallized before being subjected to morphological analysis. This characteristic method is an extraordinary tool for the direct characterization of a variety of samples with complex morphological structures. By moving a tip attached to a flexible cantilever across the sample surface, an atomic-scale measurement of surface morphology is accomplished using AFM. The deflection of cantilevers during scanning is used to determine the force acting between the tip and the sample [147].

One of the most important advantages of MNPs is their ability to selectively accumulate in tumor tissues while sparing normal, healthy cells. This property not only increases the efficacy of the therapy but also minimizes systemic toxicity and side effects associated with non-specific drug distribution. Tumor targeting by MNPs occurs through two fundamental mechanisms: passive targeting and active targeting [159]. Passive targeting exploits the abnormal architecture of tumor blood vessels, thus enhancing permeability and retention (EPR) effect. Tumors generally have leaky vasculature and inefficient lymphatic drainage, which allow NPs to passively accumulate in the tumor interstitial space over time. This forms the basic foundation for NPs-mediated drug delivery systems [160]. Active targeting, on the other hand, takes tumor specificity a step further by modifying the surface of NPs with specific ligands such as antibodies, aptamers, peptides, or small molecules. These ligands recognize and bind to receptors that are overexpressed on the surface of cancer cells, ensuring that the therapeutic agent is delivered precisely where it is needed. This strategy improves drug localization, enhances cellular uptake, and boosts the therapeutic index [161].

The surface of MNPs can be engineered to enhance their functionality and compatibility with the biological environment. Surface modification not only prolongs the circulation time of NPs in the bloodstream but also facilitates their recognition and binding to target cells. One of the most used stabilizing agents is polyethylene glycol (PEG), which helps to reduce immune system recognition and opsonization by serum proteins, thereby enhancing their half-life [162]. Further functionalization involves conjugating targeting moieties that bind selectively to tumor-associated receptors. Among these, the matrix metalloproteinase-2 (MMP-2) receptor has been a notable focus. MMP-2 is an enzyme overexpressed in many invasive and metastatic tumors. NPs functionalized with MMP-2-sensitive peptides can undergo enzyme-mediated degradation, releasing their therapeutic payload precisely in the tumor microenvironment (TME) where MMP-2 activity is elevated [163]. Another popular targeting strategy involves the folate receptor, which is abundantly expressed in a range of cancers, including breast, ovarian, and lung cancers. Folic acid, a small molecule with high affinity for the folate receptor, can be conjugated to the surface of MNPs to achieve receptor-mediated endocytosis into tumor cells. This method is particularly beneficial because folate is a vitamin that does not trigger immunogenic responses, making it a safe and effective targeting ligand [164]. HER2/neu, a receptor tyrosine kinase commonly found in aggressive breast cancers, is another important biomarker for targeted therapy. NPs can be functionalized with monoclonal antibodies such as trastuzumab to selectively target HER2-positive tumors. These antibody-coated MNPs can carry chemotherapeutic drugs or photosensitizers to the tumor site and, in the case of AuNPs, can even be used for photothermal ablation by converting light into heat, thereby killing cancer cells [165].

Targeting strategies using MNPs can be broadly divided into basic (passive) and active approaches. As discussed, basic targeting utilizes the natural tendency of NPs to accumulate in tumor tissues due to the EPR effect. While this method improves drug delivery compared to systemic administration, it does not provide the level of precision required to completely spare healthy tissues [166]. Active MNPs, in contrast, are designed to respond to internal or external stimuli for controlled drug release. These stimuli can include pH changes, redox gradients, enzyme activity, temperature shifts, or the application of external magnetic fields. For example, pH-sensitive AuNPs release their drug payload only in the acidic environment typical of tumors, thus minimizing off-target effects. Similarly, MNPs such as iron oxide (Fe₃O₄) can be guided to tumor sites using external magnets and can also be heated under alternating magnetic fields for hyperthermia-based cancer therapy [167]. These advanced MNPs offer multiple levels of control, combining targeting, therapy, and real-time imaging into a single theranostics platform.

Beyond direct targeting of cancer cells, MNPs are playing a transformative role in the field of cancer immunotherapy, which seeks to harness the body’s immune system to identify and destroy cancer cells. MNPs can serve as delivery vehicles for a wide range of immunomodulatory agents such as cytokines, immune checkpoint inhibitors, and tumor-associated antigens [168]. One of the promising applications involves using NPs as cancer vaccines. AuNPs, for instance, can be loaded with tumor antigens and adjuvants to activate dendritic cells, which in turn prime T-cells to recognize and kill cancer cells. This approach shows potential in generating a strong and long-lasting anti-tumor immune response [169]. Iron oxide NPs are also gaining traction in immunotherapy. They can be taken up by macrophages and help polarize them from the M2 (tumor-supporting) phenotype to the M1 (tumor-fighting) phenotype. By reprogramming the TEM, these MNPs reduce immunosuppression and facilitate the infiltration and activity of cytotoxic immune cells [170]. Additionally, MNPs can be engineered to block immune checkpoint pathways, such as PD-1/PD-L1 and CTLA-4, either by delivering antibodies or RNA-based inhibitors directly into tumor sites, thereby restoring immune function and enabling T-cells to eliminate cancer cells more effectively in Table 1 [171].

MNPs, including AuNPs, AgNPs, iron oxide, zinc oxide, and copper oxide, exhibit multifaceted mechanisms for tumor-specific targeting and theranostics [192]. Engineered with precise size, shape, and surface chemistry, MNPs exploit the EPR effect for passive accumulation in tumor tissues due to aberrant vasculature and impaired lymphatic drainage. Smaller particle sizes further improve tumor penetration and therapeutic efficacy in cancer [193]. Particle size plays a critical role in the anticancer efficacy of NPs, as smaller particles exhibit greater cellular uptake and deeper tumor penetration. Studies have shown that NPs below 50 nm induce higher levels of apoptosis in cancer cells due to enhanced ROS generation and DNA damage. Thus, reducing particle size significantly improves the therapeutic potential of nanocarriers against cancer [194]. For active targeting, their surfaces are functionalized with monoclonal antibodies, peptides, or aptamers that selectively bind overexpressed tumor-associated antigens or receptors [195]. AuNPs, in particular, are utilized in photothermal therapy owing to their strong surface plasmon resonance in the NIR region, enabling efficient photo-induced hyperthermia and tumor ablation (see Figure 3) [196]. AgNPs exhibit potent cytotoxicity via redox imbalance, mitochondrial dysfunction, and DNA damage through excessive ROS production [197]. Superparamagnetic iron oxide NPs (SPIONPs) allow magnetic field-guided delivery, real-time MRI tracking, and local hyperthermia induction [198]. Other MNPs like ZnO and CuO trigger endosomal escape and initiate intrinsic apoptotic cascades by disrupting redox homeostasis [199]. Additionally, MNPs serve as nanocarriers for chemotherapeutics, siRNA, or CRISPR systems, enabling TME-responsive, site-specific delivery to minimize systemic exposure [200]. Functionalization with pH- or enzyme-sensitive linkers ensures stimuli-triggered release within acidic or protease-rich TMEs. MNPs further enhance imaging modalities such as MRI, CT, and photoacoustic imaging, facilitating image-guided therapy [201]. These integrated diagnostic and therapeutic capabilities position MNPs as next-generation nanotheranostic platforms for precision oncology [202].

Figure 3 shows AgNPs functionalized with peptides, genes, or chemotherapeutic drugs like gemcitabine for targeted cancer therapy. Exploiting the EPR effect, AgNPs accumulate in tumor tissues and are internalized by cancer cells. This leads to efficient drug uptake and induction of apoptosis, enhancing anticancer efficacy.

In the current scenario, plants are used to synthesize AgNPs. It is simple to synthesize using plant extracts or even the entire plant [207, 208]. In the health sector, AgNPs are frequently used as antibacterial agents, for food preservation, textile coatings, and with significant environmental applications (see Table 2) [208, 209]. AgNPs are used against antibacterial activity, the ability of AgNPs to reduce silver ions, to more frequently attach to thiol groups in bacterial proteins, interrupting their physiological activity, and causing cell death. According to many researchers, AgNPs penetrate and then destroy the bacterial membrane, preventing proper cell function, which causes structural damage and finally cell death, as shown in Figure 4 [210].

In chemistry and materials science, the creation of reliable, recyclable, environmentally friendly catalysts is considered an enormous challenge. Understanding this fieldʼs potency, while using MNPs, is now important due to the fieldʼs reliable development. More importantly, the creation of biodegradable, reusable catalysts helps to reduce the amount of waste that must be disposed of, and these catalysts are seen as essential [219–221]. The significance of environmental protection for humans has increased in recent years, and some poisonous dye molecules, like Methylene orange, Methylene blue, Congo red, 4-nitrophenol, and eosin Y, are hazardous to the environment. Hazardous dyes can be used to reduce smaller organic molecules and non-toxic species by reductants like NaBH₄; however, the rate of reduction is particularly slow (see Table 3). High reactivity, as well as the particular surface area of AgNPs, can accelerate the reduction of dyes, improving the efficiency of the reduction process [222].

Leishmaniasis is a parasitic disease caused by parasites of the genus Leishmania [230]. AgNPs were found to exhibit larvicidal action against sandfly bites in Table 4. The current scenario causes concern due to the costly nature and limited supply of antileishmanial medications, as well as the development of resistance to these drugs. However, due to the formation of ROS, this parasite is extremely sensitive to AgNPs. Under UV light, NPs have a combinatory detrimental impact on Leishmania tropica [231–233].