Therapeutic Perspectives of Metal Nanoformulations

: In recent decades, acceptance of nanoparticles (NPs) in therapeutic applications has increased because of their outstanding physicochemical features. By overcoming the drawbacks of conventional therapy, the utilization of metal NPs, metal-oxide, or metal supported nanomaterials have shown to have signiﬁcant therapeutic applications in medicine. This is proved by a lot of clinical and laboratory investigations that show improved treatment outcomes, site-speciﬁc drug delivery, and fewer side effects compared to traditional medicine. The metal NPs interaction with living cells (animal and plant) showed many ways to develop therapeutic models with the NPs. Despite all of the advancements that science has achieved, there is still a need to ﬁnd out their performance for long-term use to solve modern challenges. In this regard, the present documentation reviews some potential metals, including silver (Ag), gold (Au), zinc (Zn), copper (Cu), iron (Fe), and nickel (Ni) NPs, as therapeutic agents in various areas such as anticancer, antimicrobial, antidiabetic, and applicable for the treatment of many other diseases. Depending on the outstanding ongoing research and practical trials, metal-based NPs can be considered the hope of prospective modern therapeutic areas.


Introduction
The implementation of nanoparticles (NPs) for the treatment and diagnosis of disease is a revolutionary concept that has been developed over the past few decades. The nanotechnological approach can be divided into two branches: one is nanodevices and the other is nanomaterials. The nanodevice can be defined as such tiny devices at the nanoscale range, which includes microarrays and some devices such as respirocytes [1][2][3].
Particles smaller than 100 nanometers (nm) in any one of the dimensions are considered nanomaterials. Biomedical science found successful result by using nanoparticles as therapeutic agents in the treatment of various diseases. As it is selective on the target organ and receptors, it overcomes several limitations of conventional therapy, such as nonspecificity, unwanted side effects, less efficiency, and low bioavailability [4].
Therefore, current research projects are considerably more focused on developing and designing new drug delivery systems, and the most promising area is ensured by NPs for their uniqueness in biological and physicochemical characteristics, as they can deliver molecules to specific locations in the body [5]. Table 1. Concentration of metals in some human organs [14].

Major Function of Metals in Human Body
Sections 1.2.1-1.2.7 aim to describe the significance of the selected metals in the general physiological functions of the human body.

Manganese (Mn)
Manganese is significant for development, metabolism, and the antioxidant system. Importantly, Mn is needed for amino acid, cholesterol, and carbohydrate metabolism and in bone and thyroxin formation.
Mn is required for the action of enzyme families, such as oxidoreductases, hydrolases, transferases, lyases, isomerases, and ligases. For normal immune protection, blood sugar control, creating cellular energy, and reproduction, Mn works with various organ systems. It is estimated by the National Research Council that for adults, 2 to 5 mg of dietary manganese per day is safe [15].
It is particularly important for the detoxification of superoxide free radicals, and it activates some metalloproteases. It assists the body in using biotin, thiamin, vitamin C, and choline. However, excessive intake can lead to a stage called manganism, which causes neuronal death and a Parkinson's-like syndrome [16].

Iron (Fe)
Fe is a vital component of hemoglobin (RBC) [17]. Fe aids in the metabolism of muscle and active connective tissue. It is needed for the synthesis of some hormones, neurological development, maintaining physical growth, and cellular functioning [18,19].
For the synthesis of DNA and electron transportation, Fe is important [20]. Fe deficiency is the reason for about 50% of the cases of anemia around the world, according to a WHO report [21].

Cobalt (Co)
Co is a part of cobalamin, or vitamin B12, and therefore, it is significant for the function of cells. Co is needed for the production of RBC and the production of antimicrobial compounds (antibacterial and antiviral). Cobalt plays a vital role in amino acid and protein generation and the formation of neurotransmitters. Co salt is used in the treatment of anemia [22].

Nickel (Ni)
For regulating the proper function of the human body, Ni is an essential micronutrient. This metal amplifies hormonal function and is also required in lipid metabolism [23]. Although the mechanism of toxicity is unknown, prolonged contact or higher intake can result in a variety of side effects, including cancer [24]. Ni is required in trace amounts for growth and reproduction [25]. It activates arginase and urease enzymes [12] and also inhibits some enzymes, for instance, acid phosphatase [26].

Copper (Cu)
Cu is highly involved in energy production, iron metabolism, the formation of connective tissue, and the activation of neuropeptides and neurotransmitters [17,18]. Ceruloplasmin (CP), a Cu-abundant enzyme involved in Fe metabolism, is mostly composed of Cu and accounts for approximately 95% of total Cu in human plasma [27]. Cu is also involved in various physiologic processes, including angiogenesis, brain development, pigmentation, neurohormone homeostasis, gene expression regulation, and immune system functioning [24], as well as providing protection against oxidative damage [28,29].

Zinc (Zn)
Zn is required in a variety of ways for cellular metabolism. Zn is mandatory for the activation of about 100 enzymes [30,31], and it has roles in the immune system, protein synthesis [32,33], wound healing [34], DNA synthesis, and cell division [35,36]. It assists the normal development of the fetus during pregnancy and is needed for further growth from the stage of childhood to adolescence [37,38]. Moreover, Prasad et al. demonstrated that Zn is responsible for a proper sense of smell and taste [39,40]. Because the human body cannot store zinc, it must be consumed on a regular basis to keep these functions running smoothly [41].

Gold (Au)
The average human body (for an average adult human weighing 70 kg) might contain about 0.2 mg of Au [42]. Significant health functions include helping to maintain our joints as well as facilitating the transmission of electrical signals throughout the body. It is necessary for the maintenance and function of the joints. Additionally, Au it is an excellent conductor of electricity, aiding in the transmission of electrical signals throughout the body [39]. Several cell-mediated immune responses to various mitogens and antigens are inhibited by gold compounds. This inhibition is accelerated by the Au's impact on macrophages [43]. The surface of the NPs can be PEGylation or another coating, which might have present linkers containing surface functional group, surface charge, and targeting ligand (antibody, peptide, aptamer, etc.); see Figure 2. Nanomaterial size, shape, and surface coating are essential parameters that influence cell uptake and/or the pace and site-specific drug delivery from the system. The shapes of nanoparticles also play a crucial role in infrared absorption, which is particularly essential in phototherapy [44]. Rods are the most absorbent, followed by spheres, cylinders, and cubes [45]. The surface of the NPs can be PEGylation or another coating, which might have present linkers containing surface functional group, surface charge, and targeting ligand (antibody, peptide, aptamer, etc.); see Figure 2. Nanomaterial size, shape, and surface coating are essential parameters that influence cell uptake and/or the pace and site-specific drug delivery from the system. The shapes of nanoparticles also play a crucial role in infrared absorption, which is particularly essential in phototherapy [44]. Rods are the most absorbent, followed by spheres, cylinders, and cubes [45].
joints as well as facilitating the transmission of electrical signals throughout the body. It is necessary for the maintenance and function of the joints. Additionally, Au it is an excellent conductor of electricity, aiding in the transmission of electrical signals throughout the body [39]. Several cell-mediated immune responses to various mitogens and antigens are inhibited by gold compounds. This inhibition is accelerated by the Au's impact on macrophages [43].

Size, Shape, Material, and Surface of Nanoparticles
NPs range from 1 to 100 nm ( Figure 1) and could be a sphere, cube, rod, plate, or star shape. The surface of the NPs can be PEGylation or another coating, which might have present linkers containing surface functional group, surface charge, and targeting ligand (antibody, peptide, aptamer, etc.); see Figure 2. Nanomaterial size, shape, and surface coating are essential parameters that influence cell uptake and/or the pace and site-specific drug delivery from the system. The shapes of nanoparticles also play a crucial role in infrared absorption, which is particularly essential in phototherapy [44]. Rods are the most absorbent, followed by spheres, cylinders, and cubes [45].

Major Nanodrug Delivery Systems
This section will familiarize you with the various types of nanomedicine and provide a general idea for further research. Based on the recent approaches, polymeric, metallic, and ceramic NP drug delivery vehicles are widely used [8], such as liposomes [46], micelles [47], dendrimers [48], etc. A large number of clinical and pre-clinical trials demonstrated their efficacy in treating various diseases [49][50][51].
A process through which cells take in foreign material by enveloping it with their membrane is known as endocytosis. Pinocytosis and phagocytosis are the two main subtypes of endocytosis. Hormonal receptors, integrins, growth factor receptors, tyrosine kinase receptors, and lipids are just a few of the proteins that are transported via the critical cellular process known as endosomal trafficking.
Pinocytosis, from the Greek "pino" meaning to drink, is the mechanism through which the cell absorbs liquids and disperses tiny molecules. The cell membrane bends and forms tiny pockets during this process, catching the cellular fluid and other dissolved materials ( Figure 3). In many cases, nanodrugs follow this endocytosis (pinocytosis) [52]. Other delivery methods include clathrin-dependent delivery [53,54]. a general idea for further research. Based on the recent approaches, polymeric, metallic, and ceramic NP drug delivery vehicles are widely used [8], such as liposomes [46], micelles [47], dendrimers [48], etc. A large number of clinical and pre-clinical trials demonstrated their efficacy in treating various diseases [49][50][51].
A process through which cells take in foreign material by enveloping it with their membrane is known as endocytosis. Pinocytosis and phagocytosis are the two main subtypes of endocytosis. Hormonal receptors, integrins, growth factor receptors, tyrosine kinase receptors, and lipids are just a few of the proteins that are transported via the critical cellular process known as endosomal trafficking.
Pinocytosis, from the Greek "pino" meaning to drink, is the mechanism through which the cell absorbs liquids and disperses tiny molecules. The cell membrane bends and forms tiny pockets during this process, catching the cellular fluid and other dissolved materials ( Figure 3). In many cases, nanodrugs follow this endocytosis (pinocytosis) [52]. Other delivery methods include clathrin-dependent delivery [53,54].

Figure 3.
Major cellular uptake methods of nanoparticles (cellular uptake occurs mainly through endosomal trafficking, through clathrin-dependent delivery, and through ion channels).
Biomedical uses of nanohydrogels are wide in drug administration, tissue engineering [55], and wound dressing and healing due to their biocompatibility [56], nontoxicity [57], and high absorption capacity [58]. Furthermore, site-specific targeted drug administration is possible with stimulus response factors such as temperature and pH-dependent upgraded hydrogels [52,59]. There is evidence of the use of nanometal-hydrogel for tissue regeneration [60].
Nanohydrogel molecules have the features of hydrophilic and hydrophobic components, disperse in the solution to form micelles [61]. Micelles are generated by self-assembly, where the process does not begin until a specific minimum concentration is reached. This concentration is frequently referred to as the crucial micellar concentration [62]. Ag- Biomedical uses of nanohydrogels are wide in drug administration, tissue engineering [55], and wound dressing and healing due to their biocompatibility [56], nontoxicity [57], and high absorption capacity [58]. Furthermore, site-specific targeted drug administration is possible with stimulus response factors such as temperature and pH-dependent upgraded hydrogels [52,59]. There is evidence of the use of nanometal-hydrogel for tissue regeneration [60].
Nanohydrogel molecules have the features of hydrophilic and hydrophobic components, disperse in the solution to form micelles [61]. Micelles are generated by self-assembly, where the process does not begin until a specific minimum concentration is reached. This concentration is frequently referred to as the crucial micellar concentration [62]. Ag-NPs form micelles to be stable in aqueous solutions [59]. Reverse micelles are used for bimetallic (Au/Pd) NP formation [63].
Dendrimers are such structures that have branches or arms like trees and are globular, nanodimensionally compact, and radially symmetric [64]. The capacity of dendrimers to distribute drugs in a regulated and targeted manner is their most promising use. Higher stability, a longer half-life, and greater bioavailability are characteristics of drugs conjugated to such delivery systems. Additionally, prolonged drug release via the drug-dendrimer combination lowers the systemic toxicity and maintains tumor tissue-specific aggregation [65,66].
A significant number of clinical and preclinical studies show how deeply the function of NPs as carriers of therapeutic agents has been studied. NPs are regarded as one of the most promising groups of medication delivery systems. NPs can bind macromolecules such as proteins, antibodies, or nucleic acids and can encapsulate both hydrophilic and hydrophobic medicines [67]. Paclitaxel has been exemplified in polymeric NPs made by impeding copolymers of mono-methoxy polyethylene glycol and poly-D,L-lactide [68].
Additionally, NPs may be programmed to react to many environmental factors, including pH, light, temperature, enzymes, and other biological and chemical agents. The most often employed of all these stimuli is pH responsiveness. The pH differential can aid in distinguishing tumor tissue (pH 5.7-7.0) from normal tissue (pH 7.4). The capacity to directly release medications at tumor sites has made this pH responsiveness valuable in a variety of cancer and tumor therapies [69].
Inorganic NPs have been studied for their potential biomedical applications in addition to polymeric NPs. There are various ways to make inorganic NPs, including the crystallization of inorganic salts, thermal breakdown, and other well-known synthetic processes [70]. Several inorganic NPs, including Au, Ag, Pt, iron oxide (FeO), cerium oxide (CeO 2 ), and zinc oxide (ZnO), have been successfully synthesized and used in numerous preclinical and clinical trials. However, because of their higher biocompatibility, higher biodegradability, and lower systemic toxicity, polymeric nanoparticles are preferred over inorganic NPs [71]. Many researchers have been interested in liposomes as a potential medication delivery technology due to their capacity to selectively transport both hydrophilic and hydrophobic medicines to their respective target sites [72].
Liposomes are capable of encapsulating and protecting both hydrophilic and hydrophobic medicines before releasing them at specific sites. Multilamellar vesicles are made up of concentric spheres of phospholipids separated by water layers, while unilamellar vesicles only have a single phospholipid bilayer encapsulating the aqueous solution [50]. Au-NPs enable green synthesis by using glycerol liposomes [73,74] and the selective release of contents from liposomes caused by light [75]. Multifunctional metallic NPs can be formed for medical imaging and micro-fluidity [76,77].
Scaffolds are important in biomedicine and tissue engineering because of their capability to foster cell adhesion, proliferation, and differentiation, all of which are necessary for tissue development. The scaffolding allows cells to develop in all the right places, which results in the production of tissue. A biocompatible matrix is required for optimal cell growth, the strategy of employing scaffolds is of utmost relevance [78,79]. Scaffolds have great biocompatibility, mechanical strength, porosity, and interconnectivity, all of which are necessary for clinical application [80]. There is evidence of a nanohybrid scaffold of glycolic acid-g-chitosan-Pt-Fe 3 O 4 being used as a drug delivery system [81].

Scopes of Metal Nanoparticles in Remedies
To ensure that the human body functions normally, specific levels of certain metals must be present. The main functions of metals are to catalyze certain reactions and act as cofactors or prosthetic groups of enzymes. The required metals for humans include Na, K, Fe, Mg, Zn, Cd, Mn, Cu, V, Cr, Mo, Co, and Ni. In the absence of certain essential metals, anemia could occur [41].
Iron deficiency causes the loss of functional blood proteins such as hemoglobin, myoglobin, etc., whose function is to carry oxygen. Iron deficiency accounts for roughly half of all anemia cases worldwide. As a first-line therapy, oral iron supplementation is recommended; however, IV iron formulation is a recent addition to anemia treatment, and hepcidin could be a future diagnostic target [42]. Vitamin B12 is made of a cobalt complex called cobalamin, and the lack of this vitamin results in pernicious anemia.
Zn is used as a catalyst for various enzymes. Importantly, it is required for red blood cell production. That is why a deficiency in this metal can cause anemia. It can heal wounds, and Zn ions (Zn 2+ ) can be used for treating the herpes virus [82]. According to one study, infant's diets which had low in Zn; had higher rates of copper anemia, which can lead to heart disease also [31]. Copper gluconate, copper chloride, or copper sulfate are used as oral or IV copper supplements in copper anemia [83]. Some potential metallic NPs are shown in Figure 4.
For the treatment of rheumatoid arthritis [84], juvenile rheumatoid arthritis, and psoriatic arthritis, gold salt complexes have been used. Though the mechanism is still uncovered, it is assumed that Au salts interact with albumin and are taken up by the immune cells, causing antimitochondrial effects and the apoptosis of cells [85][86][87]. Head and neck tumors showed specificity towards the Pt-based compounds; they might work by cross-linking the DNA in tumor cells [88]. For the treatment of manic-depressive disorder, lithium carbonate (Li 2 CO 3 ) is used [89].
To prevent the contagiousness of infection, Ag has been used for various remedies since 4000 BC. The bactericidal effect of silver is well established, and topically, it is used to prevent infection of burned skin; it is also being used for ulcerations, bone prostheses, orthopedic surgery, catheters, heart devices, and surgical apparatus [90].
Diabetes, atherosclerosis, cancer, myocardial ischemia, pulmonary TB, asthma, Alzheimer's disease (AD), and Parkinson's disease (PD) are only a few of the many chronic diseases for which drug delivery vehicles have been extensively studied and shown to be effective. A number of these medicines, including Caelyx ® , Abraxane ® , Myocet ® , Mepact ® , Rapamune ® , and Emend ® , have been marketed for human use after positive results in preclinical and clinical testing. The potential of innovative therapeutic agents, such as peptides, nucleic acids (RNA and DNA), and genes, to be exploited as nanomedicines for the treatment of numerous chronic diseases, has been demonstrated beyond that of medications and chemicals [91].
Zn is used as a catalyst for various enzymes. Importantly, it is required for red blood cell production. That is why a deficiency in this metal can cause anemia. It can heal wounds, and Zn ions (Zn 2+ ) can be used for treating the herpes virus [82]. According to one study, infant's diets which had low in Zn; had higher rates of copper anemia, which can lead to heart disease also [31]. Copper gluconate, copper chloride, or copper sulfate are used as oral or IV copper supplements in copper anemia [83]. Some potential metallic NPs are shown in Figure 4.
For the treatment of rheumatoid arthritis [84], juvenile rheumatoid arthritis, and psoriatic arthritis, gold salt complexes have been used. Though the mechanism is still uncovered, it is assumed that Au salts interact with albumin and are taken up by the immune cells, causing antimitochondrial effects and the apoptosis of cells [85][86][87]. Head and neck tumors showed specificity towards the Pt-based compounds; they might work by crosslinking the DNA in tumor cells [88]. For the treatment of manic-depressive disorder, lithium carbonate (Li2CO3) is used [89].
To prevent the contagiousness of infection, Ag has been used for various remedies since 4000 BC. The bactericidal effect of silver is well established, and topically, it is used to prevent infection of burned skin; it is also being used for ulcerations, bone prostheses, orthopedic surgery, catheters, heart devices, and surgical apparatus [90].
Diabetes, atherosclerosis, cancer, myocardial ischemia, pulmonary TB, asthma, Alzheimer's disease (AD), and Parkinson's disease (PD) are only a few of the many chronic diseases for which drug delivery vehicles have been extensively studied and shown to be effective. A number of these medicines, including Caelyx ® , Abraxane ® , Myocet ® , Mepact ® , Rapamune ® , and Emend ® , have been marketed for human use after positive results in preclinical and clinical testing. The potential of innovative therapeutic agents, such as peptides, nucleic acids (RNA and DNA), and genes, to be exploited as nanomedicines for the treatment of numerous chronic diseases, has been demonstrated beyond that of medications and chemicals [91]. NPs based on metals such as Au, Ag, Fe, Cu, Pt, Zn, and so on have attracted a lot of interest in the medical field. NPs of metals have been demonstrated to exist in aqueous solutions, as demonstrated by Faraday [11]. Metallic NPs' hue and structure were NPs based on metals such as Au, Ag, Fe, Cu, Pt, Zn, and so on have attracted a lot of interest in the medical field. NPs of metals have been demonstrated to exist in aqueous solutions, as demonstrated by Faraday [11]. Metallic NPs' hue and structure were analyzed by Kumar et al. [92] many years later. NPs can be manufactured and optimized in the present day by altering the chemical groups that aid in binding the antibodies. Ag-NPs could be utilized to treat a variety of skin ailments. Biomedical applications of noble metal NPs (Au, Ag, and Pt) include cancer treatment, drug transport, radiation therapy augmentation, thermal ablation, fungus elimination, diagnostic testing, and gene delivery, among many others. NPs of noble metals have special qualities that increase their worth. Peptides, antibodies, RNA, and DNA are just some of the functional groups that can be attached to metal NPs to make them more specific to the cells they are intended to target [93]. Some key NPs as well as their physiological applications are summarized below in Table 2.

Major Challenges of Using Nanoparticles in Medical Treatment
Firstly, though many testing procedures [102] have been developed for the evaluation of NP toxicity [103], these procedures are not universal for all NPs; they are designed for individual NPs and are not applicable for hybrid NPs. This fact leads to an undesirable outcome from the real objects and could be harmful for the body. As its effect is dependent on the size, shape, surface charging condition, and capping agents, it is really difficult to develop an accurate strategy to find out the toxicity. On the other hand, Au-NPs' effect also depends on its target receptor or organ; for example, different NPs show their effect at different concentrations. Thus, it is urgent to formulate some universal methods, such as good laboratory practice (GLP), to evaluate the safety of the NPs [103][104][105]. The adsorption of proteins to the particles also correlated with their physical characteristics (size, shape, charge, etc.) [106]. Significantly, metal oxide NPs have a high tendency to produce toxicity, this toxicity can be caused by a variety of mechanisms, including oxidative stress, coordination effects, nonhomeostatic effects, genotoxicity, and others. Size, solubility, and exposure routes all have an impact on metal oxide nanoparticles [107].
Secondly, although Au-NPs show outstanding result in tumor disease, there is a lack of studies to find out its pharmacokinetics (clearance and bio distribution) inside the human body. In vitro and in vivo studies cannot give the full picture of the biodistribution in the organism, which limits the wide use of gold NPs [108].
Third, this study discovered that in the case of tumor treatment, only 0.7% of NPs were able to reach cancer cells, with some exceptions reaching more than 5%. Moreover, when the NPs are injected in the blood circulation, they get absorbed in the mononuclear phagocytic system (MPS) and renal system, which reduces the effectiveness of the MPS day by day.
Finally, because there have been few Au-NP clinical trials, the data do not allow for comprehensive research on clearance, distribution, and protein absorption. Thus, a comprehensive trial for safety and toxicity should be carried out [109].
This review aims to highlight experiments conducted in the path of advancement in the therapeutic use of above discussed six metal NPs, such as Ag, Au, Zn, Cu, Fe, and Ni; additionally, we have used the literature to highlight the possible mechanism of action of significant effects of the selected potential metal NPs.

Therapeutic Interventions of Gold Nanoparticles (Au-NPs)
When Robert Koch discovered that gold cyanide had a bacteriostatic effect on Mycobacterium TB, the medical use of gold for the treatment of tuberculosis was established for the first time. This led to the introduction of gold as a medicine in the 1920s [110].
Au-NPs have a tendency to aggregate at tumor sites [95]. Tumor cells can be killed by Au-NPs in a variety of ways, including as drug delivery systems for mechanical damage, anticancer medicines, and photothermal ablation [111].
In particular, Au-NPs are used in drug delivery, imaging, photo-thermal therapy, sensing, catalysis, and antimicrobials [112]. The list of applications of Au-NPs is much longer because of their unique properties ( Table 3). The biocompatibility of gold nanoparticles has been well documented; however, the typical reduction procedures used to create them can leave behind harmful chemical species [113]. Consequently, Au-NPs manufactured in an environmentally friendly manner hold far more promise in a variety of settings. Although Au-NPs are not as widely used as Ag-NPs as antibacterial agents, they nonetheless have considerable impact against a wide range of diseases due to their inherent biocidal qualities [112,114].
Au-NPs of 60 nm showed a positive result in retinoblastoma treatment [115], Au nanopopcorn 28 nm in size is used to diagnose prostate and breast cancer [116], and Au nanostars (Au-NS) 30 and 60 nm in size can be used to identify brain tumors, and this same NP showed a satisfying result against bladder cancer [117].
Silica-coated Au nanorods showed effective antitumor activity, both in vivo and in vitro, against breast cancer by targeting CD44+ receptors [118]. Colloidal Au-NPs are of interest as nontoxic carriers for drug delivery [119][120][121]. In a study, it was found that the internalization of the 50 nm spherical gold nanoparticles (AuNPs) was the best of all the nanoparticles investigated [122]. TrxR (thioredoxin reductase) function can be inhibited by gold compounds, which causes tumor cells to accumulate reactive oxygen species (ROS) and experience oxidative stress, which ultimately kills the tumor cells [123,124] and the proposed anticancer mechanism of Au-NPs is illustrated in Figure 5.

Nanotherapeutic Application of Gold
Drugs Drug Candidates 2023, 2, FOR PEER REVIEW 10 Figure 5. Proposed anticancer mechanism of gold nanoparticles. Here, Au-NPs pass through the cancer cell membrane by endocytosis, and endosomal release causes ROS (reactive oxygen species) production. These ROS cause mitochondrial dysfunction and result in caspase 3, 9, and 8 activations, which results in DNA damage and finally cell death [123][124][125][126].
Nanotherapeutic Application of Gold Figure 5. Proposed anticancer mechanism of gold nanoparticles. Here, Au-NPs pass through the cancer cell membrane by endocytosis, and endosomal release causes ROS (reactive oxygen species) production. These ROS cause mitochondrial dysfunction and result in caspase 3, 9, and 8 activations, which results in DNA damage and finally cell death [123][124][125][126].

Miscellaneous Effects
Au-NPs/chalcones conjugate (2 to 12 nm) HEK293 cells 20-100 µg/mL Therapeutic development of antidiabetic drug, which is derived from H. foetidum by increasing glucose uptake and no particle shows cytotoxicity against HaCaT keratinocytes.
Helichrysetin is a potential compound for antidiabetic effect.

Therapeutic Interventions of Silver Nanoparticles (Ag-NPs)
Silver has excellent physicochemical features, such as catalytic, optical, electric, and, of course, antibacterial capabilities, and these qualities make silver nanoparticles the most marketable nanoparticles. In the presence of Ag-NPs, the synergistic impact of antibiotics such as cefotaxime, azithromycin, cefuroxime, chloramphenicol, and fosfomycin against E. coli was greatly boosted as compared to antibiotics alone [80].
Other metal NPs may exhibit equivalent efficacy against particular germs, but overall, silver is said to be the most effective material against a variety of pathogens. Ag-NPs inhibit the extracellular activity of severe acute respiratory distress syndrome coronavirus 2 (SARS-CoV-2) [154].
Ag-NPs are the preferred metal when antibacterial characteristics are required. The antibacterial, antiviral, antioxidant, and anticancer characteristics of silver are well recognized, and it has the potential to be developed into a unique therapeutic agent. Ag also has antiparasitic, antiviral, and anticancer qualities [155,156], and the mechanisms of action of these effects are illustrated in Figure 6. Ag-NPs, after entering cells by endocytosis, produce ROS that damage the endoplasmic reticulum and mitochondria. The cellular pathways NF-kB, PI3K/AKT/mTOR, Wnt/beta-catenin, MAPK/ERK, and ERK activation result in DNA fragmentation, cell cycle arrest, and cell apoptosis [157][158][159][160][161]. Table 4 shows the prominent nanotherapeutic applications of silver.

Therapeutic Interventions of Silver Nanoparticles (Ag-NPs)
Silver has excellent physicochemical features, such as catalytic, optical, electric, and, of course, antibacterial capabilities, and these qualities make silver nanoparticles the most marketable nanoparticles. In the presence of Ag-NPs, the synergistic impact of antibiotics such as cefotaxime, azithromycin, cefuroxime, chloramphenicol, and fosfomycin against E. coli was greatly boosted as compared to antibiotics alone [80].
Other metal NPs may exhibit equivalent efficacy against particular germs, but overall, silver is said to be the most effective material against a variety of pathogens. Ag-NPs inhibit the extracellular activity of severe acute respiratory distress syndrome coronavirus 2 (SARS-CoV-2) [154].
Ag-NPs are the preferred metal when antibacterial characteristics are required. The antibacterial, antiviral, antioxidant, and anticancer characteristics of silver are well recognized, and it has the potential to be developed into a unique therapeutic agent. Ag also has antiparasitic, antiviral, and anticancer qualities [155,156], and the mechanisms of action of these effects are illustrated in Figure 6. Ag-NPs, after entering cells by endocytosis, produce ROS that damage the endoplasmic reticulum and mitochondria. The cellular pathways NF-kB, PI3K/AKT/mTOR, Wnt/beta-catenin, MAPK/ERK, and ERK activation result in DNA fragmentation, cell cycle arrest, and cell apoptosis [157][158][159][160][161]. Table 4 shows the prominent nanotherapeutic applications of silver.

µg/mL
The mechanism of silver colloid particles bactericidal action on bacteria is still being investigated.

Therapeutic Interventions of Copper Nanoparticles (Cu-NPs)
Researchers and health care professionals have been drawn to cupric oxide (CuO) NPs for their physical, chemical, high temperature, and photocatalytic capabilities, but most notably for their antibacterial properties [185]. Copper nanoparticles' synergistic activity with amoxicillin, ampicillin, ciprofloxacin, and gentamicin against both Gram-positive and Gram-negative bacteria was investigated, and ampicillin showed comparatively improved activity compared to alone [186]. Cu-NPs inactivate glycosidase to provide an antidiabetic effect, and the study found that Cu-NPs showed an anticancer effect by activating BAX and p53 and by decreasing Bcl-2 expression, which result in apoptosis in cancer [187]. Cu-NPs increase ROS production in bacterial cells and cause bacterial DNA and protein destruction; on the other hand, accumulation of Cu-NPs in the bacterial cell wall causes cell wall disruption [188][189][190][191][192][193][194].
The mechanisms underlying these effects are depicted in Figure 7. Other nanotherapeutic applications of copper are presented in Table 5.

Therapeutic Interventions of Copper Nanoparticles (Cu-NPs)
Researchers and health care professionals have been drawn to cupric oxide (CuO) NPs for their physical, chemical, high temperature, and photocatalytic capabilities, but most notably for their antibacterial properties [185]. Copper nanoparticles' synergistic activity with amoxicillin, ampicillin, ciprofloxacin, and gentamicin against both Gram-positive and Gram-negative bacteria was investigated, and ampicillin showed comparatively improved activity compared to alone [186]. Cu-NPs inactivate glycosidase to provide an antidiabetic effect, and the study found that Cu-NPs showed an anticancer effect by activating BAX and p53 and by decreasing Bcl-2 expression, which result in apoptosis in cancer [187]. Cu-NPs increase ROS production in bacterial cells and cause bacterial DNA and protein destruction; on the other hand, accumulation of Cu-NPs in the bacterial cell wall causes cell wall disruption [188][189][190][191][192][193][194].
The mechanisms underlying these effects are depicted in Figure 7. Other nanotherapeutic applications of copper are presented in Table 5.   Melanoma [196] Cu-NPs (12-16 nm) Human lung carcinoma cells (A549) 20-100 µg/mL In a dose-dependent way, lung cancer cells showed extensive structural damages and increased oxidative stress indicators.

Antiviral Effects
Cu-NPs (20 nm) SARS-CoV-2 500 µL By putting virus-containing media onto copper-coated PP filters and then adding Vero cells, inactivation was assessed.

Therapeutic Interventions of Zinc Nanoparticles (Zn-NPs)
Zinc is a material that is frequently used in biomedical applications due to its unique features, such as electric conductivity, optical capabilities, and piezoelectric qualities [207]. Beyth et al. defined the method of killing bacteria using zinc oxide (ZnO) NPs as having two pathways of action [208]. The first involves cell wall penetration, and the second includes the formation of ROS. Zn-NPs follow the Bcl-2/BAX/BAK pathway to cell apoptosis by caspase-3 and -9 and ROS-induced DNA fragmentation leading to cell cycle arrest and apoptosis, and also follow the mitochondrial disruption for an anticancer effect [209][210][211], as shown in Figure 8.

Therapeutic Interventions of Zinc Nanoparticles (Zn-NPs)
Zinc is a material that is frequently used in biomedical applications due to its unique features, such as electric conductivity, optical capabilities, and piezoelectric qualities [207]. Beyth et al. defined the method of killing bacteria using zinc oxide (ZnO) NPs as having two pathways of action [208]. The first involves cell wall penetration, and the second includes the formation of ROS. Zn-NPs follow the Bcl-2/BAX/BAK pathway to cell apoptosis by caspase-3 and -9 and ROS-induced DNA fragmentation leading to cell cycle arrest and apoptosis, and also follow the mitochondrial disruption for an anticancer effect [209][210][211], as shown in Figure 8.
ZnO-NPs have antibacterial, antifungal, anticancer, antidiabetic, and antitubercular activity, and breast cancer inhibition is an optimistic property that this study observed in a number of studies (presented in Table 6). Even 100 nm Zn-NPs supplemented at 30 ppm improved growth and serum glucose levels in layer chicks [212].

Figure 8.
Proposed anticancer mechanism of Zn-NPs (ZnO-NPs create stress in endoplasmic reticulum, and produce ROS, which results DNA fragmentation and cell cycle arrest; on the other hand, produced ROS disrupts mitochondrial membrane and activates caspase 3, 7, and 9, which results in apoptosis). Figure 8. Proposed anticancer mechanism of Zn-NPs (ZnO-NPs create stress in endoplasmic reticulum, and produce ROS, which results DNA fragmentation and cell cycle arrest; on the other hand, produced ROS disrupts mitochondrial membrane and activates caspase 3, 7, and 9, which results in apoptosis).

Nanotherapeutic Applications of Zinc
ZnO-NPs have antibacterial, antifungal, anticancer, antidiabetic, and antitubercular activity, and breast cancer inhibition is an optimistic property that this study observed in a number of studies (presented in Table 6). Even 100 nm Zn-NPs supplemented at 30 ppm improved growth and serum glucose levels in layer chicks [212].  Bacterial and fungal infection [229] ZnO-NPs (45-150 nm)

Nanotherapeutic Application of Zinc
Helicobacter pylori and human mesenchymal stem cells (hMSc)

3.125-100 µg/mL
Biocompatibility to hMSC and described as safe in mammalian cells and can be used as antibiotics.

Vero cells 100 µg/mL
The gene transcription is inhibited by inhibiting the β-subunit of the bacterial RNA polymerase. Tuberculosis [231]  ZnO-NPs (12 nm) Staphylococcus aureus and Escherichia coli 5.6 µg/mL The ROS production was increased, while the cellular function and cell membrane were disrupted.

Miscellaneous Effects
Zn-NPs (1-100 nm) Layer chicks 30 ppm Increasing the level of serum glucose and alkaline phosphate, while decreasing alanine transferase.
Increased chicken growth rate [212] ZnO-NPs (48.2 nm) Xanthomonas oryzae 16.0 µg/mL The bacterial membrane is collapsed and ruptured by interacting with ZnO NPs and as a result in the leakage of bacterial cytoplasm.
ZnAA-Vac treated for 12 days at 100 and 1000 ppm It had a stronger depigmenting impact, reducing the melanin hue by 75%. Melanin treatment [259]

Therapeutic Interventions of Nickel Nanoparticles (Ni-NPs)
Ni-NPs have anticancer action [260,261]. A complex structure of Qu-PEG-NiGs (48-72 nm), green synthesized by Ocimum sanctum leaf extract, showed mitochondrialmediated apoptosis against the MCF-7 cell line [262], antimicrobial activity, antioxidant action, and activity against human ovarian cancer, liver and spleen injury [260,[263][264][265], lung inflammation [266], human lung cancer [267], lymphatic filariasis [268], and larvicidal parasitic activity [269]. Bacterial protein leakage induced by ROS activation [270] and disruption of the cell membrane [271] is one way of causing bacterial cell death. The antimicrobial mechanism is shown in Figure 9. It has numerous other therapeutic properties in a single formulation or a complex formulation, as shown in Table 7.

Therapeutic Interventions of Nickel Nanoparticles (Ni-NPs)
Ni-NPs have anticancer action [260,261]. A complex structure of Qu-PEG-NiGs (48-72 nm), green synthesized by Ocimum sanctum leaf extract, showed mitochondrial-mediated apoptosis against the MCF-7 cell line [262], antimicrobial activity, antioxidant action, and activity against human ovarian cancer, liver and spleen injury [260,[263][264][265], lung inflammation [266], human lung cancer [267], lymphatic filariasis [268], and larvicidal parasitic activity [269]. Bacterial protein leakage induced by ROS activation [270] and disruption of the cell membrane [271] is one way of causing bacterial cell death. The antimicrobial mechanism is shown in Figure 9. It has numerous other therapeutic properties in a single formulation or a complex formulation, as shown in Table 7.     [281]. Magnetite (Fe 3 O 4 ) NPs are used in biomedical applications due to their magnetic characteristics, biocompatibility, and, in particular, their superparamagnetic capabilities [282].
Superparamagnetic iron oxide nanoparticles (SPIONs) provide action against the human breast cancer cell MCF7 [284]. Different nanotherapeutic studies of Fe-NPs are arranged in Table 8. In the treatment of different types of cancer, ferroptosis, a new Fe-and ROS-dependent form of controlled cell death, has received a lot of attention. The potential of ferroptosis in combination with NPs for cancer therapy is becoming more and more clear as a result of the development of nanomaterials [293]. After cells consume Fe-based NPs, an excess of iron ions released from the lysosome in an acidic environment activates the fenton reaction, which causes ROS formation and cell ferroptosis [294].
Importantly, when antibiotic drugs are coupled with the iron nanoparticles of neem extract, the dose of traditional antibiotics can be decreased by nearly half without affecting efficiency. As a result, the use of natural antibiotics aids in the reduction of regular antibiotic doses [295]. There was also a trial of producing bimetallic NPs (Ag-Fe) that established the synergistic antibacterial (bactericidal) impact of the two metals forming the bimetallic nanoparticles when compared to the effects of the monometallic nanoparticles against yeast and both Gram-positive and Gram-negative multidrug-resistant bacteria [296]. Superparamagnetic iron oxide nanoparticles (SPIONs) provide action against the human breast cancer cell MCF7 [284]. Different nanotherapeutic studies of Fe-NPs are arranged in Table 8.

Nanotherapeutic Applications of Iron
In the treatment of different types of cancer, ferroptosis, a new Fe-and ROS-dependent form of controlled cell death, has received a lot of attention. The potential of ferroptosis in combination with NPs for cancer therapy is becoming more and more clear as a result of the development of nanomaterials [293]. After cells consume Fe-based NPs, an excess of iron ions released from the lysosome in an acidic environment activates the fenton reaction, which causes ROS formation and cell ferroptosis [294].
Importantly, when antibiotic drugs are coupled with the iron nanoparticles of neem extract, the dose of traditional antibiotics can be decreased by nearly half without affecting efficiency. As a result, the use of natural antibiotics aids in the reduction of regular antibiotic doses [295]. There was also a trial of producing bimetallic NPs (Ag-Fe) that established the synergistic antibacterial (bactericidal) impact of the two metals forming the bimetallic nanoparticles when compared to the effects of the monometallic nanoparticles against yeast and both Gram-positive and Gram-negative multidrug-resistant bacteria [296].

Metal Nanoparticles Elimination from Body
The elimination of NPs depends on their particle size, intrinsic biodegradability, core density, surface charge, and surface chemistry [307]. The liver is the major clearance organ in the oral administration of NPs. Intravenously administered NPs are cleared from the bloodstream by two main mechanisms: (i) renal elimination and (ii) hepatobiliary elimination. Choi et al. [308] reported that smaller-sized (<5.5 nm diameter) quantum dots undergo efficient urinary excretion due to the pore size limit of glomerular filtration in the kidneys. According to estimates of Si-NPs in rats, 7-8% of NPs were eliminated in urine and 75-80% were expelled in feces [309]. Nonbiodegradable and larger-sized (>5.5 nm) NPs are supposed to be eliminated through the hepatobiliary route. The hepatobiliary elimination involved the following pathways: (1) the liver sinusoid; (2) the space of Disse, a tiny perisinusoidal space containing blood plasma, nutrients, oxygen, and body waste that has become crucial in the treatment of liver disease, which is located between endothelial cells and hepatocytes; (3) hepatocytes; (4) bile ducts; (5) intestines; and finally (6) out of the body, as shown in Figure 11. In hepatobiliary elimination, the liver nonparenchymal cells (e.g., Kupffer cells and liver sinusoidal endothelial cells) influence and determine the elimination fate. The removal of Kupffer cells increased the fecal elimination of NPs by more than 10-fold [310].

Metal Nanoparticles Elimination from Body
The elimination of NPs depends on their particle size, intrinsic biodegradability, core density, surface charge, and surface chemistry [307]. The liver is the major clearance organ in the oral administration of NPs. Intravenously administered NPs are cleared from the bloodstream by two main mechanisms: (i) renal elimination and (ii) hepatobiliary elimination. Choi et al. [308] reported that smaller-sized (<5.5 nm diameter) quantum dots undergo efficient urinary excretion due to the pore size limit of glomerular filtration in the kidneys. According to estimates of Si-NPs in rats, 7-8% of NPs were eliminated in urine and 75-80% were expelled in feces [309]. Nonbiodegradable and larger-sized (>5.5 nm) NPs are supposed to be eliminated through the hepatobiliary route. The hepatobiliary elimination involved the following pathways: (1) the liver sinusoid; (2) the space of Disse, a tiny perisinusoidal space containing blood plasma, nutrients, oxygen, and body waste that has become crucial in the treatment of liver disease, which is located between endothelial cells and hepatocytes; (3) hepatocytes; (4) bile ducts; (5) intestines; and finally (6) out of the body, as shown in Figure 11. In hepatobiliary elimination, the liver nonparenchymal cells (e.g., Kupffer cells and liver sinusoidal endothelial cells) influence and determine the elimination fate. The removal of Kupffer cells increased the fecal elimination of NPs by more than 10-fold [310]. Figure 11. Proposed metal nanoparticles hepatobiliary clearance pathway (when metal NPs pass through the liver sinusoid, they enter the space of Disse via Kupffer cells, and then enter the bile duct, followed by fecal elimination.
NPs can enter the body through multiple routes, including the skin, respiratory tract, dermal exposure, mucosal, oral, intravenous, subcutaneous, intramuscular, etc., and can induce acute or chronic toxicities [311]. The anionic NPs are less toxic than the cationic NPs, which cause hemolysis and clotting [312]. Singh et al. [313] reported that ceramic NPs, commonly used for drug delivery, exhibit oxidative stress and cytotoxic activity in the lungs, liver, heart, and brain, as well as having teratogenic or carcinogenic effects. NPs have been shown, both in vivo and in vitro, to increase cellular reactive oxygen species, induce multiple minor and severe toxicities, and even disrupt host homeostasis [311]. Although NPs are useful for numerous medical applications, there are still some concerns for ecosystems and living organisms due to their uncontrollable use and discharge to the Figure 11. Proposed metal nanoparticles hepatobiliary clearance pathway (when metal NPs pass through the liver sinusoid, they enter the space of Disse via Kupffer cells, and then enter the bile duct, followed by fecal elimination.
NPs can enter the body through multiple routes, including the skin, respiratory tract, dermal exposure, mucosal, oral, intravenous, subcutaneous, intramuscular, etc., and can induce acute or chronic toxicities [311]. The anionic NPs are less toxic than the cationic NPs, which cause hemolysis and clotting [312]. Singh et al. [313] reported that ceramic NPs, commonly used for drug delivery, exhibit oxidative stress and cytotoxic activity in the lungs, liver, heart, and brain, as well as having teratogenic or carcinogenic effects. NPs have been shown, both in vivo and in vitro, to increase cellular reactive oxygen species, induce multiple minor and severe toxicities, and even disrupt host homeostasis [311]. Although NPs are useful for numerous medical applications, there are still some concerns for ecosystems and living organisms due to their uncontrollable use and discharge to the natural environment; thus, it should be considered to make the use of NPs more convenient and environmentally friendly. Preclinical studies have revealed the importance of renalclearable luminous metal NPs in cancer therapy, which offers tremendous promise for potential clinical translation [314]. The retention of NPs in the body, especially in the vital organs, usually depends on the density of the particles. In a study of gold and silver NPs by Tang et al., it was demonstrated that the lower-density metal NPs have a higher distribution and shorter retention time than the higher-density metal NPs [315].

Conclusions
Nanodrugs can be highlighted as the future of medicine, and using potential metals such as Fe, Au, Cu, Ag, Ni, and Zn in NPs showed optimistic results against various types of cancers, as well as displaying antitumor, antidiabetic, and antimicrobial activity. They are also applicable for other purposes, and it was found that metal NPs have significant synergistic activity with commercially available antibiotics. Since we already have certain levels of most metals in our bodies, they are compatible with our immune systems, which is of benefit to metal nanotherapy. Research has found that metals enhance the pharmacological activity considerably.
Despite recent advances in metal nanotherapy, the majority of nanotherapeutics are still being studied. The main concerns are not only their long-term safety for the patient, but also the ecological and toxicological aspects that need to be considered.
The generation of ROS is a significant challenge for metal NPs and metal oxide NPs. Diameter, structure, interface, content, solubility, accumulation, and particle absorption are factors that can affect ROS generation. A metallic nanomaterial's toxicity may vary based on its oxidation reaction, ligand, solubility, shape, environment, and medical factors. For example, characterization and cell type are important factors in the uptake of Au-NPs. If the Au-NPs are absorbed by a healthy cell, they will eventually be removed, but if they are absorbed by a malignant cell, they will cause cell death. More in vivo metal nanotherapeutic studies are needed to find out the toxicological conditions in normal cell lines when targeting cancer cells.