The development of silica shells on iron oxide nanoparticle surfaces was mediated by a process catalysing the hydrolysis of TEOS and subsequent condensation of silica (SiO₂) onto iron oxide nanoparticle (Fe₃O₄-NPs) surfaces [39]. This is made feasible by the fact that silica shells on top of magnetite nanoparticles generate negative charges on their surfaces, which enables them to create extremely stable dispersions in water without the need for additional surfactants [40].

Iron oxide nanoparticles and TEOS were combined in a way that was optimised to prevent homogenous spheres [41]. The growth time, the concentration of the ammonia catalyst, the concentration of water, and other factors can all be altered to control the thickness of the silica shells; however, the simplest and most repeatable method to do so is to change the ratio of the concentration of the TEOS precursor to the concentration of the magnetite nanoparticles [42]. Figure 1 explains the method of synthesizing silica-coated magnetite (Fe₃O₄@SiO₂) nanocomposites [43].

Figure 1. Synthetic route for the silica-coated magnetite (Fe₃O₄@SiO₂) nanoparticles [43].

Molecularly imprinted (MI) magnetite samples (Fe₃O₄@SiO₂/thermosensitive) were heated for 8 hours at 60˚C~65˚C in the presence of 40 ml of ethanol (C₂H₅.OH), under N₂ atmosphere, and then were made into thermosensitive agents by adding 1 g of n-vinyl caprolactam (C₈H₁₃NO) to the procedure [44]. Then, 6.25 ml of an aqueous solution of ethylene di-amine tetra acetic acid (EDTA) at 3.3% was added to this suspension. A 15 m-mole solution of acetic

Figure 2.A schematic mechanism of the reaction of EDTA-functionalized silica-coated magnetite nanoparticles (Fe₃O₄@SiO₂/Thermosensitive/EDTA) [46].

acid is required to maintain the reaction medium's pH between 4.5 and 5. The molecularly imprinted (MI) magnetite (Fe₃O₄) nanoparticles were then vacuum dried at room temperature after being washed multiple times with ethanol [45]. Azo-bis-isobutyro nitrile was applied as an activator of radicals. As a result, the ideal ratio of silica coated magnetite to EDTA (2:1) is achieved for homogenous homogeneous functionalization of EDTA as shown in Figure 2 [46].

The ferromagnetic, super para-magnetic, half-metallic and ferro-fluid features of the y-Fe₂O₃ and Fe₃O₄ (iron oxide) nanoparticles give them incredible promise. With oxygen ions forming FCCs (Face-centered cubic closed packings), the iron oxide magnetite is a member of the class of super paramagnetic iron oxides. Tetrahedral and octahedral interstitial spaces are occupied by the Fe⁺ cations. In Figure 4, the electrons in the octahedral sites can jump between the Fe²⁺ and Fe³⁺ ions when their charge is high. The classification of ferrite as a sort of half-metallic substance and of dispersion particles as a ferro-fluid would follow [52].

Figure 4.Structure and Properties of magnetite (Fe₃O₄) nanoparticles [52].

Silicon is the most common material used to create functionalized iron oxide nanoparticles in contemporary nanotechnology. Silicon plays a key role in the development of magnetic nanoparticle coatings, both for fundamental studies and practical applications [54].

3.1.1. Magnetite Nanoparticles (Fe₃O₄-NPs) Coated with Silicon (SiO₂) Have the Following Characteristics

(a) Because of the magnetic dipolar affinity between the nanoparticles, silica-coated magnetic nanoparticles prevent acidic environments from leaching. In liquid mediums, the coating also prevents the magnetic dipolar attraction of nanoparticles.

(b) These composite nanoparticles have good biocompatibility, hydrophilicity, and stability.

(c) Magnetic nanoparticles coated with silica can also be used to form a surface with different

functional groups. The abundance of silanol groups on the silica layer adds to this result.

(d) Because the methods for creating adjustable composite nanoparticle shells are well-established, controlling them is not too difficult.

Figure 5.Flow chart of a typical sol-gel process of Silica (SiO₂) [55].

Studies demonstrated that the recovery of harmful material depositions from environmental samples and actual environments may be accomplished using magnetite nanoparticles (Fe₃O₄-NPs) functionalized with various functional groups. Fe₃O₄-NPs have been coated with silica using stober method are shown in Figure 5 [55].

3.1.2. Synthetic Approach of Silica (SiO₂)

Smaller, more uniform particles can be created using the micro-emulsion method and the quantity of iron oxide and magnetic characteristics can be controlled [56]. The Stober method shows the reaction sequences below.

It is possible to express the general TEOS as a precursor that result in the creation of silica particles in the sol-gel process as follows [57]:

Stober Method—Hydrolysis and Condensation Reactions

3.1.3. Sol-Gel Process

When TEOS molecules are hydrolyzed, silanol groups are created [58]. Silica (SiO₂) is made up entirely of siloxane bridges (Si-O-Si) created by condensation or polymerization of both silanol and ethoxy groups [59]. Silica particles are formed in two stages: nucleation and growth. To explain silica growth, several models have been put forth: the addition of monomers. The aggregation model emphasizes, in contrast, that nucleation happens continuously throughout the reaction and that the resulting particles (primary particles) quickly aggregate to generate dimmer, trimmer, and larger particles (secondaries). Figure 6 illustrates how both models result in a spherical or gel network depending on the reaction circumstances [60].

Figure 6.A Schematic representation of silica (SiO₂) formation by sol-gel process [60].

3.1.4. Morphological Study of Silica-Coated Magnetite (Fe₃O₄@SiO₂) Nanocomposites

Here in Figure 7, when silica coating is applied to composite NPs, the size of the particles increases and other characteristics, like the magnetic properties, alter as well. Using the Stober method and the sol-gel process, hydroxyl groups on the surface of iron oxide NPs, quantities of ammonia (NH₃) and tetraethoxysilane (TEOS), and water (H₂O), it was simple to create silica coatings in an aqueous environment. Additionally, as predicted, silica coatings of up to 200 nm could be produced by adjusting the ammonia concentration and TEOS to water ratios [61].

Figure 7. Structural representation of Silica-coated magnetite (Fe₃O₄@SiO₂) nanoparticles [61].

3.1.5. Specific Applications of Functionalized Silica-Coated Magnetite Nanoparticles

These studies have as their main objective the functionalization of iron oxide NPs with silica in order to produce adjustable magnetic composites with tunable architectures and good dimension stability. In this scenario, research findings have also been used to improve commercial products. In this case, great progress has been made in terms of silica-coated iron oxide (Fe₃O₄@SiO₂) nanoparticles bind with functional groups (RAFT) for target selective. This demonstrates that making silica functionalized iron oxide nanoparticles is a simple process having specific application with illustration in Figure 8 [62].

In order to shield iron oxide NPs from deterioration, pure single metal or non-metal shells made of gold, silver, platinum, palladium, iron, carbon, etc. can be oxidised. Control of the single-metal or non-metal layer has a significant impact on the uses of iron oxides, particularly in the fields of biomedicine, environmental remediation, and catalysts [63]. Gold is often used as a passivate to inhibit the oxidation of Fe₃O₄-NPs.

The saturation magnetization value of iron oxide NPs will be reduced by the single-metal functionalizations of gold, silver, and carbon; however, depending on the mass magnetic susceptibility value of the materials used, the single-metal functionalizations of cobalt, platinum, copper, and palladium may have the opposite effect [64]. Additionally, the reduction and repeat periods can be adjusted to produce iron oxide NPs with metallic or non-metallic functionalization at the required diameter. Two methods are often used to create the metal functionalized iron oxide NPs. The first method involves directly reducing the single-metal ions that are present on the surface of the iron oxide NPs.

Figure 8. Silica coated magnetite (Fe₃O₄@SiO₂) nanoparticles bind with functional groups (RAFT) for target selective [62].

Figure 9.Schematic representation for the synthetic approach of nanostructured core–shell magnetic surface imprinted polymers (Fe₃O₄@SiO₂@Toluene) [68].

The surface of a small molecule, polymer, or Silica functionalized iron oxide NPs is reduced to generate metallic functionalized iron oxide NPs in the second procedure [65]. Since the magnetic coating on the surface of magnetite is more stable under heat and chemical conditions more biocompatible than that of other non-metals. For their potential use as energy storage materials, a variety of magnetite nanocomposites covered with carbon have been created and thoroughly investigated [66].

There has been a tonne of research on functionalizing magnetite nanoparticles (Fe₃O₄-NPs) with metal oxides and metal sulphides as a result of the significance of their distinct physical and chemical properties. Mesoporous Titania microspheres with a thin anatase-structured shell were generated using a straightforward approach of surfactant poly (ethylene oxide) assisted nanoparticle assembly in non-aqueous solutions. In the investigation, it was discovered that the surfaces had hydroxyl groups on them and had a smaller band gap.

Applications involving surface multi-functionalization and photo-catalysis may benefit greatly from these findings. Researchers typically utilise ZnO, MgO, CaO, SnO₂, Al₂O₃, etc. for functionalizing magnetite nanoparticles (Fe₃O₄-NPs) [67]. Magnets are usually constructed using oxides of iron such as CoO, NiO, CoFe₂O₄, etc. TiO₂, ZnS, Y₂O₃, and other optical and electrically functional elements are used to functionalize magnetic nanoparticles (Fe₃O₄-NPs). Figure 9 illustrates coating on SiO₂ that is present with toluene as a functional group [68].

To reduce agglomeration and improve their ability to be used in biological and environmental applications, magnetic nanoparticles are frequently passivated with organic compounds during or after the preparation process. The organic coating on the surface of these particles not only prevents the hydrophobic interaction of magnetic nanoparticles that could result in agglomeration, but it also offers a functional site or a reactive site for a variety of impurities in inhibited materials, making the remediation process more precise and targeted.

Recently, a lot of research has been done on nanostructure materials with particular functional characteristics. Contemporary uses can be made of the magnetite NPs covered with organic molecules. These organic compounds functionalized with iron oxide nanoparticles (Fe₃O₄-NPs) revealed biocompatibility and biodegradability in addition to exhibiting the fundamental magnetism properties of magnetic nanoparticles are shown in Figure 10 [69].

Furthermore, functional reactive group ensembles can be provided by organic molecules. To use them, their groups must be able to connect to bioactive elements such as enzymes, antibodies, proteins, DNA, and so forth [70].

The synthesis of iron oxide NPs using organic, macro, polymer, and biological components has been investigated by researchers [71]. According to their surface characteristics, magnetic nanoparticles coated with tiny organic molecules can be classified as either oil-soluble, water-soluble, or amphiphilic. The surface of the oil-soluble kind of iron oxide NPs contains compounds that have a poor affinity for the solvent environment, typically fatty acids and alkyl-phenol. Chemical groups that are attracted to the solvent environment may be present in functionalized Fe₃O₄-NPs with water-soluble surfaces. Ammonium salts, polyols, and lycine may be among these compounds. Amphiphilic iron oxides NPs are those that have had their surfaces modified to include both hydrophilic and hydrophobic groups. This could result in concurrent hydrophobic and hydrophilic patches in the main chain of the functionalized nanoparticles which revealed in Figure 11 [72].

Figure 10.The representative structure of organic compounds functionalized magnetite nanoparticles (Fe₃O₄-NPs) (If iron oxide NPs were always assumed as the core) [69].

Figure 11. The different organic functional groups attached on surface of magnetite Nanoparticles (Fe₃O₄-NPs) [72].

In the same way that sulfuric lycine does, the nanoparticles also have water and oil soluble. Magnetic nanoparticles have been modified with a range of long chain fatty acids, phosphate, and phosphonate. Researchers proposed that alkyl phosphate and phosphonate coated magnetite nanoparticles (Fe₃O₄-NPs) help them achieve thermodynamically stable dispersion [73].

Due to the advantages that polymer coating offers strengthened repulsion forces in order to poise the magnetic and Vander Waals forces of attraction. So, proposed Polymer-functionalized magnetite nanoparticles (Fe₃O₄-NPs) have recently gained a lot of attention. Because of these benefits, polymer-coated magnetite nanoparticles (Fe₃O₄-NPs) have proven in a variety of applications. Due to their distinctive physical or chemical properties, Researchers have thoroughly investigated magnetite nanoparticles (Fe₃O₄-NPs) functionalized with polymers. Natural and artificial polymers can be grafted onto magnetite nanoparticles (Fe₃O₄-NPs). The magnetite nanoparticles (Fe₃O₄-NPs) have a natural polymer, CS, grafted onto their surface clearly shown in Figure 12 [74].

Figure 12.Functionalisation of the magnetite nanoparticles (Fe₃O₄-NPs) with chitosan polymer [74].

Researchers revealed that it is feasible to produce nanoparticles with certain qualities by carefully deciding on the passivating and activating polymers and/or reaction conditions [75]. Both synthetic and natural polymers can be coated with Fe₃O₄-NPs. Polymers are grafted to comprise the absence of tiny organic molecules and surfactants to produce surface functionalized magnetite nanoparticles (Fe₃O₄-NPs).

Now, there are two main ways to synthesize polymer-functionalised magnetite nanoparticles (Fe₃O₄-NPs). Producing monodisperse nanoparticles with precise forms and controlled compositions is one way to increase the spectrum of utilizing for functionalized-nanoparticles with polymer. Although there has been substantial progress, creating functionalized magnetite nanoparticles (Fe₃O₄-NPs) with enough chemical stability and high biocompatibility is still a difficult task. The creation of well-controlled functionalized nanoparticles can be made easier to a certain extent by selecting the right organic material [76].

By chemically attaching to the surfaces of magnetite nanoparticles (Fe₃O₄-NPs) via certain functional end groups biopolymers [77]; such protein polypeptide, antibody, biotin, and avidin can also bind to them directly or indirectly [78]. This makes them specific target [79]. Molecular imaging, drug delivery, and therapeutic nanoparticles are shown in a schematic manner [80]. For specialised diagnoses and treatments, it is possible to create nanoparticles that have been designed and optionally functionalized [81]. Their biocompatibility will be improved by functionalizing molecularly imprinted (MI) core-shell nanoparticles (Fe₃O₄@SiO₂-NPs) with biological molecules [82]. Magnetic nanoparticles (Fe₃O₄-NPs) are helpful in the separation of proteins, DNA, cells, biological products, and other materials [83][84]. The construction of a multifunctional/multimodal MNP combining shell coating and surface changes is shown in Figure 13 [85][86].

Figure 13.The scheme illustrates two commonly used strategies to fabricate multifunctional magnetic nanoparticles and their potential applications [85].

One of the most frequent ways for people to become exposed is thought to be by inhalation. The size of the particles appears to first determine how much dust and air-borne particles enter, deposit, or finally travel to other places. The finer grains can enter the bronchial tree, where they can be kept and continue to cause harm or depart from there to go to other sites. Typically, the larger particles are deposited in the nose, mouth, and larynx. Epidemiological research on miners and refinery employees exposed to metal particles has shown a connection between exposure and the occurrence of respiratory tract cancer (lung and nose cancer) [88]. The tendency for nickel miners to develop nasal cavity malignancies after exposure to nickel-containing dusts was first noted in 1933 [9] [89]. A particular particle appears to be responsible for the pattern of negative effects on the retention sites. For instance, chrysolite asbestos fibres deposition concentrates on the bifurcations of airways and causes “spots” of fibrosis after acute exposure; they manifest as emphysema in “coal workers” and as fibrotic nodules in chronic beryllium illness via lymphatic pathway along airways [90].

Figure 14.The ways of translocation of NPs in Human body [87].

The majority of research on the absorption, delivery, and distribution of NPs in and through the skin indicates that the associated toxicity appears to be minimal; typically, it is reported that the skin acts as a protective barrier and NPs through the skin can only reach the upper layers of the epidermis before permeating the area close to the hair follicles [91]. However, the skin is permeable to nanomaterials and provides holes on its surface through its hair follicles and glands, which can act as channels of entrance, particularly ideal for ultra-fine materials.

In fact, it has been shown that a number of NPs can penetrate the skin and the lymphatic system to enter the systemic vascular system [92]. Millions of individuals use a variety of personal care and cosmetic items every day that include nanoparticles, including shampoo, toothpaste, deodorant, soap, sunscreen, moisturiser, foundation, face powder, perfume, and eye shadow, to mention a few. According to reports, 33 million Americans use sunscreen products every day and many more do so sporadically, and a rising number of individuals are exposed to the sun every day [93].

These strange NPs, which are frequently combined with TiO₂, have the capacity to offer an effective, long-lasting UV barrier that is not unpleasant [94]. ZnO particles with a size less than 100 nm are employed to achieve better dispersion quality and a higher UV protection level [95]. Inhaling sunscreen spray can be a significant exposure method even if, unlike skin applications, spray preparations are typically unable to transcend the stratum cornea layers and may instead find a means to translocate to the essential organs.

ZnO-NPs can offer a significant risk when inhaled, and BEAS-2B cells treated to 20 nm-sized ZnO-NPs showed elevated expression of genes related to oxidative stress and apoptosis. Numerous researches on the toxicity of ZnO-NPs have focused on the harm done to cells, such as oxidative harm, cytotoxicity, and geno-toxicity [96]. In these situations, a lot of focus has been placed on the ZnO materials’ solubility since they have the potential to release free Zn²⁺ ions inside of cells, which may interact with various binding sites in proteins and enzymes and significantly alter their functions [97]. Additionally, it has been observed that human epidermal cells exposed to 0.8 g/ml of ZnO-NPs with a diameter of 30 nm suffered DNA damage.

Compared to other routes of entrance into the body, studies on the toxicity of NPs after consumption are less common [98]. Through the nose, through water, food, or medications containing NPs, NPs can enter the body directly into the digestive tract. The primary area of concern and anxiety with regard to ingestion is specifically related to engineered NPs, which are substances created with the intention of being added to various goods like foods or medications in order to accomplish particular functions [99]. As food additives and across several food processing industries, NPs are being employed more and more. Due to this, it is important to pay close attention to the chance that they could cause numerous injuries by travelling via the digestive tract to other target organs and the blood circulation system. Despite the fact that several studies claimed that ingested NPs are quickly removed from the intestinal system because of the ongoing regeneration of the epithelium, many others provide unmistakable proof that specific NPs are translocated to target organs [99].

In reality, it has been discovered that nanoparticles can enter the bloodstream and then travel throughout the body following absorption via the gastro intestinal route [101]. Certain NPs have really been discovered in a variety of organs after oral administration, including the liver, lymph nodes, bone marrow, lungs, kidneys, brain, and both the small and large intestines [102]. The size and morphological characteristics of NPs affect how well they are absorbed from the gastrointestinal tract, as they do for other tracts as well [103]. Positively charged particles are trapped in the negatively charged mucus layer, whereas smaller, more morphologically diverse NPs are better absorbed [104]. The liver, spleen, and lymph have been observed to absorb titanium particles of sizes between 150 and 500 nm that are more pronouncedly than those typically found in sunscreen creams [105].