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Metal nanoparticles are being extensively used in biomedical fields due to their small size-to-volume ratio and extensive thermal stability. Gold nanoparticles (AuNPs) are an obvious choice for biomedical applications due to their amenability of synthesis, stabilization, and functionalization, low toxicity, and ease of detection. In the past few decades, various chemical methods have been used for the synthesis of AuNPs, but recently, newer environment friendly green approaches for the synthesis of AuNPs have gained attention. AuNPs can be conjugated with a number of functionalizing moieties including ligands, therapeutic agents, DNA, amino acids, proteins, peptides, and oligonucleotides. Recently, studies have shown that gold nanoparticles not only infiltrate the blood vessels to reach the site of tumor but also enter inside the organelles, suggesting that they can be employed as effective drug carriers. Moreover, after reaching their target site, gold nanoparticles can release their payload upon an external or internal stimulus. This review focuses on recent advances in various methods of synthesis of AuNPs. In addition, strategies of functionalization and mechanisms of application of AuNPs in drug and bio-macromolecule delivery and release of payloads at target site are comprehensively discussed.
Keywords: gold nanoparticles, drug delivery, biological synthesis, functionalization, drug releaseNanotechnology is referred to the designing and application of components which occur at the nano-scale: up to 10–1,000 nm in size. 1 Nanotechnology encompasses the study of structural properties of nano-structures at the molecular and sub-molecular level along with their electrical, optical, and magnetic attributes. At present, nanotechnology is an interdisciplinary field which takes engineering, biomedicine, chemistry, and physics under one umbrella. 2 The application of nanomaterials in different fields ranging from oil and gas and cosmetics to nanomedicine has taken this world to the new era, which is the era of nanotechnology. 3 , 4 The best investigated nanostructures include carbon nanotubes, gold nanoparticles, liposomes, and paramagnetic nanostructures. 5–8 Gold colloids are now increasingly utilized in different fields like chemistry, biology, engineering, and medicine. In the biomedical field they have vast applications in diagnostics, therapy, and immunology. 9
Gold nanoparticles provide an outstanding material for study due to the fact that they are one of the most stable, non-toxic, and easy to synthesize nanoparticles and exhibit various fascinating properties like assembly of various types and quantum size effect. 6 The optical behavior of gold nanoparticles is dependent on their surface plasmon resonance (SPR), located in a wide region ranging from visible to the infrared region of the spectrum, which is determined by collective oscillation of conducting electrons. The range of the spectrum depends on various features of gold nanoparticles, including size and shape. 9 Methods have been developed to synthesize these materials reproducibly, which can further be modified using countless chemical functional groups. Many new sensitive and specific assays are based on the gold nanoconjugates.
Gold nanoparticles have emerged as an excellent candidate for the application in delivery of various payloads to the target site. 10 , 11 These payloads range from small drug molecules including drugs to large biomolecules like DNA, RNA, and proteins. Some drugs molecules do not require modification of a monolayer of gold nanoparticles for their delivery and can be directly conjugated with gold nanoparticles through physical absorption and ionic or covalent bonding. 12 Whereas for the delivery of other payloads, gold nanoparticles require functionalization like PEGlyation, 13 peptide and amino acid conjugation, 14 , 15 or functionalization with oligonucleotides. 16 Apart from that, another prerequisite for the efficient delivery of therapeutic agents is their release. Various internal stimuli (glutathione, pH and enzymes) 17–19 and external stimuli (light, etc.) 20 have been investigated for the efficient release of these payloads from gold nanoparticles.
Due to the vast amount of information available and the level at which it is being renewed we have chosen the generalized data from the past few years to present this review encompassing the most promising application of gold nanoparticles in drug delivery.
For the synthesis of AuNPs, there are two basic strategies that are used, which are “Top-Down” and “Bottom-Up” approaches. The top-down approach involves the synthesis of AuNPs starting from bulk material and cracking it into nanoparticles using different methods. In contrast, the bottom-up approach synthesizes nanoparticles starting from atomic level. Figure 1 shows the basic steps that are involved in the top-down and bottom-up approaches. Synthesis methods that involve the top-down approach include laser ablation, 21 ion sputtering, 22 UV and IR irradiation, 23 , 24 and aerosol technology, 25 whereas the reduction of Au 3+ to Au 0 is the bottom-up approach.
Top-down and bottom-up approaches for the synthesis of NPs. The top-down approach involves the transformation of bulk material by using energy to produce the powder form which is then transformed into smaller fragments with multiple layers and then to the monolayers leading to the formation of nanoparticles. On the other hand, the bottom-up approach uses the precursor molecules which are then ionized by using energy. Radicals, ions, and electrons thus produced are condensed to form clusters which are then transformed to nanoparticles.
The formulation of AuNPs involves two main stages:
In the first stage the gold precursor, which is usually an aqueous gold salt solution, is reduced to gold nanoparticles using a specific reducing agent like citrate.
In the second stage the stabilization of gold nanoparticles is done by a specific capping agent. The capping agents hinder the agglomeration of metallic nanoparticles.
This method for the synthesis of AuNPs was first reported in 1951. It is one of the most commonly used techniques for formulation of spherical AuNPs. AuNPs prepared using this method have the size in the range of 1–2 nm. 26 The basic principle of this technique involves the reduction of gold ions (Au 3+ ) to produce gold atoms (Au 0 ) by using some reducing agents like amino acids, 27 ascorbic acid, 28 UV light, or citrate. 29 , 30 Stabilization of AuNPs is carried out by using different capping/stabilizing agents. At the beginning, the applications of Turkevich method were finite because of the limited range of AuNPs that could be synthesized by this technique. With the passage of time several advancements in the basic method have enabled researchers to extend the size range of particles synthesized using this method. In 1973, it was established that by varying the ratio of reducing as well as stabilizing agents, AuNPs of particular size with the range from 16–147 nm can be produced. 31–33 Figure 2A shows the basic method involved in the Turkevich method.
(A) Turkevich method for the synthesis of AuNPs. (B) Series of steps involved in the Burst method for the synthesis of AuNPs.
This method was first reported in 1994 and involves a two-phase reaction to synthesize AuNPs with the size range of 1.5–5.2 nm by using organic solvents. 34 The method encompasses the use of a phase transfer such as tetraoctylammonium bromide to carry out transferring of gold salt to organic solvent from its aqueous solution (eg, toluene). The gold is then reduced by the use of a reducing agent such as sodium borohydride along with an alkanethiol. The alkanethiol carries out the stabilization of AuNPs. 35 As a result of this reaction the color changes from orange to brown. 34 , 36 Figure 2B shows the schematic illustration of main steps involved in Brust method.
The previous two methods can synthesize only spherical AuNPs; however, they can also be formulated in a number of geometries and shape such as rods. 37 , 38 The most commonly used technique to synthesize rod shaped AuNPs is seed-mediated growth. This method is based on the fundamental principle which involves first synthesizing seed particles by reducing gold salts. This reaction is done in the presence of reducing agents like NaBH4. The next step involves the transferring of the seed particles to a metal salt and a weak reducing agent like ascorbic acid which prevents further nucleation and speeds up the synthesis of AuNPs of rod shape. Shape and geometry of gold nanoparticles depends on the concentration of reducing agents and seeds. Figure 3A shows schematic illustration of seed-mediated growth of short and long gold nanorods as reported by a study. 39
Series of steps involved in the synthesis of AuNPs. (A) Seed-mediated method. (B) Digestive ripening method.
Digestive ripening is considered to be a convenient method to prepare monodispersed gold nanoparticles in the presence of excessive ligands (digestive ripening agents). The basic process comprises heating a colloidal suspension at high temperatures (~138ºC) for 2 minutes and then heating at 110ºC for 5 hours by using alkanethiol, as shown in Figure 3B . Temperature is the major factor for determining the size distribution of the gold colloids. 40
In addition to these methods, other methods involve the use of ultrasonic waves for the synthesis of AuNPs. 41 , 42
Turkevich method is a fairly uncomplicated and reproducible procedure for the formulation of spherical particles with the size range 10–30 nm. But, as the size of particles increases above 30 nm, they become less spherical in shape with broader size distribution. Moreover, this reaction gives low yield and involves the use of only water as a solvent. 43 Brust method, on the other hand, involves an easy strategy for the formulation of thermal and air-stable AuNPs having controlled size and less dispersity. One possible limitation of Brust method is synthesis of AuNPs which are less dispersed and used of organic solvents immiscible with water, therefore, limiting their biological applications. 44 Seed-mediated growth is a reliable method for the synthesis of rod-shaped AuNPs, but various factors affect the size of rod and so must be carefully controlled. In a study when higher concentrations of HAuCl4 were used it produced bigger seed rods with smaller aspect ratios. Temperature also plays a significant role in the synthesis of rods and at higher temperatures rods with lower aspect ratio were produced. Also, the number of seeds added to the reaction mixture must be critically considered to stimulate the growth of rods. 45 The digestive ripening method is also an easy and valuable chemical technique to produce monodispersed nanoparticles. Another benefit of this strategy is the high yield of nanoparticles. 46 A possible disadvantage of the digestive ripening method is that controlling the shape of nanoparticles via the digestive ripening process is difficult as it involves very high temperatures. 47
In addition, chemical methods are inherited with their own limitations which include environmental and biocompatibility concerns. Some of the chemicals that are used in the synthesis of gold nanoparticles during chemical synthesis can affect our environment and are the cause of risks for administering them into the living organisms, thus limiting the biological applications of such NPs. 48 Therefore, various biological methods have been devised for the synthesis of AuNPs to limit these concerns.
Recently, efforts have been made for biological synthesis of AuNPs, which is a clean, dependable, and bio-friendly alternative to harsh chemicals used in chemical synthesis reactions. The biological resources used in synthesis of nanoparticle range from simple bacterial cells to complex eukaryotes. Interestingly, the capability of organisms in synthesis of metal nanoparticles has given rise to a new thrilling approach toward the development of these biological nano-factories. 49 A plethora of organisms have been reported to carry out successful synthesis of AuNPs, ranging from bacteria to plants, algae, and fungi.
Microorganisms have been reported to be an excellent candidate for the synthesis of both intracellular and extracellular AuNPs. 50–52 The negatively charged cell wall of bacteria can electrostatically interact with positively charged Au(III) ions. During the intracellular synthesis, gold ions are transported inside the cell where enzymes and biomolecules carry out the synthesis of AuNPs. On the other hand, during extracellular synthesis the gold ions are trapped on the cell membrane by membrane enzymes. These enzymes on the membrane or reductase enzymes secreted out by the microbial cell can carry out the synthesis process outside the bacterial cell. 53 Extracellular synthesis, however, is more fascinating as it does not require extra downstream processing steps which are required for the separation of nanoparticles from the intracellular matrix. A study has shown that, during the extracellular synthesis reaction NADPH-dependent enzymes are secreted by bacteria which can reduce Au(III) ions to Au 0 such as nitrate reductase secrete by Pseudomonas denitrificans. The results showed that the action reductase enzyme diminished once AuNPs had been synthesized. 54 Shah et al 55 reported that both NADH and NADH-dependent enzymes function as a scaffold or nucleating agent for the synthesis reaction. Singh et al reported that Rhodopseudomonas capsulate secreted NADH and NADH-dependent enzymes during extracellular synthesis of AuNPs. The transfer of electrons from NADH carried by NADH-dependent enzyme causes the reduction of Au(III) to Au 0 , resulting in the synthesis of AuNPs. 51 Thermomonospora sp. (Order: Actinomycetes) was reported to carry out intracellular enzymes mediated synthesis of AuNPs by achieving the reduction of Au(III) ions at the surface of membrane and mycelia. 56 Similarly, Shewanella algae efficiently carried out enzymes mediated bioreduction of AuCl 4− ions to AuNPs which were found to be dispersed in periplasmic membrane of bacterium. 57 Certain materials produced by microbial cells like proteins, enzymes, and organic substances can act as capping agents to stabilize nanoparticles and, hence, prevent their agglomeration. 58 Micro-organisms possess certain reductase enzymes which can reduce metal salts to metal nanoparticles with narrow size distributions and monodispersity. By altering the essential growth parameters, the shape and size of AuNPs can be controlled. Synthesis of AuNPs using bacteria is a tedious reaction and requires additional precautionary measures while handling bacteria, and also takes hours and days as bacterial cultural is a time consuming process. These drawbacks have limited the use of bacteria for the synthesis of AuNPs. 59
Fungi have also been used as a biological source for the synthesis of AuNPs. Fungi secrete a number of biomolecules, including metabolites and extracellular enzymes, such as hemicellulose, acetyl xylem esterase, 3-glucanase, cell wall lytic enzyme β-1, etc., which have been reported to play a role during the synthesis of metallic nanoparticles. 60 Numerous studies have reported the synthesis of gold nanoparticles using unicellular and multicellular fungi. 61 , 62 A fungal species Fusarium oxysporum was used in a study for the extracellular synthesis of Au-Ag alloy NPs by the reduction action of nitrate-dependent enzyme and shuttle quinone. 63 A fungal species Verticillium has also been reported to carry out intracellular synthesis of AuNPs. AuNPs were found to be trapped in the cell membrane and the cell wall of fungi, indicating that Au 3+ ions were bio-reduced by the reduction action of reductase enzymes in fungi. 64 A study on the biosynthesis of AuNPs from Phanerochaete chrysosporium proved that laccase was the enzyme secreted by the fungi for extracellular synthesis of AuNPs and, for intracellular synthesis, ligninase was found to be responsible. 65
Phytonanotechnology has gained attention with time as it comprises an eco-friendly, cheap, and rapid process for the synthesis of nanoparticles. A number of studies have reported biosynthesis of AuNPs using different plants or plant extracts involving the use of harmless bio-components from plants to carry out the reduction and capping of AuNPs, reducing the waste generation and limiting the requirement for additional purification steps. Numerous bio-components present in plants such as flavonoids, phytosterols, quinones, etc., play a role in the synthesis of AuNPs because of the functional groups which speed up the reduction and stabilization of AuNPs. 66 Although nearly every part of plants has been reported to successfully carry out the synthesis of AuNPs, leaves are most commonly used. The difference in the level of various compounds present in different plants and even in different parts of a plant affects the synthesis of AuNPs. For example, a study has reported the effect of difference in level of phenolic contents present in leaves and fruit of Garcinia mangostana plant on the synthesis of AuNPs. As the leaves are rich in phenolic content so the rate of synthesis of AuNPs was faster in the presence of leaves than fruit. 67 , 68 Moreover, recently the synthesis of gold nanoparticles using medicinal plant Acorus calamus and Cassia auriculate has been reported. 69 , 70
Reactive compounds; Lignans [(+)-pinoresinol, (+)-medioresinol], alkaloids, flavonoids, steroids (sitosterol-3-0-glucoside), and terpenoids present in the leaves of Justicia glauca have been reported to complete the synthesis reaction of AuNPs in 1 hour. AuNPs had spherical and hexagonal morphology and were 32 nm in size. 71 Leaves of the Terminalia arjuna plant also carried out the synthesis of AuNPs within 15 minutes. AuNPs synthesized in this study were 20–50 nm in size and had spherical morphology. The author claimed that the reactive compounds Arjunetin, leucoanthoc-yanidins and hydrolysable tannins present in leaves of Terminalia arjuna contributed to the synthesis of AuNPs. 72 Similarly, the leaves of olive plant and Cassia auriculata were shown to complete the synthesis reaction of AuNPs in 20 minutes and 10 minutes, respectively. The active metabolites and biomolecules in the leaves of the olive plant are proteins, oleuropein, apigenin-7-glucoside, and luteolin-7-glucoside, which resulted in the formation of spherical and anisotropic AuNPs with the size range of 50–100 nm. 73 Polysaccharides and flavonoids are the major active substances in the leaves of Cassia auriculata and AuNPs synthesized from leaves of this plant were 15–25 nm in size and had spherical and anisotropic morphology. 70 Mangifera indica leaves used by Philip 74 synthesized spherical AuNPs within 2 minutes of reaction time. The size of AuNPs was found to be in the range of 17–20 nm. Terpenoids, flavonoids, and thiamine are the active compounds present in mango fruit, which might have contributed to the synthesis of AuNPs.
Apart from leaves, various other parts of plants, including fruits, roots, stems, etc., have been used for the synthesis of AuNPs. The fruit of Citrus maxima was used in one study and synthesized spherical AuNPs with the size range of 15–35 nm within 5 minutes of reaction time. Proteins, terpenes, and ascorbic acid were the major compounds that were claimed to act as reducing agents during reaction. 75 The high phenolic content of Sambucus nigra (elderberry) was the major factor in the synthesis of AuNPs. 76 Apart from that, flowers of Lonicera Japonica contain amino acids as active compounds and successfully synthesized AuNPs of triangular and tetrahedral morphology with the size range of 8 nm in the reaction time of 1 hour. 77 Similarly flowers of the Moringa oleifera plant synthesized AuNPs of size 3–5 nm. This plant was reported to contain a high content of flavonoids, carotenoids, phenols, sterols, and amino acids, which were claimed to be responsible for carrying out the reduction reaction during the synthesis process. 78 Various types of roses have been demonstrated to possess the reducing ability for the synthesis of AuNPs. 79 , 80 Similarly, banana and mango peels can synthesize AuNPs with the sizes 50 nm and 6.03±2.77 to 18.01±3.67 nm, respectively. Banana peels synthesized spherical shaped AuNPs and mango peel synthesized quasi-spherical shaped AuNPs. The reaction time for both processes was 20 and 25 minutes, respectively. 81 , 82 Apart from the above-mentioned parts of plants, rhizomes of turmeric, 83 yam beans, 84 ginger, 85 and seeds of cocoa, 86 pulp of green pepper, 87 bark of bay cedar, 88 galls of zebra wood, 89 latex of Hevea brasiliensis, 90 nuts of Areca catechu, 91 and effluent from palm oil mill 92 were found to carry out the synthesis of AuNPs.
There are a few studies which have demonstrated the synthesis of gold NPs using algae. A few species of both marine and fresh algae were used in these studies. Among the marine red algae, Gracilaria corticata, 93 Acanthophora spicifera, 94 and Galaxaura elongata, 95 and marine brown algae, Stoechospermum marginatum, 96 Ecklonia cava, 97 Sargassum wightii, 98 Cystoseira baccata, 99 Laminaria japonica, 100 and Turbinaria conoides 101 have been previously reported to carry out the synthesis of AuNPs. On the other hand, biomass from freshwater algae including Prasiola crispa, 102 Lemanea fluviatilis, 103 and Chlorella pyrenoidusa 104 can also synthesize AuNPs. Previous studies have shown that hydroxyl and carbonyl groups present in algal biomass can act as reducing agents for carrying out the synthesis of AuNPs. It has also been shown that these group can also act as the capping agent for gold nanoparticles. 105–107 Table 1 shows the list of various organisms that have been reported to carry out successful synthesis of AuNPs.
Various Types of Living Organisms That Can Carry Either Intracellular or Extracellular Synthesis of AuNPs
| Name of Organism | Intra/Extracellular | Reaction Type | References |
|---|---|---|---|
| Bacteria | |||
| Deinococcus radiodurans | Extra/intra | Reduction | [ 311 ] |
| Bacillus cereus | Extra | Reduction | [ 50 ] |
| Pseudomonas aeruginosa | Extra | Reduction | [ 312 ] |
| Rhodopseudomonas capsulate | Extra | Reduction | [ 51 ] |
| Rhodococcus | Intra | Reduction | [ 313 ] |
| Marinobacter pelagius | Extra | Reduction | [ 52 ] |
| Bacillus megaterium | Extra | Reduction | [ 112 ] |
| Fungi | |||
| Neurospora crassa | Intra | Reduction | [ 61 ] |
| Trichothecium sp. | Extra/Intra | Reduction | [ 314 ] |
| Candida albicans | Extra | Reduction | [ 315 ] |
| Penicillium brevicompactum | Extra/Intra | Reduction | [ 316 ] |
| Algae | |||
| Rhizoclonium fontinale | Intra | Reduction | [ 317 ] |
| Sargassum wightii | Extra | Reduction | [ 98 ] |
| Tetraselmis kochinensis | Intra | Reduction | [ 318 ] |
| Prasiola crispa | Extra | Reduction | [ 102 ] |
| Shewanella | Extra | Reduction | [ 319 ] |
| Plants | |||
| Magnolia kobus | Extra | Reduction | [ 320 ] |
| Sesbania drummondii | Intra | Reduction | [ 321 ] |
| Coriandrum sativum | Extra | Reduction | [ 322 ] |
| Tanacetum vulgare | Extra | Reduction | [ 323 ] |
| Abelmoschus esculentus | Extra | Reduction | [ 324 ] |
Molecules synthesized by living organisms to speed up their biological processes of the body are known as biomolecules and these include macromolecules such as amino acids, nucleic acids, carbohydrates, and lipids. Previous studies have reported the synthesis of gold nanoparticles using chitosan which does not only act as a reducing agent but also as a stabilizing agent during synthesis reaction. 108 Apart from that, starch is another polysaccharide used for the synthesis of AuNPs. In an alkaline environment starch can be degraded into short chains having carboxyl groups and the –OH group of carboxylic acid can reduce Au 3+ ions to gold nanoparticles. 109 Among proteins, consensus sequence tetratricopeptide repeat proteins and corn protein, α-zein can be used to carry out the synthesis reaction of AuNPs. 110 , 111 The biological method of synthesis of AuNPs can conveniently overcome the complications of biosafety of the chemicals used for the generation of AuNPs.
Synthesis of AuNPs using biomass from bacteria is an advantageous process as some species of bacteria are not affected by the presence of heavy metals. Also, the extracellular synthesis approach produces pure nanoparticles as compared to the intracellular synthesis process which requires additional purification steps. Conversely, culturing of bacteria is a slow and tedious process so the synthesis reaction of AuNPs can take a long time comprising hours and even days. On the other hand, fungi produce a large quantity of proteins and reactive compounds. Therefore, the reaction process can be easily scaled up. 112–114 Moreover, as compared to bacteria it is easier to culture and grow fungi. But preparing biomass from fungi for the synthesis reaction requires careful steps as it is complicated to separate mycelia from culture filtrates. Manipulation of the genetic makeup of eukaryotes to produce desired proteins is also challenging. Also, some species of fungi are pathogenic. 115–117 Synthesis of AuNPs using plants based material is a facile and uncomplicated process. Various attributes of AuNPs such as shape and size can be regulated by controlling the reaction parameters. Additionally, the reaction process is fast and economical. The disadvantage of using plants for the synthesis of AuNPs is that the identification of reactive components is difficult as plant biomass comprises a large number of organic components. 118–120 Synthesis of AuNPs from algal biomass is also easy and simple, but algae take a lot of time to grow so the overall process can become tedious and time consuming. Biomolecules on the other hand contains various functional groups which can aid in the synthesis of AuNPs. Contrarily, as different biomaterials show different reducing ability it is imperative to first determine their reducing ability before using them in the synthesis reaction. 110 , 111 , 121 , 122
Nanoparticles can be stabilized using a stabilizing agent which basically assists in maintaining repulsive forces to overcome Wan der Vaal forces in the solution of nanoparticles to avoid agglomeration. 123 During the chemical synthesis of AuNPs sodium borohydride or sodium hydride, sodium citrate or ascorbic acid may act as capping and stabilizing agents for AuNPs. However, during the biological synthesis of AuNPs, stabilization of nanoparticles can be successfully achieved by using the bio-material rich in antioxidant properties. The large variety of reactive compounds present in the biomass can take part in the synthesis and stabilizing process. Various studies have reported the synthesis of highly stable AuNPs via green approach. AuNPs synthesized from Actinidia deliciosa showed a zeta potential value of −22.3 mV, 124 whereas two different types of AuNPs synthesized from Cannabis sativa showed zeta potential values of −12.3 mV and −20.6 mV. 125 The high values of zeta potential mean that AuNPs are highly stable due to the presence of high surface charge which prevents agglomeration. Various studies have reported that phenolic compounds, 126 terpenoids, 127 , 128 proteins, 129 and nicotinamide adenine dinucleotide 54 might act as stabilizing and capping agents during the biological synthesis of AuNPs.
Moreover, changing the concentration of gold salt used for the synthesis reaction, pH, and temperature can also provide control over the size and geometry of AuNPs. Derjaguin Landau VerweyOverbeek theory (DLVO) explains the whole process for stabilization of metallic nanoparticles. 130 , 131 The stabilization of NPs done by using various capping agents can be divided into three different categories, including steric, electrostatic, and unification of steric and electrostatic stabilization. 132
Ionic groups present in the liquid dispersion media can attach to the surface of a colloidal nanoparticle giving rise to a charged layer. As a result, an equal number of oppositely charged ions will border the colloidal nanoparticles giving rise to overall electro-neutral double layers.
This stabilization which involves an electric double layer originating from the presence of both repulsive as well as attractive forces between the nanoparticles as a result of the action of some ionic composites is shown in Figure 4A . These ions include polyoxyanions, carboxylates, as well as fluorides. This type of stabilization involving electrostatic repulsions inhibits the agglomeration of nanoparticles in the solution phase. Electrostatic stabilization is regulated by controlling certain significant variables including pH, concentration, and temperature. 133
