Nanotechnology is a rapidly growing field due to its unique functionality and a wide range of applications. Nanomedicine explores the possibilities of applying the knowledge and tools of nanotechnology for the prevention, treatment, diagnosis and control of disease. In this regard, silver nanoparticles with diameters ranging from 1 to 100 nm are considered most important due to their unique properties, ability to form diverse nanostructures, their extraordinary range of bactericidal and anticancer properties, wound healing and other therapeutic abilities and their cost-effectiveness in production. The current paper reviews various types of physical, chemical and biological methods used in the production of silver nanoparticles. It also describes approaches employing silver nanoparticles as antimicrobial and antibiofilm agents, as antitumour agents, in dentistry and dental implants, as promoters of bone healing, in cardiovascular implants and as promoters of wound healing. The paper also explores the mechanism of action, synthesis methods and morphological characterisation of silver nanoparticles to examine their role in medical treatments and disease management.
Keywords: silver nanoparticles, biological synthesis, characterisation, antimicrobial agent, antibiofilm, health management activity
Considering the epitome of scientific discoveries and inventions ever since the advent of man on Earth, the emergence of nanotechnology is a relatively recent development. However, the last 30 years have been witness to the invention of nanotechnology in the 1980s and its rise to prominence during the early 2000s with wide commercial applications in various sectors. Materials with unique properties are downsized to the level of individual atoms and molecules, collectively called nanoparticles, generally ranging from 1 to 100 nm. Potential uses for these particles include commercial, industrial, agricultural and medicinal applications. Although nanoparticles have an identical chemical composition to the parent material, physical properties including colour, strength, magnetic and thermodynamic properties and other physical aspects may differ widely. The application of nanomaterials in medicine is also a recent venture with most applications still in the research and development stage.
However, certain materials, due to their exemplary medicinal properties, have been part of the medicinal domain since time immemorial. Silver (Ag), due to its extraordinary range of bactericidal properties and therapeutic abilities, has been a part of medical treatment and management of various diseases since ancient times. It is a well-recognised fact that silver ions and silver-based compounds have a great microbial killing capacity [1,2]. However, the development of technologies and a better understanding of the mechanism of silver in disease prevention via killing of microorganisms have opened the door towards their uses in nanomedicine. Many approaches and methods have evolved for the effective synthesis of silver nanoparticles, including physical, chemical and biological techniques. While physical and chemical methods are commercially more cost-effective, the biological methods are relatively less harsh on the environment [3].
In nanomedicine, silver nanoparticles are extremely important due to their attractive physicochemical properties and biological functionality, including their high antimicrobial efficiency and relatively non-toxic, wide spectrum of bactericidal properties [4], anticancer properties and other therapeutic abilities, their unique ability to form diverse nanostructures [5] and their relatively low manufacturing cost [6].
Silver nanoparticles are intensively explored nanostructures ranging between 1 and 100 nm, primarily used for unconventional and enhanced biomedical applications in such areas as drug delivery, wound dressings, tissue scaffolding and protective coating applications. Moreover, the impressive available surface of nanosilver allows the coordination of many ligands, thus enabling tremendous possibilities with respect to the surface functionalisation of silver nanoparticles. Silver is routinely used in the form of silver nitrate (NO3 − ) for antimicrobial activity. In addition, silver nanoparticles are more beneficial as compared to free silver because their greater surface area increases the exposure of microbes. Furthermore, silver nanoparticles have emerged as a great field of interest for researchers because of their unique activity against a large range of microorganisms and due to resistance against commonly used antibiotics [7]. To date, several studies have reported applications in fields such as food processing, agriculture and agro-based industries, biomedical and medical remediation, healthcare products, consumer products, numerous industries, pharmaceuticals, in diagnostics, orthopaedics, drug delivery, imaging, filters as antitumour agents and as enhancer of tumour-killing effects of anticancer drugs.
The current review summarises the important approaches for the synthesis of silver nanoparticles as well as their various roles as antimicrobial and antibiofilm agents, antitumour agents, in dentistry, bone healing, dental implants, cardiovascular implants and wound healing.
Several procedures are employed for the manufacture of silver nanoparticles, including physical, chemical and biological syntheses. It is worth noting that each method has its own advantages and disadvantages. During biological synthesis of silver nanoparticles, the organism acts as a capping agent, reducing agent or stabilising agent and reduces Ag + to produce Ag 0 [8]. Due to their low cost, high yields and low toxicity on the human body and the environment, biological methods based on natural products obtained from microorganism and plant sources have increased in popularity in recent years [9]. Different methods for synthesis of silver nanoparticles are described in the following sections.
Various methods are available to synthesise silver nanoparticles. Chemical methods are beneficial because the equipment required is more convenient and simple than that used in biological methods. It has already been reported that silver ions receive electrons from the reducing agent and become converted into the metallic form, which finally aggregates to form silver nanoparticles. Among the silver salts used in chemical synthesis of silver nanoparticles, AgNO3 is one of the most commonly used due to properties such as low cost ( Table 1 ) [10,11]. In 2002, Sun and Xia reported the synthesis of monodispersed silver nanocubes through reducing nitrate [12]. Mukherji and Agnihotri synthesised silver nanoparticles using AgNO3 as a precursor, and sodium borohydride and trisodium citrate as stabilising agents. It has been reported that sodium borohydride is a good reducing agent for the synthesis of silver nanoparticles having a size range of 5–20 nm. In comparison, trisodium citrate is the most effective reducing agent for the synthesis of silver nanoparticles of the size range 60–100 nm [13]. Polyvinylpyrrolidone (PVP) as a size controller and a capping agent, with ethylene glycol as a solvent and a reducing agent, is reported to give rise to silver nanoparticles with an average size less than 10 nm [14]. Patil et al. confirmed the synthesis of silver nanoparticles using hydrazine hydrate as the reducing agent and polyvinyl alcohol as the stabilising agent. Their results revealed that the resultant nanoparticles had a spherical morphology and these particles showed significant applications in biotechnology and biomedical science [15]. According to another important study, the synthesised silver nanoparticles were found to be spherical with different sizes [16].
Chemical methods for the synthesis of monodispersed and quasi-spherical silver nanoparticles [11]
| Reducing agent | Precursor agent | Capping agent | Experimental conditions |
|---|---|---|---|
| Trisodium citrate | Silver nitrate | Trisodium citrate | Diameter ≈ 10–80 nm; temperature ≈ boiling point |
| Ascorbic acid | Silver nitrate | Daxad 19 | Diameter ≈ 15–26 nm; temperature ≈ boiling point |
| Alanine/NaOH | Silver nitrate | DBSA (dodecylbenzenesulfonic acid) | Diameter ≈ 8.9 nm; temperature ≈ 90°C; time ≈ 60 min |
| Ascorbic acid | Silver nitrate | Glycerol/PVP | Diameter ≈ 20–100 nm; temperature ≈ 90°C |
| Oleic acid | Silver nitrate | Sodium oleate | Diameter ≈ 5–100 nm; temperature ≈ 100–160°C; time ≈ 15–120 min |
| Trisodium citrate | Silver nitrate | Trisodium citrate | Diameter ≈ 30–96 nm; temperature ≈ boiling point; pH ≈ 5.7–11.1 |
| Trisodium citrate | Silver nitrate | Trisodium citrate/Tannic acid | Diameter ≈ 10–100 nm; temperature ≈ 90°C |
The AgNO3 solution is heated to the reaction temperature in the precursor heating method and the nanoparticle size is observed to be most affected by the ramping rate, whereas in the precursor injection method, a silver nitrate aqueous solution is injected, and the reaction temperature is a key factor for the reduction of particle size and for achieving monodispersity [17]. High yield is the main advantage of chemical methods, compared to physical methods. Chemical methods are highly expensive, and chemicals and compounds used for silver nanoparticle synthesis such as borohydride, 2-mercaptoethanol, citrate and thio-glycerol are hazardous and toxic. It is extremely difficult to produce silver nanoparticles with a definite size and it requires an additional step to prevent particle aggregation [18]. Numerous hazardous and toxic by-products are produced during synthesis. Moreover, the reducing agents used in these methods are toxic [19].
Physical methods for the preparation of silver nanoparticles include evaporation–condensation and laser ablation. The main drawbacks of these methods are the huge amount of energy required, plus long duration for completion of the whole process.
Lee and Kang have reported that thermal decomposition of Ag + –oleate complexes results in the synthesis of monodispersed silver nanocrystallites [20]. In a study conducted by Jung et al., a small ceramic heater was used to prepare metal nanoparticles through evaporation/condensation processes. It was noticed that a constant temperature of the heater surface with time generated polydispersed nanoparticles. These silver nanoparticles were spherical and non-agglomerated [21]. Recently, it has been demonstrated that the polyol process produces spherical nanoparticles with different sizes under laser ablation [17,22]. To examine the effects of laser wavelength on the particle size, silver nanoparticles were synthesised through ablation with different lasers and it was noticed that decrease in laser wavelength reduced the average diameter of particles from 29 to 12 nm [23]. Nanosized particles of silver were prepared by Tsuji et al. through laser ablation in water to compare the formation efficacy and the size of colloidal particles produced by femtosecond pulses with colloidal particles produced by nanosecond laser pulses. The formation efficiency for femtosecond pulses was significantly lower than that for nanosecond pulses. Besides this, the size of colloids prepared via femtosecond pulses was less dispersed than that of colloids prepared by nanosecond laser pulses [24]. Seigal and colleagues examined the synthesis of silver nanoparticles through a direct physical deposition of metal into the glycerol. This approach was found to be a good alternative for time-consuming chemical processes. Furthermore, consequential nanoparticles were resistant to aggregation and had a narrow size distribution [25]. Speed, no requirement for toxic reagents and radiation utilised as a reducing agent are the advantages of physical methods of production. Solvent contamination, minimal yield, non-uniform distribution and high energy consumption are the disadvantages of physical methods ( Table 2 ) [26].
Physical and chemical syntheses of silver nanoparticles
| Type | Reducing agent | Biological activity | Characterisation | Ref. |
|---|---|---|---|---|
| Polydiallyldimethylammonium chloride and polymethacrylic acid capped silver nanoparticles | Methacrylic acid polymers | Antimicrobial | UV-Vis, reflectance spectrophotometry | [27] |
| Silver nanoparticles | Ascorbic acid | Antibacterial | UV-Vis, EFTEM | [28] |
| Chitosan-loaded silver nanoparticles | Polysaccharide chitosan | Antibacterial | TEM, FTIR, XRD, DSC, TGA | [29] |
| Silver nanoparticles | Hydrazine, d -glucose | Antibacterial | UV-Vis, TEM | [30] |
| PVP-coated silver nanoparticles | Sodium borohydride | — | UV-Vis, TEM, EDS, DLS, FIFFF | [31] |
Abbreviations: UV-Vis – ultraviolet-visible spectroscopy, FIFFF – flow field-flow fractionation, DSC – differential scanning calorimetry, TEM – transmission electron microscopy, EDS – energy-dispersive spectroscopy, EFTEM – energy filtered TEM, FTI R – Fourier transform infrared, DLS – dynamic light scattering, XRD – X-ray diffraction, TGA – thermogravimetric analysis.
Production of silver nanoparticles by physical and chemical processes is expensive, time consuming and eco-unfriendly. Hence, it is very important to develop an environmentally and economically friendly method, which does not involve toxic chemicals [32] and avoids the other problems associated with chemical and physical means of production. Biological methods fill these gaps and have various applications in health management through regulation of various biological activities. Biological production methods include the use of fungi, bacteria and yeasts as well as plant sources. These sources make this approach very popular for medical applications of nanoparticles.
It has been reported that nanoparticle production methods based on microorganisms and plants are safe, economic and are relatively less harmful to the environment than chemical synthesis [33,34]. Moreover, microorganisms and plants are able to absorb and accumulate inorganic metallic ions from their surrounding environment [35]. Biological production of silver nanoparticles mainly involves the use of microorganisms and plant sources ( Figure 1 ) [36].
Different biological methods for the synthesis of silver nanoparticles.
Recently, a study was performed to produce silver nanoparticles through the reduction of aqueous Ag + ions using the culture supernatants of various bacteria. This approach was demonstrated to be fast and the interaction of silver ions with the cell filtrate generated silver nanoparticles within 5 min. Moreover, this study also reported that piperitone partially inhibited the reduction of Ag + to metallic silver nanoparticles [37]. It is important to note that the nitro reduction activity of Enterobacteriaceae is inhibited by the natural product piperitone. It is assumed that the bioreduction of silver ions to silver nanoparticles might be partially inhibited by different strains of Enterobacteriaceae such as Klebsiella pneumoniae. Korbekandi and colleagues studied the optimisation of silver nanoparticle production by Lactobacillus casei subspecies casei, confirming the bioreductive synthesis of silver nanoparticles [38]. Liu et al. showed the formation of nanoparticles from dried cells of Bacillus megaterium [39]. Das et al. have described the extracellular synthesis of silver nanoparticles through a bacterial strain. The study showed that the treatment of Bacillus strain CS 11 with AgNO3 resulted in the formation of silver nanoparticles extracellularly [40].
Various types of fungi have been reported to be involved in the production of silver nanoparticles [41]. The production of silver nanoparticles by fungi has been found to be very quick. Many researchers have studied the biosynthesis of silver nanoparticles by fungi in detail [32]. One study has shown the extracellular biosynthesis of spherical silver nanoparticles by interaction of Fusarium solani with silver nitrate [42]. Syed and colleagues have reported the biosynthesis of silver nanoparticles by the Humicola sp. It was shown that a precursor solution was reduced by the interaction between Humicola sp. and Ag + ions and extracellular nanoparticles were produced [43]. Owaid and colleagues have reported the production of silver nanoparticles by the bioreduction of silver nitrate induced by the extract of Pleurotus cornucopiae [44]. Xue et al. conducted an experiment to biosynthesise silver nanoparticles with antifungal properties using Arthroderma fulvum [45]. Vigneshwaran et al. reported that the interaction of silver nitrate solution with the fungus Aspergillus flavus resulted in the accumulation of silver nanoparticles on the surface of its cell wall [46]. Furthermore, Bhainsa and D’Souza had investigated the extracellular biosynthesis of silver nanoparticles using Aspergillus fumigatus. The results indicated that the interaction of silver ions with the cell filtrate generated silver nanoparticles in a very short time [47]. However, using Fusarium oxysporum results in an extracellular production of silver nanoparticles with a size of 5–50 nm [48]. Additionally, incubation of Phanerochaete chrysosporium mycelium with silver nitrate solution produced silver nanoparticles [49]. Korbekandi and colleagues showed the bioreductive production of silver nanoparticles by using Fusarium oxysporum [50].
This approach is a feasible substitute for physical and chemical methods of nanoparticle production because it is economic and eco-friendly [51]. Furthermore, algae have a high capacity for metal uptake. It has been seen that biological sources such as marine algae have the capacity to catalyse specific reactions. This capacity is key to modern and realistic biosynthetic plans [52]. A study based on the algae extract has shown that the change of colour from yellow to brown can indicate the reduction of silver ions to silver nanoparticles. In addition, Rajeshkumar and colleagues noticed the deep brown colour of silver nanoparticles at 32 h and it was observed that the time of incubation was directly associated with the increase in colour intensity [53]. Silver nanoparticles were synthesised through the reduction of aqueous solutions of silver nitrate with powder and solvent extracts of Padina pavonia. Additionally, the achieved nanoparticles showed high stability, fast formation and small size [54]. Salari and colleagues reported the production of silver nanoparticles through bioreduction of silver ions induced by Spirogyra varians [55].
Yeasts have been reported to have the capability to produce silver nanoparticles. In addition, silver nanoparticle production methods based on yeast are cost-effective as well as eco-friendly. In this regard, Niknejad and colleagues performed a study that was based on Saccharomyces cerevisiae. It was noted that with increasing time of incubation, the colourless sample slowly turned to reddish-brown after adding Ag + ions to the yeast culture. Furthermore, the colour of the solution changed into strong reddish-brown [56]. In 2003, Kowshik et al. have reported the extracellular synthesis of nanoparticles through the interaction of soluble silver with a silver-tolerant yeast in its log phase of growth [57].
Like other biological methods, production in plants is better than chemical and physical methods because high temperature, energy and toxic chemicals are not needed and it is cost-effective and environment-friendly [58]. Numerous active constituents are present in Aloe vera leaves. These ingredients include lignin, hemicellulose and pectins, which have been shown to have a clear role in the reduction of silver ions [59]. In a recent study, silver nanoparticles were synthesised using an aqueous solution of the plant extract of Saudi Arabia Origanum vulgare L. The result demonstrated that synthesis of silver nanoparticles occurred by reduction of Ag + ions. During this process, the colour of the reaction mixture was converted from light brown to dark brown. On the other hand, in the absence of plant extract no change in colour was observed under the same conditions [60]. The results of another study reported that the colour of the aqueous silver nitrate solution was changed from faint light to yellowish brown after the addition of different concentrations of aqueous leaf extracts of Azadirachta indica [61]. López-Miranda et al. biosynthesised silver nanoparticles rapidly by using Tamarix gallica plant extract [62].
Chinnappan et al. have reported a fast and simple method for the synthesis of silver nanoparticles using an extract of Bauhinia purpurea flower [63]. In 2016, Ibraheim et al. reported the synthesis of silver nanoparticles from silver nitrate using aqueous pomegranate juice extract as a reducing agent and their results demonstrated that the use of juice extract leads to a quick synthesis of silver nanoparticles from AgNO3 solution. It was found that the colour changed from light yellow to reddish-brown with the formation of silver nanoparticles after exposure to microwaves for a few minutes [64]. Lakshmanan et al. synthesised silver nanoparticles using Cleome viscosa plant extract and the study revealed that extract of this plant has a good ability to reduce silver nitrate into metallic silver [65].
Prasad et al. employed aqueous leaf extracts of Moringa oleifera to develop a simple and quick method for bioreduction of silver nanoparticles. Their findings concluded that Moringa oleifera had a strong potential for synthesis of silver nanoparticles via rapid reduction of silver ions [66]. In this regard, another finding indicated a fast and convenient method for the synthesis of silver nanoparticles using Ficus benghalensis leaf extract and the reduction of silver ions into silver nanoparticles occurred within short periods (5 min) of reaction time without using any hard conditions [67]. Moreover, the treatment of aqueous solutions of silver nitrate and chloroauric acid with neem leaf extract leads to the fast synthesis of stable silver and gold nanoparticles at high concentrations [68]. Earlier investigators are accountable for pioneering nanoparticle synthesis through using plant extracts [60,68,69,70,71,72].
Ponarulselvam et al. concluded that extracts of the leaves of Catharanthus roseus could be used in the synthesis of silver nanoparticles that exhibited antiplasmodial activity against Plasmodium falciparum [73]. Some studies have reported that silver ions are reduced extracellularly in the presence of fungi to generate stable silver nanoparticles in water [42,74]. Zarghar and colleagues have indicated the formation of spherical silver nanoparticles by using methanolic leaf extracts of Vitex negundo and demonstrated the antibacterial activity of these silver nanoparticles against both Gram-positive and Gram-negative bacteria [75].
DNA can be used as a reducing agent for silver nanoparticle synthesis. High affinity of silver ions with DNA base pairs makes DNA a template stabiliser. Synthesised silver nanoparticles were found at N-7 phosphate and guanine base pair on DNA strand. Another study reported the synthesis of silver nanoparticles with calf thymus DNA [36,76].
Characterisation is an important step in the green synthesis of nanoparticles. It is a pivotal step to determine the morphology, surface chemistry, surface area and disparity in the nature of any silver nanoparticle. Various techniques are used for characterisation of silver nanoparticles ( Figure 2 ).
