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REVIEW ARTICLE
Year : 2016  |  Volume : 7  |  Issue : 3  |  Page : 162-165

Silver nanoparticles in dentistry: An emerging trend


Department of Prosthodontics, Swami Devi Dyal Hospital and Dental College, Panchkula, Haryana, India

Date of Web Publication22-Aug-2016

Correspondence Address:
Palwinder Kaur
Department of Prosthodontics, Swami Devi Dyal Hospital and Dental College, Barwala, Panchkula, Haryana
India
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DOI: 10.4103/0976-433X.188808

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  Abstract 

Nanotechnology, the term currently revolutionized the research field associated to particles at nanometer scale (1–100 nm). Nanoparticles (NPs) can occur naturally be industrially engineered or exist as byproducts. With their greater surface to volume ratio, these materials are more reactive as compared to non nanoscale particles. This unique property makes these materials as fillers/modifier of choice in different products and materials, whereby they play a vital role in improving the properties. Silver NPs find use in many devices that are used in medical procedures, in therapies and molecular diagnostics, including dentistry. However, the recent studies showed concern about environmental and health associated risks with their use.

Keywords: Antimicrobial agent, nanoparticles, toxicity


How to cite this article:
Kaur P, Luthra R. Silver nanoparticles in dentistry: An emerging trend. SRM J Res Dent Sci 2016;7:162-5

How to cite this URL:
Kaur P, Luthra R. Silver nanoparticles in dentistry: An emerging trend. SRM J Res Dent Sci [serial online] 2016 [cited 2019 Sep 21];7:162-5. Available from: http://www.srmjrds.in/text.asp?2016/7/3/162/188808


  Introduction Top


Silver nanoparticles (AgNPs) are nanometer-sized particles of silver that are <100 nm in size. Over the years, silver ions and silver compounds are known to have antibacterial activity. With silver NPs, there is an enormous increase in the surface area for microbe to act on, so improving antibacterial activity. There are various theories of antibacterial effect of silver NPs. Silver ions are used in the formulation of dental resin composites; in coatings of medical devices; as a bactericidal coating in water filters; as an antimicrobial agent in air sanitizer sprays, pillows, respirators, socks, wet wipes, detergents, soaps, shampoos, toothpastes, washing machines, and many other consumer products; as bone cement; and in many wound dressings to name a few.[1] The aim of this article is to review the synthesis, mechanism of action, application of silver NPs in the dental field.


  Production of Silver Nanoparticles Top


Silver NPs can be synthesized by physical, chemical, and biological methods. Silver NPs can be produced by top-down and bottom-up methods. The top-down method involves the mechanical grinding of bulk metals and subsequent stabilization of the resulting nanosized metal particles by the addition of colloidal protecting agents.[2],[3] The bottom-up methods, on the other hand, include reduction of metals, electrochemical methods, and sonodecomposition. The simplest method involves the chemical reduction of the metal salts AgBF4 by NaBH4 in water. The NPs thus obtained are of size range between 3 and 40 nm.[4] There is the electrochemical method that involves the electroreduction of AgNO3 in aqueous solution in the presence of polyethylene glycol with NPs of 10 nm in diameter.[5] Sonodecomposition, to yield silver NPs of 20 nm in diameter, involves the usage of ultrasonic waves to induce cavitation, a phenomenon whereby the passage of ultrasonic waves through an aqueous solution yields microscopic bubbles that expand and ultimately burst.[6]

Silver NPs synthesis also involves the reduction of silver NPs using variable frequency microwave radiation. This is a faster method and gives higher concentration of silver NPs of size 15–25 nm.[7] Another method of synthesizing NPs is using aqueous foams as a template. The NPs so formed are aerosol stabilized and of 5–40 nm in diameter.[8] The physical and chemical methods employed the use of toxic, hazardous chemicals which create environmental and biological health concerns. Hence, the alternative would be the use of biological methods that are less expensive and environmentally safe. Biological methods mainly employed fungi, bacteria, and plant extracts. The production by fungi involves trapping of Ag + ions at the surface of the fungal cell and then reduction of silver ions by extracellular enzymes such as NADPH-dependent nitrate reductase for, example, Fusarium oxysporum, Fusarium semitectum, Aspergillus fumigatus, Aspergillus flavus, Aspergillus clavatus, etc.[9],[10],[11],[12],[13] The first evidence of bacteria synthesizing silver NPs was established using the Pseudomonas stutzeri AG259 strain that was isolated from silver mine.[14] The mechanism recognized for silver NPs synthesis in bacteria is the reduction of nitrate to nitrite by nitrate reductase enzyme. The silver ion thus is reduced to silver (Ag + to Ag 0) using the transferred ion. This process is found in Bacillus licheniformis that effectively convert Ag + to Ag 0.[15]

Plant extracts are also used for reducing silver ions. The main benefits are ease of availability, safe, nontoxic, and are faster than microbe in biosynthesis of silver NPs. The clear-cut mechanism in plant synthesis is varied as the phytochemicals varied in each plant species though the main method is a reduction of ions. Phytochemicals such as terpenoids, flavones, ketones, aldehydes, amides, and carboxylic acids are assisted in the reduction of the ions and thus formation of silver NPs.[16]

Antimicrobial action

At the present time, silver-based compounds are recognized to be toxic to microorganisms. This has led to the use of various medical systems that release these silver ions to attain antimicrobial activity. Reducing the size of silver particles, i.e. NPs is an effective way of increasing its efficacy as it augments the relative contact area. The mechanism of action of silver ions is a controversial topic. The various theories discussed in the literature are listed here. Silver ions (Ag +) bind to electron donor groups in biological molecules containing sulfur or nitrogen, resulting in defects in the bacterial cell membrane and leading to loss of their cell contents and to the death of bacteria.[17] Another mode is free radical formation by the silver NPs when in contact with bacteria, and these radicals have the ability to damage the cell membrane and make it porous which can ultimately lead to cell death.[18] Studies also stated that silver interact with sulfhydryl groups of proteins and with DNA, altering hydrogen bonding, respiratory processes, DNA unwinding, cell wall synthesis, and cell division.[19]

The use of silver nanoparticles in dentistry

The existing standard of oral preventive programs includes the use of antimicrobial mouthwashes, proper toothbrushing techniques, and the prophylactic antibiotics. However, it has little success because of the development of antibiotic resistant strains of bacteria and side effects due to poor patient compliance. Hence, the need of the hour is the development of broad spectrum antimicrobial that can be effectively incorporated into the dental biomaterials. The antimicrobial nature of silver NPs is well-documented in medical literature since the nineteenth century. In dentistry, there is a recent interesting trend in using silver ions in the form of silver NPs. Current uses of these silver NPs in dental field are explored here.

Stomatitis represents a challenge for the dentists, and the incorporation of silver NPs into polymers used as denture base and tissue conditioners have been an alternative solution. Studies have advocated the incorporation of silver NPs in the denture base polymers as an effective antimicrobial agent that control oral infections. In one study, after adding a silver nanoparticle suspension to the denture base resin, it was observed that the lower the volume of this suspension added, the lower the distribution and the higher the dispersion of the NPs in the polymethyl methacrylate (PMMA) matrix.[20] In another study, the modified denture base acrylic combined with silver NPs at 20.0 wt% displayed antifungal properties and synthesis of Ag-denture acrylic resulted in appropriate physical characteristics although improvement in color stability is required.[21]

The soft-liners used for lining dentures in patients with sharp alveolar ridges, irritated mucosa, and resorbed ridges are also modifying with silver NPs to halt the bacterial and fungal growth. The modification procedure of two-component silicone denture soft lining material for long-term use by AgNP has been presented in a study. Results showed that AgNP has the tendency to form large aggregations of more than several hundred nm in the material, noticeable at concentrations >80 ppm can result in a decrease in antimicrobial action of NPs.[22]

A pilot study was done to generate AgNP in situ in PMMA and bisphenol A-glycidyl methacrylate-based resins for dental and medical applications.[23]

Secondary caries is the main cause of failure of dental restorations. Several efforts have been made to improve the longevity of dental restorations by incorporating bioactive agents having antimicrobial activities. Slow-release of various low molecular weight antibacterial agents such as antibiotics, zinc ions, silver ions, iodine, and chlorhexidine was evaluated in glass ionomer cement.[24] Dental composites are increasingly used as restorative material because of esthetics and improved performance. Until now, composites containing quaternary ammonium salts (QAS) have been used as antimicrobial dental restorative material.[25],[26] Antibacterial QAS monomers such as 12-metacryloyloxydodecylpyridinium bromide (MDPB) copolymerize with other monomers in the composite to form polymer matrices that can combat bacteria. The resulting composite decreased the attachment of Streptococcus mutans and plaque accumulation and inhibited the progression of secondary caries.[27] Newly, another class of composites was introduced that used a unique approach to prepare nanocomposites with evenly dispersed Ag NPs at a mass fraction of 0.08% to achieve a 40% reduction in bacteria coverage on the nanocomposites.[28] Recently, a study was conducted to develop antibacterial and mechanically strong nanocomposites incorporating a quaternary ammonium methacrylate (QADM), NPs of silver (NAg), and NPs of amorphous calcium phosphate (NACP). It was established that the metabolic activity and lactic acid production of biofilms on NACP + QADM + NAg composite were much less than those on commercial composites.[29]

Silver diamine fluoride can also be used to halt the cariogenic process, prevent new caries with silver salts, and stimulated sclerotic or calcified dentin formation. Silver nitrate has potent germicidal effect and fluoride has ability to reduce the decay of tooth.[30]

Dental adhesives that need to bond the composite to the dentine also need modification to counteract the invading bacteria along the margins of restorations. In addition, dental primers could kill the remaining bacteria as it contacts the tooth surface directly. To strengthen this idea, antibacterial dental adhesives were developed incorporating Ag NPs and NACP into a bonding system. The incorporation of 0.1% NAg plus 20% or 30% NACP into both primer and adhesive has antibacterial, acid neutralization, and remineralization benefits without jeopardizing the bond strength.[31]

Orthodontic adhesives are also developed with added benefits of antibacterial and remineralization properties. Incorporation of silver and hydroxyapatite NPs increase the shear bond strength of orthodontic adhesives.[32]

Another utilization of silver NPs is in root canal irrigation solution which directly target pulp canal microbes. According to a study, the nanosilver-based irrigant is as potent as 5.25% NaOCl in eradication Enterococcus faecalis and Streptococcus aureus.[33]

Esthetics and durability make the dental porcelain as the choice of material for ceramic restorations. However, the problems such as fragility, chipping, and fracture of dental porcelain have forced to develop the methods to toughen porcelain. The addition of silver NPs to Noritake Super porcelain may provide clinically useful dental porcelain with significantly higher fracture toughness and increased Vickers hardness. The results suggested that effective sintering and compressive residual stress in the sintered bodies increases density and fracture toughness occurs by adding silver NPs.[34] In another study, NPs of precious metals of silver and platinum were used that found to increase the fracture toughness.[35] It was also found that silver NPs affect the behavior of subcritical crack growth (SCG) in dental porcelain. SCG in dental porcelain can be characterized by the stress corrosion susceptibility coefficient. Addition of silver NPs significantly increased the stress corrosion susceptibility coefficient of dental porcelain.[36] It was also investigated that staining slurries containing silver and/or potassium compounds has the ability to enhance mechanical properties of a leucite-reinforced glass ceramic without producing any color changes.[37]

The development of a smart coating of silver NPs that act as antimicrobial agent for surgical implants was discussed in a research. The silver limits the amount of antibiotic patients need and provides protection from infection at the implant site for the life of the implant. Moreover, the rate of silver release can be tailor rate, i.e., silver is released more quickly right after surgery and slowly while the patient is healing.[38]

Another use of silver NPs is in dental unit waterline which are contaminated by microbial growth. It was described to dose dental water with silver using electrolysis or chemical additives such as silver nitrate give levels of silver in water well below the recommended maximum of 0.1 mg/lt.[39]

Future work is still going on in this unique field of NPs. It is worth mentioning that the antimicrobial action of silver NPs is now explored further for the prospective use in future dentistry.

Safety concerns

One of the critical aspects of silver NPs in biomedical use is toxicity. Nanotoxicity is the toxicity imposed by nanomaterials.[40] The toxic effects of AgNPs are proportional to the activity of free Ag + ions released by the NPs.[41] Though these NPs have remarkable potential for a wide range of applications, their adverse effects on living cells have raised serious concern. Corrosive products and discolorations of dental materials in contact with silver NPs are also drawbacks. When these NPs with antibacterial properties are released into the environment, they pose a danger to the marine life. All these issues need to be debated and new techniques and methods are required to overcome these issues.


  Conclusion Top


Silver NPs have really been a future of dentistry. Though there are certain issues that need to be further discussed and research is required to overcome these. We are hopeful of a bright future of these small particles.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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