Elsevier

Biotechnology Advances

Volume 27, Issue 1, January–February 2009, Pages 76-83
Biotechnology Advances

Research review paper
Silver nanoparticles as a new generation of antimicrobials

https://doi.org/10.1016/j.biotechadv.2008.09.002Get rights and content

Abstract

Silver has been in use since time immemorial in the form of metallic silver, silver nitrate, silver sulfadiazine for the treatment of burns, wounds and several bacterial infections. But due to the emergence of several antibiotics the use of these silver compounds has been declined remarkably. Nanotechnology is gaining tremendous impetus in the present century due to its capability of modulating metals into their nanosize, which drastically changes the chemical, physical and optical properties of metals. Metallic silver in the form of silver nanoparticles has made a remarkable comeback as a potential antimicrobial agent. The use of silver nanoparticles is also important, as several pathogenic bacteria have developed resistance against various antibiotics. Hence, silver nanoparticles have emerged up with diverse medical applications ranging from silver based dressings, silver coated medicinal devices, such as nanogels, nanolotions, etc.

Introduction

Due to the outbreak of the infectious diseases caused by different pathogenic bacteria and the development of antibiotic resistance the pharmaceutical companies and the researchers are searching for new antibacterial agents. In the present scenario, nanoscale materials have emerged up as novel antimicrobial agents owing to their high surface area to volume ratio and the unique chemical and physical properties (Morones et al., 2005, Kim et al., 2007).

Nanotechnology is emerging as a rapidly growing field with its application in Science and Technology for the purpose of manufacturing new materials at the nanoscale level (Albrecht et al., 2006). The word “nano” is used to indicate one billionth of a meter or 10 9. The term Nanotechnology was coined by Professor Norio Taniguchi of Tokyo Science University in the year 1974 to describe precision manufacturing of materials at the nanometer level (Taniguchi, 1974). The concept of Nanotechnology was given by physicist Professor Richard P. Feynman in his lecture There’s plenty of room at the Bottom (Feynman, 1959).

Bionanotechnology has emerged up as integration between biotechnology and nanotechnology for developing biosynthetic and environmental-friendly technology for synthesis of nanomaterials.

Nanoparticles are clusters of atoms in the size range of 1–100 nm. “ Nano” is a Greek word synonymous to dwarf meaning extremely small. The use of nanoparticles is gaining impetus in the present century as they posses defined chemical, optical and mechanical properties. The metallic nanoparticles are most promising as they show good antibacterial properties due to their large surface area to volume ratio, which is coming up as the current interest in the researchers due to the growing microbial resistance against metal ions, antibiotics and the development of resistant strains (Gong et al., 2007).

Different types of nanomaterials like copper, zinc, titanium (Retchkiman-Schabes et al., 2006), magnesium, gold (Gu et al., 2003), alginate (Ahmad et al., 2005) and silver have come up but silver nanoparticles have proved to be most effective as it has good antimicrobial efficacy against bacteria, viruses and other eukaryotic micro-organisms (Gong et al., 2007). Silver nanoparticles used as drug disinfectant have some risks as the exposure to silver can cause agyrosis and argyria also; it is toxic to mammalian cells (Gong et al., 2007).

The current investigation supports that use of silver ion or metallic silver as well as silver nanoparticles can be exploited in medicine for burn treatment, dental materials, coating stainless steel materials, textile fabrics, water treatment, sunscreen lotions, etc. and posses low toxicity to human cells, high thermal stability and low volatility (Duran et al., 2007).

Section snippets

Silver as antimicrobial agent

For centuries silver has been in use for the treatment of burns and chronic wounds. As early as 1000 B.C. silver was used to make water potable (Richard et al., 2002, Castellano et al., 2007). Silver nitrate was used in its solid form and was known by different terms like, “Lunar caustic” in English, “Lapis infernale” in Latin and “Pierre infernale” in French (Klasen, 2000). In 1700, silver nitrate was used for the treatment of venereal diseases, fistulae from salivary glands, and bone and

Metallic silver

The antimicrobial property of silver is related to the amount of silver and the rate of silver released. Silver in its metallic state is inert but it reacts with the moisture in the skin and the fluid of the wound and gets ionized. The ionized silver is highly reactive, as it binds to tissue proteins and brings structural changes in the bacterial cell wall and nuclear membrane leading to cell distortion and death. Silver also binds to bacterial DNA and RNA by denaturing and inhibits bacterial

Silver sulfadiazine

Silver sulfadiazine (AgSD) is a combination of silver and sulfadiazine. AgSD is used as a 1% water-soluble cream. AgSD works as a broad-spectrum antibiotic. It is used especially for the treatment of burn wounds. AgSD serves as reservoir of silver in the wound and slowly liberates silver ions. All kinds of sulfa drugs have been tested in combination with silver but sulphadiazine was found to be most effective. AgSD binds to cell components including DNA and cause membrane damage (Atiyeh et al.,

Silver zeolite

Silver zeolite is made by complexing alkaline earth metal with crystal aluminosilicate, which is partially replaced by silver ions using ion exchange method. In Japan, ceramics are manufactured coated with silver zeolite to apply antimicrobial property to their products. These ceramics are used for food preservation, disinfection of medical products, decontamination of materials (Kourai et al., 1994, Kawahara et al., 2000, Matsumura et al., 2003).

The state-of-the-art

Feng et al. (2000) reported mechanistic study of inhibition of silver ions against two strains of bacteria, S. aureus and E. coli. For the experiment, both bacteria E. coli and S. aureus were inoculated on Luria Bertoni (LB) medium and incubated at 37 °C on rotary shaker (200 rpm) for 16 h. After that 10 µg/ml of silver nitrate was added to the liquid culture and allowed to grow for 4–12 h. Five milliliters of the above culture was removed, centrifuged and the subsequent biomass obtained was

Mechanism of action

The exact mechanism of action of silver on the microbes is still not known but the possible mechanism of action of metallic silver, silver ions and silver nanoparticles have been suggested according to the morphological and structural changes found in the bacterial cells.

Effect of size and shape on the antimicrobial activity of nanoparticles

The surface plasmon resonance plays a major role in the determination of optical absorption spectra of metal nanoparticles, which shifts to a longer wavelength with increase in particle size. The size of the nanoparticle implies that it has a large surface area to come in contact with the bacterial cells and hence, it will have a higher percentage of interaction than bigger particles (Kreibig and Vollmer, 1995, Mulvaney, 1996, Morones et al., 2005, Pal et al., 2007). The nanoparticles smaller

Silver coated medical devices

Silver nanoparticles are also considered as candidate for coating medical devices. Medical devices coated with silver ions or metallic silver proved to be disappointing in clinical tests. The reason for this might be the inactivation of metallic silver when it comes in contact with blood plasma and the lack of durability of the coatings. The metallic silver also failed to improve the antimicrobial activity (Riley et al., 1995, Everaet et al., 1998). Furno et al. (2004) demonstrated the use of

Applications

Silver has been known to possess strong antimicrobial properties both in its metallic and nanoparticle forms hence, it has found variety of application in different fields.

  • The Fe3O4 attached Ag nanoparticles can be used for the treatment of water and easily removed using magnetic field to avoid contamination of the environment (Gong et al., 2007).

  • Silver sulfadazine depicts better healing of burn wounds due to its slow and steady reaction with serum and other body fluids (Fox and Modak, 1974).

Conclusion and future prospects

In summary, it can be concluded that among the different antimicrobial agents, silver has been most extensively studied and used since ancient times to fight infections and prevent spoilage. The antibacterial, antifungal and antiviral properties of silver ions, silver compounds and silver nanoparticles have been extensively studied. Silver is also found to be non-toxic to humans in minute concentrations. The microorganisms are unlikely to develop resistance against silver as compared to

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