Silver nanoparticles: Green synthesis and their antimicrobial activities

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

Abstract

This review presents an overview of silver nanoparticles (Ag NPs) preparation by green synthesis approaches that have advantages over conventional methods involving chemical agents associated with environmental toxicity. Green synthetic methods include mixed-valence polyoxometallates, polysaccharide, Tollens, irradiation, and biological. The mixed-valence polyoxometallates method was carried out in water, an environmentally-friendly solvent. Solutions of AgNO3 containing glucose and starch in water gave starch-protected Ag NPs, which could be integrated into medical applications. Tollens process involves the reduction of Ag(NH3)2+ by saccharides forming Ag NP films with particle sizes from 50–200 nm, Ag hydrosols with particles in the order of 20–50 nm, and Ag colloid particles of different shapes. The reduction of Ag(NH3)2+ by HTAB (n-hexadecyltrimethylammonium bromide) gave Ag NPs of different morphologies: cubes, triangles, wires, and aligned wires. Ag NPs synthesis by irradiation of Ag+ ions does not involve a reducing agent and is an appealing procedure. Eco-friendly bio-organisms in plant extracts contain proteins, which act as both reducing and capping agents forming stable and shape-controlled Ag NPs. The synthetic procedures of polymer-Ag and TiO2–Ag NPs are also given. Both Ag NPs and Ag NPs modified by surfactants or polymers showed high antimicrobial activity against Gram-positive and Gram-negative bacteria. The mechanism of the Ag NP bactericidal activity is discussed in terms of Ag NP interaction with the cell membranes of bacteria. Silver-containing filters are shown to have antibacterial properties in water and air purification. Finally, human and environmental implications of Ag NPs to the ecology of aquatic environment are briefly discussed.

Introduction

The application of nanoscale materials and structures, usually ranging from 1 to 100 nanometers (nm), is an emerging area of nanoscience and nanotechnology. Nanomaterials may provide solutions to technological and environmental challenges in the areas of solar energy conversion, catalysis, medicine, and water treatment [1], [2]. This increasing demand must be accompanied by “green” synthesis methods. In the global efforts to reduce generated hazardous waste, “green” chemistry and chemical processes are progressively integrating with modern developments in science and industry. Implementation of these sustainable processes should adopt the 12 fundamental principles of green chemistry [3], [4], [5], [6], [7]. These principles are geared to guide in minimizing the use of unsafe products and maximizing the efficiency of chemical processes. Hence, any synthetic route or chemical process should address these principles by using environmentally benign solvents and nontoxic chemicals [3].

Nanomaterials often show unique and considerably changed physical, chemical and biological properties compared to their macro scaled counterparts [8]. Synthesis of noble metal nanoparticles for applications such as catalysis, electronics, optics, environmental, and biotechnology is an area of constant interest [9], [10], [11], [12], [13], [14], [15]. Gold, silver, and copper have been used mostly for the synthesis of stable dispersions of nanoparticles, which are useful in areas such as photography, catalysis, biological labeling, photonics, optoelectronics and surface-enhanced Raman scattering (SERS) detection [16], [17]. Additionally, metal nanoparticles have a surface plasmon resonance absorption in the UV–Visible region. The surface plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size [18], [19]. The band shift is dependent on the particle size, chemical surrounding, adsorbed species on the surface, and dielectric constant [20]. A unique characteristic of these synthesized metal particles is that a change in the absorbance or wavelength gives a measure of the particle size, shape, and interparticle properties [20], [21]. Moreover, functionalized, biocompatible and inert nanomaterials have potential applications in cancer diagnosis and therapy [22], [23], [24], [25], [26]. The target delivery of anticancer drugs has been done using nanomaterials [22]. With the use of fluorescent and magnetic nanocrystals, the detection and monitoring of tumor biomakers have been demonstrated [24], [25].

Generally, metal nanoparticles can be prepared and stabilized by physical and chemical methods; the chemical approach, such as chemical reduction, electrochemical techniques, and photochemical reduction is most widely used [27], [28]. Studies have shown that the size, morphology, stability and properties (chemical and physical) of the metal nanoparticles are strongly influenced by the experimental conditions, the kinetics of interaction of metal ions with reducing agents, and adsorption processes of stabilizing agent with metal nanoparticles [21], [22]. Hence, the design of a synthesis method in which the size, morphology, stability and properties are controlled has become a major field of interest [29].

Section snippets

Silver nanoparticles

Silver is widely known as a catalyst for the oxidation of methanol to formaldehyde and ethylene to ethylene oxide [30]. In the United States, more than 4 × 106 tons of silver were consumed in 2000. Colloidal silver is of particular interest because of distinctive properties, such as good conductivity, chemical stability, catalytic and antibacterial activity [31]. For example, silver colloids are useful substrates for surface enhanced spectroscopy (SERS), since it partly requires an electrically

Polysaccharide method

In this method, Ag NPs are prepared using water as an environmentally benign solvent and polysaccharides as a capping agent, or in some cases polysaccharides serve as both a reducing and a capping agent. For instance, synthesis of starch-Ag NPs was carried out with starch as a capping agent and β-d-glucose as a reducing agent in a gently heated system [7]. The starch in the solution mixture avoids use of relatively toxic organic solvents [56]. Additionally, the binding interactions between

Ag NPs and their incorporation into other materials

The unique properties of Ag NPs have been extended into a broader range of applications. Incorporation of Ag NPs with other materials is an attractive method of increasing compatibility for specific applications.

Antimicrobial activities

Silver is known for its antimicrobial properties and has been used for years in the medical field for antimicrobial applications and even has shown to prevent HIV binding to host cells [178], [183], [184], [185], [186]. Additionally, silver has been used in water and air filtration to eliminate microorganisms [187], [188], [189].

Human Health

Nanoparticles may have different effects on human health relative to bulk material from which they are produced [15]. Increase in biological activity of nanoparticles can be beneficial, detrimental or both. Many nanoparticles are small enough to have an access to skin, lungs, and brain [15], [231], [232]. Currently, no sufficient information is available on the adverse effects of nanoparticles on human health [233], but studies are forthcoming to address this subject [234], [235], [236], [237],

Concluding remarks

Several synthetic methods for Ag NPs using inexpensive and nontoxic compounds under water environments were summarized and experimental approaches under different conditions were given to control the morphology of the Ag particles. Rapid and green synthetic methods using extracts of bio-organisms have shown a great potential in Ag NP synthesis. However, understanding the mechanism by which biomolecules of these organisms are involved in synthesis is lacking. A progress in this area will give

Acknowledgment

We wish to thank three anonymous reviewers for their useful comments that greatly improved this paper.

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