**Silver Nanoparticles: Real Antibacterial Bullets**

G. Thirumurugan and M. D. Dhanaraju *Research Lab, GIET School of Pharmacy, Chaitanya Nagar, Rajahmundry, AP India* 

#### **1. Introduction**

406 Antimicrobial Agents

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One of the first and most natural questions to ask when starting to deal with nanoparticles is: "why are nanoparticles so interesting"? Why even bother to work with these extremely small structures when handling and synthesis is much more complicated than that of their macroscopic counterparts. The answer lies in the nature of and unique properties possessed by nanostructures. Nanoparticles possess a very high surface to volume ratio. This can be utilized in areas where high surface areas are critical for success. Over the past few decades, Metal nanoparticles, whose structures exhibit significantly novel and distinct physical, chemical, and biological properties, and functionality due to their nanoscale size, have elicited much interest. Especially in biological and pharmaceutical sector nanostructure materials are attracting a great deal of attention because of their potential for achieving specific processes and selectivity. Decreasing the dimension of nanoparticles has pronounced effect on the physical properties that significantly differ from the bulk material. Moreover, there are several reasons for the use of silver nanoparticles in nanotechnology as well as in medical and pharmaceutical field. (i) First of all, silver compounds have been used in medicine throughout the history of civilization. (Patra, 2008; Klasen, 2000; Lansdown, 2002) (ii) It is easy to synthesize silver nanoparticles by several simple, economically cheap, safe and reliable methods such as wet chemical, physical and biological; (iii) it can be synthesized from sizes of 2–500 nm by changing the reaction parameters; (iv) it can be easily synthesized with different shapes (spheres, rods, tubes, wires, ribbons, plate, cubic, hexagonal, triangular) using templates and changing reaction conditions; (v) due to the presence of a negative charge on the surface, they are highly reactive, which helps to modify the surface of silver nanoparticles using several biomolecules. Due to the strong interaction between the metal surface and thiol/amine containing molecules (organic molecules, DNA, protein, enzyme etc.) the surface of SNPs can be easily modified; (Bhattacharya, 2007) (vi) SNPs can be easily characterized due to the presence of the characteristic surface plasmon resonance (SPR) bands; (Daniel and Astruc, 2004) due to the presence of a unique optical as well as electronic behavior, these metal particles can be used in biosensors and molecular imaging; (Oghabian, 2010) due to its strong antimicrobial activity, it has found variety of application in different fields (Fig. 1).

Silver Nanoparticles: Real Antibacterial Bullets 409

Most frequently preparation of silver nanoparticles is carried out by chemical reduction method. Borohydrate, citrate, ascorbate, and elemental hydrogen are commonly used reductants for the synthesis of silver nanoparticles. The reduction of metal ions (Me+) like silver (Ag+ or gold (Au+) in aqueous solution generally yields colloidal metal with particle diameters of several nanometers (Wiley, 2005). Initially, the reduction of various complexes with metal (Ag+) ions leads to the formation of metal atoms (Ag0), which is followed by agglomeration into oligomeric clusters (Kapoor, 2002). These clusters eventually lead to the formation of colloidal Metal particles (Kapoor, 2002). For example, while formation of colloidal silver particles, when the colloidal particles are much smaller than the wavelength of visible light, the solutions have a yellow color with an intense band in the 380–400 nm range and other less intense or smaller bands at longer wavelength in the absorption spectrum (Cao, 2002). This band is attributed to collective excitation of the electron gas in the particles, with a periodic change in electron density at the surface (surface Plasmon

The synthesis of silver nanoparticles in this project will be based on a wet chemical method. The starting point of the synthesis is the production of a silver nitrate (AgNO3) solution. When silver nitrate is dissolved it splits into a positive silver ion (Ag+) and a negative nitrate ion (NO3-). In order to turn the silver ions into solid silver, the ions have to be reduced by receiving an electron from a donator. A flowchart illustrating the reduction of the silver ions by addition of an electron can be seen in Equation 1. The flowchart of Equation 2 illustrates the reduction of (Ag+) in a solution of ethanol. After the silver germ has been formed it starts to grow and continue the growth until the equilibrium between the final nanoparticles and

Ag(aq) (s) e Ag + − + →

(aq) 2 5 2 (s) 2 5 2 2Ag C H OH+H O 2Ag C H O 3H <sup>+</sup> − + + →+ +

3 4 2 26 3 AgNO NaBH Ag+ H B H NaNO 1 1 2 2 +→ + +

The preparation of silver nanoparticles in briefly, A 10-mL volume of 1.0 mM silver nitrate was added dropwise (about 1 dropsecond) to 30 mL of 2.0 mM sodium borohydride solution that had been chilled in an ice bath. The reaction mixture was stirred vigorously on a magnetic stir plate. The solution turned light yellow after the addition of 2 mL of silver nitrate and a brighter yellow, when all of the silver nitrate had been added. The entire addition took about three minutes, after which the stirring was stopped and the stir bar removed. The clear yellow colloidal silver is stable at room temperature stored in a transparent vial for as long as several weeks or months. Reaction conditions including stirring time and relative quantities of reagents (both the absolute number of moles of each reactant as well as their relative molarities) must be carefully controlled to obtain stable yellow colloidal silver. A large excess of sodium borohydride is needed both to reduce the ionic silver and to stabilize the silver nanoparticles. The possibility of aggregation during the synthesis, colloidal silver solution turns darker

The chemical reaction is the sodium borohydride reduction of silver nitrate:

(1)

(2)

(3)

absorption), (Gutiérrez, 1993).

the (Ag+) of the solution is reached (Chou, 2005).

Fig. 1. Applications of metal nanoparticles in various fields (G.Thirumurugan, 2011).

The synthesis of monodispersed metal nanoparticles with different size and shape has been challenge in nanotechnology. Although various physical and chemical methods are extensively used to produce monodispersed nanoparticles, the stability and the use of toxic chemicals is the subject of paramount concern. Moreover, the use of toxic chemicals on the surface of nanoparticles and non-polar solvents in the synthesis procedure limits their applications in clinical and pharmaceutical field (Oghabian, 2010) Therefore, development of clean, biocompatible, non-toxic and eco-friendly methods for silver nanoparticles synthesis deserves merit.

In this chapter, we will discuss an overview of silver nanoparticle preparation involving physical, chemical method and biological method, mechanism and advantages and disadvantages of the above methods. We provide various antibacterial mechanisms of silver nanoparticles to reduce antibiotic resistance and incorporation of SNPs on cotton fabrics and conjugation of SNPs on pharmaceutical compounds. Finally, we will discuss site-specific, antibacterial drug delivery of SNPs due to its unique surface modification, photo-thermal properties.
