We describe a simple one-pot thermal decomposition method for the production of a stable colloidal suspension of narrowly dispersed superparamagnetic Fe 3 O 4 -Ag core-shell nanostructures. These biocompatible nanostructures are highly toxic to microorganisms. Antimicrobial activity studies were carried out on both Gram negative (Escherichia coli and Proteus vulgaris) and Gram positive (Bacillus megaterium and Staphylococcus aureus) bacterial strains. Efforts have been made to understand the underlying molecular mechanism of such antibacterial actions. The effect of the core-shell nanostructures on Gram negative strains was found to be better than that observed for silver nanoparticles. The minimum inhibitory concentration (MIC) values of these nanostructures were found to be considerably lower than those of commercially available antibiotics. We attribute this enhanced antibacterial effect of the nanostructures to their stability as a colloid in the medium, which modulates the phosphotyrosine profile of the bacterial proteins and arrests bacterial growth. We also demonstrate that these core-shell nanostructures can be removed from the medium by means of an external magnetic field which provides a mechanism to prevent uncontrolled waste disposal of these potentially hazardous nanostructures.

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Silver shows the highest antimicrobial activities amongst all metals. It is better than many first line antibiotics. The antimicrobial properties of silver can be tuned by altering its physical and surface properties. Researchers have demonstrated enhancement in the antibacterial properties of silver with decreasing particle size from bulk to nano. In the present article, we study the effect of particle size of silver at nanoscale on their antimicrobial properties. Two samples of silver nanoparticles (SNPs) of same physical size (≈8 nm) but different hydrodynamic size (59 and 83 nm) are prepared by chemical reduction of AgNO3 with oleylamine followed by phase transfer with triblock copolymer Pluronic F-127. Their antimicrobial properties are investigated by microdilution method against clinically important strains of gram positive (S. aureus and B. megaterium) and gram negative (P. vulgaris and S. sonnei) bacteria. Nearly 38-50% enhancement in the antibacterial action of SNPs was observed when their hydrodynamic size was reduced to 59 nm from 83 nm. It has been observed that the antibacterial action of SNPs was governed by their hydrodynamic size and not by their crystallite and physical size. The phenomenological model was also proposed which makes an attempt to explain the microscopic mechanism responsible for the size dependent antibacterial activities of silver.

In this article we report the growth kinetics of ultrafine monodispersed silver nanoparticles prepared via thermal reduction of silver nitrate with oleylamine. Effect of nucleation and growth temperature and time on the quality and quantity of silver nanoparticles was monitored in terms of product yield, crystal phase, morphology, aggregation, particle size and size distribution. To understand the effect of kinetic parameters, purified silver nanoparticles were characterized by UV-visible, FTIR and photon correlation spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetry (TG).Irrespective of the kinetic conditions, oleylamine always reduces AgNO 3 into spherical Ag nanoparticles with simple cubic structure. The nanoparticle yield is highest for 21 mM oleylamine. Its size decreases with increasing oleylamine concentration and levels off at 3.5 nm with a polydispersity of 0.12. When concentration of oleylamine is ,15 mM, agglomerated silver nanoparticles resulted while they selfassembled into hexagonal close pack structure when oleylamine is ¢15 mM. Nucleation at 200 uC for 30 min and growth at 150 uC for 4 h are the optimum processing parameters for highest nanoparticle yield (60%), lowest particle size (3.5 nm) and polydispersity index (0.12) with no or very little agglomeration.

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