Controllable synthesis of silver nanoparticles and its size-dependent antimicrobial activities

Recent advances in technology demand complete control of the size of nanoparticles to enable efficient application in various engineering fields. Specifically, noble metal nanoparticles such as silver and copper, which are known to possess strong antibacterial effects that can be tapped and utilized in tackling bacterial-related infections, better than their macro-sized counterparts. Recently published research work has already established that the antimicrobial activity of metal nanoparticles is highly dependent on size, hence the recent spike in metal nanoparticles-related research activity, that is aimed at achieving size-controllable preparation of metal nanoparticles. Furthermore, a recent discovery depicting silver nanoparticles as strong dependents of size, size distribution, the variance of the synthetic technique, stabilizers and reducing agents employed, has intensified research in this field. Therefore, it is a significant challenge to achieve size-tunable synthesis of silver nanoparticles with large surface area and surface activity as well as poor stability and strong aggregation tendency.

To this note, a team of researchers led by professor Lei Sun from the Engineering Research Center for Nanomaterials at Henan University in China established a simple route to prepare a series of size-tunable water-soluble silver nanoparticles with highly uniform morphologies and narrow size distributions. They purposed to obtain silver nanoparticles whose size could be precisely tuned to the desired sizes. Their work is currently published in the research journal, Advanced Powder Technology.

Briefly, the research technique employed entailed the preparation and characterization of silver nanoparticles of averages sizes of 2, 12 and 32 nm, and of highly uniform morphologies and narrow size distributions. Next, they assessed the antimicrobial effect of the as-prepared silver nanoparticles with different particles size by broth dilution and disk diffusion as well as measurement of optical density. Lastly, the researchers analyzed the antibacterial mechanism with regard to morphology observation of microorganism by scanning electron microscopy and to concentration detection of silver ions by stripping voltammetry.

The authors observed that parameters such as reactant molar ratio, reaction time, dropping speed, and most of all, pH of the reactant solutions, had substantial influences on size-regulation of silver nanoparticles. Above of all, it was found that pH played a key role with regard to controlling the size of the nanoparticles. Furthermore, the as-prepared silver nanoparticles exhibited excellent antibacterial properties against both Gram-negative bacteria of E. coli and Gram-positive bacteria of S. aureus.

The Lei Sun and colleagues study presented the development of a facile simple method for preparing silver nanoparticles by using sodium borohydride and sodium citrate as reducing and modification agents, respectively. The results obtained indicated that the antibacterial activities increased with the decrease of particles size. Altogether, it has been demonstrated that the antibacterial activities are attributed to the contact action of silver nanoparticles with microbes and the released silver ions which have strong bonding abilities with functional groups of cellular contents.