Stir away for safe drinking water

If you are afraid of drinking tap water because of harmful bacteria, stirring a 'magic wand' coated in a special film might help. Scientists at the University of Hyderabad say it's an easy way to kill bacteria without the need for expensive water purifiers.

Their magic wand is actually a glass rod coated with film of silver nanoparticles. "The nanoparticles simply kill the bacteria on contact," says the report¹. The film-coated glass stirrer can be used several times.

Silver nanoparticles are well-established antibacterial agents and are actually used in some ceramic candle filters available in the market. But fabrication of these filters is rather involved, requiring multiple steps. In many cases, silver nanoparticles must be prepared first by traditional routes, and then embedded in the ceramic material which is subsequently extruded to make the candles.

In contrast, the process developed by the Hyderabad team, led by chemist Thavarool Puthiyedath Radhakrishnan together with life scientist Aparna Dutta-Gupta, is claimed to be relatively easier.

In simple terms it involves dissolving in water, silver nitrate (AgNO3) and 'poly vinyl alcohol' or PVA — a commonly available non-toxic, biodegradable polymer and a known reducing agent that converts metal ions into metal atoms.

Next, the glass stirrer is dipped into the solution to coat it with a thin film of AgNO3-PVA. The film coated stirrer is then heated to 130°C for one hour in a hot air oven. This thermal annealing, says Radhakrishnan, results in 'soft-chemical synthesis', wherein the silver ions from AgNO3 get reduced by the hydroxyl groups of the PVA macromolecule to silver atoms, which aggregate to form the silver nanoparticles. "In other words, the silver nanoparticles responsible for anti-bacterial action are synthesized inside the film rather than physically incorporated as in several other methods," explains Radhakrishnan.

The thermal treatment also makes the final Ag-PVA film insoluble in water, the researchers say. "In essence, once you complete the thermal treatment, you have a film insoluble in water with the silver nanoparticles formed inside," says Radhakrishnan. This methodology developed by his group has been used in several other applications already.

According to the researchers, in situ generation of silver nanoparticles within a polymer film is an efficient approach to immobilize the silver nanoparticles in the polymer matrix. The thin film is adaptable for applications as coatings not just on glass rods but on a variety of surfaces. The "antibacterial effect can be realized by simple stirring or even static contact," says the report.

The researchers confirmed the bactericidal action of the film coated rods on Escherichia coli in different sources of water including ordinary tap water. According to Radhakrishnan, the Ag-PVA film is particularly interesting for bactericidal application in view of the low extent of leaching. The researchers estimate that the maximum silver concentration left in the treated water is about 0.05 µg/ml which is well below the upper permissible limit of 0.1 µg/ml in drinking water.

The researchers admit that the detailed mechanism behind the bactericidal action is not clear at the moment. "This could result from the direct contact of bacteria with the silver nanoparticles inside the Ag-PVA film or the interaction of bacteria with the silver ions released from the nanoparticles," the study says.

According to the researchers, being a hydrogel, PVA is likely to swell in water, facilitating contact between the bacteria and the nanoparticles. "The silver nanoparticles remain within the polymer and the polymer swells giving access to bacteria," explains Radhakrishnan.

"The inherent antibacterial efficacy of silver augmented by the large surface-to-volume ratio of the nanoparticles is believed to lead to the potent bactericidal action of the Ag-PVA film," he says and adds that further experiments and optimization of the PVA film matrix are needed "to improve the efficacy of the thin film bactericidal agent and to draw deeper insights into the mechanism of action involved."

Extensive reusability, low cost and feasibility of synthesizing the nano-composite thin film even in a domestic environment make it attractive, especially for applications in a rural scenario, the researchers conclude.

Commenting on this work, chemistry professor Talappil Pradeep of the Indian Institute of Technology (Madras) says a polymer offers great potential for low cost application in the form of paints, coatings and thin films. "The key here is in-situ growth and how that can be employed in the field will decide the application of this method," he told Nature India .

"There is a need to understand the exact reason behind the antibacterial action."

Radhakrishnan says that this is a "proof of principle" study and some questions still remain to be answered before extending this approach to practical applications. "We have to further minimize leaching to make it safer and still cheaper, and mechanical stirring may be required to handle large volumes of water", he said. "Our method also has a drawback — no filtration takes place and so the killed bacteria remain in the water." The general applicability of the approach to different kinds of bacteria remains to be tested.

The researchers have filed an Indian patent application. "The idea is not to commercialize it, but to facilitate non-profit organizations to take it up further", Radhakrishnan says.