New titanium dioxide structure reacts with visible light

Coating created in New Zealand uses intricate titanium dioxide nanostructures to kill bacteria on metal surfaces as well as on glass

Titanium dioxide is commonly used as an antibacterial surface, but not in areas like hospitals. This is owing to the UV light needed to activate the antibacterial properties of the surface and the impracticality of this in a hospital setting.

The drive for this type of antibacterial is the need for cheaper coatings that can kill bacteria without contributing to the development of antibiotic resistance.

A new coating for stainless steel could be the solution to this. The coating is made of intricate titanium dioxide nanostructures and kills bacteria under visible light without using antibiotics.

In this regard titanium dioxide is perfect, it kills by breaks down water vapour in the air to produce free radicals from oxygen that will attack whatever is touching it.

Structure of a killer

The new coating kills in a similar way but has different properties due to its new structure. Mechanical engineer Susan Krumdieck and her colleagues at the University of Canterbury, New Zealand, designed the technology, publishing a paper on it in Nature.

The team grew titania coating directly on stainless steel using a pulsed chemical vapour deposition technique Krumdieck developed. Normally, vapours of the target molecule mixed with an inert carrier gas are injected into a vacuum chamber containing the heated substrate to be coated, but this dilutes the starting material.

Krumdieck’s method uses an ultrasonic atomiser to make a fine mist of a titanium dioxide precursor and sprays pulses of it into the chamber, which increases the rate at which the titania crystals grow.

The result involved thick single crystals of titania interspersed with pine cone-like polycrystalline structures of the compound. The key factor, however, was the carbon present on the crystal surfaces in spaces between the nanostructures. This structure increases the surface area 100-fold and creates a pathway for the carbon to ‘feed’ the titanium dioxide the visible-light photons it has absorbed.

To test the coating, the researchers spread E. coli over it, then exposing it to UV light and visible light. UV exposure killed all the bacteria as expected, but the visible light sample killed over 99%. Although not quite up to UV light’s standard, this does beat the next antimicrobial surface by a landslide.

In terms of the practical usage of the technology, Krumdieck said it should work on other metal surfaces as well as on glass. As doorknobs, bed-rails, and faucet handles are key areas that perpetuate the spread of bacteria, these were emphasised as possible locations to implement this new surface.