7/29/2019 Build Magazine

1/244 BUILD February/March 2007

ntibacterial nanomaterials have

been used in a number of textiles,

plastics and ceramics coming on the

market in recent years. Antibacterial

fridges, bathroom tiles and paints are now

advertised. Antibacterial surfaces are seenas a second line of defense against disease,

although normal good hygiene practices,

such as wiping down benches, need to

be maintained.

NaNotechNology helpsbattle the bugsgd niri qii v d mr nnrrd mri ininrdd in vri f rd. Wi iin r i in dvd, f nimiri n in idin mri rd mn nfi.By Dr Larry Jordan, Program Manager Nanostructured Materials, Nanotechnology Victoria Ltd, Melbourne

A

PREVENTINGDISEASE

Nanoparticles (particles less than 100 nm

in diameter) are much more active than

larger particles. This is because a given

weight of nanoparticles has a much higher

surface area than the same weight of larger

particles. A kilogram of nanoparticles couldhave a surface area of one square kilometre,

if the surface areas of each particle were

added up. It is this high activity which makes

nanoparticles suitable for hygienic surfaces.

The use of antimicrobial agents in building

materials has a number of potential benets:

enhanced hygienic properties

odour control in gymnasium equipment

prevention of staining and discolourationfrom microbial attacks

retention of mechanical properties, as

micro-organisms can degrade compounds

such as plasticisers, bres, and some

polymer materials.

There are two key nanoparticles which have

been used in antibacterial materials: silver

nanoparticles; and titania nanoparticles. As

well as being antibacterial, nanoparticle

silver has some antifungal activity. Other

nanoparticles, such as copper and zinc-

containing particles, have been used also for

producing antifungal surfaces.

Silver nanoparticles

Nanoparticle silver is a broad spectrum

antibacterial additive that works at very low

concentrations. Silver nanoparticles can be

mixed into a building material as an additive

at typical silver concentrations of between 1%

and 0.01%. As the particles are so small and

added at a low concentration, they do not affect

the appearance of the material. However, some

silver compounds may react with light causing

discolouration and these should not be used

where this is an issue. Light stability should be

guaranteed by the manufacturer.

The additives can be either silver particles,

silver particles mixed into a supporting media

such as silica, or silver ions attached to a host

molecule. Particles mixed into a supporting

media have the advantage of longevity and

prevention of nanoparticle aggregation.

The action is similar, whatever type of

additive is used. Each relies on the additive

Laboratory bacteria culture tests.

Fluorescent images of bacteria on a glass sur face.

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The manufacturer should also have

demonstrated that discolouration of the silver

additive under sunlight does not occur, as

some silver compounds are known to darken

under sunlight.

While many applications of nanosilver are

under development, the main benets may

emerge in materials used in hospitals andfood processing areas, as well as domestic

kitchen and bathroom sur faces. Interestingly,

silver nanoparticles have been used by

health care companies in wound dressings

and bandages recently.

Titania nanoparticles

The use of titania nanoparticles as an

antibacterial additive relies on UV light

interacting with the titania in the presence of

moisture to form free radicals. Free radicals

are very strong oxidizing agents stronger

even than peroxide or bleach, and they kill

bacteria in a similar way.

The limitation with such surfaces, such as

when used in bathroom tiles, is that the level

of UV light indoors can be too low to result

in antibacterial action, and claims that such

surfaces are antibacterial should be treated

with caution.

reacting with moisture present in the

building material to release silver ions (Ag+).

Although normal silver metal would release

silver ions, the benet of nanoparticles

is that their high surface area results in a

much faster rate of release, resulting in a

dilute solution of silver ions to maintain

antibacterial activity.

Uses of silver

With concerns about the release of

antibacterial compounds to the environment,

silver is said to have an advantage in that

it is deactivated in the environment and in

waste-water treatment plants by reaction

with sulde or other reactive ions. Silver has

other advantages, such as thermal stability,

which makes it suitable for use with high

temperatures. Silver also has FDA food

contact approval for concentrations of up

to 3%. Nanosilver is suitable for indoor and

outdoor uses.

The manufacturers guarantee of an

antibacterial building material is important,

as the item needs to contain a suitable

concentration of antimicrobial agent in the

right form to cover its life-time. It should also

have passed antibacterial test specications.

In outdoor applications, the level of UV is

high enough that titania nanoparticles can

kill bacteria and mould, and degrade organic

pollutants. Because of this, nanoparticle titania

materials have been used for self-cleaning

windows and building faades. Titania with

dopants, which enable the titania to work under

visible indoor light, is a topic of much research even then, the need for a certain intensity of

light for activation would limit the surfaces that

could be commercially developed.

Uses of titania

One area in which titania nanoparticles

have had success is in air-conditioning

units in which a UV light can be installed,

guaranteeing activation of the titania surface.

As well as killing bacteria in such units, titania

nanoparticle surfaces can remove smells

by oxidizing the odour-causing molecules.Titania nanoparticle coatings have even been

demonstrated for use in aquariums, in which

the titania interacts with UV light to break

down algae on the glass surface, reducing

the need for cleaning.

For more information contact Dr Larry Jordan

on larry.jordan@nanovic.com.au or visit

NanoVic at www.nanovic.com.au.