Microbiological investigations into photocatalytic surfaces – a field trial

Figure 1: Growth of a biofilm of fungi and algae on the facade of a house.

Facades of houses, traffic signs, public transport bus shelters and noise barriers: fungi, algae, mosses and lichens settle on them and colonize the man-made and man-used surfaces (Fig. 1). In many cases the actual construction material used to create these surfaces is irrelevant.

Often it is only a question of time before the colonization with macroscopically noticeable biofilms starts. Apart from the unattractive appearance they incur costs for cleaning, restoration or replacement. If the functionality of the objects concerned is greatly impaired by microbial growth, this may, in some cases, even cause safety hazards for users or the environment.


Figure 2: Example of an armrest after 9-months exposure: fungi and algae have colonized the armrest and become clearly visible by forming unattractive stains.
Figure 3: Plastic armrest treated with a photocatalyst after the same exposure time: the visible stains are very small.

To research the exposure of plastic surfaces under practical conditions a trial plant was constructed on the north-western side of the Fraunhofer IGB within the scope of a joint research project funded by the BMBF. Various plastic surfaces which had been photocatalytically coated by our project partners were investigated. Armrests for garden furniture had been selected as model objects with practical relevance. The monitored time was 10 to 12 months and will be continued for a further winter season. Photocatalytically coated samples were compared with untreated ones. By complementing lab investigations these experiments serve to analyze the capabilities and limitations of photocatalytic layers when used as a barrier to estimate the formation of biofilms over long periods of time.


Figure 4: Microbiological analysis after sampling with contact agar plates: fungus colonies prevail.
Figure 5: Image of surfaces after 10 months of weathering taken with a scanning electron microscope. Fungus hyphae are noticeable.

Photocatalytic layers reinforced by a catalyst (commonly TiO2) integrated into the layer, degrade organic material by means of radiation, thus preventing the formation of biofilms. The experiment was set up in such a way that a contamination of fungi, algae and bacteria was purposely applied to the surfaces and inspected regularly both macroscopically and microbiologically. In Figures 2 and 3 the difference between a coated and an uncoated armrest (here: plastic material PA6) after 9 months of weathering is clearly visible. Figure 4 shows fungus colonies detected after several months of weathering of the samples on special agars. The inhomogeneously strong growth was tested by means of chemical analyses and evaluated by means of a scanning electron microscope (Fig. 5).


Photocatalytically equipped surfaces considerably reduce microbiological growth on outdoor surfaces thus providing an opportunity to reduce the number of cleaning cycles. The question of whether the formation of a biofilm can be prevented, depends on the material composition and the photocatalytic layers applied. It is also greatly dependent on the intensity of radiation at the exposed site.