Enzyme-polymer hybrid layers for self-cleaning surfaces

摘要

This work focused on the generation and characterization of ultrathin coatings with a self-cleaning ability. The general idea was to immobilize a typical detergent enzyme on model surfaces in a way that retains the activity of the enzyme. The specific system under investigation was a hydrogel coating which was chemically anchored on the surface and which also served as a carrier for the enzyme. The hydrogel coating was prepared from water-soluble polymers that carry a small percentage of photo-reactivegroups suitable for reaction with neighbouring chains in a way that leads to crosslinking. Similar groups on the surface provide a surface anchoring of the layers by theestablishment of covalent bonds between the two components. Amylase was chosen as the enzyme because it is readily available and it cleaves carbohydrates which are typically present in many surface stains. The resulting molecules are smaller and typicallymore soluble in water such that a stain containing such components is more easily rinsed off the surface.This enzyme was to be incorporated into the hydrogel carrier coating by means of co-deposition via dip coating. Thesubsequent UV crosslinking whichlead to a surface anchoring not only of the film but also of the enzyme by means of either physical entrapment or chemical anchoring through the surface reaction.Previous finding suggested an insufficient anchoring of enzymes via this route as the enzyme needs to be deposited from buffer solutions which contain high amounts of salt. This is also deposited and keeps much of the enzyme from being incorporated into the hydrogel coating. For this reason the enzyme was modified with another hydrophilic polymer, Polyethylene glycol chains were chemically attached to the amylase via an active ester. Using this procedure the amylase was rendered soluble in ethanol without losing anyactivity and overall increased in enzymatic activity as compared to the native enzyme.An investigation of the coating procedure by various techniques revealed indeed that the route via PEGylated amylase and deposition from ethanol in the absence of any salt yielded much smoother layers in which the enzyme was rather homogenously distributed. Control coatings generated from native amylase and deposited from PBS buffer gave very rough coatings that were covered with salt crystals.A comparison of the enzymatic activity via colorimetric measurements first demonstrated that enzymaticallycoatings could be generated via both routes. The coatings prepared from PEG-conjugated amylase however were always more active than those prepared from native amylase and they also retained this active character better if exposed to UV light or heat.A simple self-cleaning test also showed that coatings prepared from native amylase lost their initially good self-cleaning character after a first rinse. Repeated use of such samples was not possible. The coatings carrying PEGylated enzyme also lost much of their initial activity after the first test but repeated use was possible and clear degradation of starch in a test involving mayonnaise was clearly visible.The general concept of enzyme carrying coatings based on surface-attached hydrogels was successfully demonstrated. Further investigations should concentrate on the analysis of the influence of the deposition conditions on the enzyme activity. Such investigations should include a more thorough characterization of the resulting layers. Such research may eventually lead to self-cleaning surfaces that are stable on a time scale which is suitable at least for delicate applications e.g. in a biomedical environment.