Natural dye-sensitized ZnO nano-particles as photo-catalysts in complete degradation of E. coli bacteria and their organic content

Schematic showing how anthocyanin-sensitized ZnO particles catalyze solar disinfection processes. [Display omitted] •Natural dye anthocyanin sensitizes ZnO particles to visible light in phot-catalytic processes.•ZnO/Dye kills E. coli bacteria and completely mineralizes its organic content under solar simulator light.•ZnO/Dye catalyst can be recovered and re-activated by re-sensitizing with dye. This communication describes for the first time how nano-size particles, sensitized with natural dye molecules of anthocyanin, can be used as catalysts in photo-degradation of gram negative Escherichia coli bacteria in water. The naked ZnO nano-particles degraded up to 83% of the bacteria under solar simulator light, while the dye-sensitized particles increased the bacterial loss by ∼10%. Solar simulator light includes about 5% of UV tail (shorter than 400nm) which means that both UV and visible light (longer than 400nm) radiations could be involved. When a cut-off filter was used, the naked ZnO caused only 40% bacterial loss, in accordance with earlier literature that described killing of bacteria with ZnO particles both in the dark and under light. With the cut-off filter, the sensitized ZnO particles caused higher than 90% bacterial loss, which confirms sensitization of the ZnO particles to visible light. Moreover, the results show that the catalyzed photo-degradation process causes mineralization of the bacteria and their organic internal components which leach out by killing. The catalyst can be recovered and reused losing ∼10% of its activity each time due to mineralization of the dye molecules. However, catalyst activity can be totally regained by re-sensitizing it with the anthocyanin dye. The effects of different experimental conditions, such as reaction temperature, pH, bacterial concentration and catalyst amount together with nutrient broth and saline media, will be discussed together with the role of the sensitizer.