UVC and blue light-based tratments for inactivation of bacteria

Disinfection using the short-wavelength Ultra-Violet (the UVC, 200-280 nm) has been known for over a century but is gaining increasing popularity in these years thanks to the combined effect of the COVID19 pandemic outbreak, the stronger emerging of antibiotic resistant bacteria, and the availability of new compact LED sources in replacement for the traditional mercury vapor lamps, which are being banned for their toxic mercury content. UVC irradiation can be exploited both for its direct damaging action on the bacteria RNA or DNA and for its indirect mechanism, the so-called Photo-Dynamic Therapy (PDT), in which it triggers the generation of cytotoxic Reactive Oxygen Species (ROSs) from a non-toxic photosensitizer (PS). Long-term UVC exposure, however, can also be dangerous and have carcinogenic effects on human cells; an alternative is represented by the generation of ROSs from suitable photosensitizes that can be excited with the less harmful blue light (400-470 nm). This thesis focuses on the study of the use of UVC and blue light for sanitization of water. The microbiological safety of water is essential to prevent waterborne diseases, which are one of the leading causes of human deaths, affecting millions of people worldwide. “Clean water and sanitation”, distribution of safe drinking water to everyone and hygiene are the sixth of the Sustainable Development Goals of the United Nations. In the first part of this thesis, the effectiveness of the mechanism of UVC has been studied and some water sanitization solutions have been analyzed. A simplified model of the LED irradiation based on Lambertian sources has been developed and implemented in Matlab to analyze various configurations and evaluate the optimal geometry that guarantees a uniform irradiation dose for bacteria inactivation. The developed simulation code has been validated using a commercial ray-tracing software, Tracepro. Then, the UVC germicidal action has been experimentally proven with extensive tests on contaminated water at different exposure times, using culture media sensitive to Enterobacteriaceae. The results show the relevant antimicrobial UVC effect with exposure times of 1-2 s with a final bacteria concentration lower than 1E3 CFU/ml. In the second part of the work, the UVC disinfection has been compared against the effect of blue light plus a suitable photosensitizer. Based on evidences in the literature, curcumin has been chosen as natural photosensitizer because of its extended antimicrobial activity with blue light and its safety evaluated by clinical trials on humans; indeed, it is commonly used in dentistry for oral disinfection. This part has been mainly experimental and different curcumin concentrations and exposure times have been studied. In general, it has been found that longer exposure times are required than when using UVC. This prolonged treatment could lead to an increase of temperature, which could be responsible of part of the antimicrobial action; therefore, to assess the specific efficacy of the PDT and isolate the thermal effect contribution, Pulsed Wave (PW) irradiation has been used, while measuring the induced temperature with Fibre Bragg Grating (FBG) sensors.