Development of novel methods for the determination of antimicrobial capabilities of photocatalytic titania coatings

Scimone, Antony Damien (2018) Development of novel methods for the determination of antimicrobial capabilities of photocatalytic titania coatings. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Pathogenic and food spoilage organisms can survive on surfaces for extended periods of time and must be removed or inactivated to avoid product spoilage and disease. Current interventions include chemical and physical cleaning protocols, which can be costly and environmentally unsustainable. Coating surfaces with photocatalytic titanium dioxide may represent a solution to these issues, due to its stability, sustainability, low cost and significant self-cleaning and antimicrobial properties. However, only UV can activate the photocatalyst, making the technology impractical for many desired applications. By doping the titania with other materials, it is possible to achieve activation using visible light. The focus of this thesis was to produce visible light-activated photocatalytic coatings and assess their antimicrobial efficacy. Furthermore, the current tests used to assess antimicrobial efficacy are complex, time-consuming and unattractive for non-specialists. Therefore, this work also aimed to develop simple, rapid tests for the antimicrobial potential of photocatalytic coatings. Titanium dioxide films doped with molybdenum or niobium at a range of concentrations were produced on stainless steel and glass substrates using magnetron sputtering techniques, annealed at a range of times and temperatures and characterised via EDX, SEM, XRD and optical profiling. The photocatalytic activity of the films was measured via the degradation of methylene blue while the photocatalytic-antimicrobial properties were determined using an adaptation of the BS ISO 27447:2009 test. It was found that Mo-TiO2 had improved visible light activity and photocatalytic-antimicrobial properties; antimicrobial activity was also present in the absence of irradiation, while Nb-TiO2 did not show improved activity compared to undoped TiO2. The substrate greatly affected the antimicrobial activity of Mo-TiO2 and this was ascribed to differences in roughness and surface area; substrates with lower surfaces areas consistently had reduced antimicrobial activity. Two existing methods were identified in the literature and redeveloped as assays for photocatalytic-antimicrobial efficacy. The first method was based on real time in situ light absorbance of bacteria in suspension using inexpensive spectrophotometric equipment. This method showed promise, approximating the viability dynamics of a planktonic suspension well. However, assessment of decreases in viability could not be made. The second method was based on the time taken to detect changes in the colour of resazurin dye to indicate the viability of bacteria after a period of irradiation. The time to detect a colour change was inversely proportional to the viable population size and thus represents a rapid alternative for enumeration of viable cells following photocatalytic treatment. Furthermore, the test was stratified into three tiers of sensitivity and complexity and so can be used as a first-stage screening for putative photocatalytic-antimicrobial properties, or a fully quantitive enumeration technique. It is hoped that that this alternative test will enable non-specialists to further develop their photocatalytic coatings for antimicrobial use.