Silver has been shown to inhibit energy production by inhibition of the respiratory chain of Escherichia coli.Indirect toxicity may also arise from salt formation with silver ions that results in a chloride or anion limitation within the cell. Nanocrystalline silver also releases Ag0 and has been shown to rapidly kill bacteria and fungi. Although Ag+ is rapidly inactivated by interaction with organic matter, Ag0 is much more stable.
Titanium dioxide has received considerable interest as a semiconductor photocatalyst, When irradiated with UV light titania is able to photomineralise surface contaminants to carbon dioxide and water, thus TiO2 is also an effective biocide , also under UV radiation TiO2 exhibits enhanced surface wetability, a synergism exists when coupled with photomineralisation to provide self-cleaning surfaces.
silver-titania composite films
The combination of Ag and TiO2 for thin film photocatalysis has been studied predominantly for sol-gel grown materials , however there have been few studies investigating the antimicrobial properties of such these films. The biocidal effectiveness of a Ag films are limited to around one layer of cellular matter, (unless enhanced by diffusion). The dead/deactivated bacteria cells eventually block biocidal activity. The photo-active titania (with UV) photomineralises these CO2 and water thus regenerating the biocidal property of the surface.
Photo-activated films are biocidal. The films produce active species which for self clean destroy organics. These same active species kill bacteria. The same process can be used to clean the surface of dead bacteria and bacteria residues which often de-activate conventional biocidal surfaces.
CVD is an attractive route to depositing Photo-catalytic films. APCVD is capable of a high degree of structural control (crystallinity and the ratio of anatase to rutile) which are crucial for high performance photoactivity.
The novel combination of Thermal and Flame Assisted CVD, has allowed both new material composites and novel thin film structures to be explored. The resultant films were shown to be highly biocidal to (Gram negative) E Coli. The nano-structure of the grown films is proposed as an important factor in the high levels of activity seen
Furthermore, CVD is the only approach which has been approved for use on external window surfaces. (where 20 year life is required). CVD thus is capable of producing highly durable photo-active surfaces.
CVD allows for the introduction of additional film components which can be used to enhance activity, such as silver and silver nano-particles into the photo-active titania. This gives a high degree of "passive" biocidal activity in addition to the "active" biocidal properties and self clean action. Furthermore, the silver does not require light activation thus allowing for activity in wider range of settings