Thin film coating technology has been widely employed for various purposes in many areas. Up to now, many surface modification techniques have been reported to produce antibacterial surfaces coated polymers. TiO2 photo-catalysts, known as physicochemical stability, non- toxicity, bio-compatibility, and low cost, have been investigated throughout for the growth inhibition of pathogenic microorganisms and other contaminants [1].
TiO2 is an n-type semiconductor who has been known since 1985 as the publication of a Japanese scientist. Since then, numerous studies on the photo-catalytic property of titanium dioxide had been carried out. The main concerns of those researches were producing anti-infective biomaterial in medicine and purification of contaminated water and air through various approaches with the application of Titanium Dioxide.
PRINCIPLES BEHIND THE PROCESS
Many studies have shown that TiO2 films coated substances are testified antibacterial functions under weak ultraviolet light in living areas.
Titanium dioxide particles can crystallize in two forms, which are anatase, and rutile. When illuminated with a light of a proper wavelength, crystalline titania (TiO2) behaves like n-type photoconductors. Once being excited by UV light whose energy is more than that of band gap, an electron-hole pair was engendered on the TiO2 surface. As a result, the hole in the valence band can react with H2O or hydroxide ions adsorbed on the surface to produce hydroxyl radicals (OH•), at the same time, the Oxygen molecule was reduced to superoxide ions (O2 −) by the electron in the conduction band can reduce O2. Both the holes and (OH•) are an organic containing aqueous solution, hydroxyl radicals oxidizes the organic substance, such as proteins inside the bacterial cell, leading to bacteria death.
Figure1: Titania semiconductor photo catalyst process.
The antibacterial efficiency of Titania depends on the concentration of OH radicals, crystalline structure, and particle size. The difference in titania films could be credited to the different photo-catalytic activities of the films. The primary attraction site by free radicals produced by the photo-catalytic activity is the microorganism cell membranes, following by their entry into the cell where bio-molecules such as lipids and proteins which are then destroyed by free radical.
SCIENCE AND ENGINEERING APPLICATIONS
Nano TiO2 Photo-catalyst coating is considered a significant purifying and disinfecting coating technology. This property indicates the coated material a good candidate to apply as medical devices, food preparation surfaces, air conditioning filters, and sanitary ware surfaces.
To create disinfection or purification systems based on nano-particulate TiO2 , various approaches can be chosen, such as directly attaching nanoparticles to a surface or to use the sol-gel process, to apply a thin layer of particles on a substrate. Possible substrates available are metal, alloys, glass, glass fibre, cellulose, plastics, ceramics and wood.
All though Titania is neither medicine nor health chemical or liquid disinfectant; it shows an important role in medicine. One of its application as antibacterial coating was the usage in ophthalmic intraocular lens.
Polymethyl methacrylate (PMMA) is the common medical polymers, which is widely utilized in manufacturing various implants, especially in the fabrication of an ophthalmic intraocular lens. Coating with TiO2 thin-films which has high transmittance in the visible region could be favourable to PMMA application in ophthalmic[4].
After Transparent TiO2 films were successfully coated on PMMA by sol-gel dip coating, the surface antibacterial behaviour enhances. This plasma technique provides number of various high-energy species such as electrons, atoms, and radicals for reactions in a non-equilibrium condition, leading to helpful modifications of the surface functionality. Consequently, the TiO2 coating on PMMA improves the anti-infective ability of the implant material.
Beside employment in medicines, TiO2 has also been widely used in air and water purification, in addition remediation of hazardous wastes.
Methyl-tertiary-butyl-ether (MTBE) is considered as one of the most dangerous environmental hazardous for human. Usually, MTBE was detected in wastewaters, groundwater, watercourse, drinking water and soil. Because of a high MTBE solubility in water and its low volatility, the treatment and purification of MTBE is an intractable work. The EPA suggested a concentration limit of 20-40 μg/L for MTBE in drinking water [5], the California Department of Health Services has assigned 5μg/L as maximum concentration levels of MTBE [6].
According to the experiments conducted by scientists, photo catalytic degradation was proven an efficient method to reduce concentration of MTBE. It was concluded that the highest MTBE degradation rate could be achieved in the photo catalytic degradation with titanium dioxide doped with LaCl3・7H2O, synthesized by sol gel procedure.
RECENT PROCESS
Research has shown that titanium dioxide in the anatase phase is a more powerful photo-catalyst than rutile in environmental purification. It is noticed that anatase titanium dioxide can be used to remove not only the bacterial but also the harmful organic toxics and pollutants from the air and aqueous environments under ultraviolet light.
Therefore, recent studies have attempted to employ it as concrete pavement coating to purify the air .To accomplish this, a thin surface layer coated tconcrete pavements is produced by mixing nano-titanium anatase particles with cement and water. In the presence of light, the pollutants are oxidized by the free radicals and precipitated on the pavement surface. These pollutants are then removed from the surface by rain. The process is shown in the figure below.
http://www.mchnanosolutions.com/images/photocatalyst.gif
Figure 2. Air pollution cleaning by Titania comparing with photosynthesis
FUTURE OUTLOOK AND CHALLENGES
Titanium dioxide coating is considered a significant improvement and support to further evaluation of anti-infective technology. Let's imagine, if it could be applied any surface, such as the furniture in our house, transform the furniture into pathogen-free surface, also let it purify the surrounding air and protect us from environmental contamination. We do not need to use "Dettol" to clean our home any longer. Moreover, the probability of illness maybe reduced.
There is no end in Science research; the study in the area of Titania photo-catalytic coating still has huge potential for research to be carried on. Further research is recommended to consider other aspects in the analysis such as maintenance and rehabilitation activities, and variation of experimental data adopted with trail scale and different physical conditions. Research is also essential to quantify side effects of any by-products developed from process; durability and effect on substances, in addition the long-term effectiveness of photo-catalytic process.
