**6.1 TiO2 in sunscreen**

The sunlight reaching the earth's surface contains UV, visible and infrared wavelength. The Sun releases ultraviolet (UV) radiation in three different wavelengths, and all are harmful in different ways. These wavelengths in sunlight are called UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm) [119]. Because the earth's atmosphere blocks most UVC rays, UVC does not generally reach the earth's surface to a significant degree. Therefore, they are not thought to be important contributors to the biological effects on human skin [120]. UVA wavelength penetrates more deeply into the skin causing photo-aging and the formation of skin cancer. UVB is shorter, and damages the surface of the skin. The damage from UVB can cause sunburn and cancer [121–124].

TiO2 is a semiconducting material with very high refractive index. The high refractive index is what allows the substance to scatter visible light. The current method of preventive treatment again harmful UV radiation involves suspending a substance that either absorbs or scatters UV radiation in a thick emulsion, called sunscreen. Titanium dioxide (TiO2) is an ingredient in sunscreens where its loading is frequently 2–15%. Sunscreen typically contain chemical filters that are organic compounds that absorb strongly the UV (most often UVB) and physical filters such as TiO2 and ZnO that block UVA and UVB sunlight through absorption, reflection and scattering.

### **6.2 TiO2 for cancer treatment**

In biomedicine, TiO2 nanoparticles with their extraordinary stability, exceptional photo-reactivity, and biocompatibility have a special place in biomedical solutions. The therapeutic potential of TiO2 lies in the ability of these particles in response to light to produce reactive oxygen species (ROS). Production of ROS is the main factor in causing detrimental effects on cells. This effect was first applied by Cai *et al* in the immortal HeLa cell lines [125]. Distinct cell death was detected after HeLa cells were illuminated with UV light in the presence of TiO2 (100 μg/ mL). TiO2 particles in the absence of light showed little cytotoxicity for concentration as high as 360 μg/mL. This demonstrated that the cells were killed by radicals produced from water upon illumination of TiO2 particles and also oxidized by the photogenerated holes in TiO2. Because the size and shape of TiO2 nanoparticles have strongly influence in crystallinity, surface characteristics, electron/hole transportation and charge separation, it is important to be able to control the shape and size of TiO2 nanoparticles to optimize their electronic and chemical properties, resulting in more efficient site-selective reactions. Various functionalized TiO2 nanoparticles have been designed to be used in nanomedicine, as agents for photosensitization or sonosensitization and as drug carriers [126].

In both photodynamic therapy (PDT) and sonodynamic therapy (SDT), nanostructured titanium dioxide is used as an agent to produce reactive oxygen species (ROS). Photodynamic therapy (PDT) is an anti-tumor method in which photosensitive agent is applied and target area is illuminated for the activation of the agent. TiO2 is normally a photocatalyst that produces oxidizing radicals by reacting with water during UV exposure and can damage nearby cells [127, 128]. Titanium dioxide and zinc oxide are two of the most effective photosensitizers for PDT applications. In sonodynamic therapy, TiO2 acts as a sonocatalyst. Studies have shown that TiO2 particles can promote the production of hydroxyl (OH• ) radicals by ultrasound irradiation even in dark conditions [129, 130]. Ultrasound technology has been already used for some cancer therapies, either by generating localized heating using high intensity ultrasound or by activating a drug release using low intensity ultrasound. Ultrasound can penetrate inches below the skin. Therefore, it can be used to activate TiO2 nanoparticles deep below the skin surface.

TiO2 has been considered to be a good material for the design of drug carriers, for the reasons that the shape and size of TiO2 nanoparticles can be engineered to control their electronic and chemical properties, and the surface of TiO2 nanoparticles can be functionalized with various drug molecules [131, 132]. These capabilities bring new opportunities for more efficient site-selective chemistry of TiO2, and form the vehicles for drug delivery applications.
