Nanoscale titania Structure

Nanocrystalline titanium dioxide (nano-TiO2) is an important material used in commerce today. When designed appropriately it can generate reactive species (RS) quite efficiently, particularly under ultraviolet (UV) illumination; this feature is exploited in applications ranging from self-cleaning glass to low-cost solar cells. In this study, we characterize the toxicity of this important class of nanomaterials under ambient (e.g., no significant light illumination) conditions in cell culture. Only at relatively high concentrations (100 µg/ml) of nanoscale titania did we observe cytotoxicity and inflammation; these cellular responses exhibited classic dose-response behavior, and the effects increased with time of exposure. The extent to which nanoscale titania affected cellular behavior was not dependent on sample surface area in this study; smaller nanoparticlulate materials had effects comparable to larger nanoparticle materials. What did correlate strongly to cytotoxicity, however, was the phase composition of the nanoscale titania. Anatase TiO2, for example, was 100 times more toxic than an equivalent sample of rutile TiO2. The most cytotoxic nanoparticle samples were also the most effective at generating reactive oxygen species; ex vivo RS species generation under UV illumination correlated well with the observed biological response. These data suggest that nano-TiO2 samples optimized for RS production in photocatalysis are also more likely to generate damaging RS species in cell culture. The result highlights the important role that ex vivo measures of RS production can play in developing screens for cytotoxicity.

..........Studies of size-dependent effects in nanoscale titania are thus intrinsically confounded with changes in sample structure and photoactivity. For these reasons, we focus here on the phase composition of nanoscale titania as the
primary variable. We hypothesize that this parameter will have the greatest effect on toxicity because it is so strongly correlated with chemical reactivity. Future studies will evaluate what effect, if any, grain size has on the toxicity of nanotitania of constant phase composition.


.....Anatase TiO2 with dimensions under 100 nm is a high refractive index material with low scattering and strong absorption of ultraviolet (UV) radiation. This material is currently used in products such as sunscreens and coatings for self-cleaning windows. TiO2, in its larger micron-scale form, is generally inert; however, nano-TiO2 particles under illumination are strong oxidizing agents capable of reacting with a wide range of organic and biological molecules. Their reactivity is generally ascribed to the dissociative adsorption of water to titania surfaces; under illumination, highly reactive carriers trap onto nanotitania surfaces and facilitate the transformation of chemisorbed water into OH-. This process can be leveraged in photocatalysis where nanotitania materials are used to remove organic compounds in water. Interest in the use of titania in photocatalysis has driven extensive work on optimization of nanotitania samples for reactive species (RS) generation; phase composition, even more than nanoparticle size, has emerged as the key material parameter.

Interestingly,TiO2 particles have also long been of interest in particle toxicology, where they have largely served as a negative control material in pulmonary studies; however, some systematic evaluations of how particle structure influences biological response have been reported. Several studies found that ultrafine particles of titania are more toxic than equivalent larger fine particles of the same chemical (e.g., TiO2) composition. However, titania can exist as several different phases (anatase, rutile, and brookite) and control over this phase in nanoparticle systems is challenging. When produced with grain sizes under ~ 10 nm, titania is generally in the more photoactive anatase form, while larger nanocrystalline titania can be generated as either pure anatase or rutile, or a particularly photoactive mixture of the two.

The studies presented here focus specifically on nano-TiO2 spherical nanoparticles of ~ 3–10 nm diameter (specific surface area [SSA] 1/4 110–155 m2/g).The high degree of material control afforded by these materials makes it possible to correlate the photocatalytic and in vitro toxicological behavior and relate both back to the basic phase composition of nano-TiO2 particles. We evaluated the photocatalytic properties of various nano-TiO2 particles (anatase, anatase/rutile, and rutile) and compared these properties with biological endpoints such as cytotoxicity and inflammatory indices of human dermal fibroblasts (HDF) and immortalized human lung epithelial cells (A549). Biological endpoints including lactate dehydrogenase (LDH) release, metabolic activity, and production of inflammation mediators were all evaluated in exposed cells. We found that more photoactive titania materials exhibited more substantial toxicological effects on cells. Both of these results are correlated to the ability for nanotitania to generate RS species, and we confirmed this using ex vivo chemiluminescence studies and the degradation of an azo dye. This paper not only shows the photochemical and cytotoxicological differences in the anatase and rutile phases of TiO2 nanoparticles, but also demonstrates that for these systems ex vivo methods for detecting RS species can correlate well with in vitro toxicological data.




Christie M. Sayes et al (2006); Correlating Nanoscale Titania Structure with Toxicity: A Cytotoxicity and Inflammatory Response Study with Human Dermal Fibroblasts and Human Lung Epithelial Cells

www.oxfordjournals.org



Nanocrystalline titania thin films

The wide-band-gap semiconductor TiO2 is often used as an electron acceptor in composite solar cells (CSC’s) consisting of conjugated polymer acting both as a light absorber and as a hole conductor. Excitons photogenerated in the conjugated polymer can dissociate at the polymer/titania interfaces injecting electrons into the TiO2. Hence high quantum efficiency in a CSC requires a large organic-inorganic interface area, which in turn suggests a need for a high porosity in the titania. In addition to the exciton and charge transport properties of the polymer, CSC efficiency strongly depends on the efficiency with which the injected electrons can pass through the porous nanocrystalline titania film to reach the anode. Electron transport is heavily affected by both the crystalline imperfections in the TiO2 nanocrystals2,3 and the film morphology,4,5 such as effective surface area. Techniques used to explore the transport properties include time of flight,6 photocurrent transients,7 spectroelectrochemical method,8 and intensity-modulated photocurrent spectroscopy.9 The main parameters of interest are electron mobility and diffusion coefficient. Assuming the concentration of free electrons to be in the range of 1018–1019 cm−3, the electron drift mobility is found to be in the range of 10^−7 10^−4 cm2 V−1 s−1.
Consider an intrinsic semiconductor with microstructurally induced random fluctuations of local electronic energy. These fluctuations in energy can be associated with fluctuations of electrostatic potential about an average level, which
we set to zero. The negative fluctuations of the potential can be associated with negatively charged traps for holes. According to the charge neutrality of the intrinsic semiconductor, an average charge density associated with these traps is
totally compensated by that associated with positive fluctuations of the potential. The electron mobility in TiO2 is four orders of magnitude higher than the hole mobility.8 This suggests that localization of electrons on positive fluctuations of
the potential is much weaker than localization of holes on negative fluctuations of the potential, so that holes spend much longer time on localization centers than electrons do. The number of localized electrons is therefore expected to be
much lower than that of localized holes.

Intensity-dependent relaxation of photoconductivity in nanocrystalline titania thin films; 2006
ora.ouls.ox.ac.uk


Nanoparticles safety

Because of their unique physicochemical properties, engineered nanoparticles have the potential to significantly impact respiratory research and medicine by means of improving imaging capability and drug delivery, among other applications. These same properties, however, present potential safety concerns, and there is accumulating evidence to suggest that nanoparticles may exert adverse effects on pulmonary structure and function.


Card J W et al, Pulmonary applications and toxicity of engineered nanoparticles; Cantox Health Sciences International, Mississauga, Ontario, Canada

source: www.oxfordjournals.org