Titanium Dioxide: Structure, Impact, and Toxicity - PMC

Titanium dioxide, first manufactured a century ago, is significant in industry due to its chemical inertness, low cost, and availability. The white mineral has a wide range of applications in photocatalysis, in the pharmaceutical industry, and in food processing sectors. Its practical uses stem from its dual feature to act as both a semiconductor and light scatterer. Optical performance is therefore of relevance in understanding how titanium dioxide impacts these industries. Recent breakthroughs are summarised herein, focusing on whether restructuring the surface properties of titanium dioxide either enhances or inhibits its reactivity, depending on the required application. Its recent exposure as a potential carcinogen to humans has been linked to controversies around titanium dioxide&#;s toxicity; this is discussed by illustrating discrepancies between experimental protocols of toxicity assays and their results. In all, it is important to review the latest achievements in fast-growing industries where titanium dioxide prevails, while keeping in mind insights into its disputed toxicity.

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1. Introduction

Titanium dioxide (TiO2) is a white powder extensively used to decontaminate water and food, ensuring environmental and industrial safety, while also serving to protect the skin against harmful radiation [1,2,3,4,5]. To better understand how this metal oxide functions, it is relevant to describe its polymorphic crystal structure [1,2].

1.1. Polymorphism

Titanium dioxide exists in three phases: as rutile [1], anatase [1], and brookite [2]. These crystal phases assemble as octahedra, where six oxygen anions are shared by three titanium (IV) cations [2], hence the formula TiO6/3, which equals TiO2. While expanding in a three-dimensional space, these octahedral units arrange and distort differently for each polymorph, which leads to distinct patterns of crystallinity [2]. As such, the three polymorphs differ in shape, structure [1,2,3], density [1], and refractive index [1]. Rutile has a comparatively higher structural stability [1,2,4,5,6], given that transitions of this phase during synthesis and use are rare [1]. The metastable anatase and brookite can be thermally restructured into the more thermodynamically stable rutile, depending on the mineral&#;s industrial purpose [4,5]. Brookite is a rarely encountered crystal phase and challenging to manufacture in industry [2].

1.3. Nanoarchitecture Achievements

Titanium dioxide nanoparticles are part of the top five nanoparticles used in industry [12], owing to their versatility in applications&#;as photocatalysts [5], in pharmaceuticals [13,14], processed foods [15,16,17,18] and household products [13,17,19], cosmetic white pigments [17,18], fabrics [18], and paints, and sunscreens [19].

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Compared to microparticles, titanium dioxide nanoparticles display enhanced catalytic activity [3,5]. This is because a decrease in size leads to an increased surface area available for catalysis [5,10,12,20,21]. Recent breakthroughs have been achieved in medicinal applications of photocatalysis, by testing nano-titania as an anticancer agent [14,20,21]. Balachandran et al. reported that irradiated TiO2 particles below 20 nm are an efficient &#;photo-killer&#; of pulmonary cancer cells [20]. Valence band holes, with their strong oxidant character, lead to the formation of reactive oxygen species; these will interact with defective cells, causing significant intracellular damage, to finally induce their necrosis [20,21].

Modern breakthroughs are also seen in nanobiotechnology. Although many synthetic routes have been designed for nano-TiO2, their cost is significant and often associated with environmental hazards [22,23,24,25]. In contrast, the "green" syntheses of nano-TiO2 from plants and seeds extracts have been extensively researched, as they prove to be safer, cost-effective, and less toxic [23,24,25]. In general, these methods require TiO2 precursors, such as titanium isopropoxide [24] or titanium trichloride [25], which are centrifuged with natural extracts in aqueous solutions [23,24,25]. Interestingly, nanoparticle formation is accelerated by stabilizing interactions with these natural biocomponents [24]. Lingaraju et al. recently tailored the synthesis of anatase titania nanoparticles from fungal biomass [13]. The publication highlights an improved activity of UV-irradiated nano-TiO2 against the proliferation of microbial pathogens [13]. Moreover, the metal oxide&#;s cytotoxic character was assessed, by monitoring the induction of apoptosis in lung and breast cancer cells [13]. Another medicinal attribute observed is the oxide&#;s role as an anticoagulant, by limiting the formation of blood clots and preventing heart and brain damage [13]. While the interaction mechanisms with cells have yet to be explored, the novel synthesis was facile, cost-effective, and environmentally benign [13].

Titania&#;s nanoform is also valuable in preventing skin cancer caused by overexposure to ultraviolet radiation [26,27]. TiO2 nanoparticles scatter UV photons more efficiently than microparticles [26]. This ability enhances the sun protection factor (SPF) of sun creams, a measure of dermal shielding against photodamage [28,29,30]. Moreover, nano-titania&#;s photoprotective behaviour [28], coupled with its ability to preserve aliments [31], have seen a multitude of applications in the food industry [15,16,31].

Given the complexity of titania&#;s features, this review highlights the impact of the metal oxide&#;s optical properties on the environmental safety sector, and on the pharmaceutical and food industries. Further on, the focus will be on growing concerns in the scientific community regarding titanium dioxide&#;s nanotoxicity. Then, the discrepancies between toxicity assays will be elaborated on.

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