Assessment of Crystal Morphology on Uptake, Particle Dissolution, and Toxicity of Nanoscale Titanium Dioxide on Artemia Salina

Introduction

Our understanding about the environmental and human health effects of nanoscale materials is still in its infancy despite growing research exploring the environmental and biological fate and toxicity of nanomaterials (Scown, van Aerle & Tyler, 2010). This is mainly because the physico-chemical and toxicological properties of materials at nanoscale are a complex phenomenon governed by numerous parameters. Nanomaterials possess very large surfaces (e.g., more atoms per unit area) owing to their extremely small size (1-100 nm), which evidently results in larger contact surfaces and liberation of more toxic elements and ions upon degradation, respectively. Particle size, shape/morphology, surface charge and methods of synthesis also impart significant differences in toxicological properties of nanomaterials (Hu et. al, 2009; Huang, Wu & Aronstam, 2010). Moreover, the physiochemical conditions of test environment (e.g., salinity, pH, temperature, light etc.) and the resilience of model organism may alter observed effects.

Titanium dioxide nanoparticles (n-TiO₂) exhibit photocatalytic and antibacterial properties under UV-light (Lai et al., 2008). In recent years, n-TiO₂ have been extensively used in various consumer products, including sunscreens, toothpaste, food additives, paints, surface coatings, water disinfection and degradation of pollutants in air and soil (Nowack & Bucheli, 2007; Battin et al., 2009; Weir et al., 2012; Jovanovi, 2015). n-TiO₂ is considered an aquatic pollutant due to numerous exterior uses and applications in water treatment. Nevertheless, the information about safety of n-TiO₂ to aquatic environments and species is controversial though it is among the earliest nanoscale compounds investigated extensively so far (Jovanovi, 2015). Some groups did not observe any adverse effects on different biological species (Lovern & Klaper, 2006; Lai et al., 2008; Aruoja et al., 2009, Zhu et al., 2011, Fang et al., 2015), while some others reported dose dependent toxic effects (Heinlaan et al., 2008; Zhu et al., 2010; Ates et al., 2013a; Mansfield et al., 2015). For instance, n-TiO₂ was reported to be more toxic to microalgae Pseudokirchneriella subcapitata (LC₅₀ = 5.83 mg L^–1) than bulk TiO₂ (LC₅₀ = 35.9 mg L^–1) (Aruoja et al., 2009). In contrast, no apparent toxicity was observed from n-TiO₂ to crustaceans Daphnia magna and Thamnocephalus paltyurus, and the bacteria Vibrio fischeri (Heinlaan et al., 2008), but previously exposed Daphnia magna offsprings were found more sensitive to n-TiO₂ (Bundschuh et al., 2012). Similarly, aqueous suspensions of n-TiO₂ were not acutely toxic but induced oxidative stress on marine abalone (Haliotis diversicolor supertexta) (Zhu et al., 2011). Japanese medaka (Oryzias latipes) embryos showed premature hatching and high mortality from chronic exposure to n-TiO₂ (Paterson et al., 2011). Toxicity to rainbow trout (Oncorhynchus mykiss) varied with the route of exposure (Handy et al., 2008). Direct exposure to n-TiO₂ colloids caused severe adverse effects whereas no acute toxicity was detected from dietary exposure.