Acute toxicity of zinc oxide nanoparticles to the rat olfactory system after intranasal instillation Lifeng Gao a,b† , Sheng-Tao Yang c† , Shaorui Li a,b , Yuguang Meng a,b , Haifang Wang d * and Hao Lei a * ABSTRACT: With the increased applications of zinc oxide (ZnO) nanoparticles (NPs), the toxicity of ZnO NPs arouses great concerns from the nano community and the general public. In this study, we report the toxicity of ZnO NPs (30 nm) to the rat olfactory system after intranasal instillation revealed by non-invasive magnetic resonance imaging (MRI). MRI scans were performed on a 4.7-T scanner at 1, 2, 3 and 7 days post-exposure, and the histological changes of the rat olfactory epithelium were evaluated. The influences of chemical component and dispersant of the NPs were also investigated. In addition, an olfactory behavior test was performed. The MRI and histological results indicated that ZnO NPs induced significant damages to the olfactory epithelium, including disruption of the olfactory epithelial structures and inflammation. The destruction of mitochondria in epithelial cells was observed under transmission electron microscopy (TEM), suggesting that the possible toxicological mechanism might involve cellular energy metabolic dysfunction. Further, the lesion of the olfactory epithelium disturbed sniffing behaviors of the treated animals. The results suggest that MRI is potentially useful as a screening tool to assess the consequence of occupational exposure of ZnO NPs. Caution should therefore be taken during the use and disposal of ZnO NPs to prevent the unintended public health impacts. Copyright © 2012 John Wiley & Sons, Ltd. Additional Supporting Information may be found in the online version of this article. Keywords: zinc oxide; nanoparticles; toxicity; olfactory epithelium; magnetic resonance imaging Introduction Biosafety issues of manufactured nanomaterials have recently attracted significant public attention and research interest. Zinc oxide (ZnO) nanoparticles (NPs) are mass-produced nanomaterials and have been widely used in the industrial production of sun- screen, antibacterial reagents, rubber additives and pigments (Kotecha et al., 2006). ZnO NPs have also found applications in the production of solar cells, photocatalysts, transparent conduc- tive films and ultraviolet photodetectors (Beek et al., 2004; Berber et al., 2005; Jing et al., 2004; Jun et al., 2009). Previous studies have demonstrated that ZnO NPs are toxic to microorganisms, cells, plants, aquatic biota and rodents (Brayner et al., 2006; Lin & Xing, 2007; Premanathan et al., 2011; Reddy et al., 2007; Wang et al., 2008a; Zhu et al., 2009). Many previous studies have suggested that the high solubility of ZnO NPs (i.e. the ability to release a large number of free zinc ions) might play an important role in the cytotoxicity. Xia et al. (2008) measured the solubility of ZnO NPs in the culture medium of human lung epithelial cells (BEAS-2B) and rat alveo- lar macrophage cells (RAW 264.7) to be 15.5 and 18.3 mg ml –1 , respectively. The authors attributed the cytotoxicity of ZnO NPs to free Zn 2+ released in the culture media as well as the cellular uptake of ZnO NPs. Brunner et al. (2006) observed that soluble ZnO NPs were more toxic than other insoluble metal oxide NPs (i.e. TiO 2 , CeO 2 and ZrO 2 ) to mesothelioma (MSTO) or 3T3 cells. Our own previous work demonstrated that in NIH/3T3 cells, intracellular Zn 2+ levels reached saturation upon exposure to 240 mM ZnO NPs within 0.5 h, and the solubilization promoted by the microenvironment (i.e. the CO 2 atmosphere under cell culture conditions) aggravated the cytotoxicity of ZnO NPs (Yang et al., 2010). Tannic acids have also been shown to be capable of effectively reducing the bactericidal effects of ZnO NPs to Bacillus subtilis and Pseudomonas putida by forming stable complexes with Zn 2+ ions released from the NPs (Li et al., 2011). The inhalation of ZnO is known to cause pulmonary inflamma- tion (Kuschner et al., 1997; Lam et al., 1985) and systemic effects such as metal fume fever (Fine et al., 1997). According to the Inter- national Commission on Radiological Protection (ICRP, 1994) model, approximately 90% of the inhaled ultrafine particles of ~1-nm diameter deposit in the nasopharyngeal region, whereas 20–30% of 5–10-nm ultrafine particles and 10% of approximately 20-nm ultrafine particles deposit in this region. Therefore, in addition to the lung, there is a significant likelihood that inhaled NPs deposit in the nasal cavity and upper respiratory tract and cause damage in these regions (Oberdörster et al., 2004). *Correspondence to: Hao Lei, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, China. Email: leihao@wipm.ac.cn Haifang Wang, Institute of Nanochemistry and Nanobiology, Shanghai University, China. Email: hwang@shu.edu.cn † These two authors have contributed equally to the manuscript. a Wuhan Center for Magnetic Resonance, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China b Graduate School, Chinese Academy of Sciences, Beijing 100049, China c College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, China d Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, China J. Appl. Toxicol. 2013; 33: 1079–1088 Copyright © 2012 John Wiley & Sons, Ltd. Research Article Received: 11 March 2012, Revised: 24 October 2012, Accepted: 24 October 2012 Published online in Wiley Online Library: 11 January 2013 (wileyonlinelibrary.com) DOI 10.1002/jat.2842 1079