Role of size scale of ZnO nanoparticles and microparticles on toxicity toward bacteria and osteoblast cancer cells Shantikumar Nair Æ Abhilash Sasidharan Æ V. V. Divya Rani Æ Deepthy Menon Æ Seema Nair Æ K. Manzoor Æ Satish Raina Received: 20 November 2007 / Accepted: 18 July 2008 / Published online: 21 August 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The specific role of size scale, surface capping, and aspect ratio of zinc oxide (ZnO) particles on toxicity toward prokaryotic and eukaryotic cells was investigated. ZnO nano and microparticles of controlled size and mor- phology were synthesized by wet chemical methods. Cytotoxicity toward mammalian cells was studied using a human osteoblast cancer cell line and antibacterial activity using Gram-negative bacteria (Escherichia coli) as well as using Gram-positive bacteria (Staphylococcus aureus). Scanning electron microscopy (SEM) was conducted to characterize any visual features of the biocidal action of ZnO. We observed that antibacterial activity increased with reduction in particle size. Toxicity toward the human cancer cell line was considerably higher than previously observed by other researchers on the corresponding pri- mary cells, suggesting selective toxicity of the ZnO to cancer cells. Surface capping was also found to profoundly influence the toxicity of ZnO nanoparticles toward the cancer cell line, with the toxicity of starch-capped ZnO being the lowest. Our results are found to be consistent with a membrane-related mechanism for nanoparticle toxicity toward microbes. 1 Introduction Zinc oxide (ZnO) is currently being investigated as an antibacterial agent in both microscale and nanoscale for- mulations. Results have indicated that ZnO nanoparticles show antibacterial activity [1–11] apparently greater than for microparticles [1]. While the exact mechanisms of the antibacterial action have not yet been clearly elucidated, suggested mechanisms include, the role of reactive oxygen species (ROS) generated on the surface of the particles [2– 4], zinc ion release [5], membrane dysfunction [5, 6], and nanoparticle internalization [7]. The role of ROS needs further study because the influence of light on antibacterial effect related to ROS production is not conclusive. Although one study reported substantial inhibition of bac- terial growth under dark conditions, another showed significant antibacterial effect under dark conditions, both studies being on Escherichia coli. Furthermore, there is no effect of illumination on the antibacterial effect in certain Gram-positive bacteria [8]. Nevertheless, the excellent study by Sawai et al. [4] clearly showing that ROS con- centrations increased with the ZnO content of slurries makes this mechanism worthy of further detailed evalua- tions. With regard to the role of cell membrane versus cell internalization, one transmission electron microscopy study showed that many particles of 10–14 nm ZnO were inter- nalized [7] after overnight exposure, but membrane damage was also observed. The effect of particle size on internalization of ZnO is not known. Another aspect of relevance to completed studies is the role of the medium in which the exposures are carried out. ZnO can be processed through diethylene glycol (DEG) or aqueous routes. In the former case, DEG can cause damage to bacterial mem- branes [7], which complicates the interpretation of the role of ZnO. S. Nair (&) Á A. Sasidharan Á V. V. Divya Rani Á D. Menon Á S. Nair Á K. Manzoor Amrita Center for Nanosciences, Amrita Vishwa Vidyapeetham, Kochi 682 026, Kerala, India e-mail: shantinair@aims.amrita.edu S. Raina Institute of Molecular Medicine, Amrita Institute of Medical Sciences, Kochi 682 026, Kerala, India 123 J Mater Sci: Mater Med (2009) 20:S235–S241 DOI 10.1007/s10856-008-3548-5