ZnS nanoparticles of different sizes are synthesized employing a simple wet chemical method. These nanoparticles are used to study their impact on the Daphnia sp. through traditional toxicity tests. The percentage of mortality is found to increase initially with increasing nanoparticle concentration or exposure time and is finally found to saturate for higher concentrations or exposure times. Mortality is found to be higher for smaller particles. Hopping frequency and heart rate are also found to increase with increasing nanoparticle exposure time for a fixed nanoparticle concentration. These observations can be attributed to the enhanced surface photooxidation property of the ZnS nanoparticles. Thus the present study will help people to understand the hitherto unknown harmful impact of ZnS nanoparticles on aquatic organisms in the western part of West Bengal (Bankura and Purulia districts), India. 1. Introduction Nanoparticles (1–100？nm) comprise the latest technological advances designed for various applications [1]. Many nanoparticle compounds occur naturally and are used in many vital life processes [1]. Because of their very small size, they have chemical properties that differ from those of their bulk counterparts [2]. As the size of the particle decreases, the percentage of atoms exposed on the surface increases. Because of their increased reactivity, the interaction of the particle with its environment also changes. The change in reactivity increases the potential of nanoparticles for use in industry and pharmaceuticals [2], but this also is the reason for environmental concern. Recently, nanoparticles have come under scrutiny for their potential to cause environmental damage [3, 4]. Because of their increased reactivity, nanoparticles can be detrimental to the environment. Therefore, it is very important to understand the potential impacts of nanoparticles upon environments. The increased production of nanoparticles is making it more likely that such materials will end up in watercourses, either as medical or industrial waste, or when used as ecological tools, with unknown consequences for aquatic life. Therefore, it is gradually becoming very important to identify the most appropriate route of nanotechnology that will preserve the aquatic environment while also advancing industrial technology. Recent studies [5–9] have revealed some of the detrimental effects of different nanoparticles on animals. Inorganic nanoparticles such as TiO2, SiO2, and ZnO had a toxic effect on bacteria, and the presence of light was a significant factor increasing
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